diff --git a/Cargo.lock b/Cargo.lock
index edca5ee..2ef4d1f 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -679,6 +679,7 @@ dependencies = [
  "thiserror 2.0.18",
  "tokio",
  "tokio-util",
+ "tracing",
 ]
 
 [[package]]
@@ -723,12 +724,14 @@ dependencies = [
  "async-stream",
  "async-trait",
  "bytes",
+ "dirs",
  "eventsource-stream",
  "futures",
  "harness-core",
  "reqwest",
  "serde",
  "serde_json",
+ "tempfile",
  "thiserror 2.0.18",
  "tokio",
  "tokio-util",
diff --git a/crates/harness-app/Cargo.toml b/crates/harness-app/Cargo.toml
index ceb4f67..2eb3ea2 100644
--- a/crates/harness-app/Cargo.toml
+++ b/crates/harness-app/Cargo.toml
@@ -14,6 +14,7 @@ tokio = { workspace = true }
 tokio-util = { workspace = true }
 dirs = { workspace = true }
 thiserror = { workspace = true }
+tracing = { workspace = true }
 
 [dev-dependencies]
 tempfile = { workspace = true }
diff --git a/crates/harness-app/src/lib.rs b/crates/harness-app/src/lib.rs
index 999bcb3..44073c2 100644
--- a/crates/harness-app/src/lib.rs
+++ b/crates/harness-app/src/lib.rs
@@ -19,7 +19,7 @@ use harness_core::tool::ToolRegistry;
 use harness_core::types::{
     Message, MessageId, ModelRef, Part, PartBody, PartId, Session, SessionId,
 };
-use harness_providers::{AnthropicProvider, ProviderRegistry};
+use harness_providers::{AnthropicProvider, ModelCatalog, OpenAiProvider, ProviderRegistry};
 use tokio::task::JoinHandle;
 use tokio_util::sync::CancellationToken;
 
@@ -83,6 +83,7 @@ struct EngineInner {
     permissions: Arc<PermissionService>,
     tools: ToolRegistry,
     providers: ProviderRegistry,
+    catalog: ModelCatalog,
     cwd: PathBuf,
     data_dir: PathBuf,
     runs: Mutex<HashMap<SessionId, RunHandle>>,
@@ -97,7 +98,9 @@ impl EngineHandle {
     pub fn init(cwd: PathBuf) -> Result<Self, AppError> {
         let path = db_path(&cwd);
         let store = Store::open(&path)?;
-        Self::new(cwd, store)
+        let handle = Self::new(cwd, store)?;
+        handle.spawn_catalog_refresh();
+        Ok(handle)
     }
 
     /// In-memory store — useful for tests and ephemeral sessions.
@@ -122,12 +125,31 @@ impl EngineHandle {
         {
             providers.register(Arc::new(AnthropicProvider::new(key)));
         }
+        if let Some(key) = config
+            .providers
+            .get("openai")
+            .and_then(|p| p.api_key.clone())
+        {
+            let provider = match config
+                .providers
+                .get("openai")
+                .and_then(|p| p.base_url.clone())
+            {
+                Some(base_url) => OpenAiProvider::with_base_url(key, base_url),
+                None => OpenAiProvider::new(key),
+            };
+            providers.register(Arc::new(provider));
+        }
 
         let data_dir = dirs::data_dir()
             .unwrap_or_else(std::env::temp_dir)
             .join("ai-harness")
             .join("tool-output");
 
+        // No network at construction: use the cached copy if fresh, else the baked snapshot.
+        // `init` warms the cache in the background for the next launch.
+        let catalog = ModelCatalog::load_cached_or_baked(&ModelCatalog::default_cache_path());
+
         Ok(Self {
             inner: Arc::new(EngineInner {
                 config,
@@ -136,6 +158,7 @@ impl EngineHandle {
                 permissions,
                 tools,
                 providers,
+                catalog,
                 cwd,
                 data_dir,
                 runs: Mutex::new(HashMap::new()),
@@ -143,6 +166,15 @@ impl EngineHandle {
         })
     }
 
+    /// Fire-and-forget refresh of the models.dev cache so the next launch has current pricing.
+    fn spawn_catalog_refresh(&self) {
+        tokio::spawn(async {
+            if let Err(e) = ModelCatalog::refresh_default_cache().await {
+                tracing::debug!(error = %e, "models.dev refresh failed; using cached/baked metadata");
+            }
+        });
+    }
+
     pub fn bus(&self) -> EventBus {
         self.inner.bus.clone()
     }
@@ -239,6 +271,7 @@ impl EngineHandle {
             temperature: None,
             max_steps: DEFAULT_MAX_STEPS,
             instructions: self.inner.config.instructions.clone(),
+            cost: Some(self.inner.catalog.cost(provider_id, model_id)),
         };
 
         // Reserve the run slot *before* spawning. If we inserted after spawning, a run that
diff --git a/crates/harness-core/src/engine/loop.rs b/crates/harness-core/src/engine/loop.rs
index e495bba..2b8423a 100644
--- a/crates/harness-core/src/engine/loop.rs
+++ b/crates/harness-core/src/engine/loop.rs
@@ -20,6 +20,33 @@ pub struct RunConfig {
     pub temperature: Option<f32>,
     pub max_steps: u32,
     pub instructions: Vec<String>,
+    /// Pricing for `model`, from models.dev metadata. `None` leaves cost at 0.
+    pub cost: Option<crate::types::ModelCost>,
+}
+
+/// Adds a step's usage/cost onto the persisted session and republishes it. Cost accounting is
+/// best-effort: a store error here is logged, not surfaced as a run failure.
+async fn accumulate_session_usage(
+    ctx: &StepContext,
+    usage: &crate::types::TokenUsage,
+    cost: f64,
+    now: i64,
+) {
+    match ctx.store.session(ctx.session_id.clone()).await {
+        Ok(Some(mut session)) => {
+            session.usage.add(usage);
+            session.cost += cost;
+            session.updated_at = now;
+            if let Err(e) = ctx.store.upsert_session(session.clone()).await {
+                tracing::warn!(error = %e, "failed to persist session usage");
+                return;
+            }
+            ctx.bus
+                .publish(crate::event::AppEvent::SessionUpdated { session });
+        }
+        Ok(None) => {}
+        Err(e) => tracing::warn!(error = %e, "failed to load session for usage accounting"),
+    }
 }
 
 /// opencode's exit condition: keep stepping while the last assistant turn asked for more
@@ -221,17 +248,24 @@ pub async fn run_session(
             &ctx,
             run_config.model.clone(),
             &run_config.agent_name,
+            run_config.cost,
             &mut doomloop,
         )
         .await;
 
         match step {
-            Ok(outcome) if outcome.aborted => return RunOutcome::Aborted,
-            Ok(outcome) => match outcome.result {
-                StepResult::Continue => continue,
-                StepResult::Stop => return RunOutcome::Stopped,
-                StepResult::Compact => return RunOutcome::Stopped, // stub until M6
-            },
+            Ok(outcome) if outcome.aborted => {
+                accumulate_session_usage(&ctx, &outcome.usage, outcome.cost, now_fn()).await;
+                return RunOutcome::Aborted;
+            }
+            Ok(outcome) => {
+                accumulate_session_usage(&ctx, &outcome.usage, outcome.cost, now_fn()).await;
+                match outcome.result {
+                    StepResult::Continue => continue,
+                    StepResult::Stop => return RunOutcome::Stopped,
+                    StepResult::Compact => return RunOutcome::Stopped, // stub until M6
+                }
+            }
             Err(step_err) if matches!(step_err.source, ProviderError::Cancelled) => {
                 return RunOutcome::Aborted;
             }
@@ -422,11 +456,28 @@ mod tests {
             temperature: None,
             max_steps: 10,
             instructions: Vec::new(),
+            // $3/1M input, $15/1M output.
+            cost: Some(crate::types::ModelCost {
+                input: 3.0,
+                output: 15.0,
+                ..Default::default()
+            }),
         };
 
         let outcome = run_session(Arc::new(provider), ctx, &run_config, || 2).await;
         assert!(matches!(outcome, RunOutcome::Stopped));
 
+        // Both steps' usage and cost accumulate onto the session.
+        let session = store.session(session_id.clone()).await.unwrap().unwrap();
+        assert_eq!(session.usage.input, 30);
+        assert_eq!(session.usage.output, 13);
+        // (10*3 + 5*15)/1e6 + (20*3 + 8*15)/1e6 = 0.000105 + 0.00018
+        assert!(
+            (session.cost - 0.000_285).abs() < 1e-9,
+            "cost = {}",
+            session.cost
+        );
+
         let messages = store.messages(session_id.clone()).await.unwrap();
         assert_eq!(
             messages.len(),
@@ -517,6 +568,7 @@ mod tests {
             temperature: None,
             max_steps: 10,
             instructions: Vec::new(),
+            cost: None,
         };
 
         let outcome = run_session(Arc::new(provider), ctx, &run_config, || 2).await;
diff --git a/crates/harness-core/src/engine/processor.rs b/crates/harness-core/src/engine/processor.rs
index a4ff597..bdc2839 100644
--- a/crates/harness-core/src/engine/processor.rs
+++ b/crates/harness-core/src/engine/processor.rs
@@ -26,6 +26,8 @@ pub struct StepOutcome {
     pub result: StepResult,
     pub message_id: Option<MessageId>,
     pub usage: TokenUsage,
+    /// Dollar cost of this step's usage (0.0 when no pricing is available).
+    pub cost: f64,
     pub aborted: bool,
 }
 
@@ -513,6 +515,7 @@ impl<'a> Run<'a> {
         &mut self,
         reason: FinishReason,
         usage: TokenUsage,
+        cost: f64,
     ) -> Result<StepResult, ProviderError> {
         let part = Part {
             id: PartId::new(),
@@ -521,7 +524,7 @@ impl<'a> Run<'a> {
             idx: self.next_idx,
             body: PartBody::StepFinish {
                 usage,
-                cost: 0.0,
+                cost,
                 reason: reason.clone(),
             },
         };
@@ -567,10 +570,12 @@ pub async fn process_step(
     ctx: &StepContext,
     model: crate::types::ModelRef,
     agent: &str,
+    cost: Option<crate::types::ModelCost>,
     doomloop: &mut DoomLoopGuard,
 ) -> Result<StepOutcome, StepError> {
     let mut run = Run::new(ctx);
     let mut usage = TokenUsage::default();
+    let mut step_cost = 0.0;
     let mut result = StepResult::Stop;
 
     loop {
@@ -582,6 +587,7 @@ pub async fn process_step(
                     result: StepResult::Stop,
                     message_id: run.assistant.as_ref().map(|m| m.id.clone()),
                     usage,
+                    cost: step_cost,
                     aborted: true,
                 });
             }
@@ -630,7 +636,8 @@ pub async fn process_step(
                 usage: finish_usage,
             } => {
                 usage = finish_usage;
-                match run.on_finish(reason, finish_usage).await {
+                step_cost = cost.map(|c| c.cost_of(&finish_usage)).unwrap_or(0.0);
+                match run.on_finish(reason, finish_usage, step_cost).await {
                     Ok(r) => {
                         result = r;
                         Ok(())
@@ -652,6 +659,7 @@ pub async fn process_step(
         result,
         message_id: run.assistant.as_ref().map(|m| m.id.clone()),
         usage,
+        cost: step_cost,
         aborted: false,
     })
 }
diff --git a/crates/harness-core/src/store/actor.rs b/crates/harness-core/src/store/actor.rs
index 098c550..05671de 100644
--- a/crates/harness-core/src/store/actor.rs
+++ b/crates/harness-core/src/store/actor.rs
@@ -13,6 +13,7 @@ pub enum StoreCmd {
     UpsertSession(Session, Reply<()>),
     UpsertMessage(Message, Reply<()>),
     UpsertPart(Part, Reply<()>),
+    Session(SessionId, Reply<Option<Session>>),
     Sessions(Reply<Vec<Session>>),
     Messages(SessionId, Reply<Vec<Message>>),
     Parts(MessageId, Reply<Vec<Part>>),
@@ -69,6 +70,15 @@ fn upsert_part(conn: &Connection, part: &Part) -> Result<(), StoreError> {
     Ok(())
 }
 
+fn get_session(conn: &Connection, id: &SessionId) -> Result<Option<Session>, StoreError> {
+    let mut stmt = conn.prepare("SELECT data FROM session WHERE id = ?1")?;
+    let mut rows = stmt.query_map(params![id.as_ref()], |row| row.get::<_, String>(0))?;
+    match rows.next() {
+        Some(data) => Ok(Some(serde_json::from_str(&data?)?)),
+        None => Ok(None),
+    }
+}
+
 fn list_sessions(conn: &Connection) -> Result<Vec<Session>, StoreError> {
     let mut stmt = conn.prepare("SELECT data FROM session ORDER BY id")?;
     let rows = stmt
@@ -116,6 +126,9 @@ pub fn run(conn: Connection, mut rx: tokio::sync::mpsc::Receiver<StoreCmd>) {
             StoreCmd::UpsertPart(part, reply) => {
                 let _ = reply.send(upsert_part(&conn, &part));
             }
+            StoreCmd::Session(id, reply) => {
+                let _ = reply.send(get_session(&conn, &id));
+            }
             StoreCmd::Sessions(reply) => {
                 let _ = reply.send(list_sessions(&conn));
             }
diff --git a/crates/harness-core/src/store/api.rs b/crates/harness-core/src/store/api.rs
index 0a6b29f..3b00129 100644
--- a/crates/harness-core/src/store/api.rs
+++ b/crates/harness-core/src/store/api.rs
@@ -77,6 +77,11 @@ impl Store {
         self.call(|reply| StoreCmd::UpsertPart(part, reply)).await
     }
 
+    pub async fn session(&self, session_id: SessionId) -> Result<Option<Session>, StoreError> {
+        self.call(|reply| StoreCmd::Session(session_id, reply))
+            .await
+    }
+
     pub async fn sessions(&self) -> Result<Vec<Session>, StoreError> {
         self.call(StoreCmd::Sessions).await
     }
diff --git a/crates/harness-core/src/types/mod.rs b/crates/harness-core/src/types/mod.rs
index ed9e940..24c4b6d 100644
--- a/crates/harness-core/src/types/mod.rs
+++ b/crates/harness-core/src/types/mod.rs
@@ -6,6 +6,6 @@ pub mod session;
 
 pub use ids::{MessageId, PartId, SessionId};
 pub use message::{Message, MessageError, Role};
-pub use model::{ModelInfo, ModelRef, TokenUsage};
+pub use model::{ModelCost, ModelInfo, ModelRef, TokenUsage};
 pub use part::{Part, PartBody, ToolState};
 pub use session::Session;
diff --git a/crates/harness-core/src/types/model.rs b/crates/harness-core/src/types/model.rs
index ac2c491..675dac6 100644
--- a/crates/harness-core/src/types/model.rs
+++ b/crates/harness-core/src/types/model.rs
@@ -34,11 +34,71 @@ impl TokenUsage {
     }
 }
 
-// Full cost/context-limit metadata is populated by harness-providers (models.dev) in M1/M3;
-// this placeholder only carries what harness-core needs to key on.
+/// Per-model pricing in USD per **one million** tokens. Populated from models.dev metadata
+/// by `harness-providers`.
+#[derive(Debug, Clone, Copy, Default, PartialEq, Serialize, Deserialize)]
+pub struct ModelCost {
+    pub input: f64,
+    pub output: f64,
+    pub cache_read: f64,
+    pub cache_write: f64,
+}
+
+impl ModelCost {
+    /// Dollar cost of a usage sample. Reasoning tokens are billed within `output` by the
+    /// providers we support (OpenAI reports them as a subset of `output_tokens`; Anthropic
+    /// counts thinking in output), so they are intentionally not charged separately here.
+    pub fn cost_of(&self, usage: &TokenUsage) -> f64 {
+        let per_million = |tokens: u64, rate: f64| (tokens as f64) * rate / 1_000_000.0;
+        per_million(usage.input, self.input)
+            + per_million(usage.output, self.output)
+            + per_million(usage.cache_read, self.cache_read)
+            + per_million(usage.cache_write, self.cache_write)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn cost_of_sums_components_per_million_excluding_reasoning() {
+        let cost = ModelCost {
+            input: 3.0,
+            output: 15.0,
+            cache_read: 0.30,
+            cache_write: 3.75,
+        };
+        let usage = TokenUsage {
+            input: 1_000_000,
+            output: 1_000_000,
+            reasoning: 500_000, // billed within output — must not add extra cost
+            cache_read: 1_000_000,
+            cache_write: 1_000_000,
+        };
+        // 3 + 15 + 0.30 + 3.75, with reasoning contributing nothing.
+        assert!((cost.cost_of(&usage) - 22.05).abs() < 1e-9);
+    }
+
+    #[test]
+    fn default_cost_is_zero() {
+        assert_eq!(ModelCost::default().cost_of(&TokenUsage::default()), 0.0);
+    }
+}
+
+/// Model metadata, keyed on `model`. Cost/limits are populated from models.dev by
+/// `harness-providers`; `reasoning`/`tool_call`/`attachment` are capability flags.
 #[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
 pub struct ModelInfo {
     pub model: ModelRef,
     pub context_limit: u64,
     pub output_limit: u64,
+    #[serde(default)]
+    pub cost: ModelCost,
+    #[serde(default)]
+    pub reasoning: bool,
+    #[serde(default)]
+    pub tool_call: bool,
+    #[serde(default)]
+    pub attachment: bool,
 }
diff --git a/crates/harness-providers/Cargo.toml b/crates/harness-providers/Cargo.toml
index 9972f4c..ea6b8a7 100644
--- a/crates/harness-providers/Cargo.toml
+++ b/crates/harness-providers/Cargo.toml
@@ -18,9 +18,11 @@ serde = { workspace = true }
 serde_json = { workspace = true }
 thiserror = { workspace = true }
 tracing = { workspace = true }
+dirs = { workspace = true }
 
 [dev-dependencies]
 tokio = { workspace = true, features = ["test-util", "macros"] }
+tempfile = { workspace = true }
 
 [lints]
 workspace = true
diff --git a/crates/harness-providers/assets/models-snapshot.json b/crates/harness-providers/assets/models-snapshot.json
new file mode 100644
index 0000000..932efa7
--- /dev/null
+++ b/crates/harness-providers/assets/models-snapshot.json
@@ -0,0 +1,67 @@
+{
+  "anthropic": {
+    "id": "anthropic",
+    "name": "Anthropic",
+    "models": {
+      "claude-sonnet-4-5": {
+        "id": "claude-sonnet-4-5",
+        "reasoning": true,
+        "tool_call": true,
+        "attachment": true,
+        "cost": { "input": 3, "output": 15, "cache_read": 0.3, "cache_write": 3.75 },
+        "limit": { "context": 200000, "output": 64000 }
+      },
+      "claude-opus-4-1": {
+        "id": "claude-opus-4-1",
+        "reasoning": true,
+        "tool_call": true,
+        "attachment": true,
+        "cost": { "input": 15, "output": 75, "cache_read": 1.5, "cache_write": 18.75 },
+        "limit": { "context": 200000, "output": 32000 }
+      },
+      "claude-haiku-4-5": {
+        "id": "claude-haiku-4-5",
+        "reasoning": true,
+        "tool_call": true,
+        "attachment": true,
+        "cost": { "input": 1, "output": 5, "cache_read": 0.1, "cache_write": 1.25 },
+        "limit": { "context": 200000, "output": 64000 }
+      }
+    }
+  },
+  "openai": {
+    "id": "openai",
+    "name": "OpenAI",
+    "models": {
+      "gpt-4o": {
+        "id": "gpt-4o",
+        "tool_call": true,
+        "attachment": true,
+        "cost": { "input": 2.5, "output": 10, "cache_read": 1.25 },
+        "limit": { "context": 128000, "output": 16384 }
+      },
+      "gpt-4o-mini": {
+        "id": "gpt-4o-mini",
+        "tool_call": true,
+        "attachment": true,
+        "cost": { "input": 0.15, "output": 0.6, "cache_read": 0.075 },
+        "limit": { "context": 128000, "output": 16384 }
+      },
+      "gpt-5": {
+        "id": "gpt-5",
+        "reasoning": true,
+        "tool_call": true,
+        "attachment": true,
+        "cost": { "input": 1.25, "output": 10, "cache_read": 0.125 },
+        "limit": { "context": 400000, "output": 128000 }
+      },
+      "o3": {
+        "id": "o3",
+        "reasoning": true,
+        "tool_call": true,
+        "cost": { "input": 2, "output": 8, "cache_read": 0.5 },
+        "limit": { "context": 200000, "output": 100000 }
+      }
+    }
+  }
+}
diff --git a/crates/harness-providers/src/auth.rs b/crates/harness-providers/src/auth.rs
new file mode 100644
index 0000000..4340081
--- /dev/null
+++ b/crates/harness-providers/src/auth.rs
@@ -0,0 +1,228 @@
+//! Credential storage for providers that authenticate outside the config file.
+//!
+//! A single JSON file (`~/.local/share/ai-harness/auth.json`, mode `0600`) keyed by provider
+//! id. OAuth providers (Copilot) store the token triple and refresh it in place; API-key
+//! providers store the bare key. Config-supplied keys take precedence over this file — this
+//! is only for interactively-obtained credentials.
+
+use std::collections::HashMap;
+use std::io;
+use std::path::{Path, PathBuf};
+
+use serde::{Deserialize, Serialize};
+
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
+#[serde(tag = "type", rename_all = "snake_case")]
+pub enum AuthRecord {
+    /// OAuth credentials. `expires` is a unix-ms timestamp; `0` means the token never expires
+    /// (opencode's direct-Bearer Copilot mode).
+    OAuth {
+        access: String,
+        refresh: String,
+        expires: i64,
+    },
+    Api {
+        key: String,
+    },
+}
+
+impl AuthRecord {
+    /// True when an OAuth token is at or past `expires` (with a safety skew), given `now_ms`.
+    /// API keys and never-expiring OAuth tokens (`expires == 0`) are never considered expired.
+    pub fn is_expired(&self, now_ms: i64, skew_ms: i64) -> bool {
+        match self {
+            AuthRecord::OAuth { expires, .. } if *expires > 0 => now_ms + skew_ms >= *expires,
+            _ => false,
+        }
+    }
+}
+
+/// Read/modify/write access to the auth file. Cheap to construct; every operation reloads so
+/// concurrent writers (a refresh in one provider, a login in another) don't clobber each other.
+#[derive(Debug, Clone)]
+pub struct AuthStorage {
+    path: PathBuf,
+}
+
+impl AuthStorage {
+    pub fn new(path: PathBuf) -> Self {
+        Self { path }
+    }
+
+    /// `~/.local/share/ai-harness/auth.json` (falling back to the temp dir if there is no
+    /// data directory).
+    pub fn default_path() -> PathBuf {
+        dirs::data_dir()
+            .unwrap_or_else(std::env::temp_dir)
+            .join("ai-harness")
+            .join("auth.json")
+    }
+
+    pub fn with_default_path() -> Self {
+        Self::new(Self::default_path())
+    }
+
+    /// Loads all records. A missing file yields an empty map; a corrupt file is an error.
+    pub fn load(&self) -> io::Result<HashMap<String, AuthRecord>> {
+        match std::fs::read(&self.path) {
+            Ok(bytes) => serde_json::from_slice(&bytes)
+                .map_err(|e| io::Error::new(io::ErrorKind::InvalidData, e)),
+            Err(e) if e.kind() == io::ErrorKind::NotFound => Ok(HashMap::new()),
+            Err(e) => Err(e),
+        }
+    }
+
+    pub fn get(&self, provider: &str) -> io::Result<Option<AuthRecord>> {
+        Ok(self.load()?.remove(provider))
+    }
+
+    pub fn set(&self, provider: &str, record: AuthRecord) -> io::Result<()> {
+        let mut records = self.load()?;
+        records.insert(provider.to_string(), record);
+        self.write(&records)
+    }
+
+    pub fn remove(&self, provider: &str) -> io::Result<()> {
+        let mut records = self.load()?;
+        if records.remove(provider).is_some() {
+            self.write(&records)?;
+        }
+        Ok(())
+    }
+
+    /// Serializes `records` and writes them with `0600` permissions via a temp-file rename so
+    /// a partial write can never leave a truncated auth file behind.
+    fn write(&self, records: &HashMap<String, AuthRecord>) -> io::Result<()> {
+        if let Some(parent) = self.path.parent() {
+            std::fs::create_dir_all(parent)?;
+        }
+        let json = serde_json::to_vec_pretty(records)
+            .map_err(|e| io::Error::new(io::ErrorKind::InvalidData, e))?;
+
+        let tmp = self.path.with_extension("json.tmp");
+        std::fs::write(&tmp, &json)?;
+        set_owner_only(&tmp)?;
+        std::fs::rename(&tmp, &self.path)?;
+        Ok(())
+    }
+}
+
+#[cfg(unix)]
+fn set_owner_only(path: &Path) -> io::Result<()> {
+    use std::os::unix::fs::PermissionsExt;
+    std::fs::set_permissions(path, std::fs::Permissions::from_mode(0o600))
+}
+
+#[cfg(not(unix))]
+fn set_owner_only(_path: &Path) -> io::Result<()> {
+    Ok(())
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn storage() -> (tempfile::TempDir, AuthStorage) {
+        let dir = tempfile::tempdir().unwrap();
+        let path = dir.path().join("nested").join("auth.json");
+        (dir, AuthStorage::new(path))
+    }
+
+    #[test]
+    fn missing_file_loads_empty() {
+        let (_dir, store) = storage();
+        assert!(store.load().unwrap().is_empty());
+        assert_eq!(store.get("anthropic").unwrap(), None);
+    }
+
+    #[test]
+    fn set_get_roundtrip_and_overwrite() {
+        let (_dir, store) = storage();
+        store
+            .set("anthropic", AuthRecord::Api { key: "k1".into() })
+            .unwrap();
+        assert_eq!(
+            store.get("anthropic").unwrap(),
+            Some(AuthRecord::Api { key: "k1".into() })
+        );
+
+        store
+            .set("anthropic", AuthRecord::Api { key: "k2".into() })
+            .unwrap();
+        assert_eq!(
+            store.get("anthropic").unwrap(),
+            Some(AuthRecord::Api { key: "k2".into() })
+        );
+    }
+
+    #[test]
+    fn multiple_providers_coexist() {
+        let (_dir, store) = storage();
+        store
+            .set("openai", AuthRecord::Api { key: "sk".into() })
+            .unwrap();
+        store
+            .set(
+                "github-copilot",
+                AuthRecord::OAuth {
+                    access: "a".into(),
+                    refresh: "r".into(),
+                    expires: 0,
+                },
+            )
+            .unwrap();
+        let all = store.load().unwrap();
+        assert_eq!(all.len(), 2);
+        assert!(all.contains_key("openai"));
+        assert!(all.contains_key("github-copilot"));
+    }
+
+    #[test]
+    fn remove_deletes_only_named_provider() {
+        let (_dir, store) = storage();
+        store
+            .set("openai", AuthRecord::Api { key: "sk".into() })
+            .unwrap();
+        store
+            .set("anthropic", AuthRecord::Api { key: "an".into() })
+            .unwrap();
+        store.remove("openai").unwrap();
+        assert_eq!(store.get("openai").unwrap(), None);
+        assert!(store.get("anthropic").unwrap().is_some());
+        // Removing an absent provider is a no-op, not an error.
+        store.remove("openai").unwrap();
+    }
+
+    #[test]
+    fn oauth_expiry_respects_skew_and_never_expires() {
+        let never = AuthRecord::OAuth {
+            access: "a".into(),
+            refresh: "r".into(),
+            expires: 0,
+        };
+        assert!(!never.is_expired(i64::MAX, 0));
+
+        let expiring = AuthRecord::OAuth {
+            access: "a".into(),
+            refresh: "r".into(),
+            expires: 1_000,
+        };
+        assert!(!expiring.is_expired(800, 120));
+        assert!(expiring.is_expired(880, 120)); // within skew window
+        assert!(expiring.is_expired(1_000, 0));
+
+        assert!(!AuthRecord::Api { key: "k".into() }.is_expired(i64::MAX, 0));
+    }
+
+    #[cfg(unix)]
+    #[test]
+    fn file_is_written_owner_only() {
+        use std::os::unix::fs::PermissionsExt;
+        let (_dir, store) = storage();
+        store
+            .set("openai", AuthRecord::Api { key: "sk".into() })
+            .unwrap();
+        let mode = std::fs::metadata(&store.path).unwrap().permissions().mode();
+        assert_eq!(mode & 0o777, 0o600);
+    }
+}
diff --git a/crates/harness-providers/src/codec/mod.rs b/crates/harness-providers/src/codec/mod.rs
index e529997..daffdf8 100644
--- a/crates/harness-providers/src/codec/mod.rs
+++ b/crates/harness-providers/src/codec/mod.rs
@@ -1 +1,3 @@
 pub mod anthropic;
+pub mod openai_chat;
+pub mod openai_responses;
diff --git a/crates/harness-providers/src/codec/openai_chat.rs b/crates/harness-providers/src/codec/openai_chat.rs
new file mode 100644
index 0000000..a9310eb
--- /dev/null
+++ b/crates/harness-providers/src/codec/openai_chat.rs
@@ -0,0 +1,502 @@
+//! Request builder + SSE decoder for OpenAI's `/chat/completions` streaming API.
+//!
+//! Also used as the fallback codec for OpenAI-compatible endpoints (including Copilot models
+//! whose `supported_endpoints` list `/chat/completions`).
+
+use std::collections::HashMap;
+
+use async_stream::try_stream;
+use eventsource_stream::Eventsource;
+use futures::Stream;
+use harness_core::llm::{
+    FinishReason, LlmEvent, LlmEventStream, LlmRequest, ProviderError, ReasoningEffort, Role,
+    WireContent,
+};
+use harness_core::types::TokenUsage;
+use serde_json::{json, Value};
+
+fn effort_str(effort: ReasoningEffort) -> &'static str {
+    match effort {
+        ReasoningEffort::Low => "low",
+        ReasoningEffort::Medium => "medium",
+        ReasoningEffort::High => "high",
+    }
+}
+
+fn role_str(role: Role) -> &'static str {
+    match role {
+        Role::System => "system",
+        Role::User => "user",
+        Role::Assistant => "assistant",
+        Role::Tool => "tool",
+    }
+}
+
+/// Flattens our grouped `WireMessage`s into the flat chat-completions message list. Tool
+/// results become their own `role: "tool"` messages (one per result), which is what the API
+/// expects regardless of how the engine grouped them.
+fn build_messages(system: &[String], messages: &[harness_core::llm::WireMessage]) -> Vec<Value> {
+    let mut out: Vec<Value> = Vec::new();
+
+    if !system.is_empty() {
+        out.push(json!({"role": "system", "content": system.join("\n\n")}));
+    }
+
+    for m in messages {
+        // Tool results are always emitted as standalone `tool` messages.
+        for c in &m.content {
+            if let WireContent::ToolResult {
+                call_id, output, ..
+            } = c
+            {
+                out.push(json!({
+                    "role": "tool",
+                    "tool_call_id": call_id,
+                    "content": output,
+                }));
+            }
+        }
+
+        let mut text = String::new();
+        let mut images: Vec<Value> = Vec::new();
+        let mut tool_calls: Vec<Value> = Vec::new();
+        for c in &m.content {
+            match c {
+                WireContent::Text { text: t } => {
+                    if !text.is_empty() {
+                        text.push('\n');
+                    }
+                    text.push_str(t);
+                }
+                WireContent::ToolCall {
+                    call_id,
+                    name,
+                    input,
+                } => tool_calls.push(json!({
+                    "id": call_id,
+                    "type": "function",
+                    "function": {"name": name, "arguments": input.to_string()},
+                })),
+                WireContent::Image { mime_type, data } => images.push(json!({
+                    "type": "image_url",
+                    "image_url": {"url": format!("data:{mime_type};base64,{data}")},
+                })),
+                WireContent::ToolResult { .. } => {} // handled above
+            }
+        }
+
+        // A message that carried nothing but tool results contributes no further entry.
+        if text.is_empty() && images.is_empty() && tool_calls.is_empty() {
+            continue;
+        }
+
+        let mut msg = json!({"role": role_str(m.role)});
+        if images.is_empty() {
+            msg["content"] = json!(text);
+        } else {
+            let mut parts = vec![json!({"type": "text", "text": text})];
+            parts.extend(images);
+            msg["content"] = json!(parts);
+        }
+        if !tool_calls.is_empty() {
+            msg["tool_calls"] = json!(tool_calls);
+        }
+        out.push(msg);
+    }
+
+    out
+}
+
+pub fn build_request(req: &LlmRequest) -> Value {
+    let mut body = json!({
+        "model": req.model,
+        "messages": build_messages(&req.system, &req.messages),
+        "stream": true,
+        "stream_options": {"include_usage": true},
+    });
+
+    if !req.tools.is_empty() {
+        body["tools"] = Value::Array(
+            req.tools
+                .iter()
+                .map(|t| {
+                    json!({
+                        "type": "function",
+                        "function": {
+                            "name": t.name,
+                            "description": t.description,
+                            "parameters": t.parameters,
+                        },
+                    })
+                })
+                .collect(),
+        );
+    }
+    if let Some(temp) = req.temperature {
+        body["temperature"] = json!(temp);
+    }
+    if let Some(max) = req.max_tokens {
+        body["max_completion_tokens"] = json!(max);
+    }
+    if let Some(effort) = req.reasoning.as_ref().and_then(|r| r.effort) {
+        body["reasoning_effort"] = json!(effort_str(effort));
+    }
+    body
+}
+
+fn map_finish_reason(reason: &str) -> FinishReason {
+    match reason {
+        "stop" => FinishReason::Stop,
+        "tool_calls" | "function_call" => FinishReason::ToolCalls,
+        "length" => FinishReason::Length,
+        "content_filter" => FinishReason::ContentFilter,
+        other => FinishReason::Unknown(other.to_string()),
+    }
+}
+
+#[derive(Default)]
+struct ToolAccum {
+    call_id: String,
+    name: String,
+    args: String,
+}
+
+/// Decodes a chat-completions SSE byte stream into normalized `LlmEvent`s. Tool-call deltas
+/// are accumulated by their `index` and flushed as `ToolCall`s once the stream ends.
+pub fn decode<S, E>(byte_stream: S) -> LlmEventStream
+where
+    S: Stream<Item = Result<bytes::Bytes, E>> + Send + 'static,
+    E: std::error::Error + Send + Sync + 'static,
+{
+    let events = byte_stream.eventsource();
+    Box::pin(try_stream! {
+        futures::pin_mut!(events);
+        let mut usage = TokenUsage::default();
+        let mut reason = FinishReason::Stop;
+        let mut text_open = false;
+        let mut reasoning_open = false;
+        let mut tools: HashMap<u64, ToolAccum> = HashMap::new();
+        let mut tool_order: Vec<u64> = Vec::new();
+
+        while let Some(item) = futures::StreamExt::next(&mut events).await {
+            let event = item.map_err(|e| ProviderError::Decode(e.to_string()))?;
+            let data = event.data.trim();
+            if data.is_empty() {
+                continue;
+            }
+            if data == "[DONE]" {
+                break;
+            }
+            let value: Value = serde_json::from_str(data)
+                .map_err(|e| ProviderError::Decode(format!("{e}: {data}")))?;
+
+            if let Some(u) = value.get("usage").filter(|u| u.is_object()) {
+                usage.input = u["prompt_tokens"].as_u64().unwrap_or(usage.input);
+                usage.output = u["completion_tokens"].as_u64().unwrap_or(usage.output);
+                usage.reasoning = u["completion_tokens_details"]["reasoning_tokens"]
+                    .as_u64()
+                    .unwrap_or(usage.reasoning);
+                usage.cache_read = u["prompt_tokens_details"]["cached_tokens"]
+                    .as_u64()
+                    .unwrap_or(usage.cache_read);
+            }
+
+            let choice = &value["choices"][0];
+            let delta = &choice["delta"];
+
+            if let Some(rc) = delta["reasoning_content"].as_str().filter(|s| !s.is_empty()) {
+                if !reasoning_open {
+                    reasoning_open = true;
+                    yield LlmEvent::ReasoningStart { id: "reasoning".into() };
+                }
+                yield LlmEvent::ReasoningDelta { id: "reasoning".into(), text: rc.to_string() };
+            }
+
+            if let Some(text) = delta["content"].as_str().filter(|s| !s.is_empty()) {
+                if reasoning_open {
+                    reasoning_open = false;
+                    yield LlmEvent::ReasoningEnd { id: "reasoning".into(), signature: None };
+                }
+                if !text_open {
+                    text_open = true;
+                    yield LlmEvent::TextStart { id: "0".into() };
+                }
+                yield LlmEvent::TextDelta { id: "0".into(), text: text.to_string() };
+            }
+
+            if let Some(calls) = delta["tool_calls"].as_array() {
+                for call in calls {
+                    let index = call["index"].as_u64().unwrap_or(0);
+                    let entry = tools.entry(index).or_insert_with(|| {
+                        tool_order.push(index);
+                        ToolAccum::default()
+                    });
+                    if let Some(id) = call["id"].as_str().filter(|s| !s.is_empty()) {
+                        entry.call_id = id.to_string();
+                    }
+                    if let Some(name) = call["function"]["name"].as_str().filter(|s| !s.is_empty()) {
+                        entry.name = name.to_string();
+                        yield LlmEvent::ToolInputStart {
+                            call_id: entry.call_id.clone(),
+                            name: entry.name.clone(),
+                        };
+                    }
+                    if let Some(args) = call["function"]["arguments"].as_str().filter(|s| !s.is_empty()) {
+                        entry.args.push_str(args);
+                        yield LlmEvent::ToolInputDelta {
+                            call_id: entry.call_id.clone(),
+                            json: args.to_string(),
+                        };
+                    }
+                }
+            }
+
+            if let Some(fr) = choice["finish_reason"].as_str() {
+                reason = map_finish_reason(fr);
+            }
+        }
+
+        if reasoning_open {
+            yield LlmEvent::ReasoningEnd { id: "reasoning".into(), signature: None };
+        }
+        if text_open {
+            yield LlmEvent::TextEnd { id: "0".into() };
+        }
+        for index in tool_order {
+            if let Some(entry) = tools.remove(&index) {
+                let input: Value = if entry.args.trim().is_empty() {
+                    json!({})
+                } else {
+                    serde_json::from_str(&entry.args).unwrap_or(Value::Null)
+                };
+                yield LlmEvent::ToolCall { call_id: entry.call_id, name: entry.name, input };
+            }
+        }
+        yield LlmEvent::Finish { reason, usage };
+    })
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use futures::StreamExt;
+    use harness_core::llm::{Initiator, ReasoningOpts, ToolSchema, WireMessage};
+
+    fn sse_stream(raw: &'static str) -> LlmEventStream {
+        let chunks: Vec<Result<bytes::Bytes, std::io::Error>> =
+            vec![Ok(bytes::Bytes::from_static(raw.as_bytes()))];
+        decode(futures::stream::iter(chunks))
+    }
+
+    fn req(model: &str) -> LlmRequest {
+        LlmRequest {
+            model: model.into(),
+            system: vec![],
+            messages: vec![],
+            tools: vec![],
+            temperature: None,
+            max_tokens: None,
+            reasoning: None,
+            initiator: Initiator::User,
+        }
+    }
+
+    #[tokio::test]
+    async fn decodes_text_only_response() {
+        let raw = concat!(
+            "data: {\"choices\":[{\"delta\":{\"content\":\"Hel\"},\"finish_reason\":null}]}\n\n",
+            "data: {\"choices\":[{\"delta\":{\"content\":\"lo\"},\"finish_reason\":null}]}\n\n",
+            "data: {\"choices\":[{\"delta\":{},\"finish_reason\":\"stop\"}]}\n\n",
+            "data: {\"choices\":[],\"usage\":{\"prompt_tokens\":10,\"completion_tokens\":5}}\n\n",
+            "data: [DONE]\n\n",
+        );
+        let events: Vec<LlmEvent> = sse_stream(raw).map(|e| e.unwrap()).collect().await;
+        assert_eq!(
+            events,
+            vec![
+                LlmEvent::TextStart { id: "0".into() },
+                LlmEvent::TextDelta {
+                    id: "0".into(),
+                    text: "Hel".into()
+                },
+                LlmEvent::TextDelta {
+                    id: "0".into(),
+                    text: "lo".into()
+                },
+                LlmEvent::TextEnd { id: "0".into() },
+                LlmEvent::Finish {
+                    reason: FinishReason::Stop,
+                    usage: TokenUsage {
+                        input: 10,
+                        output: 5,
+                        ..Default::default()
+                    },
+                },
+            ]
+        );
+    }
+
+    #[tokio::test]
+    async fn decodes_tool_call_accumulated_by_index() {
+        let raw = concat!(
+            "data: {\"choices\":[{\"delta\":{\"tool_calls\":[{\"index\":0,\"id\":\"call_1\",\"function\":{\"name\":\"read\",\"arguments\":\"\"}}]}}]}\n\n",
+            "data: {\"choices\":[{\"delta\":{\"tool_calls\":[{\"index\":0,\"function\":{\"arguments\":\"{\\\"file\\\"\"}}]}}]}\n\n",
+            "data: {\"choices\":[{\"delta\":{\"tool_calls\":[{\"index\":0,\"function\":{\"arguments\":\":\\\"a.txt\\\"}\"}}]}}]}\n\n",
+            "data: {\"choices\":[{\"delta\":{},\"finish_reason\":\"tool_calls\"}],\"usage\":{\"prompt_tokens\":1,\"completion_tokens\":8}}\n\n",
+            "data: [DONE]\n\n",
+        );
+        let events: Vec<LlmEvent> = sse_stream(raw).map(|e| e.unwrap()).collect().await;
+        assert_eq!(
+            events,
+            vec![
+                LlmEvent::ToolInputStart {
+                    call_id: "call_1".into(),
+                    name: "read".into()
+                },
+                LlmEvent::ToolInputDelta {
+                    call_id: "call_1".into(),
+                    json: "{\"file\"".into()
+                },
+                LlmEvent::ToolInputDelta {
+                    call_id: "call_1".into(),
+                    json: ":\"a.txt\"}".into()
+                },
+                LlmEvent::ToolCall {
+                    call_id: "call_1".into(),
+                    name: "read".into(),
+                    input: json!({"file": "a.txt"}),
+                },
+                LlmEvent::Finish {
+                    reason: FinishReason::ToolCalls,
+                    usage: TokenUsage {
+                        input: 1,
+                        output: 8,
+                        ..Default::default()
+                    },
+                },
+            ]
+        );
+    }
+
+    #[tokio::test]
+    async fn decodes_reasoning_content_before_text() {
+        let raw = concat!(
+            "data: {\"choices\":[{\"delta\":{\"reasoning_content\":\"hmm\"}}]}\n\n",
+            "data: {\"choices\":[{\"delta\":{\"content\":\"answer\"}}]}\n\n",
+            "data: {\"choices\":[{\"delta\":{},\"finish_reason\":\"stop\"}]}\n\n",
+            "data: [DONE]\n\n",
+        );
+        let events: Vec<LlmEvent> = sse_stream(raw).map(|e| e.unwrap()).collect().await;
+        assert_eq!(
+            events,
+            vec![
+                LlmEvent::ReasoningStart {
+                    id: "reasoning".into()
+                },
+                LlmEvent::ReasoningDelta {
+                    id: "reasoning".into(),
+                    text: "hmm".into()
+                },
+                LlmEvent::ReasoningEnd {
+                    id: "reasoning".into(),
+                    signature: None
+                },
+                LlmEvent::TextStart { id: "0".into() },
+                LlmEvent::TextDelta {
+                    id: "0".into(),
+                    text: "answer".into()
+                },
+                LlmEvent::TextEnd { id: "0".into() },
+                LlmEvent::Finish {
+                    reason: FinishReason::Stop,
+                    usage: TokenUsage::default()
+                },
+            ]
+        );
+    }
+
+    #[test]
+    fn build_request_wraps_tools_in_function_envelope() {
+        let mut r = req("gpt-4o");
+        r.tools = vec![ToolSchema {
+            name: "read".into(),
+            description: "reads a file".into(),
+            parameters: json!({"type": "object"}),
+        }];
+        let body = build_request(&r);
+        assert_eq!(body["tools"][0]["type"], "function");
+        assert_eq!(body["tools"][0]["function"]["name"], "read");
+        assert_eq!(
+            body["tools"][0]["function"]["parameters"],
+            json!({"type": "object"})
+        );
+        assert_eq!(body["stream_options"]["include_usage"], true);
+    }
+
+    #[test]
+    fn build_request_prepends_system_message() {
+        let mut r = req("gpt-4o");
+        r.system = vec!["env".into(), "agent".into()];
+        r.messages = vec![WireMessage {
+            role: Role::User,
+            content: vec![WireContent::Text { text: "hi".into() }],
+        }];
+        let body = build_request(&r);
+        let msgs = body["messages"].as_array().unwrap();
+        assert_eq!(msgs[0]["role"], "system");
+        assert_eq!(msgs[0]["content"], "env\n\nagent");
+        assert_eq!(msgs[1]["role"], "user");
+        assert_eq!(msgs[1]["content"], "hi");
+    }
+
+    #[test]
+    fn build_request_expands_tool_results_to_tool_messages() {
+        let mut r = req("gpt-4o");
+        r.messages = vec![WireMessage {
+            role: Role::Tool,
+            content: vec![WireContent::ToolResult {
+                call_id: "call_1".into(),
+                output: "contents".into(),
+                is_error: false,
+            }],
+        }];
+        let body = build_request(&r);
+        let msgs = body["messages"].as_array().unwrap();
+        assert_eq!(msgs.len(), 1);
+        assert_eq!(msgs[0]["role"], "tool");
+        assert_eq!(msgs[0]["tool_call_id"], "call_1");
+        assert_eq!(msgs[0]["content"], "contents");
+    }
+
+    #[test]
+    fn build_request_emits_assistant_tool_calls() {
+        let mut r = req("gpt-4o");
+        r.messages = vec![WireMessage {
+            role: Role::Assistant,
+            content: vec![WireContent::ToolCall {
+                call_id: "call_1".into(),
+                name: "read".into(),
+                input: json!({"file": "a.txt"}),
+            }],
+        }];
+        let body = build_request(&r);
+        let msgs = body["messages"].as_array().unwrap();
+        assert_eq!(msgs[0]["tool_calls"][0]["id"], "call_1");
+        assert_eq!(msgs[0]["tool_calls"][0]["function"]["name"], "read");
+        assert_eq!(
+            msgs[0]["tool_calls"][0]["function"]["arguments"],
+            "{\"file\":\"a.txt\"}"
+        );
+    }
+
+    #[test]
+    fn build_request_sets_reasoning_effort() {
+        let mut r = req("o3");
+        r.reasoning = Some(ReasoningOpts {
+            effort: Some(ReasoningEffort::High),
+            budget_tokens: None,
+        });
+        let body = build_request(&r);
+        assert_eq!(body["reasoning_effort"], "high");
+    }
+}
diff --git a/crates/harness-providers/src/codec/openai_responses.rs b/crates/harness-providers/src/codec/openai_responses.rs
new file mode 100644
index 0000000..fc5de23
--- /dev/null
+++ b/crates/harness-providers/src/codec/openai_responses.rs
@@ -0,0 +1,402 @@
+//! Request builder + SSE decoder for OpenAI's `/responses` streaming API.
+//!
+//! The responses API is the preferred surface for `gpt-*` / `o-*` models: it carries
+//! reasoning items natively and reports reasoning-token usage separately.
+
+use std::collections::HashMap;
+
+use async_stream::try_stream;
+use eventsource_stream::Eventsource;
+use futures::Stream;
+use harness_core::llm::{
+    FinishReason, LlmEvent, LlmEventStream, LlmRequest, ProviderError, ReasoningEffort, Role,
+    WireContent,
+};
+use harness_core::types::TokenUsage;
+use serde_json::{json, Value};
+
+fn effort_str(effort: ReasoningEffort) -> &'static str {
+    match effort {
+        ReasoningEffort::Low => "low",
+        ReasoningEffort::Medium => "medium",
+        ReasoningEffort::High => "high",
+    }
+}
+
+/// Builds the `input` item list. Text/image content become `message` items; tool calls and
+/// their results become `function_call` / `function_call_output` items (the responses API
+/// keeps these as top-level items rather than nesting them inside messages).
+fn build_input(messages: &[harness_core::llm::WireMessage]) -> Vec<Value> {
+    let mut out: Vec<Value> = Vec::new();
+    for m in messages {
+        let (role, text_type) = match m.role {
+            Role::Assistant => ("assistant", "output_text"),
+            _ => ("user", "input_text"),
+        };
+        let mut content_parts: Vec<Value> = Vec::new();
+        for c in &m.content {
+            match c {
+                WireContent::Text { text } => {
+                    content_parts.push(json!({"type": text_type, "text": text}));
+                }
+                WireContent::Image { mime_type, data } => {
+                    content_parts.push(json!({
+                        "type": "input_image",
+                        "image_url": format!("data:{mime_type};base64,{data}"),
+                    }));
+                }
+                WireContent::ToolCall {
+                    call_id,
+                    name,
+                    input,
+                } => out.push(json!({
+                    "type": "function_call",
+                    "call_id": call_id,
+                    "name": name,
+                    "arguments": input.to_string(),
+                })),
+                WireContent::ToolResult {
+                    call_id, output, ..
+                } => out.push(json!({
+                    "type": "function_call_output",
+                    "call_id": call_id,
+                    "output": output,
+                })),
+            }
+        }
+        if !content_parts.is_empty() {
+            out.push(json!({"type": "message", "role": role, "content": content_parts}));
+        }
+    }
+    out
+}
+
+pub fn build_request(req: &LlmRequest) -> Value {
+    let mut body = json!({
+        "model": req.model,
+        "input": build_input(&req.messages),
+        "stream": true,
+        "store": false,
+    });
+
+    if !req.system.is_empty() {
+        body["instructions"] = json!(req.system.join("\n\n"));
+    }
+    if !req.tools.is_empty() {
+        body["tools"] = Value::Array(
+            req.tools
+                .iter()
+                .map(|t| {
+                    json!({
+                        "type": "function",
+                        "name": t.name,
+                        "description": t.description,
+                        "parameters": t.parameters,
+                    })
+                })
+                .collect(),
+        );
+    }
+    if let Some(temp) = req.temperature {
+        body["temperature"] = json!(temp);
+    }
+    if let Some(max) = req.max_tokens {
+        body["max_output_tokens"] = json!(max);
+    }
+    if let Some(effort) = req.reasoning.as_ref().and_then(|r| r.effort) {
+        body["reasoning"] = json!({"effort": effort_str(effort), "summary": "auto"});
+    }
+    body
+}
+
+fn map_status(status: Option<&str>, had_tool_call: bool) -> FinishReason {
+    match status {
+        Some("completed") if had_tool_call => FinishReason::ToolCalls,
+        Some("completed") => FinishReason::Stop,
+        Some("incomplete") => FinishReason::Length,
+        Some(other) => FinishReason::Unknown(other.to_string()),
+        None if had_tool_call => FinishReason::ToolCalls,
+        None => FinishReason::Stop,
+    }
+}
+
+/// Tracks which normalized stream an `item_id` belongs to so deltas route correctly.
+enum ItemKind {
+    Text,
+    Reasoning,
+    FunctionCall { call_id: String, name: String },
+}
+
+/// Decodes a `/responses` SSE byte stream into normalized `LlmEvent`s.
+pub fn decode<S, E>(byte_stream: S) -> LlmEventStream
+where
+    S: Stream<Item = Result<bytes::Bytes, E>> + Send + 'static,
+    E: std::error::Error + Send + Sync + 'static,
+{
+    let events = byte_stream.eventsource();
+    Box::pin(try_stream! {
+        futures::pin_mut!(events);
+        let mut items: HashMap<String, ItemKind> = HashMap::new();
+        let mut fn_args: HashMap<String, String> = HashMap::new();
+        let mut usage = TokenUsage::default();
+        let mut had_tool_call = false;
+
+        while let Some(item) = futures::StreamExt::next(&mut events).await {
+            let event = item.map_err(|e| ProviderError::Decode(e.to_string()))?;
+            if event.data.trim().is_empty() {
+                continue;
+            }
+            let value: Value = serde_json::from_str(&event.data)
+                .map_err(|e| ProviderError::Decode(format!("{e}: {}", event.data)))?;
+            let kind = value["type"].as_str().unwrap_or_default();
+
+            match kind {
+                "response.output_item.added" => {
+                    let item = &value["item"];
+                    let id = item["id"].as_str().unwrap_or_default().to_string();
+                    match item["type"].as_str().unwrap_or_default() {
+                        "message" => {
+                            items.insert(id.clone(), ItemKind::Text);
+                            yield LlmEvent::TextStart { id };
+                        }
+                        "reasoning" => {
+                            items.insert(id.clone(), ItemKind::Reasoning);
+                            yield LlmEvent::ReasoningStart { id };
+                        }
+                        "function_call" => {
+                            let call_id = item["call_id"].as_str().unwrap_or_default().to_string();
+                            let name = item["name"].as_str().unwrap_or_default().to_string();
+                            fn_args.insert(id.clone(), String::new());
+                            items.insert(id, ItemKind::FunctionCall { call_id: call_id.clone(), name: name.clone() });
+                            had_tool_call = true;
+                            yield LlmEvent::ToolInputStart { call_id, name };
+                        }
+                        _ => {}
+                    }
+                }
+                "response.output_text.delta" => {
+                    let id = value["item_id"].as_str().unwrap_or_default().to_string();
+                    let text = value["delta"].as_str().unwrap_or_default().to_string();
+                    yield LlmEvent::TextDelta { id, text };
+                }
+                "response.reasoning_summary_text.delta" => {
+                    let id = value["item_id"].as_str().unwrap_or_default().to_string();
+                    let text = value["delta"].as_str().unwrap_or_default().to_string();
+                    yield LlmEvent::ReasoningDelta { id, text };
+                }
+                "response.function_call_arguments.delta" => {
+                    let id = value["item_id"].as_str().unwrap_or_default().to_string();
+                    let delta = value["delta"].as_str().unwrap_or_default();
+                    if let (Some(buf), Some(ItemKind::FunctionCall { call_id, .. })) =
+                        (fn_args.get_mut(&id), items.get(&id))
+                    {
+                        buf.push_str(delta);
+                        yield LlmEvent::ToolInputDelta { call_id: call_id.clone(), json: delta.to_string() };
+                    }
+                }
+                "response.output_item.done" => {
+                    let id = value["item"]["id"].as_str().unwrap_or_default().to_string();
+                    match items.remove(&id) {
+                        Some(ItemKind::Text) => yield LlmEvent::TextEnd { id },
+                        Some(ItemKind::Reasoning) => yield LlmEvent::ReasoningEnd { id, signature: None },
+                        Some(ItemKind::FunctionCall { call_id, name }) => {
+                            let raw = fn_args.remove(&id).unwrap_or_default();
+                            let input: Value = if raw.trim().is_empty() {
+                                json!({})
+                            } else {
+                                serde_json::from_str(&raw).unwrap_or(Value::Null)
+                            };
+                            yield LlmEvent::ToolCall { call_id, name, input };
+                        }
+                        None => {}
+                    }
+                }
+                "response.completed" | "response.incomplete" => {
+                    let response = &value["response"];
+                    let u = &response["usage"];
+                    usage.input = u["input_tokens"].as_u64().unwrap_or(0);
+                    usage.output = u["output_tokens"].as_u64().unwrap_or(0);
+                    usage.reasoning = u["output_tokens_details"]["reasoning_tokens"].as_u64().unwrap_or(0);
+                    usage.cache_read = u["input_tokens_details"]["cached_tokens"].as_u64().unwrap_or(0);
+                    let status = response["status"].as_str();
+                    yield LlmEvent::Finish { reason: map_status(status, had_tool_call), usage };
+                }
+                "error" | "response.failed" => {
+                    let message = value["response"]["error"]["message"]
+                        .as_str()
+                        .or_else(|| value["message"].as_str())
+                        .unwrap_or("unknown error")
+                        .to_string();
+                    Err(ProviderError::Http { status: 0, body: message })?;
+                }
+                _ => {}
+            }
+        }
+    })
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use futures::StreamExt;
+    use harness_core::llm::{Initiator, ReasoningOpts, ToolSchema, WireMessage};
+
+    fn sse_stream(raw: &'static str) -> LlmEventStream {
+        let chunks: Vec<Result<bytes::Bytes, std::io::Error>> =
+            vec![Ok(bytes::Bytes::from_static(raw.as_bytes()))];
+        decode(futures::stream::iter(chunks))
+    }
+
+    fn req(model: &str) -> LlmRequest {
+        LlmRequest {
+            model: model.into(),
+            system: vec![],
+            messages: vec![],
+            tools: vec![],
+            temperature: None,
+            max_tokens: None,
+            reasoning: None,
+            initiator: Initiator::User,
+        }
+    }
+
+    #[tokio::test]
+    async fn decodes_text_response() {
+        let raw = concat!(
+            "data: {\"type\":\"response.output_item.added\",\"item\":{\"id\":\"msg_1\",\"type\":\"message\"}}\n\n",
+            "data: {\"type\":\"response.output_text.delta\",\"item_id\":\"msg_1\",\"delta\":\"Hello\"}\n\n",
+            "data: {\"type\":\"response.output_item.done\",\"item\":{\"id\":\"msg_1\"}}\n\n",
+            "data: {\"type\":\"response.completed\",\"response\":{\"status\":\"completed\",\"usage\":{\"input_tokens\":10,\"output_tokens\":5}}}\n\n",
+        );
+        let events: Vec<LlmEvent> = sse_stream(raw).map(|e| e.unwrap()).collect().await;
+        assert_eq!(
+            events,
+            vec![
+                LlmEvent::TextStart { id: "msg_1".into() },
+                LlmEvent::TextDelta {
+                    id: "msg_1".into(),
+                    text: "Hello".into()
+                },
+                LlmEvent::TextEnd { id: "msg_1".into() },
+                LlmEvent::Finish {
+                    reason: FinishReason::Stop,
+                    usage: TokenUsage {
+                        input: 10,
+                        output: 5,
+                        ..Default::default()
+                    },
+                },
+            ]
+        );
+    }
+
+    #[tokio::test]
+    async fn decodes_function_call_with_reasoning_tokens() {
+        let raw = concat!(
+            "data: {\"type\":\"response.output_item.added\",\"item\":{\"id\":\"fc_1\",\"type\":\"function_call\",\"call_id\":\"call_1\",\"name\":\"read\"}}\n\n",
+            "data: {\"type\":\"response.function_call_arguments.delta\",\"item_id\":\"fc_1\",\"delta\":\"{\\\"file\\\":\"}\n\n",
+            "data: {\"type\":\"response.function_call_arguments.delta\",\"item_id\":\"fc_1\",\"delta\":\"\\\"a.txt\\\"}\"}\n\n",
+            "data: {\"type\":\"response.output_item.done\",\"item\":{\"id\":\"fc_1\"}}\n\n",
+            "data: {\"type\":\"response.completed\",\"response\":{\"status\":\"completed\",\"usage\":{\"input_tokens\":3,\"output_tokens\":9,\"output_tokens_details\":{\"reasoning_tokens\":4}}}}\n\n",
+        );
+        let events: Vec<LlmEvent> = sse_stream(raw).map(|e| e.unwrap()).collect().await;
+        assert_eq!(
+            events,
+            vec![
+                LlmEvent::ToolInputStart {
+                    call_id: "call_1".into(),
+                    name: "read".into()
+                },
+                LlmEvent::ToolInputDelta {
+                    call_id: "call_1".into(),
+                    json: "{\"file\":".into()
+                },
+                LlmEvent::ToolInputDelta {
+                    call_id: "call_1".into(),
+                    json: "\"a.txt\"}".into()
+                },
+                LlmEvent::ToolCall {
+                    call_id: "call_1".into(),
+                    name: "read".into(),
+                    input: json!({"file": "a.txt"}),
+                },
+                LlmEvent::Finish {
+                    reason: FinishReason::ToolCalls,
+                    usage: TokenUsage {
+                        input: 3,
+                        output: 9,
+                        reasoning: 4,
+                        ..Default::default()
+                    },
+                },
+            ]
+        );
+    }
+
+    #[tokio::test]
+    async fn failed_response_surfaces_as_err() {
+        let raw = "data: {\"type\":\"response.failed\",\"response\":{\"error\":{\"message\":\"boom\"}}}\n\n";
+        let events: Vec<Result<LlmEvent, ProviderError>> = sse_stream(raw).collect().await;
+        assert!(
+            matches!(events.last(), Some(Err(ProviderError::Http { body, .. })) if body == "boom")
+        );
+    }
+
+    #[test]
+    fn build_request_puts_system_in_instructions() {
+        let mut r = req("gpt-5");
+        r.system = vec!["env".into(), "agent".into()];
+        let body = build_request(&r);
+        assert_eq!(body["instructions"], "env\n\nagent");
+        assert!(body.get("messages").is_none());
+    }
+
+    #[test]
+    fn build_request_flattens_tool_calls_and_results_to_items() {
+        let mut r = req("gpt-5");
+        r.messages = vec![
+            WireMessage {
+                role: Role::Assistant,
+                content: vec![WireContent::ToolCall {
+                    call_id: "call_1".into(),
+                    name: "read".into(),
+                    input: json!({"file": "a.txt"}),
+                }],
+            },
+            WireMessage {
+                role: Role::Tool,
+                content: vec![WireContent::ToolResult {
+                    call_id: "call_1".into(),
+                    output: "contents".into(),
+                    is_error: false,
+                }],
+            },
+        ];
+        let body = build_request(&r);
+        let input = body["input"].as_array().unwrap();
+        assert_eq!(input[0]["type"], "function_call");
+        assert_eq!(input[0]["call_id"], "call_1");
+        assert_eq!(input[0]["arguments"], "{\"file\":\"a.txt\"}");
+        assert_eq!(input[1]["type"], "function_call_output");
+        assert_eq!(input[1]["output"], "contents");
+    }
+
+    #[test]
+    fn build_request_uses_flat_function_tool_shape_and_reasoning() {
+        let mut r = req("o3");
+        r.tools = vec![ToolSchema {
+            name: "read".into(),
+            description: "reads a file".into(),
+            parameters: json!({"type": "object"}),
+        }];
+        r.reasoning = Some(ReasoningOpts {
+            effort: Some(ReasoningEffort::Medium),
+            budget_tokens: None,
+        });
+        let body = build_request(&r);
+        assert_eq!(body["tools"][0]["type"], "function");
+        assert_eq!(body["tools"][0]["name"], "read");
+        assert_eq!(body["reasoning"]["effort"], "medium");
+        assert_eq!(body["reasoning"]["summary"], "auto");
+    }
+}
diff --git a/crates/harness-providers/src/copilot/device_flow.rs b/crates/harness-providers/src/copilot/device_flow.rs
new file mode 100644
index 0000000..406c58e
--- /dev/null
+++ b/crates/harness-providers/src/copilot/device_flow.rs
@@ -0,0 +1,209 @@
+//! GitHub OAuth device flow (RFC 8628) for Copilot login.
+//!
+//! ai-harness must register its own GitHub OAuth app and supply its client id (we do not
+//! hardcode opencode's). The client id is passed in by the caller — see [`request_device_code`].
+
+use std::time::Duration;
+
+use serde::Deserialize;
+use serde_json::Value;
+
+const DEVICE_CODE_URL: &str = "https://github.com/login/device/code";
+const ACCESS_TOKEN_URL: &str = "https://github.com/login/oauth/access_token";
+const GRANT_TYPE: &str = "urn:ietf:params:oauth:grant-type:device_code";
+/// Minimum scope needed to call the Copilot token-exchange endpoint.
+pub const SCOPE: &str = "read:user";
+
+#[derive(Debug, Clone, Deserialize)]
+pub struct DeviceCode {
+    pub device_code: String,
+    pub user_code: String,
+    pub verification_uri: String,
+    /// Seconds between polls; GitHub requires honoring this and any `slow_down` bumps.
+    #[serde(default = "default_interval")]
+    pub interval: u64,
+    #[serde(default)]
+    pub expires_in: u64,
+}
+
+fn default_interval() -> u64 {
+    5
+}
+
+/// Result of one poll of the access-token endpoint.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub enum PollOutcome {
+    /// The user hasn't authorized yet; keep polling at the current interval.
+    Pending,
+    /// GitHub asked us to slow down; add 5s to the interval (RFC 8628).
+    SlowDown,
+    /// Authorization complete.
+    Success { access_token: String },
+    /// Terminal failure (expired code, denied, unknown error) with a human-readable reason.
+    Failed(String),
+}
+
+#[derive(Debug, thiserror::Error)]
+pub enum DeviceFlowError {
+    #[error("network: {0}")]
+    Network(String),
+    #[error("unexpected response: {0}")]
+    Unexpected(String),
+}
+
+/// Pure mapping of an access-token poll response body to a [`PollOutcome`], so the state
+/// machine is testable without a live GitHub.
+pub fn parse_poll_response(body: &Value) -> PollOutcome {
+    if let Some(token) = body["access_token"].as_str() {
+        return PollOutcome::Success {
+            access_token: token.to_string(),
+        };
+    }
+    match body["error"].as_str() {
+        Some("authorization_pending") => PollOutcome::Pending,
+        Some("slow_down") => PollOutcome::SlowDown,
+        Some(other) => {
+            let desc = body["error_description"].as_str().unwrap_or(other);
+            PollOutcome::Failed(desc.to_string())
+        }
+        None => PollOutcome::Failed("no access_token and no error in response".to_string()),
+    }
+}
+
+/// Applies a `slow_down` to a poll interval per RFC 8628 (+5s).
+pub fn bump_interval(interval: u64) -> u64 {
+    interval + 5
+}
+
+/// Step 1: request a device + user code for `client_id`.
+pub async fn request_device_code(
+    client: &reqwest::Client,
+    client_id: &str,
+) -> Result<DeviceCode, DeviceFlowError> {
+    let resp = client
+        .post(DEVICE_CODE_URL)
+        .header("accept", "application/json")
+        .json(&serde_json::json!({"client_id": client_id, "scope": SCOPE}))
+        .send()
+        .await
+        .map_err(|e| DeviceFlowError::Network(e.to_string()))?;
+    let value: Value = resp
+        .json()
+        .await
+        .map_err(|e| DeviceFlowError::Network(e.to_string()))?;
+    serde_json::from_value(value.clone())
+        .map_err(|_| DeviceFlowError::Unexpected(value.to_string()))
+}
+
+/// Step 2 (single poll): exchange the device code for an access token, once.
+pub async fn poll_once(
+    client: &reqwest::Client,
+    client_id: &str,
+    device_code: &str,
+) -> Result<PollOutcome, DeviceFlowError> {
+    let resp = client
+        .post(ACCESS_TOKEN_URL)
+        .header("accept", "application/json")
+        .json(&serde_json::json!({
+            "client_id": client_id,
+            "device_code": device_code,
+            "grant_type": GRANT_TYPE,
+        }))
+        .send()
+        .await
+        .map_err(|e| DeviceFlowError::Network(e.to_string()))?;
+    let value: Value = resp
+        .json()
+        .await
+        .map_err(|e| DeviceFlowError::Network(e.to_string()))?;
+    Ok(parse_poll_response(&value))
+}
+
+/// Step 2 (full loop): polls until success or terminal failure, honoring `interval` and
+/// `slow_down`. `sleep` is injected so tests can drive it without real time.
+pub async fn poll_for_token<S, Fut>(
+    client: &reqwest::Client,
+    client_id: &str,
+    device: &DeviceCode,
+    sleep: S,
+) -> Result<String, DeviceFlowError>
+where
+    S: Fn(Duration) -> Fut,
+    Fut: std::future::Future<Output = ()>,
+{
+    let mut interval = device.interval;
+    loop {
+        sleep(Duration::from_secs(interval)).await;
+        match poll_once(client, client_id, &device.device_code).await? {
+            PollOutcome::Pending => {}
+            PollOutcome::SlowDown => interval = bump_interval(interval),
+            PollOutcome::Success { access_token } => return Ok(access_token),
+            PollOutcome::Failed(reason) => return Err(DeviceFlowError::Unexpected(reason)),
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use serde_json::json;
+
+    #[test]
+    fn parses_success() {
+        let outcome =
+            parse_poll_response(&json!({"access_token": "gho_abc", "token_type": "bearer"}));
+        assert_eq!(
+            outcome,
+            PollOutcome::Success {
+                access_token: "gho_abc".into()
+            }
+        );
+    }
+
+    #[test]
+    fn parses_pending_and_slow_down() {
+        assert_eq!(
+            parse_poll_response(&json!({"error": "authorization_pending"})),
+            PollOutcome::Pending
+        );
+        assert_eq!(
+            parse_poll_response(&json!({"error": "slow_down", "interval": 10})),
+            PollOutcome::SlowDown
+        );
+    }
+
+    #[test]
+    fn parses_terminal_errors_with_description() {
+        match parse_poll_response(
+            &json!({"error": "expired_token", "error_description": "code expired"}),
+        ) {
+            PollOutcome::Failed(msg) => assert_eq!(msg, "code expired"),
+            other => panic!("expected Failed, got {other:?}"),
+        }
+        assert!(matches!(
+            parse_poll_response(&json!({"error": "access_denied"})),
+            PollOutcome::Failed(_)
+        ));
+        assert!(matches!(
+            parse_poll_response(&json!({})),
+            PollOutcome::Failed(_)
+        ));
+    }
+
+    #[test]
+    fn slow_down_adds_five_seconds() {
+        assert_eq!(bump_interval(5), 10);
+        assert_eq!(bump_interval(10), 15);
+    }
+
+    #[test]
+    fn device_code_defaults_interval() {
+        let dc: DeviceCode = serde_json::from_value(json!({
+            "device_code": "d",
+            "user_code": "WXYZ-1234",
+            "verification_uri": "https://github.com/login/device"
+        }))
+        .unwrap();
+        assert_eq!(dc.interval, 5);
+    }
+}
diff --git a/crates/harness-providers/src/copilot/mod.rs b/crates/harness-providers/src/copilot/mod.rs
new file mode 100644
index 0000000..7416c0a
--- /dev/null
+++ b/crates/harness-providers/src/copilot/mod.rs
@@ -0,0 +1,10 @@
+//! GitHub Copilot: OAuth device-flow login, token exchange, and a provider that multiplexes
+//! the chat/responses/anthropic codecs behind `api.githubcopilot.com`.
+
+pub mod device_flow;
+pub mod provider;
+pub mod token;
+
+pub use device_flow::{DeviceCode, PollOutcome};
+pub use provider::{CopilotCodec, CopilotModel, CopilotProvider};
+pub use token::{CopilotToken, TokenProvider};
diff --git a/crates/harness-providers/src/copilot/provider.rs b/crates/harness-providers/src/copilot/provider.rs
new file mode 100644
index 0000000..f2e754b
--- /dev/null
+++ b/crates/harness-providers/src/copilot/provider.rs
@@ -0,0 +1,296 @@
+//! Copilot provider: multiplexes all three wire codecs behind `api.githubcopilot.com`.
+//!
+//! Each model's `supported_endpoints` (from `GET /models`) decides which codec/endpoint to
+//! use. The token comes from [`super::token::TokenProvider`]. Network paths here are the
+//! flagged live-verification risk — the routing/header logic is unit-tested; end-to-end
+//! streaming must be checked against the live API.
+
+use std::collections::HashMap;
+use std::time::{SystemTime, UNIX_EPOCH};
+
+use async_trait::async_trait;
+use harness_core::llm::{
+    Initiator, LlmEventStream, LlmRequest, Provider, ProviderError, WireContent,
+};
+use harness_core::types::ModelInfo;
+use serde_json::Value;
+use tokio_util::sync::CancellationToken;
+
+use crate::codec::{anthropic, openai_chat, openai_responses};
+
+use super::token::TokenProvider;
+
+pub const DEFAULT_BASE_URL: &str = "https://api.githubcopilot.com";
+const API_VERSION: &str = "2026-06-01";
+const ANTHROPIC_BETA: &str = "interleaved-thinking-2025-05-14";
+const USER_AGENT: &str = concat!("ai-harness/", env!("CARGO_PKG_VERSION"));
+
+/// Which wire codec a Copilot model speaks.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum CopilotCodec {
+    Chat,
+    Responses,
+    Anthropic,
+}
+
+impl CopilotCodec {
+    fn path(self) -> &'static str {
+        match self {
+            CopilotCodec::Chat => "/chat/completions",
+            CopilotCodec::Responses => "/responses",
+            CopilotCodec::Anthropic => "/v1/messages",
+        }
+    }
+
+    fn build_request(self, req: &LlmRequest) -> Value {
+        match self {
+            CopilotCodec::Chat => openai_chat::build_request(req),
+            CopilotCodec::Responses => openai_responses::build_request(req),
+            CopilotCodec::Anthropic => anthropic::build_request(req),
+        }
+    }
+
+    fn decode(
+        self,
+        stream: impl futures::Stream<Item = reqwest::Result<bytes::Bytes>> + Send + 'static,
+    ) -> LlmEventStream {
+        match self {
+            CopilotCodec::Chat => openai_chat::decode(stream),
+            CopilotCodec::Responses => openai_responses::decode(stream),
+            CopilotCodec::Anthropic => anthropic::decode(stream),
+        }
+    }
+}
+
+/// A single model as advertised by `GET /models`.
+#[derive(Debug, Clone)]
+pub struct CopilotModel {
+    pub id: String,
+    pub codec: CopilotCodec,
+    /// Only `model_picker_enabled` models are offered in the UI picker.
+    pub picker_enabled: bool,
+}
+
+/// Picks a codec from a model's `supported_endpoints`, preferring the responses API, then the
+/// Anthropic messages API, then chat completions.
+pub fn codec_for_endpoints(endpoints: &[String]) -> CopilotCodec {
+    let has = |needle: &str| endpoints.iter().any(|e| e.contains(needle));
+    if has("/responses") {
+        CopilotCodec::Responses
+    } else if has("/v1/messages") || has("/messages") {
+        CopilotCodec::Anthropic
+    } else {
+        CopilotCodec::Chat
+    }
+}
+
+/// Parses a Copilot `GET /models` response body.
+pub fn parse_models(body: &Value) -> Vec<CopilotModel> {
+    body["data"]
+        .as_array()
+        .map(|arr| {
+            arr.iter()
+                .filter_map(|m| {
+                    let id = m["id"].as_str()?.to_string();
+                    let endpoints: Vec<String> = m["supported_endpoints"]
+                        .as_array()
+                        .map(|a| {
+                            a.iter()
+                                .filter_map(|e| e.as_str().map(String::from))
+                                .collect()
+                        })
+                        .unwrap_or_default();
+                    Some(CopilotModel {
+                        id,
+                        codec: codec_for_endpoints(&endpoints),
+                        picker_enabled: m["model_picker_enabled"].as_bool().unwrap_or(false),
+                    })
+                })
+                .collect()
+        })
+        .unwrap_or_default()
+}
+
+fn has_images(req: &LlmRequest) -> bool {
+    req.messages
+        .iter()
+        .flat_map(|m| &m.content)
+        .any(|c| matches!(c, WireContent::Image { .. }))
+}
+
+fn initiator_header(initiator: Initiator) -> &'static str {
+    match initiator {
+        Initiator::User => "user",
+        Initiator::Agent => "agent",
+    }
+}
+
+pub struct CopilotProvider {
+    tokens: TokenProvider,
+    base_url: String,
+    client: reqwest::Client,
+    /// model id → codec, from `GET /models`. Unknown models default to chat completions.
+    codecs: HashMap<String, CopilotCodec>,
+}
+
+impl CopilotProvider {
+    pub fn new(oauth_token: impl Into<String>, models: Vec<CopilotModel>) -> Self {
+        Self::with_base_url(oauth_token, DEFAULT_BASE_URL, models)
+    }
+
+    pub fn with_base_url(
+        oauth_token: impl Into<String>,
+        base_url: impl Into<String>,
+        models: Vec<CopilotModel>,
+    ) -> Self {
+        let client = reqwest::Client::new();
+        let codecs = models.into_iter().map(|m| (m.id, m.codec)).collect();
+        Self {
+            tokens: TokenProvider::new(oauth_token, client.clone()),
+            base_url: base_url.into(),
+            client,
+            codecs,
+        }
+    }
+
+    fn codec_for(&self, model: &str) -> CopilotCodec {
+        self.codecs
+            .get(model)
+            .copied()
+            .unwrap_or(CopilotCodec::Chat)
+    }
+
+    fn classify_error(status: reqwest::StatusCode, body: String) -> ProviderError {
+        match status.as_u16() {
+            401 | 403 => ProviderError::Auth(body),
+            429 => ProviderError::RateLimited { retry_after: None },
+            s if (500..600).contains(&s) => ProviderError::Overloaded,
+            s => ProviderError::Http { status: s, body },
+        }
+    }
+}
+
+#[async_trait]
+impl Provider for CopilotProvider {
+    fn id(&self) -> &str {
+        "github-copilot"
+    }
+
+    async fn list_models(&self) -> Result<Vec<ModelInfo>, ProviderError> {
+        // Cost/limit metadata is layered on by models.dev; routing metadata lives in `codecs`.
+        Ok(Vec::new())
+    }
+
+    async fn stream(
+        &self,
+        req: LlmRequest,
+        cancel: CancellationToken,
+    ) -> Result<LlmEventStream, ProviderError> {
+        let codec = self.codec_for(&req.model);
+        let body = codec.build_request(&req);
+        let url = format!("{}{}", self.base_url, codec.path());
+
+        let now_secs = SystemTime::now()
+            .duration_since(UNIX_EPOCH)
+            .unwrap_or_default()
+            .as_secs() as i64;
+        let token = self.tokens.token(now_secs).await;
+
+        let mut builder = self
+            .client
+            .post(&url)
+            .header("authorization", format!("Bearer {}", token.token))
+            .header("user-agent", USER_AGENT)
+            .header("x-github-api-version", API_VERSION)
+            .header("openai-intent", "conversation-edits")
+            .header("x-initiator", initiator_header(req.initiator));
+        if has_images(&req) {
+            builder = builder.header("copilot-vision-request", "true");
+        }
+        if codec == CopilotCodec::Anthropic {
+            builder = builder.header("anthropic-beta", ANTHROPIC_BETA);
+        }
+
+        let send = builder.json(&body).send();
+        let response = tokio::select! {
+            result = send => result.map_err(|e| ProviderError::Network(e.to_string()))?,
+            _ = cancel.cancelled() => return Err(ProviderError::Cancelled),
+        };
+
+        if !response.status().is_success() {
+            let status = response.status();
+            let body = response.text().await.unwrap_or_default();
+            return Err(Self::classify_error(status, body));
+        }
+
+        Ok(codec.decode(response.bytes_stream()))
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use serde_json::json;
+
+    #[test]
+    fn codec_routing_prefers_responses_then_anthropic_then_chat() {
+        assert_eq!(
+            codec_for_endpoints(&["/chat/completions".into(), "/responses".into()]),
+            CopilotCodec::Responses
+        );
+        assert_eq!(
+            codec_for_endpoints(&["/v1/messages".into()]),
+            CopilotCodec::Anthropic
+        );
+        assert_eq!(
+            codec_for_endpoints(&["/chat/completions".into()]),
+            CopilotCodec::Chat
+        );
+        assert_eq!(codec_for_endpoints(&[]), CopilotCodec::Chat);
+    }
+
+    #[test]
+    fn parses_models_with_endpoints_and_picker_flag() {
+        let body = json!({
+            "data": [
+                {"id": "gpt-4o", "supported_endpoints": ["/chat/completions"], "model_picker_enabled": true},
+                {"id": "claude-sonnet-4-5", "supported_endpoints": ["/v1/messages"], "model_picker_enabled": true},
+                {"id": "o3", "supported_endpoints": ["/responses"], "model_picker_enabled": false},
+                {"id": "no-endpoints"}
+            ]
+        });
+        let models = parse_models(&body);
+        assert_eq!(models.len(), 4);
+        assert_eq!(models[0].codec, CopilotCodec::Chat);
+        assert!(models[0].picker_enabled);
+        assert_eq!(models[1].codec, CopilotCodec::Anthropic);
+        assert_eq!(models[2].codec, CopilotCodec::Responses);
+        assert!(!models[2].picker_enabled);
+        // Missing supported_endpoints → default chat, picker false.
+        assert_eq!(models[3].codec, CopilotCodec::Chat);
+        assert!(!models[3].picker_enabled);
+    }
+
+    #[test]
+    fn unknown_model_defaults_to_chat_codec() {
+        let provider = CopilotProvider::new("oauth", vec![]);
+        assert_eq!(provider.codec_for("whatever"), CopilotCodec::Chat);
+        assert_eq!(provider.id(), "github-copilot");
+    }
+
+    #[test]
+    fn known_model_uses_mapped_codec() {
+        let provider = CopilotProvider::new(
+            "oauth",
+            vec![CopilotModel {
+                id: "claude-sonnet-4-5".into(),
+                codec: CopilotCodec::Anthropic,
+                picker_enabled: true,
+            }],
+        );
+        assert_eq!(
+            provider.codec_for("claude-sonnet-4-5"),
+            CopilotCodec::Anthropic
+        );
+    }
+}
diff --git a/crates/harness-providers/src/copilot/token.rs b/crates/harness-providers/src/copilot/token.rs
new file mode 100644
index 0000000..e4165a7
--- /dev/null
+++ b/crates/harness-providers/src/copilot/token.rs
@@ -0,0 +1,144 @@
+//! Copilot API token strategy (flagged-risk area — verify against the live API).
+//!
+//! opencode sends the GitHub OAuth token directly as the Bearer (`expires: 0`). The classic
+//! Copilot API instead wants a short-lived token from `copilot_internal/v2/token`. We try the
+//! exchange first and cache it until shortly before expiry; if the endpoint rejects us, we fall
+//! back to the direct-Bearer behavior. Refresh is single-flight under a `tokio::Mutex`.
+
+use serde::Deserialize;
+use tokio::sync::Mutex;
+
+const EXCHANGE_URL: &str = "https://api.github.com/copilot_internal/v2/token";
+/// Refresh this many seconds before the reported expiry.
+const EXPIRY_SKEW_SECS: i64 = 120;
+const USER_AGENT: &str = concat!("ai-harness/", env!("CARGO_PKG_VERSION"));
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct CopilotToken {
+    pub token: String,
+    /// Unix seconds; `0` means "never expires" (direct-Bearer fallback).
+    pub expires_at: i64,
+}
+
+impl CopilotToken {
+    /// True when the token should be refreshed at `now_secs` (never, for `expires_at == 0`).
+    pub fn needs_refresh(&self, now_secs: i64) -> bool {
+        self.expires_at != 0 && now_secs + EXPIRY_SKEW_SECS >= self.expires_at
+    }
+}
+
+#[derive(Debug, thiserror::Error)]
+pub enum TokenError {
+    #[error("network: {0}")]
+    Network(String),
+    /// The exchange endpoint is unavailable/unauthorized — the caller should fall back to the
+    /// direct-Bearer strategy.
+    #[error("exchange unsupported (status {0})")]
+    ExchangeUnsupported(u16),
+    #[error("unexpected response: {0}")]
+    Unexpected(String),
+}
+
+#[derive(Deserialize)]
+struct ExchangeResponse {
+    token: String,
+    #[serde(default)]
+    expires_at: i64,
+}
+
+/// Performs the token exchange once. `ExchangeUnsupported` signals the caller to fall back.
+pub async fn exchange(
+    client: &reqwest::Client,
+    oauth_token: &str,
+) -> Result<CopilotToken, TokenError> {
+    let resp = client
+        .get(EXCHANGE_URL)
+        .header("authorization", format!("token {oauth_token}"))
+        .header("accept", "application/json")
+        .header("user-agent", USER_AGENT)
+        .send()
+        .await
+        .map_err(|e| TokenError::Network(e.to_string()))?;
+
+    let status = resp.status();
+    if !status.is_success() {
+        // 401/403/404 → this deployment doesn't support the exchange; fall back to direct Bearer.
+        if matches!(status.as_u16(), 401 | 403 | 404) {
+            return Err(TokenError::ExchangeUnsupported(status.as_u16()));
+        }
+        return Err(TokenError::Network(format!("status {}", status.as_u16())));
+    }
+
+    let parsed: ExchangeResponse = resp
+        .json()
+        .await
+        .map_err(|e| TokenError::Unexpected(e.to_string()))?;
+    Ok(CopilotToken {
+        token: parsed.token,
+        expires_at: parsed.expires_at,
+    })
+}
+
+/// Caches the exchanged Copilot token and refreshes it single-flight. Falls back to using the
+/// OAuth token directly (never-expiring) when the exchange endpoint rejects the request.
+pub struct TokenProvider {
+    oauth_token: String,
+    client: reqwest::Client,
+    cached: Mutex<Option<CopilotToken>>,
+}
+
+impl TokenProvider {
+    pub fn new(oauth_token: impl Into<String>, client: reqwest::Client) -> Self {
+        Self {
+            oauth_token: oauth_token.into(),
+            client,
+            cached: Mutex::new(None),
+        }
+    }
+
+    /// Returns a usable Bearer token, refreshing/exchanging as needed. `now_secs` is injected
+    /// for testability.
+    pub async fn token(&self, now_secs: i64) -> CopilotToken {
+        let mut guard = self.cached.lock().await;
+        if let Some(tok) = guard.as_ref() {
+            if !tok.needs_refresh(now_secs) {
+                return tok.clone();
+            }
+        }
+        let fresh = match exchange(&self.client, &self.oauth_token).await {
+            Ok(tok) => tok,
+            Err(_) => CopilotToken {
+                // Direct-Bearer fallback: use the OAuth token itself, never expiring.
+                token: self.oauth_token.clone(),
+                expires_at: 0,
+            },
+        };
+        *guard = Some(fresh.clone());
+        fresh
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn never_expiring_token_does_not_refresh() {
+        let tok = CopilotToken {
+            token: "t".into(),
+            expires_at: 0,
+        };
+        assert!(!tok.needs_refresh(i64::MAX));
+    }
+
+    #[test]
+    fn refresh_triggers_within_skew_window() {
+        let tok = CopilotToken {
+            token: "t".into(),
+            expires_at: 1_000,
+        };
+        assert!(!tok.needs_refresh(800)); // 800 + 120 < 1000
+        assert!(tok.needs_refresh(881)); // 881 + 120 >= 1000
+        assert!(tok.needs_refresh(1_000));
+    }
+}
diff --git a/crates/harness-providers/src/lib.rs b/crates/harness-providers/src/lib.rs
index 63c8f2d..d7f8fb0 100644
--- a/crates/harness-providers/src/lib.rs
+++ b/crates/harness-providers/src/lib.rs
@@ -1,6 +1,14 @@
 pub mod anthropic;
+pub mod auth;
 pub mod codec;
+pub mod copilot;
+pub mod modelsdev;
+pub mod openai;
 pub mod registry;
 
 pub use anthropic::AnthropicProvider;
+pub use auth::{AuthRecord, AuthStorage};
+pub use copilot::CopilotProvider;
+pub use modelsdev::ModelCatalog;
+pub use openai::OpenAiProvider;
 pub use registry::ProviderRegistry;
diff --git a/crates/harness-providers/src/modelsdev.rs b/crates/harness-providers/src/modelsdev.rs
new file mode 100644
index 0000000..6d11bc2
--- /dev/null
+++ b/crates/harness-providers/src/modelsdev.rs
@@ -0,0 +1,296 @@
+//! models.dev metadata: per-model context/output limits, pricing, and capability flags.
+//!
+//! At runtime we prefer a cached copy of `https://models.dev/api.json` (refreshed every 24h);
+//! if the cache is missing/stale and the network is unavailable, we fall back to a baked
+//! snapshot so cost display and limits still work offline.
+
+use std::collections::HashMap;
+use std::path::{Path, PathBuf};
+use std::time::{Duration, SystemTime};
+
+use harness_core::types::{ModelCost, ModelInfo, ModelRef};
+use serde::Deserialize;
+
+const API_URL: &str = "https://models.dev/api.json";
+const CACHE_TTL: Duration = Duration::from_secs(24 * 60 * 60);
+/// Baked fallback, refreshed manually. Keeps cost/limits working with no cache and no network.
+const SNAPSHOT: &str = include_str!("../assets/models-snapshot.json");
+
+/// models.dev top-level shape: `{ provider_id: { models: { model_id: {...} } } }`. Unknown
+/// keys (provider metadata, per-model extras) are ignored.
+#[derive(Deserialize)]
+struct ApiProvider {
+    #[serde(default)]
+    models: HashMap<String, ApiModel>,
+}
+
+#[derive(Deserialize)]
+struct ApiModel {
+    #[serde(default)]
+    reasoning: bool,
+    #[serde(default)]
+    tool_call: bool,
+    #[serde(default)]
+    attachment: bool,
+    #[serde(default)]
+    cost: ApiCost,
+    #[serde(default)]
+    limit: ApiLimit,
+}
+
+#[derive(Deserialize, Default)]
+struct ApiCost {
+    #[serde(default)]
+    input: f64,
+    #[serde(default)]
+    output: f64,
+    #[serde(default)]
+    cache_read: f64,
+    #[serde(default)]
+    cache_write: f64,
+}
+
+#[derive(Deserialize, Default)]
+struct ApiLimit {
+    #[serde(default)]
+    context: u64,
+    #[serde(default)]
+    output: u64,
+}
+
+/// Parsed metadata keyed by `(provider_id, model_id)`.
+#[derive(Debug, Clone, Default)]
+pub struct ModelCatalog {
+    models: HashMap<(String, String), ModelInfo>,
+}
+
+impl ModelCatalog {
+    /// Parses a models.dev `api.json` document.
+    pub fn from_api_json(bytes: &[u8]) -> Result<Self, serde_json::Error> {
+        let raw: HashMap<String, ApiProvider> = serde_json::from_slice(bytes)?;
+        let mut models = HashMap::new();
+        for (provider_id, provider) in raw {
+            for (model_id, m) in provider.models {
+                let info = ModelInfo {
+                    model: ModelRef::new(provider_id.clone(), model_id.clone()),
+                    context_limit: m.limit.context,
+                    output_limit: m.limit.output,
+                    cost: ModelCost {
+                        input: m.cost.input,
+                        output: m.cost.output,
+                        cache_read: m.cost.cache_read,
+                        cache_write: m.cost.cache_write,
+                    },
+                    reasoning: m.reasoning,
+                    tool_call: m.tool_call,
+                    attachment: m.attachment,
+                };
+                models.insert((provider_id.clone(), model_id), info);
+            }
+        }
+        Ok(Self { models })
+    }
+
+    /// The baked fallback snapshot — always available, never fails.
+    pub fn baked() -> Self {
+        Self::from_api_json(SNAPSHOT.as_bytes()).expect("baked models snapshot must parse")
+    }
+
+    pub fn get(&self, provider: &str, model: &str) -> Option<&ModelInfo> {
+        self.models.get(&(provider.to_string(), model.to_string()))
+    }
+
+    /// Pricing for a model, or zero-cost if unknown.
+    pub fn cost(&self, provider: &str, model: &str) -> ModelCost {
+        self.get(provider, model)
+            .map(|m| m.cost)
+            .unwrap_or_default()
+    }
+
+    pub fn len(&self) -> usize {
+        self.models.len()
+    }
+
+    pub fn is_empty(&self) -> bool {
+        self.models.is_empty()
+    }
+
+    /// Default cache location: `~/.cache/ai-harness/models.json`.
+    pub fn default_cache_path() -> PathBuf {
+        dirs::cache_dir()
+            .unwrap_or_else(std::env::temp_dir)
+            .join("ai-harness")
+            .join("models.json")
+    }
+
+    /// Loads from the cache if present and younger than [`CACHE_TTL`]; otherwise returns the
+    /// baked snapshot. Never touches the network — call [`ModelCatalog::refresh`] for that.
+    pub fn load_cached_or_baked(cache_path: &Path) -> Self {
+        if cache_fresh(cache_path, CACHE_TTL, SystemTime::now()) {
+            if let Ok(bytes) = std::fs::read(cache_path) {
+                if let Ok(catalog) = Self::from_api_json(&bytes) {
+                    if !catalog.is_empty() {
+                        return catalog;
+                    }
+                }
+            }
+        }
+        Self::baked()
+    }
+
+    /// Refreshes the default cache location with a fresh HTTP client. Convenience wrapper for
+    /// callers (the app) that don't want to depend on `reqwest` directly.
+    pub async fn refresh_default_cache() -> Result<Self, RefreshError> {
+        let client = reqwest::Client::new();
+        Self::refresh(&client, &Self::default_cache_path()).await
+    }
+
+    /// Fetches the latest metadata and writes it to `cache_path` (best-effort). Returns the
+    /// freshly-parsed catalog. Intended to run in the background so the *next* launch is current.
+    pub async fn refresh(
+        client: &reqwest::Client,
+        cache_path: &Path,
+    ) -> Result<Self, RefreshError> {
+        let bytes = client
+            .get(API_URL)
+            .send()
+            .await
+            .map_err(|e| RefreshError::Network(e.to_string()))?
+            .error_for_status()
+            .map_err(|e| RefreshError::Network(e.to_string()))?
+            .bytes()
+            .await
+            .map_err(|e| RefreshError::Network(e.to_string()))?;
+
+        let catalog =
+            Self::from_api_json(&bytes).map_err(|e| RefreshError::Parse(e.to_string()))?;
+
+        if let Some(parent) = cache_path.parent() {
+            let _ = std::fs::create_dir_all(parent);
+        }
+        if let Err(e) = std::fs::write(cache_path, &bytes) {
+            tracing::warn!(error = %e, "failed to cache models.dev metadata");
+        }
+        Ok(catalog)
+    }
+}
+
+#[derive(Debug, thiserror::Error)]
+pub enum RefreshError {
+    #[error("network: {0}")]
+    Network(String),
+    #[error("parse: {0}")]
+    Parse(String),
+}
+
+/// True when `path` exists and was modified within `ttl` of `now`.
+fn cache_fresh(path: &Path, ttl: Duration, now: SystemTime) -> bool {
+    let Ok(modified) = std::fs::metadata(path).and_then(|m| m.modified()) else {
+        return false;
+    };
+    match now.duration_since(modified) {
+        Ok(age) => age < ttl,
+        Err(_) => true, // modified in the future (clock skew) — treat as fresh
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    const FIXTURE: &str = r#"{
+        "anthropic": {
+            "id": "anthropic",
+            "name": "Anthropic",
+            "models": {
+                "claude-sonnet-4-5": {
+                    "id": "claude-sonnet-4-5",
+                    "reasoning": true,
+                    "tool_call": true,
+                    "attachment": true,
+                    "cost": {"input": 3, "output": 15, "cache_read": 0.3, "cache_write": 3.75},
+                    "limit": {"context": 200000, "output": 64000}
+                }
+            }
+        },
+        "openai": {
+            "id": "openai",
+            "models": {
+                "gpt-4o": {
+                    "id": "gpt-4o",
+                    "tool_call": true,
+                    "cost": {"input": 2.5, "output": 10},
+                    "limit": {"context": 128000, "output": 16384}
+                }
+            }
+        }
+    }"#;
+
+    #[test]
+    fn parses_providers_and_models() {
+        let catalog = ModelCatalog::from_api_json(FIXTURE.as_bytes()).unwrap();
+        assert_eq!(catalog.len(), 2);
+
+        let sonnet = catalog.get("anthropic", "claude-sonnet-4-5").unwrap();
+        assert_eq!(sonnet.context_limit, 200_000);
+        assert_eq!(sonnet.output_limit, 64_000);
+        assert_eq!(sonnet.cost.input, 3.0);
+        assert_eq!(sonnet.cost.cache_write, 3.75);
+        assert!(sonnet.reasoning && sonnet.tool_call && sonnet.attachment);
+    }
+
+    #[test]
+    fn missing_cost_fields_default_to_zero() {
+        let catalog = ModelCatalog::from_api_json(FIXTURE.as_bytes()).unwrap();
+        let gpt = catalog.get("openai", "gpt-4o").unwrap();
+        assert_eq!(gpt.cost.cache_read, 0.0);
+        assert_eq!(gpt.cost.cache_write, 0.0);
+        assert!(!gpt.reasoning); // absent → false
+        assert!(gpt.tool_call);
+    }
+
+    #[test]
+    fn cost_helper_is_zero_for_unknown_model() {
+        let catalog = ModelCatalog::from_api_json(FIXTURE.as_bytes()).unwrap();
+        assert_eq!(catalog.cost("openai", "nonexistent"), ModelCost::default());
+        assert_eq!(catalog.cost("anthropic", "claude-sonnet-4-5").input, 3.0);
+    }
+
+    #[test]
+    fn baked_snapshot_parses_and_is_nonempty() {
+        let catalog = ModelCatalog::baked();
+        assert!(!catalog.is_empty());
+    }
+
+    #[test]
+    fn cache_freshness_respects_ttl() {
+        let dir = tempfile::tempdir().unwrap();
+        let path = dir.path().join("models.json");
+        assert!(!cache_fresh(&path, CACHE_TTL, SystemTime::now())); // missing
+
+        std::fs::write(&path, "{}").unwrap();
+        let now = SystemTime::now();
+        assert!(cache_fresh(&path, CACHE_TTL, now));
+        // A "now" far in the future makes the file look stale.
+        let future = now + Duration::from_secs(48 * 60 * 60);
+        assert!(!cache_fresh(&path, CACHE_TTL, future));
+    }
+
+    #[test]
+    fn load_cached_or_baked_falls_back_when_cache_absent() {
+        let dir = tempfile::tempdir().unwrap();
+        let path = dir.path().join("models.json");
+        let catalog = ModelCatalog::load_cached_or_baked(&path);
+        assert!(!catalog.is_empty()); // baked fallback
+    }
+
+    #[test]
+    fn load_cached_or_baked_reads_fresh_cache() {
+        let dir = tempfile::tempdir().unwrap();
+        let path = dir.path().join("models.json");
+        std::fs::write(&path, FIXTURE).unwrap();
+        let catalog = ModelCatalog::load_cached_or_baked(&path);
+        assert_eq!(catalog.len(), 2);
+        assert!(catalog.get("anthropic", "claude-sonnet-4-5").is_some());
+    }
+}
diff --git a/crates/harness-providers/src/openai.rs b/crates/harness-providers/src/openai.rs
new file mode 100644
index 0000000..9961d17
--- /dev/null
+++ b/crates/harness-providers/src/openai.rs
@@ -0,0 +1,173 @@
+use std::time::Duration;
+
+use async_trait::async_trait;
+use harness_core::llm::{LlmEventStream, LlmRequest, Provider, ProviderError};
+use harness_core::types::ModelInfo;
+use tokio_util::sync::CancellationToken;
+
+use crate::codec::{openai_chat, openai_responses};
+
+const DEFAULT_BASE_URL: &str = "https://api.openai.com/v1";
+
+/// Which OpenAI wire format to speak for a given model.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+enum ApiFlavor {
+    Responses,
+    Chat,
+}
+
+/// `gpt-*` and `o-*` models use the responses API; everything else (including
+/// OpenAI-compatible third-party endpoints) falls back to chat completions.
+fn flavor_for(model: &str) -> ApiFlavor {
+    let is_native = model.starts_with("gpt-")
+        || model.starts_with("o1")
+        || model.starts_with("o3")
+        || model.starts_with("o4")
+        || model == "o1"
+        || model == "o3";
+    if is_native {
+        ApiFlavor::Responses
+    } else {
+        ApiFlavor::Chat
+    }
+}
+
+pub struct OpenAiProvider {
+    api_key: String,
+    base_url: String,
+    client: reqwest::Client,
+}
+
+impl OpenAiProvider {
+    pub fn new(api_key: impl Into<String>) -> Self {
+        Self {
+            api_key: api_key.into(),
+            base_url: DEFAULT_BASE_URL.to_string(),
+            client: reqwest::Client::new(),
+        }
+    }
+
+    pub fn with_base_url(api_key: impl Into<String>, base_url: impl Into<String>) -> Self {
+        Self {
+            api_key: api_key.into(),
+            base_url: base_url.into(),
+            client: reqwest::Client::new(),
+        }
+    }
+
+    fn classify_error(
+        status: reqwest::StatusCode,
+        body: String,
+        retry_after: Option<Duration>,
+    ) -> ProviderError {
+        match status.as_u16() {
+            400 if body.contains("context_length_exceeded")
+                || body.contains("maximum context length") =>
+            {
+                ProviderError::ContextOverflow
+            }
+            401 | 403 => ProviderError::Auth(body),
+            429 => ProviderError::RateLimited { retry_after },
+            s if (500..600).contains(&s) => ProviderError::Overloaded,
+            s => ProviderError::Http { status: s, body },
+        }
+    }
+}
+
+#[async_trait]
+impl Provider for OpenAiProvider {
+    fn id(&self) -> &str {
+        "openai"
+    }
+
+    async fn list_models(&self) -> Result<Vec<ModelInfo>, ProviderError> {
+        // models.dev metadata is layered on top by the caller (see `modelsdev.rs`).
+        Ok(Vec::new())
+    }
+
+    async fn stream(
+        &self,
+        req: LlmRequest,
+        cancel: CancellationToken,
+    ) -> Result<LlmEventStream, ProviderError> {
+        let (path, body) = match flavor_for(&req.model) {
+            ApiFlavor::Responses => ("/responses", openai_responses::build_request(&req)),
+            ApiFlavor::Chat => ("/chat/completions", openai_chat::build_request(&req)),
+        };
+        let url = format!("{}{}", self.base_url, path);
+
+        let send = self
+            .client
+            .post(&url)
+            .header("authorization", format!("Bearer {}", self.api_key))
+            .json(&body)
+            .send();
+
+        let response = tokio::select! {
+            result = send => result.map_err(|e| ProviderError::Network(e.to_string()))?,
+            _ = cancel.cancelled() => return Err(ProviderError::Cancelled),
+        };
+
+        if !response.status().is_success() {
+            let status = response.status();
+            let retry_after = response
+                .headers()
+                .get("retry-after")
+                .and_then(|v| v.to_str().ok())
+                .and_then(|s| s.parse::<u64>().ok())
+                .map(Duration::from_secs);
+            let body = response.text().await.unwrap_or_default();
+            return Err(Self::classify_error(status, body, retry_after));
+        }
+
+        Ok(match flavor_for(&req.model) {
+            ApiFlavor::Responses => openai_responses::decode(response.bytes_stream()),
+            ApiFlavor::Chat => openai_chat::decode(response.bytes_stream()),
+        })
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn routes_gpt_and_o_series_to_responses() {
+        assert_eq!(flavor_for("gpt-4o"), ApiFlavor::Responses);
+        assert_eq!(flavor_for("gpt-5"), ApiFlavor::Responses);
+        assert_eq!(flavor_for("o1"), ApiFlavor::Responses);
+        assert_eq!(flavor_for("o3-mini"), ApiFlavor::Responses);
+        assert_eq!(flavor_for("o4-mini"), ApiFlavor::Responses);
+    }
+
+    #[test]
+    fn routes_other_models_to_chat() {
+        assert_eq!(flavor_for("llama-3.1-70b"), ApiFlavor::Chat);
+        assert_eq!(flavor_for("deepseek-chat"), ApiFlavor::Chat);
+    }
+
+    #[test]
+    fn id_is_openai() {
+        assert_eq!(OpenAiProvider::new("k").id(), "openai");
+    }
+
+    #[test]
+    fn classifies_context_overflow_from_400_body() {
+        assert!(matches!(
+            OpenAiProvider::classify_error(
+                reqwest::StatusCode::BAD_REQUEST,
+                "context_length_exceeded".into(),
+                None,
+            ),
+            ProviderError::ContextOverflow
+        ));
+        assert!(matches!(
+            OpenAiProvider::classify_error(
+                reqwest::StatusCode::BAD_REQUEST,
+                "some other error".into(),
+                None,
+            ),
+            ProviderError::Http { status: 400, .. }
+        ));
+    }
+}
diff --git a/crates/harness-tui/src/input.rs b/crates/harness-tui/src/input.rs
index 26ac273..ee8097c 100644
--- a/crates/harness-tui/src/input.rs
+++ b/crates/harness-tui/src/input.rs
@@ -187,6 +187,8 @@ pub async fn apply_action(action: InputAction, state: &mut AppState, engine: &En
                 state.session_id = Some(id);
                 state.messages.clear();
                 state.scroll_offset = 0;
+                state.session_cost = 0.0;
+                state.session_tokens = 0;
                 state.dirty = true;
             }
             Err(e) => tracing::error!(error = %e, "failed to create session"),
diff --git a/crates/harness-tui/src/render.rs b/crates/harness-tui/src/render.rs
index 8cd3ffd..43994da 100644
--- a/crates/harness-tui/src/render.rs
+++ b/crates/harness-tui/src/render.rs
@@ -171,11 +171,25 @@ fn render_status(frame: &mut Frame, state: &AppState, area: Rect) {
     } else {
         "Ctrl+S: sessions | Esc: abort | Ctrl+C: quit"
     };
-    let text = format!("{spinner}{status} · {hints}");
+    let usage = format!(
+        "{} · ${:.4}",
+        format_tokens(state.session_tokens),
+        state.session_cost
+    );
+    let text = format!("{spinner}{status} · {usage} · {hints}");
     let paragraph = Paragraph::new(text).style(Style::new().fg(Color::Gray));
     frame.render_widget(paragraph, area);
 }
 
+/// Compact token count: `1234` → `1.2k`, `2_000_000` → `2.0M`.
+fn format_tokens(n: u64) -> String {
+    match n {
+        0..=999 => format!("{n} tok"),
+        1_000..=999_999 => format!("{:.1}k tok", n as f64 / 1_000.0),
+        _ => format!("{:.1}M tok", n as f64 / 1_000_000.0),
+    }
+}
+
 fn render_permission_modal(
     frame: &mut Frame,
     request: &harness_core::event::PermissionRequest,
diff --git a/crates/harness-tui/src/snapshots/harness__render__tests__snapshot_empty_session.snap b/crates/harness-tui/src/snapshots/harness__render__tests__snapshot_empty_session.snap
index de758c7..bc0e440 100644
--- a/crates/harness-tui/src/snapshots/harness__render__tests__snapshot_empty_session.snap
+++ b/crates/harness-tui/src/snapshots/harness__render__tests__snapshot_empty_session.snap
@@ -1,6 +1,6 @@
 ---
 source: crates/harness-tui/src/render.rs
-assertion_line: 277
+assertion_line: 300
 expression: buffer_to_string(terminal.backend())
 ---
             new session · orchestrator · anthropic/claude-sonnet-4-5
@@ -26,4 +26,4 @@ expression: buffer_to_string(terminal.backend())
 ┌ input ───────────────────────────────────────────────────────────────────────┐
 │                                                                              │
 └──────────────────────────────────────────────────────────────────────────────┘
-idle · Ctrl+S: sessions | Esc: abort | Ctrl+C: quit
+idle · 0 tok · $0.0000 · Ctrl+S: sessions | Esc: abort | Ctrl+C: quit
diff --git a/crates/harness-tui/src/snapshots/harness__render__tests__snapshot_permission_modal.snap b/crates/harness-tui/src/snapshots/harness__render__tests__snapshot_permission_modal.snap
index e83d418..348638e 100644
--- a/crates/harness-tui/src/snapshots/harness__render__tests__snapshot_permission_modal.snap
+++ b/crates/harness-tui/src/snapshots/harness__render__tests__snapshot_permission_modal.snap
@@ -1,6 +1,6 @@
 ---
 source: crates/harness-tui/src/render.rs
-assertion_line: 397
+assertion_line: 430
 expression: buffer_to_string(terminal.backend())
 ---
             new session · orchestrator · anthropic/claude-sonnet-4-5
@@ -26,4 +26,4 @@ expression: buffer_to_string(terminal.backend())
 ┌ input ───────────────────────────────────────────────────────────────────────┐
 │                                                                              │
 └──────────────────────────────────────────────────────────────────────────────┘
-idle · Ctrl+S: sessions | Esc: abort | Ctrl+C: quit
+idle · 0 tok · $0.0000 · Ctrl+S: sessions | Esc: abort | Ctrl+C: quit
diff --git a/crates/harness-tui/src/snapshots/harness__render__tests__snapshot_session_picker_modal.snap b/crates/harness-tui/src/snapshots/harness__render__tests__snapshot_session_picker_modal.snap
index 82c4456..7411661 100644
--- a/crates/harness-tui/src/snapshots/harness__render__tests__snapshot_session_picker_modal.snap
+++ b/crates/harness-tui/src/snapshots/harness__render__tests__snapshot_session_picker_modal.snap
@@ -1,6 +1,6 @@
 ---
 source: crates/harness-tui/src/render.rs
-assertion_line: 449
+assertion_line: 487
 expression: buffer_to_string(terminal.backend())
 ---
             new session · orchestrator · anthropic/claude-sonnet-4-5
@@ -26,4 +26,4 @@ expression: buffer_to_string(terminal.backend())
 ┌ input ───────────────────────────────────────────────────────────────────────┐
 │                                                                              │
 └──────────────────────────────────────────────────────────────────────────────┘
-idle · Ctrl+S: sessions | Esc: abort | Ctrl+C: quit
+idle · 0 tok · $0.0000 · Ctrl+S: sessions | Esc: abort | Ctrl+C: quit
diff --git a/crates/harness-tui/src/snapshots/harness__render__tests__snapshot_tool_card_completed.snap b/crates/harness-tui/src/snapshots/harness__render__tests__snapshot_tool_card_completed.snap
index c35c01c..f7c0a66 100644
--- a/crates/harness-tui/src/snapshots/harness__render__tests__snapshot_tool_card_completed.snap
+++ b/crates/harness-tui/src/snapshots/harness__render__tests__snapshot_tool_card_completed.snap
@@ -1,6 +1,6 @@
 ---
 source: crates/harness-tui/src/render.rs
-assertion_line: 330
+assertion_line: 357
 expression: buffer_to_string(terminal.backend())
 ---
             new session · orchestrator · anthropic/claude-sonnet-4-5
@@ -26,4 +26,4 @@ expression: buffer_to_string(terminal.backend())
 ┌ input ───────────────────────────────────────────────────────────────────────┐
 │                                                                              │
 └──────────────────────────────────────────────────────────────────────────────┘
-idle · Ctrl+S: sessions | Esc: abort | Ctrl+C: quit
+idle · 0 tok · $0.0000 · Ctrl+S: sessions | Esc: abort | Ctrl+C: quit
diff --git a/crates/harness-tui/src/snapshots/harness__render__tests__snapshot_tool_card_error.snap b/crates/harness-tui/src/snapshots/harness__render__tests__snapshot_tool_card_error.snap
index 24d9403..c0ec904 100644
--- a/crates/harness-tui/src/snapshots/harness__render__tests__snapshot_tool_card_error.snap
+++ b/crates/harness-tui/src/snapshots/harness__render__tests__snapshot_tool_card_error.snap
@@ -1,6 +1,6 @@
 ---
 source: crates/harness-tui/src/render.rs
-assertion_line: 376
+assertion_line: 407
 expression: buffer_to_string(terminal.backend())
 ---
             new session · orchestrator · anthropic/claude-sonnet-4-5
@@ -26,4 +26,4 @@ expression: buffer_to_string(terminal.backend())
 ┌ input ───────────────────────────────────────────────────────────────────────┐
 │                                                                              │
 └──────────────────────────────────────────────────────────────────────────────┘
-idle · Ctrl+S: sessions | Esc: abort | Ctrl+C: quit
+idle · 0 tok · $0.0000 · Ctrl+S: sessions | Esc: abort | Ctrl+C: quit
diff --git a/crates/harness-tui/src/snapshots/harness__render__tests__snapshot_tool_card_running.snap b/crates/harness-tui/src/snapshots/harness__render__tests__snapshot_tool_card_running.snap
index 0b90d93..f5f4ce9 100644
--- a/crates/harness-tui/src/snapshots/harness__render__tests__snapshot_tool_card_running.snap
+++ b/crates/harness-tui/src/snapshots/harness__render__tests__snapshot_tool_card_running.snap
@@ -1,6 +1,6 @@
 ---
 source: crates/harness-tui/src/render.rs
-assertion_line: 353
+assertion_line: 382
 expression: buffer_to_string(terminal.backend())
 ---
             new session · orchestrator · anthropic/claude-sonnet-4-5
@@ -26,4 +26,4 @@ expression: buffer_to_string(terminal.backend())
 ┌ input ───────────────────────────────────────────────────────────────────────┐
 │                                                                              │
 └──────────────────────────────────────────────────────────────────────────────┘
-idle · Ctrl+S: sessions | Esc: abort | Ctrl+C: quit
+idle · 0 tok · $0.0000 · Ctrl+S: sessions | Esc: abort | Ctrl+C: quit
diff --git a/crates/harness-tui/src/snapshots/harness__render__tests__snapshot_user_and_assistant_messages.snap b/crates/harness-tui/src/snapshots/harness__render__tests__snapshot_user_and_assistant_messages.snap
index f4cb1ee..4cc4add 100644
--- a/crates/harness-tui/src/snapshots/harness__render__tests__snapshot_user_and_assistant_messages.snap
+++ b/crates/harness-tui/src/snapshots/harness__render__tests__snapshot_user_and_assistant_messages.snap
@@ -1,6 +1,6 @@
 ---
 source: crates/harness-tui/src/render.rs
-assertion_line: 306
+assertion_line: 331
 expression: buffer_to_string(terminal.backend())
 ---
             new session · orchestrator · anthropic/claude-sonnet-4-5
@@ -26,4 +26,4 @@ expression: buffer_to_string(terminal.backend())
 ┌ input ───────────────────────────────────────────────────────────────────────┐
 │                                                                              │
 └──────────────────────────────────────────────────────────────────────────────┘
-idle · Ctrl+S: sessions | Esc: abort | Ctrl+C: quit
+idle · 0 tok · $0.0000 · Ctrl+S: sessions | Esc: abort | Ctrl+C: quit
diff --git a/crates/harness-tui/src/state.rs b/crates/harness-tui/src/state.rs
index 1b00b18..cb507e1 100644
--- a/crates/harness-tui/src/state.rs
+++ b/crates/harness-tui/src/state.rs
@@ -105,6 +105,10 @@ pub struct AppState {
     pub ctrl_c_pressed: bool,
     /// Width the currently cached part lines were wrapped to; a change invalidates them.
     pub render_width: u16,
+    /// Accumulated session cost in USD, from `SessionCreated`/`SessionUpdated` events.
+    pub session_cost: f64,
+    /// Accumulated session tokens (input + output), for the status bar.
+    pub session_tokens: u64,
 }
 
 impl AppState {
@@ -125,6 +129,8 @@ impl AppState {
             dirty: true,
             ctrl_c_pressed: false,
             render_width: 0,
+            session_cost: 0.0,
+            session_tokens: 0,
         }
     }
 
@@ -147,6 +153,8 @@ impl AppState {
         self.session_title = session.title;
         self.agent = session.agent;
         self.model_ref = format!("{}/{}", session.model.provider_id, session.model.model_id);
+        self.session_cost = session.cost;
+        self.session_tokens = session.usage.input + session.usage.output;
         self.messages.clear();
 
         let mut messages = messages;
@@ -171,6 +179,8 @@ impl AppState {
                     self.agent = session.agent;
                     self.model_ref =
                         format!("{}/{}", session.model.provider_id, session.model.model_id);
+                    self.session_cost = session.cost;
+                    self.session_tokens = session.usage.input + session.usage.output;
                     self.dirty = true;
                 }
             }