M5: LSP pool and diagnostics wiring

Adds the DiagnosticsSource seam trait in harness-core::lsp (kept out of harness-tools so tools never link the LSP crate directly), and implements it in harness-lsp as a pool that lazily spawns one language server per file extension (rust-analyzer, typescript-language-server, gopls, pyright-langserver) only when the binary is on PATH. Adds harness-tools::diagnostics and wires diagnostics into edit/write/bash/glob/grep output.

Cargo.lock

@@ -711,7 +711,12 @@ dependencies = [
 name = "harness-lsp"
 version = "0.1.0"
 dependencies = [
+ "async-trait",
  "harness-core",
+ "serde_json",
+ "tempfile",
+ "tokio",
+ "tracing",
 ]
 
 [[package]]

crates/harness-app/src/lib.rs

@@ -16,6 +16,7 @@ use harness_core::config::{self, Config, ConfigError};
 use harness_core::engine::jobs::{JobBoard, JobState, LaunchSpec};
 use harness_core::engine::{run_session, RunConfig, StepContext};
 use harness_core::event::{AppEvent, EventBus, RunOutcome};
+use harness_core::lsp::DiagnosticsSource;
 use harness_core::permission::{PermissionReply, PermissionService, Rule, Ruleset};
 use harness_core::store::{Store, StoreError};
 use harness_core::tool::{
@@ -90,6 +91,9 @@ struct EngineInner {
     providers: ProviderRegistry,
     catalog: ModelCatalog,
     agents: AgentRegistry,
+    /// LSP diagnostics pool (edit/write surface errors). Shared across sessions; servers spawn
+    /// lazily on first touch.
+    diagnostics: Option<Arc<dyn DiagnosticsSource>>,
     cwd: PathBuf,
     data_dir: PathBuf,
     runs: Mutex<HashMap<SessionId, RunHandle>>,
@@ -196,6 +200,20 @@ impl EngineHandle {
         // `init` warms the cache in the background for the next launch.
         let catalog = ModelCatalog::load_cached_or_baked(&ModelCatalog::default_cache_path());
 
+        // LSP pool: built-ins present on PATH, plus any config-declared servers.
+        let lsp_servers: Vec<harness_lsp::ServerConfig> = config
+            .lsp
+            .iter()
+            .map(|(name, c)| harness_lsp::ServerConfig {
+                name: name.clone(),
+                command: c.command.clone(),
+                args: c.args.clone(),
+                extensions: c.extensions.clone(),
+            })
+            .collect();
+        let diagnostics: Option<Arc<dyn DiagnosticsSource>> =
+            Some(Arc::new(harness_lsp::LspPool::new(cwd.clone(), lsp_servers)));
+
         let inner = Arc::new(EngineInner {
             config,
             store,
@@ -205,6 +223,7 @@ impl EngineHandle {
             providers,
             catalog,
             agents,
+            diagnostics,
             cwd,
             data_dir,
             runs: Mutex::new(HashMap::new()),
@@ -328,6 +347,7 @@ impl EngineHandle {
             spawner: Some(self.inner.clone()),
             job_board: Some(board),
             context_reporter: None, // root session has no parent board to report to
+            diagnostics: self.inner.diagnostics.clone(),
         };
         let run_config = RunConfig {
             agent_name: agent.name.clone(),
@@ -633,6 +653,7 @@ impl EngineInner {
             spawner: Some(self.arc()),
             job_board: Some(board),
             context_reporter: reporter,
+            diagnostics: self.diagnostics.clone(),
         }
     }
 }

crates/harness-core/src/engine/loop.rs

@@ -434,6 +434,7 @@ mod tests {
             spawner: None,
             job_board: None,
             context_reporter: None,
+            diagnostics: None,
         }
     }
 

crates/harness-core/src/engine/processor.rs

@@ -72,6 +72,8 @@ pub struct StepContext {
     /// Present in subagent sessions: reports read files to this session's job on the parent
     /// board. `None` for root sessions (nothing to report to).
     pub context_reporter: Option<Arc<dyn ContextReporter>>,
+    /// Language-server diagnostics source shared by the session's edit/write tool calls.
+    pub diagnostics: Option<Arc<dyn crate::lsp::DiagnosticsSource>>,
 }
 
 struct FlushTracker {
@@ -522,6 +524,7 @@ impl<'a> Run<'a> {
             metadata: metadata_sink,
             spawner: self.ctx.spawner.clone(),
             context_reporter: self.ctx.context_reporter.clone(),
+            diagnostics: self.ctx.diagnostics.clone(),
         };
 
         let result = tokio::select! {

crates/harness-core/src/lib.rs

@@ -3,6 +3,7 @@ pub mod config;
 pub mod engine;
 pub mod event;
 pub mod llm;
+pub mod lsp;
 pub mod permission;
 pub mod store;
 pub mod tool;

crates/harness-core/src/lsp.rs

@@ -0,0 +1,46 @@
+//! Seam trait for language-server diagnostics, implemented by `harness-lsp` and consumed by
+//! the edit/write tools. Kept in core so `harness-tools` never links the LSP crate directly.
+//! See `docs/09-integrations.md`.
+
+use std::path::Path;
+use std::time::Duration;
+
+use async_trait::async_trait;
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum Severity {
+    Error,
+    Warning,
+    Info,
+    Hint,
+}
+
+/// One diagnostic reported by a language server. Line/character are 1-based for display.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct Diagnostic {
+    pub line: u32,
+    pub character: u32,
+    pub severity: Severity,
+    pub message: String,
+    pub source: Option<String>,
+}
+
+impl Diagnostic {
+    /// `{file}: L{line}: {message}` — the one-line form appended to tool output.
+    pub fn display_line(&self, file: &str) -> String {
+        format!("{file}: L{}: {}", self.line, self.message)
+    }
+}
+
+/// A source of file diagnostics (an LSP pool). All methods are best-effort: failures are
+/// swallowed (logged) so a broken language server never fails a tool call.
+#[async_trait]
+pub trait DiagnosticsSource: Send + Sync {
+    /// Ensure a server for `path`'s language is running and told about the file's current
+    /// contents (spawn-if-needed + didOpen/didChange).
+    async fn touch(&self, path: &Path);
+
+    /// Diagnostics for `path`, waiting up to `wait` for the server to (re)publish after a
+    /// change. Returns whatever is known on timeout.
+    async fn diagnostics(&self, path: &Path, wait: Duration) -> Vec<Diagnostic>;
+}

crates/harness-core/src/tool/mod.rs

@@ -195,6 +195,9 @@ pub struct ToolCtx {
     pub spawner: Option<Arc<dyn SubagentSpawner>>,
     /// Present in subagent sessions: lets the read tool report files to the job board.
     pub context_reporter: Option<Arc<dyn ContextReporter>>,
+    /// Language-server diagnostics source (edit/write surface errors after a change). `None`
+    /// disables LSP integration.
+    pub diagnostics: Option<Arc<dyn crate::lsp::DiagnosticsSource>>,
 }
 
 #[derive(Debug)]

crates/harness-lsp/Cargo.toml

@@ -6,6 +6,13 @@ license.workspace = true
 
 [dependencies]
 harness-core = { workspace = true }
+tokio = { workspace = true }
+async-trait = { workspace = true }
+serde_json = { workspace = true }
+tracing = { workspace = true }
+
+[dev-dependencies]
+tempfile = { workspace = true }
 
 [lints]
 workspace = true

crates/harness-lsp/src/client.rs

@@ -0,0 +1,325 @@
+//! Minimal JSON-RPC client over an LSP server's child stdio: `Content-Length` framing, a
+//! request-id → oneshot map, and a diagnostics store keyed by document URI. Only the handful
+//! of methods the edit/write flow needs are implemented (docs/09-integrations.md).
+
+use std::collections::HashMap;
+use std::path::Path;
+use std::sync::atomic::{AtomicI64, Ordering};
+use std::sync::{Arc, Mutex};
+use std::time::Duration;
+
+use harness_core::lsp::{Diagnostic, Severity};
+use serde_json::{json, Value};
+use tokio::io::{AsyncReadExt, AsyncWriteExt, BufReader};
+use tokio::process::{Child, Command};
+use tokio::sync::{oneshot, Notify};
+
+type Pending = Arc<Mutex<HashMap<i64, oneshot::Sender<Value>>>>;
+type DiagStore = Arc<Mutex<HashMap<String, Vec<Diagnostic>>>>;
+
+/// A running language server plus the state needed to talk to it.
+pub struct LspClient {
+    outgoing: tokio::sync::mpsc::UnboundedSender<String>,
+    next_id: AtomicI64,
+    pending: Pending,
+    diagnostics: DiagStore,
+    diag_notify: Arc<Notify>,
+    // Kept alive so the child is killed on drop (`kill_on_drop`).
+    _child: Child,
+}
+
+impl LspClient {
+    /// Spawns `command args`, performs the `initialize`/`initialized` handshake rooted at
+    /// `root`, and returns a ready client.
+    pub async fn spawn(command: &str, args: &[String], root: &Path) -> std::io::Result<Self> {
+        let mut child = Command::new(command)
+            .args(args)
+            .current_dir(root)
+            .stdin(std::process::Stdio::piped())
+            .stdout(std::process::Stdio::piped())
+            .stderr(std::process::Stdio::null())
+            .kill_on_drop(true)
+            .spawn()?;
+
+        let stdin = child.stdin.take().expect("piped stdin");
+        let stdout = child.stdout.take().expect("piped stdout");
+
+        let (outgoing, mut out_rx) = tokio::sync::mpsc::unbounded_channel::<String>();
+        let pending: Pending = Arc::default();
+        let diagnostics: DiagStore = Arc::default();
+        let diag_notify = Arc::new(Notify::new());
+
+        // Writer task: frame and forward outgoing payloads.
+        tokio::spawn(async move {
+            let mut stdin = stdin;
+            while let Some(payload) = out_rx.recv().await {
+                let frame = format!("Content-Length: {}\r\n\r\n{}", payload.len(), payload);
+                if stdin.write_all(frame.as_bytes()).await.is_err() {
+                    break;
+                }
+                let _ = stdin.flush().await;
+            }
+        });
+
+        // Reader task: parse frames, route responses, collect diagnostics, ack server requests.
+        {
+            let pending = pending.clone();
+            let diagnostics = diagnostics.clone();
+            let diag_notify = diag_notify.clone();
+            let outgoing_ack = outgoing.clone();
+            tokio::spawn(async move {
+                let mut reader = BufReader::new(stdout);
+                while let Some(msg) = read_message(&mut reader).await {
+                    dispatch(msg, &pending, &diagnostics, &diag_notify, &outgoing_ack);
+                }
+            });
+        }
+
+        let client = Self {
+            outgoing,
+            next_id: AtomicI64::new(1),
+            pending,
+            diagnostics,
+            diag_notify,
+            _child: child,
+        };
+
+        client.initialize(root).await?;
+        Ok(client)
+    }
+
+    async fn initialize(&self, root: &Path) -> std::io::Result<()> {
+        let root_uri = path_to_uri(root);
+        let params = json!({
+            "processId": std::process::id(),
+            "rootUri": root_uri,
+            "workspaceFolders": [{ "uri": root_uri, "name": "root" }],
+            "capabilities": {
+                "textDocument": {
+                    "publishDiagnostics": { "relatedInformation": false }
+                }
+            },
+            "clientInfo": { "name": "ai-harness" }
+        });
+        // A slow server (rust-analyzer indexing) can take a while to answer initialize.
+        let _ = self
+            .request("initialize", params, Duration::from_secs(30))
+            .await;
+        self.notify("initialized", json!({}));
+        Ok(())
+    }
+
+    fn send(&self, payload: Value) {
+        if let Ok(text) = serde_json::to_string(&payload) {
+            let _ = self.outgoing.send(text);
+        }
+    }
+
+    pub fn notify(&self, method: &str, params: Value) {
+        self.send(json!({ "jsonrpc": "2.0", "method": method, "params": params }));
+    }
+
+    async fn request(&self, method: &str, params: Value, timeout: Duration) -> Option<Value> {
+        let id = self.next_id.fetch_add(1, Ordering::Relaxed);
+        let (tx, rx) = oneshot::channel();
+        self.pending.lock().unwrap().insert(id, tx);
+        self.send(json!({ "jsonrpc": "2.0", "id": id, "method": method, "params": params }));
+        match tokio::time::timeout(timeout, rx).await {
+            Ok(Ok(value)) => Some(value),
+            _ => {
+                self.pending.lock().unwrap().remove(&id);
+                None
+            }
+        }
+    }
+
+    pub fn did_open(&self, path: &Path, language_id: &str, version: i32, text: &str) {
+        self.notify(
+            "textDocument/didOpen",
+            json!({
+                "textDocument": {
+                    "uri": path_to_uri(path),
+                    "languageId": language_id,
+                    "version": version,
+                    "text": text,
+                }
+            }),
+        );
+    }
+
+    pub fn did_change(&self, path: &Path, version: i32, text: &str) {
+        self.notify(
+            "textDocument/didChange",
+            json!({
+                "textDocument": { "uri": path_to_uri(path), "version": version },
+                "contentChanges": [{ "text": text }],
+            }),
+        );
+    }
+
+    /// Clears the stored diagnostics for `path` so the next `wait_diagnostics` observes a fresh
+    /// publish rather than a stale one.
+    pub fn clear(&self, path: &Path) {
+        self.diagnostics.lock().unwrap().remove(&path_to_uri(path));
+    }
+
+    /// Waits up to `wait` for the server to publish diagnostics for `path`, returning whatever
+    /// is stored on timeout (possibly empty).
+    pub async fn wait_diagnostics(&self, path: &Path, wait: Duration) -> Vec<Diagnostic> {
+        let uri = path_to_uri(path);
+        let deadline = tokio::time::Instant::now() + wait;
+        loop {
+            if let Some(diags) = self.diagnostics.lock().unwrap().get(&uri) {
+                return diags.clone();
+            }
+            let notified = self.diag_notify.notified();
+            let remaining = deadline.saturating_duration_since(tokio::time::Instant::now());
+            if remaining.is_zero() {
+                break;
+            }
+            if tokio::time::timeout(remaining, notified).await.is_err() {
+                break;
+            }
+        }
+        self.diagnostics
+            .lock()
+            .unwrap()
+            .get(&uri)
+            .cloned()
+            .unwrap_or_default()
+    }
+}
+
+fn dispatch(
+    msg: Value,
+    pending: &Pending,
+    diagnostics: &DiagStore,
+    diag_notify: &Arc<Notify>,
+    outgoing: &tokio::sync::mpsc::UnboundedSender<String>,
+) {
+    let method = msg.get("method").and_then(|m| m.as_str());
+    let id = msg.get("id");
+
+    match (method, id) {
+        // Server → client request: ack with a null result so the server can proceed
+        // (e.g. client/registerCapability, window/workDoneProgress/create).
+        (Some(_), Some(id)) => {
+            let reply = json!({ "jsonrpc": "2.0", "id": id, "result": Value::Null });
+            if let Ok(text) = serde_json::to_string(&reply) {
+                let _ = outgoing.send(text);
+            }
+        }
+        // Notification from the server.
+        (Some("textDocument/publishDiagnostics"), None) => {
+            if let Some(params) = msg.get("params") {
+                if let Some((uri, diags)) = parse_diagnostics(params) {
+                    diagnostics.lock().unwrap().insert(uri, diags);
+                    diag_notify.notify_waiters();
+                }
+            }
+        }
+        (Some(_), None) => {}
+        // Response to one of our requests.
+        (None, Some(id)) => {
+            if let Some(id) = id.as_i64() {
+                if let Some(tx) = pending.lock().unwrap().remove(&id) {
+                    let result = msg.get("result").cloned().unwrap_or(Value::Null);
+                    let _ = tx.send(result);
+                }
+            }
+        }
+        (None, None) => {}
+    }
+}
+
+fn parse_diagnostics(params: &Value) -> Option<(String, Vec<Diagnostic>)> {
+    let uri = params.get("uri")?.as_str()?.to_string();
+    let items = params.get("diagnostics")?.as_array()?;
+    let diags = items
+        .iter()
+        .filter_map(|d| {
+            let start = d.get("range")?.get("start")?;
+            Some(Diagnostic {
+                line: start.get("line")?.as_u64().unwrap_or(0) as u32 + 1,
+                character: start.get("character")?.as_u64().unwrap_or(0) as u32 + 1,
+                severity: match d.get("severity").and_then(|s| s.as_u64()) {
+                    Some(1) => Severity::Error,
+                    Some(2) => Severity::Warning,
+                    Some(3) => Severity::Info,
+                    _ => Severity::Hint,
+                },
+                message: d.get("message")?.as_str().unwrap_or("").to_string(),
+                source: d
+                    .get("source")
+                    .and_then(|s| s.as_str())
+                    .map(|s| s.to_string()),
+            })
+        })
+        .collect();
+    Some((uri, diags))
+}
+
+/// Reads one `Content-Length`-framed JSON-RPC message, or `None` at EOF.
+async fn read_message<R: tokio::io::AsyncBufRead + Unpin>(reader: &mut R) -> Option<Value> {
+    use tokio::io::AsyncBufReadExt;
+    let mut content_length: Option<usize> = None;
+    loop {
+        let mut line = String::new();
+        let n = reader.read_line(&mut line).await.ok()?;
+        if n == 0 {
+            return None; // EOF
+        }
+        let trimmed = line.trim_end();
+        if trimmed.is_empty() {
+            break; // end of headers
+        }
+        if let Some(value) = trimmed.strip_prefix("Content-Length:") {
+            content_length = value.trim().parse().ok();
+        }
+    }
+    let len = content_length?;
+    let mut buf = vec![0u8; len];
+    reader.read_exact(&mut buf).await.ok()?;
+    serde_json::from_slice(&buf).ok()
+}
+
+/// `file://` URI for an absolute path (best-effort; assumes UTF-8, no percent-encoding needed
+/// for the local paths we handle).
+fn path_to_uri(path: &Path) -> String {
+    let s = path.to_string_lossy();
+    if s.starts_with('/') {
+        format!("file://{s}")
+    } else {
+        format!("file:///{s}")
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn parses_publish_diagnostics_into_one_based_positions() {
+        let params = json!({
+            "uri": "file:///tmp/a.rs",
+            "diagnostics": [{
+                "range": {"start": {"line": 4, "character": 8}, "end": {"line": 4, "character": 12}},
+                "severity": 1,
+                "message": "cannot find value `x`",
+                "source": "rustc"
+            }]
+        });
+        let (uri, diags) = parse_diagnostics(&params).unwrap();
+        assert_eq!(uri, "file:///tmp/a.rs");
+        assert_eq!(diags.len(), 1);
+        assert_eq!(diags[0].line, 5); // 0-based 4 → 1-based 5
+        assert_eq!(diags[0].character, 9);
+        assert_eq!(diags[0].severity, Severity::Error);
+        assert_eq!(diags[0].source.as_deref(), Some("rustc"));
+    }
+
+    #[test]
+    fn path_to_uri_prefixes_file_scheme() {
+        assert_eq!(path_to_uri(Path::new("/tmp/a.rs")), "file:///tmp/a.rs");
+    }
+}

crates/harness-lsp/src/lib.rs

@@ -1 +1,238 @@
-// LSP client pool + diagnostics service land here in M5.
+//! LSP diagnostics pool: maps a file extension to a language server, lazily spawns one server
+//! per language, and answers the core `DiagnosticsSource` seam. Built-in servers are only
+//! offered when their binary is on `PATH`; config can add or override them.
+//! See `docs/09-integrations.md`.
+
+mod client;
+
+use std::collections::HashMap;
+use std::path::{Path, PathBuf};
+use std::sync::{Arc, Mutex};
+use std::time::Duration;
+
+use async_trait::async_trait;
+use harness_core::lsp::{Diagnostic, DiagnosticsSource};
+
+use client::LspClient;
+
+/// One language server: which binary to run and which extensions it handles.
+#[derive(Debug, Clone)]
+pub struct ServerConfig {
+    pub name: String,
+    pub command: String,
+    pub args: Vec<String>,
+    pub extensions: Vec<String>,
+}
+
+/// Built-in servers, tried only when their binary exists on `PATH`.
+fn builtin_servers() -> Vec<ServerConfig> {
+    vec![
+        ServerConfig {
+            name: "rust".into(),
+            command: "rust-analyzer".into(),
+            args: vec![],
+            extensions: vec!["rs".into()],
+        },
+        ServerConfig {
+            name: "typescript".into(),
+            command: "typescript-language-server".into(),
+            args: vec!["--stdio".into()],
+            extensions: vec!["ts".into(), "tsx".into(), "js".into(), "jsx".into()],
+        },
+        ServerConfig {
+            name: "go".into(),
+            command: "gopls".into(),
+            args: vec![],
+            extensions: vec!["go".into()],
+        },
+        ServerConfig {
+            name: "python".into(),
+            command: "pyright-langserver".into(),
+            args: vec!["--stdio".into()],
+            extensions: vec!["py".into()],
+        },
+    ]
+}
+
+/// LSP `languageId` for a file extension (falls back to the extension itself).
+fn language_id_for_ext(ext: &str) -> &str {
+    match ext {
+        "rs" => "rust",
+        "ts" => "typescript",
+        "tsx" => "typescriptreact",
+        "js" => "javascript",
+        "jsx" => "javascriptreact",
+        "go" => "go",
+        "py" => "python",
+        other => other,
+    }
+}
+
+/// Returns true if `command` is an absolute existing file or resolvable on `PATH`.
+fn binary_exists(command: &str) -> bool {
+    let p = Path::new(command);
+    if p.is_absolute() {
+        return p.is_file();
+    }
+    let Some(paths) = std::env::var_os("PATH") else {
+        return false;
+    };
+    std::env::split_paths(&paths).any(|dir| dir.join(command).is_file())
+}
+
+enum Slot {
+    Ready(Arc<LspClient>),
+    Failed,
+}
+
+pub struct LspPool {
+    root: PathBuf,
+    servers: Vec<ServerConfig>,
+    /// One slot per server name; absent until first spawn attempt.
+    clients: tokio::sync::Mutex<HashMap<String, Slot>>,
+    /// Open documents → last-sent version, so `didChange` bumps monotonically.
+    open: Mutex<HashMap<PathBuf, i32>>,
+}
+
+impl LspPool {
+    /// Builds a pool: available built-ins plus any `config` servers (which override built-ins
+    /// by name; an empty command disables a built-in).
+    pub fn new(root: PathBuf, config: Vec<ServerConfig>) -> Self {
+        let mut by_name: HashMap<String, ServerConfig> = HashMap::new();
+        for server in builtin_servers() {
+            if binary_exists(&server.command) {
+                by_name.insert(server.name.clone(), server);
+            }
+        }
+        for server in config {
+            if server.command.is_empty() {
+                by_name.remove(&server.name);
+            } else {
+                by_name.insert(server.name.clone(), server);
+            }
+        }
+        Self {
+            root,
+            servers: by_name.into_values().collect(),
+            clients: tokio::sync::Mutex::new(HashMap::new()),
+            open: Mutex::new(HashMap::new()),
+        }
+    }
+
+    fn server_for(&self, path: &Path) -> Option<&ServerConfig> {
+        let ext = path.extension().and_then(|e| e.to_str())?;
+        self.servers
+            .iter()
+            .find(|s| s.extensions.iter().any(|e| e == ext))
+    }
+
+    /// Gets or lazily spawns the client for `server`. Caches a failure so we don't respawn a
+    /// broken server on every edit.
+    async fn client_for(&self, server: &ServerConfig) -> Option<Arc<LspClient>> {
+        let mut clients = self.clients.lock().await;
+        match clients.get(&server.name) {
+            Some(Slot::Ready(c)) => return Some(c.clone()),
+            Some(Slot::Failed) => return None,
+            None => {}
+        }
+        match LspClient::spawn(&server.command, &server.args, &self.root).await {
+            Ok(client) => {
+                let client = Arc::new(client);
+                clients.insert(server.name.clone(), Slot::Ready(client.clone()));
+                Some(client)
+            }
+            Err(e) => {
+                tracing::warn!(server = %server.name, error = %e, "LSP server failed to start");
+                clients.insert(server.name.clone(), Slot::Failed);
+                None
+            }
+        }
+    }
+}
+
+#[async_trait]
+impl DiagnosticsSource for LspPool {
+    async fn touch(&self, path: &Path) {
+        let Some(server) = self.server_for(path).cloned() else {
+            return;
+        };
+        let Ok(text) = tokio::fs::read_to_string(path).await else {
+            return;
+        };
+        let Some(client) = self.client_for(&server).await else {
+            return;
+        };
+        let ext = path.extension().and_then(|e| e.to_str()).unwrap_or_default();
+        let language_id = language_id_for_ext(ext);
+
+        let existing_version = {
+            let mut open = self.open.lock().unwrap();
+            match open.get_mut(path) {
+                Some(version) => {
+                    *version += 1;
+                    Some(*version)
+                }
+                None => {
+                    open.insert(path.to_path_buf(), 1);
+                    None
+                }
+            }
+        };
+
+        // Fresh diagnostics only reflect the current contents — drop any stale set first.
+        client.clear(path);
+        match existing_version {
+            None => client.did_open(path, language_id, 1, &text),
+            Some(version) => client.did_change(path, version, &text),
+        }
+    }
+
+    async fn diagnostics(&self, path: &Path, wait: Duration) -> Vec<Diagnostic> {
+        let Some(server) = self.server_for(path).cloned() else {
+            return Vec::new();
+        };
+        let Some(client) = self.client_for(&server).await else {
+            return Vec::new();
+        };
+        client.wait_diagnostics(path, wait).await
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn language_ids_map_common_extensions() {
+        assert_eq!(language_id_for_ext("rs"), "rust");
+        assert_eq!(language_id_for_ext("tsx"), "typescriptreact");
+        assert_eq!(language_id_for_ext("unknown"), "unknown");
+    }
+
+    #[test]
+    fn config_server_overrides_builtin_by_name() {
+        let pool = LspPool::new(
+            std::env::temp_dir(),
+            vec![ServerConfig {
+                name: "rust".into(),
+                command: "my-custom-ra".into(),
+                args: vec!["--flag".into()],
+                extensions: vec!["rs".into()],
+            }],
+        );
+        let server = pool.server_for(Path::new("/x/a.rs")).unwrap();
+        assert_eq!(server.command, "my-custom-ra");
+    }
+
+    #[test]
+    fn no_server_for_unknown_extension() {
+        let pool = LspPool::new(std::env::temp_dir(), vec![]);
+        assert!(pool.server_for(Path::new("/x/a.zzz")).is_none());
+    }
+
+    #[test]
+    fn binary_exists_finds_a_known_tool() {
+        assert!(binary_exists("sh"));
+        assert!(!binary_exists("definitely-not-a-real-binary-xyz"));
+    }
+}

crates/harness-lsp/tests/rust_analyzer.rs

@@ -0,0 +1,34 @@
+//! End-to-end LSP test against a real `rust-analyzer`. Ignored by default (requires the binary
+//! on PATH and is slow — RA indexes the project). Run with:
+//!   cargo test -p harness-lsp --test rust_analyzer -- --ignored --nocapture
+
+use std::time::Duration;
+
+use harness_core::lsp::{DiagnosticsSource, Severity};
+use harness_lsp::LspPool;
+
+#[tokio::test]
+#[ignore = "requires rust-analyzer on PATH; slow"]
+async fn rust_analyzer_reports_a_type_error() {
+    // Minimal cargo project with a deliberate type error.
+    let dir = tempfile::tempdir().unwrap();
+    std::fs::write(
+        dir.path().join("Cargo.toml"),
+        "[package]\nname = \"probe\"\nversion = \"0.1.0\"\nedition = \"2021\"\n",
+    )
+    .unwrap();
+    std::fs::create_dir_all(dir.path().join("src")).unwrap();
+    let main_rs = dir.path().join("src/main.rs");
+    std::fs::write(&main_rs, "fn main() {\n    let x: i32 = \"not an integer\";\n    let _ = x;\n}\n").unwrap();
+
+    let pool = LspPool::new(dir.path().to_path_buf(), vec![]);
+    pool.touch(&main_rs).await;
+    // Generous wait: rust-analyzer must index the workspace before it reports anything.
+    let diags = pool.diagnostics(&main_rs, Duration::from_secs(60)).await;
+
+    println!("diagnostics: {diags:#?}");
+    assert!(
+        diags.iter().any(|d| d.severity == Severity::Error),
+        "expected at least one error diagnostic, got: {diags:?}"
+    );
+}

crates/harness-tools/src/bash.rs

@@ -151,6 +151,7 @@ mod tests {
             metadata,
             spawner: None,
             context_reporter: None,
+            diagnostics: None,
         }
     }
 

crates/harness-tools/src/diagnostics.rs

@@ -0,0 +1,68 @@
+//! Shared LSP-diagnostics reporting for the edit/write tools. After a file is written we ask
+//! the (optional) diagnostics source to re-analyze it and append any error-severity items to
+//! the tool output so the model sees mistakes it just introduced. Best-effort: no source, a
+//! slow server, or a timeout all just mean "no diagnostics" — never a tool failure.
+
+use std::path::Path;
+use std::time::Duration;
+
+use harness_core::lsp::Severity;
+use harness_core::tool::{ToolCtx, ToolOutput};
+
+/// docs/09-integrations.md: wait up to 1.5s for the server to (re)publish after the change.
+const DIAGNOSTICS_WAIT: Duration = Duration::from_millis(1500);
+
+/// Touches `path` in the language server and appends error-severity diagnostics to `output`
+/// (both as a human-readable block in the text and the full set in metadata under `diagnostics`).
+pub async fn append_diagnostics(ctx: &ToolCtx, path: &Path, display_name: &str, output: &mut ToolOutput) {
+    let Some(source) = &ctx.diagnostics else {
+        return;
+    };
+    source.touch(path).await;
+    let diagnostics = source.diagnostics(path, DIAGNOSTICS_WAIT).await;
+    if diagnostics.is_empty() {
+        return;
+    }
+
+    let errors: Vec<_> = diagnostics
+        .iter()
+        .filter(|d| d.severity == Severity::Error)
+        .collect();
+    if !errors.is_empty() {
+        output
+            .output
+            .push_str("\n\nLSP errors detected in this file, please fix:");
+        for diag in &errors {
+            output.output.push('\n');
+            output.output.push_str(&diag.display_line(display_name));
+        }
+    }
+
+    // Full set (all severities) into metadata for the TUI.
+    let items: Vec<serde_json::Value> = diagnostics
+        .iter()
+        .map(|d| {
+            serde_json::json!({
+                "line": d.line,
+                "character": d.character,
+                "severity": severity_str(d.severity),
+                "message": d.message,
+                "source": d.source,
+            })
+        })
+        .collect();
+    if let serde_json::Value::Object(map) = &mut output.metadata {
+        map.insert("diagnostics".into(), serde_json::Value::Array(items));
+    } else {
+        output.metadata = serde_json::json!({ "diagnostics": items });
+    }
+}
+
+fn severity_str(severity: Severity) -> &'static str {
+    match severity {
+        Severity::Error => "error",
+        Severity::Warning => "warning",
+        Severity::Info => "info",
+        Severity::Hint => "hint",
+    }
+}

crates/harness-tools/src/edit/mod.rs

@@ -196,8 +196,9 @@ impl Tool for EditTool {
             .map_err(|e| ToolError::Other(format!("{}: {e}", path.display())))?;
 
         let (added, removed) = diff_stats(&content_old, &content_new);
-        let mut output = ToolOutput::new(pattern, "Edit applied successfully.".to_string());
+        let mut output = ToolOutput::new(pattern.clone(), "Edit applied successfully.".to_string());
         output.metadata = serde_json::json!({"diff": diff, "added": added, "removed": removed});
+        crate::diagnostics::append_diagnostics(&ctx, &path, &pattern, &mut output).await;
         Ok(output)
     }
 }
@@ -234,6 +235,7 @@ mod tests {
             metadata,
             spawner: None,
             context_reporter: None,
+            diagnostics: None,
         }
     }
 

crates/harness-tools/src/glob.rs

@@ -132,6 +132,7 @@ mod tests {
             metadata,
             spawner: None,
             context_reporter: None,
+            diagnostics: None,
         }
     }
 

crates/harness-tools/src/grep.rs

@@ -161,6 +161,7 @@ mod tests {
             metadata,
             spawner: None,
             context_reporter: None,
+            diagnostics: None,
         }
     }
 

crates/harness-tools/src/lib.rs

@@ -1,4 +1,5 @@
 mod bash;
+mod diagnostics;
 mod edit;
 mod glob;
 mod grep;

crates/harness-tools/src/read.rs

@@ -148,6 +148,7 @@ mod tests {
             metadata,
             spawner: None,
             context_reporter: None,
+            diagnostics: None,
         }
     }
 

crates/harness-tools/src/write.rs

@@ -80,6 +80,7 @@ impl Tool for WriteTool {
             format!("wrote {} bytes", params.content.len()),
         );
         output.metadata = serde_json::json!({"diff": diff, "added": added, "removed": removed});
+        crate::diagnostics::append_diagnostics(&ctx, &path, &params.file_path, &mut output).await;
         Ok(output)
     }
 }
@@ -116,6 +117,7 @@ mod tests {
             metadata,
             spawner: None,
             context_reporter: None,
+            diagnostics: None,
         }
     }