diff --git a/hosts/mars/hermes-agent.nix b/hosts/mars/hermes-agent.nix
index 417c32d..0d84e48 100644
--- a/hosts/mars/hermes-agent.nix
+++ b/hosts/mars/hermes-agent.nix
@@ -74,6 +74,9 @@ let
     builtins.readFile ./gitea-pr-review-prompt.md
   );
 
+  # Mnemosyne memory provider: third-party plugin, not in the image.
+  mnemosyneEnv = pkgs.callPackage ../../pkgs/mnemosyne-env.nix { };
+
   # Wire event names (X-GitHub-Event) each route accepts — NOT the
   # subscription names the gitea hooks in services/dev/gitea.nix use. The two
   # namespaces collide; see the long comment on the route unit below.
@@ -418,4 +421,33 @@ in
       mv -f "$tmp" "$conf"
     '';
   };
+
+  # Hermes discovers memory providers under $HERMES_HOME/plugins; the target
+  # is a store path, readable in the container via the /nix/store ro mount.
+  # wantedBy, not requiredBy: a failure leaves Hermes on built-in memory.
+  systemd.services.hermes-agent-mnemosyne-plugin = {
+    description = "Link Mnemosyne provider into the Hermes plugin dir";
+    before = [ "podman-hermes-agent.service" ];
+    wantedBy = [ "podman-hermes-agent.service" ];
+    after = [ "hermes-agent-prepare-dirs.service" ];
+    requires = [ "hermes-agent-prepare-dirs.service" ];
+    path = [ pkgs.coreutils ];
+    serviceConfig = {
+      Type = "oneshot";
+      RemainAfterExit = true;
+    };
+    script = ''
+      set -euo pipefail
+      pluginsDir=${hermesHome}/plugins
+      pluginDir=$pluginsDir/mnemosyne
+      target=${mnemosyneEnv}/${mnemosyneEnv.sitePackages}/hermes_memory_provider
+      [ -d "$target" ] || { echo "$target missing" >&2; exit 1; }
+
+      mkdir -p "$pluginsDir"
+      chown ${hermesUid}:${hermesGid} "$pluginsDir"
+      ln -sfn "$target" "$pluginDir.new"
+      mv -Tf "$pluginDir.new" "$pluginDir"
+      chown -h ${hermesUid}:${hermesGid} "$pluginDir"
+    '';
+  };
 }
diff --git a/pkgs/mnemosyne-env.nix b/pkgs/mnemosyne-env.nix
new file mode 100644
index 0000000..bc57508
--- /dev/null
+++ b/pkgs/mnemosyne-env.nix
@@ -0,0 +1,56 @@
+# Mnemosyne memory provider for Hermes. Built here rather than pip-installed:
+# the image's Python has no pip and is PEP 668 managed.
+#
+# Core deps only: the embeddings extra (fastembed/onnxruntime) is optional at
+# runtime, and recall falls back to FTS5.
+{
+  python3,
+  fetchPypi,
+}:
+
+let
+  mnemosyneMemory = python3.pkgs.buildPythonPackage rec {
+    pname = "mnemosyne-memory";
+    version = "3.15.1";
+    pyproject = true;
+
+    src = fetchPypi {
+      pname = "mnemosyne_memory";
+      inherit version;
+      hash = "sha256-lspUMxc0pUSkhSUrNdiiO5OJ1NMC/S853EYSanXtXKM=";
+    };
+
+    build-system = with python3.pkgs; [ setuptools ];
+    dependencies = with python3.pkgs; [ pyyaml ];
+
+    doCheck = false; # tests want a live Hermes + LLM key
+    # Ships the Hermes plugin package too, not just the core.
+    pythonImportsCheck = [
+      "mnemosyne"
+      "hermes_memory_provider"
+    ];
+  };
+
+  mnemosyneHermes = python3.pkgs.buildPythonPackage rec {
+    pname = "mnemosyne-hermes";
+    version = "0.5.0";
+    pyproject = true;
+
+    src = fetchPypi {
+      pname = "mnemosyne_hermes";
+      inherit version;
+      hash = "sha256-CzEvnUw5oPFtT5bHQQ/GBdy2C/E7qShQn32irIRYKqw=";
+    };
+
+    build-system = with python3.pkgs; [ setuptools ];
+    # Upstream asks for mnemosyne-memory[embeddings]; see the header.
+    dependencies = [ mnemosyneMemory ] ++ (with python3.pkgs; [ pyyaml ]);
+
+    doCheck = false;
+    pythonImportsCheck = [ "mnemosyne_hermes" ];
+  };
+in
+python3.withPackages (_: [
+  mnemosyneMemory
+  mnemosyneHermes
+])