diff --git a/apps/server/src/core/ai-chat/tools/docmost-client.loader.ts b/apps/server/src/core/ai-chat/tools/docmost-client.loader.ts
index 5b740cfe..e27bfb9a 100644
--- a/apps/server/src/core/ai-chat/tools/docmost-client.loader.ts
+++ b/apps/server/src/core/ai-chat/tools/docmost-client.loader.ts
@@ -5,6 +5,34 @@ import { pathToFileURL } from 'node:url';
  * ESM-only `@docmost/mcp` package. We only need the constructor + the read/write
  * methods used by the per-user tool adapter; the full client surface lives in
  * `packages/mcp/src/client.ts`. Signatures here mirror that file exactly.
+ *
+ * DRIFT GUARD: the method NAMES below are runtime-checked against the real
+ * `DocmostClient` by `packages/mcp/test/unit/client-host-contract.test.mjs`
+ * (which can import the ESM class directly). If you rename/remove a method here
+ * or in client.ts, that test fails — so a stale mirror cannot silently ship a
+ * runtime "x is not a function" into an agent tool call. Keep the two in sync.
+ *
+ * STAGED PLAN — full derivation `DocmostClientLike = <real DocmostClient type>`
+ * (issue #193, layer 3) is intentionally NOT done; it stays a hand-mirror for
+ * now because of two verified blockers across the ESM(mcp)/CJS(server) boundary:
+ *   1. `@docmost/mcp` emits NO declaration files (its tsconfig has no
+ *      `declaration`, package.json has no `types`/types-export) and the server
+ *      tsconfig has no path mapping for it — the server only loads it via the
+ *      runtime `import()` trick below, so there is no type to import today.
+ *   2. The real client methods have inferred, CONCRETE return types; the in-app
+ *      tool adapter reads results through loose `Record<string,unknown>` returns
+ *      + `as` casts (e.g. `(result?.data ?? {}) as { title?: string }`).
+ *      Deriving the exact type would make those casts non-overlapping ("may be a
+ *      mistake") and break the build, and `Partial<DocmostClientLike>` test stubs
+ *      would have to satisfy the full concrete surface.
+ * To do it safely later (incrementally): (a) turn on `declaration: true` in
+ * packages/mcp/tsconfig.json + add a `types` export condition and commit the
+ * emitted `.d.ts`; (b) `import type { DocmostClient } from '@docmost/mcp'` here
+ * and replace this interface with a `Pick<DocmostClient, ...>` of the consumed
+ * methods; (c) audit every `as` cast in ai-chat-tools.service.ts against the now
+ * concrete return types (double-cast through `unknown` only where genuinely
+ * needed); (d) keep the runtime guard test as a belt-and-braces check. Until
+ * then the guard test above is the cheap, behaviour-neutral protection.
  */
 export interface DocmostClientLike {
   // --- read ---
diff --git a/packages/mcp/test/unit/client-host-contract.test.mjs b/packages/mcp/test/unit/client-host-contract.test.mjs
new file mode 100644
index 00000000..c6ca8978
--- /dev/null
+++ b/packages/mcp/test/unit/client-host-contract.test.mjs
@@ -0,0 +1,100 @@
+import { test } from "node:test";
+import assert from "node:assert/strict";
+
+import { DocmostClient } from "../../build/index.js";
+
+// Drift guard for the THIRD hand-written layer of the AI tool set (issue #193,
+// layer 3): the in-app server hand-mirrors the DocmostClient method signatures
+// it consumes as the `DocmostClientLike` interface in
+// apps/server/src/core/ai-chat/tools/docmost-client.loader.ts ("Signatures here
+// mirror that file exactly"). That mirror lives across the ESM(mcp)/CJS(server)
+// boundary and the package ships NO .d.ts, so the server typecheck cannot verify
+// the names against the real class — a rename/removal in client.ts would surface
+// only as a runtime "x is not a function" inside an agent tool call.
+//
+// This test pins the contract from the mcp side (ESM, where the real class is
+// directly importable): every method the embedding host depends on MUST exist as
+// a function on a real DocmostClient instance. If you rename/remove a client
+// method, this fails here AND you must update DocmostClientLike to match.
+//
+// Keep HOST_CONTRACT_METHODS in sync with the methods declared in the server's
+// DocmostClientLike interface (the in-app per-user tool adapter only — it is the
+// superset of what either transport calls). Full type-derivation of
+// DocmostClientLike from this class is deferred (see the staged plan in
+// docmost-client.loader.ts): the package emits no declarations and the real
+// (inferred, concrete) return types conflict with the host's loose
+// `Record<string,unknown>` + `as`-cast result handling.
+const HOST_CONTRACT_METHODS = [
+  // read
+  "search",
+  "getPage",
+  "getWorkspace",
+  "getSpaces",
+  "listPages",
+  "listSidebarPages",
+  "getOutline",
+  "getPageJson",
+  "getNode",
+  "getTable",
+  "listComments",
+  "getComment",
+  "checkNewComments",
+  "listShares",
+  "listPageHistory",
+  "getPageHistory",
+  "diffPageVersions",
+  "exportPageMarkdown",
+  // write (page)
+  "createPage",
+  "updatePage",
+  "renamePage",
+  "movePage",
+  "deletePage",
+  "editPageText",
+  "patchNode",
+  "insertNode",
+  "deleteNode",
+  "updatePageJson",
+  "tableInsertRow",
+  "tableDeleteRow",
+  "tableUpdateCell",
+  "copyPageContent",
+  "importPageMarkdown",
+  "sharePage",
+  "unsharePage",
+  "restorePageVersion",
+  "transformPage",
+  // write (comment)
+  "createComment",
+  "resolveComment",
+];
+
+test("DocmostClient implements every method the in-app DocmostClientLike mirror declares", () => {
+  // The constructor is side-effect-free (no network/login on construction): it
+  // only stores config and creates an axios instance, so it is safe to build a
+  // throwaway instance here with a dummy token provider.
+  const client = new DocmostClient({
+    apiUrl: "http://127.0.0.1:1/api",
+    getToken: async () => "test-token",
+  });
+
+  const missing = HOST_CONTRACT_METHODS.filter(
+    (name) => typeof client[name] !== "function",
+  );
+
+  assert.deepEqual(
+    missing,
+    [],
+    `DocmostClient is missing host-contract method(s): ${missing.join(", ")}. ` +
+      `Update packages/mcp/src/client.ts and/or the server's DocmostClientLike ` +
+      `interface (apps/server/src/core/ai-chat/tools/docmost-client.loader.ts) ` +
+      `so the hand-mirrored signatures stay in sync.`,
+  );
+});
+
+test("HOST_CONTRACT_METHODS has no duplicates", () => {
+  assert.equal(
+    new Set(HOST_CONTRACT_METHODS).size,
+    HOST_CONTRACT_METHODS.length,
+  );
+});