fix: accept Grok ACP extension notifications without Method not found

Grok emits `_x.ai/session_notification` / `_x.ai/session/update` for
hook_execution status. The ACP SDK routes those to Client.extNotification;
without it, every successful post_tool_use hook logged -32601 Method not
found. Implement no-op extMethod/extNotification on default and bridging
handlers in acp-runtime and the Grok vendored copy.

plugins/fusion-plugin-acp-runtime/src/__tests__/provider-handshake.test.ts

@@ -5,6 +5,7 @@ import {
   IncompatibleProtocolError,
   HandshakeTimeoutError,
   createDefaultClientHandler,
+  createBridgingClientHandler,
 } from "../provider.js";
 import { killAllProcesses, activeProcessCount } from "../process-manager.js";
 
@@ -93,4 +94,34 @@ describe("connect() handshake", () => {
     const res = await handler.requestPermission({} as never);
     expect(res).toEqual({ outcome: { outcome: "cancelled" } });
   });
+
+  /*
+  FNXC:GrokAcp 2026-07-12-07:00:
+  Grok sends `_x.ai/session_notification` for hook_execution; without
+  extNotification the ACP SDK logs Method not found (-32601).
+  */
+  it("default and bridging handlers accept x.ai extension notifications without Method not found", async () => {
+    const defaultHandler = createDefaultClientHandler();
+    await expect(
+      defaultHandler.extNotification?.("_x.ai/session_notification", {
+        sessionId: "s1",
+        update: {
+          sessionUpdate: "hook_execution",
+          event_name: "post_tool_use",
+          tool_name: "read_file",
+          runs: [{ name: "global/settings:post_tool_use[0].hooks[0]", status: { status: "success", elapsed_ms: 9 } }],
+        },
+      }),
+    ).resolves.toBeUndefined();
+    await expect(defaultHandler.extMethod?.("_x.ai/example", { a: 1 })).resolves.toEqual({});
+
+    const { handler } = createBridgingClientHandler({});
+    await expect(
+      handler.extNotification?.("_x.ai/session/update", {
+        sessionId: "s1",
+        update: { sessionUpdate: "hook_execution", event_name: "stop", runs: [] },
+      }),
+    ).resolves.toBeUndefined();
+    await expect(handler.extMethod?.("x.ai/session_notification", {})).resolves.toEqual({});
+  });
 });

plugins/fusion-plugin-acp-runtime/src/provider.ts

@@ -67,6 +67,30 @@ export class HandshakeTimeoutError extends Error {
   }
 }
 
+/*
+FNXC:GrokAcp 2026-07-12-07:00:
+Grok ACP emits vendor extension notifications such as `_x.ai/session_notification`
+and `_x.ai/session/update` for hook_execution status (post_tool_use, etc.).
+@agentclientprotocol/sdk routes non-spec methods to Client.extNotification /
+extMethod; when those are absent the SDK logs Method not found (-32601) for
+every hook even though hooks themselves succeeded. Accept extensions as no-ops
+so Fusion clients stay forward-compatible without claiming Grok-only features.
+Do not forward into sessionUpdate: hook_execution is not a standard
+sessionUpdate tag and would fail zSessionNotification validation.
+*/
+
+/** No-op ACP extension handlers so vendor notifications do not Method-not-found. */
+function acceptAcpExtensions(): Pick<Client, "extMethod" | "extNotification"> {
+  return {
+    async extMethod(_method: string, _params: Record<string, unknown>) {
+      return {};
+    },
+    async extNotification(_method: string, _params: Record<string, unknown>) {
+      // Intentionally empty — informational Grok hooks / x.ai session extensions.
+    },
+  };
+}
+
 /**
  * Minimal default client handler. Later units (U3/U4/U5/U7) supply the real one
  * that bridges `session/update` into Fusion callbacks and routes permission
@@ -81,6 +105,7 @@ export function createDefaultClientHandler(): Client {
     async requestPermission() {
       return { outcome: { outcome: "cancelled" } };
     },
+    ...acceptAcpExtensions(),
   };
 }
 
@@ -182,6 +207,9 @@ export function createBridgingClientHandler(
         );
       });
     },
+    // FNXC:GrokAcp 2026-07-12-07:00: swallow `_x.ai/*` extension notifications
+    // (hook_execution, session admin, …) so ACP SDK does not log -32601.
+    ...acceptAcpExtensions(),
   };
 
   // Register fs handlers ONLY when enabled, so the advertised capability and the

plugins/fusion-plugin-grok-runtime/src/acp/provider.ts

@@ -68,6 +68,30 @@ export class HandshakeTimeoutError extends Error {
   }
 }
 
+/*
+FNXC:GrokAcp 2026-07-12-07:00:
+Grok ACP emits vendor extension notifications such as `_x.ai/session_notification`
+and `_x.ai/session/update` for hook_execution status (post_tool_use, etc.).
+@agentclientprotocol/sdk routes non-spec methods to Client.extNotification /
+extMethod; when those are absent the SDK logs Method not found (-32601) for
+every hook even though hooks themselves succeeded. Accept extensions as no-ops
+so Fusion clients stay forward-compatible without claiming Grok-only features.
+Do not forward into sessionUpdate: hook_execution is not a standard
+sessionUpdate tag and would fail zSessionNotification validation.
+*/
+
+/** No-op ACP extension handlers so vendor notifications do not Method-not-found. */
+function acceptAcpExtensions(): Pick<Client, "extMethod" | "extNotification"> {
+  return {
+    async extMethod(_method: string, _params: Record<string, unknown>) {
+      return {};
+    },
+    async extNotification(_method: string, _params: Record<string, unknown>) {
+      // Intentionally empty — informational Grok hooks / x.ai session extensions.
+    },
+  };
+}
+
 /**
  * Minimal default client handler. Later units (U3/U4/U5/U7) supply the real one
  * that bridges `session/update` into Fusion callbacks and routes permission
@@ -82,6 +106,7 @@ export function createDefaultClientHandler(): Client {
     async requestPermission() {
       return { outcome: { outcome: "cancelled" } };
     },
+    ...acceptAcpExtensions(),
   };
 }
 
@@ -183,6 +208,9 @@ export function createBridgingClientHandler(
         );
       });
     },
+    // FNXC:GrokAcp 2026-07-12-07:00: swallow `_x.ai/*` extension notifications
+    // (hook_execution, session admin, …) so ACP SDK does not log -32601.
+    ...acceptAcpExtensions(),
   };
 
   // Register fs handlers ONLY when enabled, so the advertised capability and the