diff --git a/src/App.tsx b/src/App.tsx
index ac8dddf4..717538b9 100644
--- a/src/App.tsx
+++ b/src/App.tsx
@@ -1850,6 +1850,7 @@ function MainApp() {
     connectWorkspace,
     sendUserMessageToThread,
     setSelectedCollaborationModeId,
+    persistThreadCodexParams,
   });
 
   const { handleMoveWorkspace } = useWorkspaceOrderingOrchestration({
diff --git a/src/features/app/hooks/usePlanReadyActions.test.tsx b/src/features/app/hooks/usePlanReadyActions.test.tsx
new file mode 100644
index 00000000..9155f511
--- /dev/null
+++ b/src/features/app/hooks/usePlanReadyActions.test.tsx
@@ -0,0 +1,260 @@
+// @vitest-environment jsdom
+import { act, renderHook } from "@testing-library/react";
+import { describe, expect, it, vi } from "vitest";
+import type { CollaborationModeOption, WorkspaceInfo } from "@/types";
+import { usePlanReadyActions } from "@app/hooks/usePlanReadyActions";
+
+function makeMode(id: string): CollaborationModeOption {
+  return {
+    id,
+    label: id,
+    mode: id,
+    model: "",
+    reasoningEffort: null,
+    developerInstructions: null,
+    value: {},
+  };
+}
+
+const connectedWorkspace: WorkspaceInfo = {
+  id: "ws-1",
+  name: "Workspace",
+  path: "/tmp/workspace",
+  connected: true,
+  settings: { sidebarCollapsed: false },
+};
+
+const disconnectedWorkspace: WorkspaceInfo = {
+  ...connectedWorkspace,
+  connected: false,
+};
+
+function renderPlanReadyActions(overrides?: {
+  activeWorkspace?: WorkspaceInfo | null;
+  collaborationModes?: CollaborationModeOption[];
+  resolvedModel?: string | null;
+  resolvedEffort?: string | null;
+}) {
+  const connectWorkspace = vi.fn().mockResolvedValue(undefined);
+  const sendUserMessageToThread = vi.fn().mockResolvedValue(undefined);
+  const setSelectedCollaborationModeId = vi.fn();
+  const persistThreadCodexParams = vi.fn();
+
+  const options = {
+    activeWorkspace: connectedWorkspace as WorkspaceInfo | null,
+    activeThreadId: "thread-1" as string | null,
+    collaborationModes: [makeMode("plan"), makeMode("default"), makeMode("code")],
+    resolvedModel: "gpt-5.2" as string | null,
+    resolvedEffort: "high" as string | null,
+    ...overrides,
+    connectWorkspace,
+    sendUserMessageToThread,
+    setSelectedCollaborationModeId,
+    persistThreadCodexParams,
+  };
+
+  const hook = renderHook(() => usePlanReadyActions(options));
+  return {
+    ...hook,
+    connectWorkspace,
+    sendUserMessageToThread,
+    setSelectedCollaborationModeId,
+    persistThreadCodexParams,
+  };
+}
+
+describe("usePlanReadyActions", () => {
+  it("uses default mode when implementing a plan and persists selection", async () => {
+    const {
+      result,
+      setSelectedCollaborationModeId,
+      persistThreadCodexParams,
+      sendUserMessageToThread,
+    } = renderPlanReadyActions();
+
+    await act(async () => {
+      await result.current.handlePlanAccept();
+    });
+
+    expect(setSelectedCollaborationModeId).toHaveBeenCalledWith("default");
+    expect(persistThreadCodexParams).toHaveBeenCalledWith({
+      collaborationModeId: "default",
+    });
+    expect(sendUserMessageToThread).toHaveBeenCalledWith(
+      connectedWorkspace,
+      "thread-1",
+      "[[cm_plan_ready:accept]] Implement this plan.",
+      [],
+      {
+        collaborationMode: {
+          mode: "default",
+          settings: {
+            developer_instructions: null,
+            model: "gpt-5.2",
+            reasoning_effort: "high",
+          },
+        },
+      },
+    );
+  });
+
+  it("falls back to first non-plan mode when default/code are unavailable", async () => {
+    const {
+      result,
+      setSelectedCollaborationModeId,
+      persistThreadCodexParams,
+      sendUserMessageToThread,
+    } = renderPlanReadyActions({
+      collaborationModes: [makeMode("plan"), makeMode("review")],
+    });
+
+    await act(async () => {
+      await result.current.handlePlanAccept();
+    });
+
+    expect(setSelectedCollaborationModeId).toHaveBeenCalledWith("review");
+    expect(persistThreadCodexParams).toHaveBeenCalledWith({
+      collaborationModeId: "review",
+    });
+    expect(sendUserMessageToThread).toHaveBeenCalledWith(
+      connectedWorkspace,
+      "thread-1",
+      "[[cm_plan_ready:accept]] Implement this plan.",
+      [],
+      expect.objectContaining({
+        collaborationMode: expect.objectContaining({
+          mode: "review",
+        }),
+      }),
+    );
+  });
+
+  it("forces neutral mode override when only plan mode is available", async () => {
+    const {
+      result,
+      setSelectedCollaborationModeId,
+      persistThreadCodexParams,
+      sendUserMessageToThread,
+    } = renderPlanReadyActions({
+      collaborationModes: [makeMode("plan")],
+    });
+
+    await act(async () => {
+      await result.current.handlePlanAccept();
+    });
+
+    expect(setSelectedCollaborationModeId).toHaveBeenCalledWith(null);
+    expect(persistThreadCodexParams).toHaveBeenCalledWith({
+      collaborationModeId: null,
+    });
+    expect(sendUserMessageToThread).toHaveBeenCalledWith(
+      connectedWorkspace,
+      "thread-1",
+      "[[cm_plan_ready:accept]] Implement this plan.",
+      [],
+      { collaborationMode: null },
+    );
+  });
+
+  it("forces neutral mode override when no collaboration modes are available", async () => {
+    const {
+      result,
+      setSelectedCollaborationModeId,
+      persistThreadCodexParams,
+      sendUserMessageToThread,
+    } = renderPlanReadyActions({
+      collaborationModes: [],
+    });
+
+    await act(async () => {
+      await result.current.handlePlanAccept();
+    });
+
+    expect(setSelectedCollaborationModeId).toHaveBeenCalledWith(null);
+    expect(persistThreadCodexParams).toHaveBeenCalledWith({
+      collaborationModeId: null,
+    });
+    expect(sendUserMessageToThread).toHaveBeenCalledWith(
+      connectedWorkspace,
+      "thread-1",
+      "[[cm_plan_ready:accept]] Implement this plan.",
+      [],
+      { collaborationMode: null },
+    );
+  });
+
+  it("connects workspace before sending plan accept message", async () => {
+    const { result, connectWorkspace, sendUserMessageToThread } =
+      renderPlanReadyActions({
+        activeWorkspace: disconnectedWorkspace,
+      });
+
+    await act(async () => {
+      await result.current.handlePlanAccept();
+    });
+
+    expect(connectWorkspace).toHaveBeenCalledWith(disconnectedWorkspace);
+    expect(sendUserMessageToThread).toHaveBeenCalledTimes(1);
+  });
+
+  it("keeps plan mode for plan-change follow-up and persists it", async () => {
+    const {
+      result,
+      setSelectedCollaborationModeId,
+      persistThreadCodexParams,
+      sendUserMessageToThread,
+    } = renderPlanReadyActions({
+      collaborationModes: [makeMode("default"), makeMode("plan")],
+    });
+
+    await act(async () => {
+      await result.current.handlePlanSubmitChanges("  Add tests  ");
+    });
+
+    expect(setSelectedCollaborationModeId).toHaveBeenCalledWith("plan");
+    expect(persistThreadCodexParams).toHaveBeenCalledWith({
+      collaborationModeId: "plan",
+    });
+    expect(sendUserMessageToThread).toHaveBeenCalledWith(
+      connectedWorkspace,
+      "thread-1",
+      "[[cm_plan_ready:changes]] Update the plan with these changes:\n\nAdd tests",
+      [],
+      {
+        collaborationMode: {
+          mode: "plan",
+          settings: {
+            developer_instructions: null,
+            model: "gpt-5.2",
+            reasoning_effort: "high",
+          },
+        },
+      },
+    );
+  });
+
+  it("uses neutral mode override for plan-change follow-up when plan mode is unavailable", async () => {
+    const {
+      result,
+      setSelectedCollaborationModeId,
+      persistThreadCodexParams,
+      sendUserMessageToThread,
+    } = renderPlanReadyActions({
+      collaborationModes: [makeMode("default")],
+    });
+
+    await act(async () => {
+      await result.current.handlePlanSubmitChanges("  Add tests  ");
+    });
+
+    expect(setSelectedCollaborationModeId).not.toHaveBeenCalled();
+    expect(persistThreadCodexParams).not.toHaveBeenCalled();
+    expect(sendUserMessageToThread).toHaveBeenCalledWith(
+      connectedWorkspace,
+      "thread-1",
+      "[[cm_plan_ready:changes]] Update the plan with these changes:\n\nAdd tests",
+      [],
+      { collaborationMode: null },
+    );
+  });
+});
diff --git a/src/features/app/hooks/usePlanReadyActions.ts b/src/features/app/hooks/usePlanReadyActions.ts
index e9c028ed..a54e6163 100644
--- a/src/features/app/hooks/usePlanReadyActions.ts
+++ b/src/features/app/hooks/usePlanReadyActions.ts
@@ -25,7 +25,8 @@ type UsePlanReadyActionsOptions = {
   resolvedEffort: string | null;
   connectWorkspace: (workspace: WorkspaceInfo) => Promise<void>;
   sendUserMessageToThread: SendUserMessageToThread;
-  setSelectedCollaborationModeId: (modeId: string) => void;
+  setSelectedCollaborationModeId: (modeId: string | null) => void;
+  persistThreadCodexParams: (patch: { collaborationModeId?: string | null }) => void;
 };
 
 export function usePlanReadyActions({
@@ -37,6 +38,7 @@ export function usePlanReadyActions({
   connectWorkspace,
   sendUserMessageToThread,
   setSelectedCollaborationModeId,
+  persistThreadCodexParams,
 }: UsePlanReadyActionsOptions) {
   const findCollaborationMode = useCallback(
     (wanted: string) => {
@@ -57,6 +59,28 @@ export function usePlanReadyActions({
     [collaborationModes],
   );
 
+  const isPlanMode = useCallback((mode: CollaborationModeOption | null) => {
+    if (!mode) {
+      return false;
+    }
+    const modeValue = (mode.mode || mode.id).trim().toLowerCase();
+    return modeValue === "plan";
+  }, []);
+
+  const findImplementationMode = useCallback(() => {
+    const defaultMode = findCollaborationMode("default");
+    if (defaultMode && !isPlanMode(defaultMode)) {
+      return defaultMode;
+    }
+
+    const codeMode = findCollaborationMode("code");
+    if (codeMode && !isPlanMode(codeMode)) {
+      return codeMode;
+    }
+
+    return collaborationModes.find((mode) => !isPlanMode(mode)) ?? null;
+  }, [collaborationModes, findCollaborationMode, isPlanMode]);
+
   const buildCollaborationModePayloadFor = useCallback(
     (mode: CollaborationModeOption | null) => {
       if (!mode) {
@@ -93,31 +117,28 @@ export function usePlanReadyActions({
       await connectWorkspace(activeWorkspace);
     }
 
-    const defaultMode =
-      findCollaborationMode("default") ??
-      findCollaborationMode("code") ??
-      collaborationModes[0] ??
-      null;
-
-    if (defaultMode?.id) {
-      setSelectedCollaborationModeId(defaultMode.id);
-    }
+    const implementationMode = findImplementationMode();
+    const implementationModeId = implementationMode?.id ?? null;
+    setSelectedCollaborationModeId(implementationModeId);
+    persistThreadCodexParams({
+      collaborationModeId: implementationModeId,
+    });
 
-    const collaborationMode = buildCollaborationModePayloadFor(defaultMode);
+    const collaborationMode = buildCollaborationModePayloadFor(implementationMode);
     await sendUserMessageToThread(
       activeWorkspace,
       activeThreadId,
       makePlanReadyAcceptMessage(),
       [],
-      collaborationMode ? { collaborationMode } : undefined,
+      { collaborationMode: collaborationMode ?? null },
     );
   }, [
     activeThreadId,
     activeWorkspace,
     buildCollaborationModePayloadFor,
-    collaborationModes,
     connectWorkspace,
-    findCollaborationMode,
+    findImplementationMode,
+    persistThreadCodexParams,
     sendUserMessageToThread,
     setSelectedCollaborationModeId,
   ]);
@@ -136,6 +157,9 @@ export function usePlanReadyActions({
       const planMode = findCollaborationMode("plan");
       if (planMode?.id) {
         setSelectedCollaborationModeId(planMode.id);
+        persistThreadCodexParams({
+          collaborationModeId: planMode.id,
+        });
       }
       const collaborationMode = buildCollaborationModePayloadFor(planMode);
       const message = makePlanReadyChangesMessage(trimmed);
@@ -144,7 +168,7 @@ export function usePlanReadyActions({
         activeThreadId,
         message,
         [],
-        collaborationMode ? { collaborationMode } : undefined,
+        { collaborationMode: collaborationMode ?? null },
       );
     },
     [
@@ -153,6 +177,7 @@ export function usePlanReadyActions({
       buildCollaborationModePayloadFor,
       connectWorkspace,
       findCollaborationMode,
+      persistThreadCodexParams,
       sendUserMessageToThread,
       setSelectedCollaborationModeId,
     ],