diff --git a/apps/web/src/components/ChatView.logic.test.ts b/apps/web/src/components/ChatView.logic.test.ts
index cc0c7dd35753..bf8d32f1daf3 100644
--- a/apps/web/src/components/ChatView.logic.test.ts
+++ b/apps/web/src/components/ChatView.logic.test.ts
@@ -1786,6 +1786,26 @@ describe("proactive completed diff guard", () => {
       }),
     ).toBe("ignore");
   });
+
+  it("leaves an already open diff and its chosen scope alone", () => {
+    const largeCheckpoint = {
+      status: "ready",
+      files: Array.from({ length: 3 }, (_, index) => ({
+        path: `src/app-${index}.ts`,
+        kind: "modified" as const,
+        additions: 20,
+        deletions: 0,
+      })),
+    } satisfies Pick<TurnDiffSummary, "status" | "files">;
+
+    expect(
+      resolveProactiveTurnDiffAction({
+        checkpoint: largeCheckpoint,
+        isGitRepo: true,
+        activeSurfaceKind: "diff",
+      }),
+    ).toBe("ignore");
+  });
 });
 
 describe("shouldRefocusComposerOnWindowFocus", () => {
diff --git a/apps/web/src/components/ChatView.logic.ts b/apps/web/src/components/ChatView.logic.ts
index dcbd4ab2f343..f36e54d334fe 100644
--- a/apps/web/src/components/ChatView.logic.ts
+++ b/apps/web/src/components/ChatView.logic.ts
@@ -185,7 +185,10 @@ export function resolveProactiveTurnDiffAction(input: {
   isGitRepo: boolean | undefined;
   activeSurfaceKind: RightPanelSurface["kind"] | null;
 }): "defer" | "ignore" | "open" {
-  if (input.activeSurfaceKind === "pull-request") return "ignore";
+  // An open diff already shows the work; reopening it would reset the chosen scope.
+  if (input.activeSurfaceKind === "pull-request" || input.activeSurfaceKind === "diff") {
+    return "ignore";
+  }
   if (input.checkpoint === undefined || input.checkpoint.status === "missing") return "defer";
   if (input.isGitRepo === undefined) return "defer";
   if (
diff --git a/apps/web/src/components/ChatView.tsx b/apps/web/src/components/ChatView.tsx
index 068b69ce0632..89b74c063da4 100644
--- a/apps/web/src/components/ChatView.tsx
+++ b/apps/web/src/components/ChatView.tsx
@@ -173,6 +173,7 @@ import { AsyncResult } from "effect/reactivity";
 import { isElectron } from "../env";
 import { readLocalApi } from "../localApi";
 import { useDiffPanelStore } from "../diffPanelStore";
+import { useActiveThreadRef } from "../hooks/useActiveThreadRef";
 import {
   type ComposerSubmissionIntent,
   collapseExpandedComposerCursor,
@@ -2192,10 +2193,7 @@ export default function ChatView(props: ChatViewProps) {
     }
     return labels;
   }, [activeThreadKnownSessions]);
-  const activeThreadRef = useMemo(
-    () => (activeThread ? scopeThreadRef(activeThread.environmentId, activeThread.id) : null),
-    [activeThread],
-  );
+  const activeThreadRef = useActiveThreadRef(activeThread);
   const activeThreadKey = activeThreadRef ? scopedThreadKey(activeThreadRef) : null;
   const activeEnvironmentServerBrowser = useEnvironmentSupportsServerBrowser(
     activeThreadRef?.environmentId ?? null,
diff --git a/apps/web/src/hooks/useActiveThreadRef.test.tsx b/apps/web/src/hooks/useActiveThreadRef.test.tsx
new file mode 100644
index 000000000000..4afc4071f1bd
--- /dev/null
+++ b/apps/web/src/hooks/useActiveThreadRef.test.tsx
@@ -0,0 +1,61 @@
+import type { EnvironmentId, ScopedThreadRef, ThreadId } from "@t3tools/contracts";
+import { act } from "react";
+import { create, type ReactTestRenderer } from "react-test-renderer";
+import { afterEach, beforeEach, describe, expect, it, vi } from "vite-plus/test";
+
+import { useActiveThreadRef } from "./useActiveThreadRef";
+
+type ThreadLike = { environmentId: EnvironmentId; id: ThreadId; status: string } | null;
+
+let renderer: ReactTestRenderer | null = null;
+let observed: Array<ScopedThreadRef | null> = [];
+
+function Probe({ thread }: { thread: ThreadLike }) {
+  observed.push(useActiveThreadRef(thread));
+  return null;
+}
+
+const thread = (id: string, status = "idle"): ThreadLike => ({
+  environmentId: "env-1" as EnvironmentId,
+  id: id as ThreadId,
+  status,
+});
+
+async function render(value: ThreadLike) {
+  await act(() => {
+    if (renderer) renderer.update(<Probe thread={value} />);
+    else renderer = create(<Probe thread={value} />);
+  });
+  return observed.at(-1);
+}
+
+beforeEach(() => {
+  observed = [];
+  vi.stubGlobal("IS_REACT_ACT_ENVIRONMENT", true);
+});
+
+afterEach(async () => {
+  await act(() => renderer?.unmount());
+  renderer = null;
+  vi.unstubAllGlobals();
+});
+
+describe("useActiveThreadRef", () => {
+  it("keeps the same ref while the thread object is replaced during a run", async () => {
+    const first = await render(thread("t1", "running"));
+    const afterUpdate = await render(thread("t1", "waiting"));
+
+    expect(first).toEqual({ environmentId: "env-1", threadId: "t1" });
+    expect(afterUpdate).toBe(first);
+  });
+
+  it("returns a new ref when the thread changes or goes away", async () => {
+    const first = await render(thread("t1"));
+    const switched = await render(thread("t2"));
+    const gone = await render(null);
+
+    expect(switched).not.toBe(first);
+    expect(switched).toEqual({ environmentId: "env-1", threadId: "t2" });
+    expect(gone).toBeNull();
+  });
+});
diff --git a/apps/web/src/hooks/useActiveThreadRef.ts b/apps/web/src/hooks/useActiveThreadRef.ts
new file mode 100644
index 000000000000..9333f6f1ea4d
--- /dev/null
+++ b/apps/web/src/hooks/useActiveThreadRef.ts
@@ -0,0 +1,20 @@
+import { scopeThreadRef } from "@t3tools/client-runtime/environment";
+import type { EnvironmentId, ScopedThreadRef, ThreadId } from "@t3tools/contracts";
+import { useMemo } from "react";
+
+/**
+ * The scoped ref of the thread a view shows, stable for as long as it shows that thread.
+ * Takes the thread object but keys on its ids: the shell changes identity on every update
+ * during a run, and effects keyed on this ref must not re-run for that.
+ */
+export function useActiveThreadRef(
+  thread: { readonly environmentId: EnvironmentId; readonly id: ThreadId } | null | undefined,
+): ScopedThreadRef | null {
+  const environmentId = thread?.environmentId ?? null;
+  const threadId = thread?.id ?? null;
+  return useMemo(
+    () =>
+      environmentId !== null && threadId !== null ? scopeThreadRef(environmentId, threadId) : null,
+    [environmentId, threadId],
+  );
+}