diff --git a/apps/web/src/components/ChatView.logic.test.ts b/apps/web/src/components/ChatView.logic.test.ts
index 43c970053e77..54960558aaa6 100644
--- a/apps/web/src/components/ChatView.logic.test.ts
+++ b/apps/web/src/components/ChatView.logic.test.ts
@@ -10,7 +10,13 @@ import {
   type ServerProvider,
 } from "@t3tools/contracts";
 import { deriveProviderInstanceEntries, NO_PROVIDER_MODEL_SELECTION } from "../providerInstances";
-import type { RightPanelSurface } from "../rightPanelStore";
+import { selectThreadDiffPanelSelection, useDiffPanelStore } from "../diffPanelStore";
+import {
+  selectActiveRightPanel,
+  selectThreadRightPanelState,
+  useRightPanelStore,
+  type RightPanelSurface,
+} from "../rightPanelStore";
 import {
   CommandId,
   EnvironmentId,
@@ -20,6 +26,7 @@ import {
   ProviderInstanceId,
   ThreadId,
   RunId,
+  type ScopedThreadRef,
   TurnItemId,
   type OrchestrationV2ProjectedTurnItem,
   type WorktreeSetupSnapshot,
@@ -27,7 +34,7 @@ import {
 import type { CodexArtifactTemplate } from "@t3tools/shared/codexArtifactTemplates";
 import * as DateTime from "effect/DateTime";
 import * as Option from "effect/Option";
-import { afterEach, describe, expect, it, vi } from "vite-plus/test";
+import { afterEach, beforeEach, describe, expect, it, vi } from "vite-plus/test";
 import { Atom, AsyncResult } from "effect/reactivity";
 import { appAtomRegistry } from "../rpc/atomRegistry";
 import { environmentThreadDetails } from "../state/threads";
@@ -44,6 +51,7 @@ import {
   resolveComposerProviderSelection,
   resolveProactiveTurnDiffAction,
   resolveDraftHeroState,
+  resolveExplicitDiffOpenOnActivate,
   resolveWorktreeSetupProgress,
   isPaintOnlyThreadTimeline,
   peekHeldThreadTimeline,
@@ -1805,6 +1813,106 @@ describe("proactive completed diff guard", () => {
   });
 });
 
+describe("Diff scope across right-panel tab switches", () => {
+  const threadRef: ScopedThreadRef = { environmentId, threadId };
+  const otherThreadRef: ScopedThreadRef = { environmentId, threadId: ThreadId.make("thread-2") };
+  let explicitDiffOpen: ScopedThreadRef | null = null;
+
+  const diffIsActive = (ref: ScopedThreadRef) =>
+    selectActiveRightPanel(useRightPanelStore.getState().byThreadKey, ref) === "diff";
+  const diffScope = (ref: ScopedThreadRef) =>
+    selectThreadDiffPanelSelection(useDiffPanelStore.getState().byThreadKey, ref).kind;
+  const settle = (ref: ScopedThreadRef) => {
+    const explicit = explicitDiffOpen;
+    explicitDiffOpen = null;
+    if (diffIsActive(ref) && explicit !== ref) {
+      useDiffPanelStore.getState().selectGitScope(ref, "branch");
+    }
+  };
+  const clickTab = (ref: ScopedThreadRef, surfaceId: string) => {
+    const surface = selectThreadRightPanelState(
+      useRightPanelStore.getState().byThreadKey,
+      ref,
+    ).surfaces.find((entry) => entry.id === surfaceId);
+    explicitDiffOpen = resolveExplicitDiffOpenOnActivate({
+      surfaceKind: surface!.kind,
+      diffOpen: diffIsActive(ref),
+      activeThreadRef: ref,
+    });
+    useRightPanelStore.getState().activateSurface(ref, surfaceId);
+    settle(ref);
+  };
+
+  beforeEach(() => {
+    explicitDiffOpen = null;
+    useRightPanelStore.setState({
+      byThreadKey: {},
+      userActionRevisionByThreadKey: {},
+      closeRevisionByThreadKey: {},
+    });
+    useDiffPanelStore.setState({ byThreadKey: {}, branchBaseRefByThreadKey: {} });
+  });
+
+  it("keeps Uncommitted when the user clicks back to the Diff tab", () => {
+    const panels = useRightPanelStore.getState();
+    panels.open(threadRef, "diff");
+    settle(threadRef);
+    useDiffPanelStore.getState().selectGitScope(threadRef, "unstaged");
+
+    panels.open(threadRef, "files");
+    settle(threadRef);
+    clickTab(threadRef, "diff");
+
+    expect(diffScope(threadRef)).toBe("unstaged");
+  });
+
+  it("still opens on Changes for generic openings", () => {
+    const panels = useRightPanelStore.getState();
+    const diffStore = useDiffPanelStore.getState();
+
+    panels.open(threadRef, "diff");
+    settle(threadRef);
+    expect(diffScope(threadRef)).toBe("branch");
+
+    diffStore.selectGitScope(threadRef, "unstaged");
+    panels.open(threadRef, "files");
+    settle(threadRef);
+    panels.closeSurface(threadRef, "files");
+    settle(threadRef);
+    expect(diffScope(threadRef)).toBe("branch");
+
+    diffStore.selectGitScope(threadRef, "unstaged");
+    panels.open(otherThreadRef, "diff");
+    diffStore.selectGitScope(otherThreadRef, "unstaged");
+    settle(otherThreadRef);
+    expect(diffScope(otherThreadRef)).toBe("branch");
+  });
+
+  it("marks only a click that brings the Diff tab on screen", () => {
+    expect(
+      resolveExplicitDiffOpenOnActivate({
+        surfaceKind: "diff",
+        diffOpen: false,
+        activeThreadRef: threadRef,
+      }),
+    ).toBe(threadRef);
+    expect(
+      resolveExplicitDiffOpenOnActivate({
+        surfaceKind: "diff",
+        diffOpen: true,
+        activeThreadRef: threadRef,
+      }),
+    ).toBeNull();
+    expect(
+      resolveExplicitDiffOpenOnActivate({
+        surfaceKind: "files",
+        diffOpen: false,
+        activeThreadRef: threadRef,
+      }),
+    ).toBeNull();
+  });
+});
+
 describe("shouldRefocusComposerOnWindowFocus", () => {
   function element(
     tagName: string,
diff --git a/apps/web/src/components/ChatView.logic.ts b/apps/web/src/components/ChatView.logic.ts
index f01fa3227571..efea4822b1fb 100644
--- a/apps/web/src/components/ChatView.logic.ts
+++ b/apps/web/src/components/ChatView.logic.ts
@@ -202,6 +202,20 @@ export function resolveProactiveTurnDiffAction(input: {
   return input.checkpoint.files.length >= 3 || changedLines >= 50 ? "open" : "ignore";
 }
 
+/**
+ * Clicking back to an existing Diff tab resumes the review in progress, so the
+ * generic-open reset to Changes must skip it. A diff that is already active is
+ * left unmarked: the reset never runs for it, and the marker would linger into
+ * the next generic open.
+ */
+export function resolveExplicitDiffOpenOnActivate(input: {
+  readonly surfaceKind: RightPanelSurface["kind"];
+  readonly diffOpen: boolean;
+  readonly activeThreadRef: ScopedThreadRef;
+}): ScopedThreadRef | null {
+  return input.surfaceKind === "diff" && !input.diffOpen ? input.activeThreadRef : null;
+}
+
 export function codexArtifactTemplatePromptToAppend(
   currentDraft: string,
   template: CodexArtifactTemplate,
diff --git a/apps/web/src/components/ChatView.tsx b/apps/web/src/components/ChatView.tsx
index d5599f8c46a3..d2991f5fcca0 100644
--- a/apps/web/src/components/ChatView.tsx
+++ b/apps/web/src/components/ChatView.tsx
@@ -17,6 +17,7 @@ import {
   peekHeldThreadTimeline,
   peekRememberedThreadTimeline,
   rememberReadyThreadTimeline,
+  resolveExplicitDiffOpenOnActivate,
   resolveThreadSwitchTimeline,
   timelineHasEphemeralPreviewUrls,
   recallCheckoutIsRepo,
@@ -5764,6 +5765,11 @@ export default function ChatView(props: ChatViewProps) {
   const requestClosePanelTerminalStable = useStableCallback(requestClosePanelTerminal);
   const activateRightPanelSurface = (surface: RightPanelSurface) => {
     if (!activeThreadRef) return;
+    explicitDiffOpenRef.current = resolveExplicitDiffOpenOnActivate({
+      surfaceKind: surface.kind,
+      diffOpen,
+      activeThreadRef,
+    });
     useRightPanelStore.getState().activateSurface(activeThreadRef, surface.id);
     if (surface.kind === "preview" && surface.resourceId) {
       setActivePreviewTab(activeThreadRef, surface.resourceId);