diff --git a/apps/extension/src/entrypoints/background.ts b/apps/extension/src/entrypoints/background.ts
index d75aca1..c70ae6a 100644
--- a/apps/extension/src/entrypoints/background.ts
+++ b/apps/extension/src/entrypoints/background.ts
@@ -169,6 +169,9 @@ export default defineBackground(() => {
     if (!sessions.findByWindowId(tab.windowId)) return;
     void pushOverlayStateForTab(tab.id, tab.windowId);
   });
+  chrome.tabs.onRemoved.addListener((tabId) => {
+    sessions.forgetAgentCreatedTab(tabId);
+  });
   // Re-sync the storage.session flag on SW startup so a previous SW's
   // stale `true` does not keep waking us on every page load until the
   // first mutation (review M4/M5 round 3 m-R3-1).
@@ -215,6 +218,9 @@ export default defineBackground(() => {
     recording: recordDeps,
     onSessionsChanged: onOverlaySessionStateChanged,
     onBrowserControlResumed,
+    onAgentTabClaimed: (tabId, windowId) => {
+      void pushOverlayStateForTab(tabId, windowId);
+    },
     approveBorrow: (ctx) =>
       requestBorrowConfirmation(ctx.tabId, {
         ...(ctx.signal !== undefined ? { signal: ctx.signal } : {}),
@@ -341,12 +347,9 @@ export default defineBackground(() => {
     if (!msg || typeof msg !== "object" || !("kind" in msg)) return false;
 
     if (msg.kind === OVERLAY_MSG_WHO_AM_I) {
-      const tabId = sender.tab?.id;
       const windowId = sender.tab?.windowId;
       const ctx = typeof windowId === "number" ? sessions.findByWindowId(windowId) : null;
-      const sessionId =
-        ctx && typeof tabId === "number" && isAgentControlledTab(ctx, tabId) ? ctx.sessionId : null;
-      sendResponse({ sessionId });
+      sendResponse({ sessionId: ctx?.sessionId ?? null });
       return false;
     }
 
diff --git a/apps/extension/src/session-manager/__tests__/manager.test.ts b/apps/extension/src/session-manager/__tests__/manager.test.ts
index fbbbf7e..ab231af 100644
--- a/apps/extension/src/session-manager/__tests__/manager.test.ts
+++ b/apps/extension/src/session-manager/__tests__/manager.test.ts
@@ -175,6 +175,15 @@ describe("SessionManager", () => {
       reservation.commit({ tabId: 42, originalWindowId: 7, originalIndex: 3 });
       expect(isAgentControlledTab(ctx, 42)).toBe(true);
     });
+
+    it("forgets an agent-created tab after Chrome removes it", async () => {
+      const sm = new SessionManager({ agentWindow: fakeAgentWindow() });
+      const ctx = await sm.start("aa11");
+      ctx.agentCreatedTabs.add(42);
+      sm.forgetAgentCreatedTab(42);
+      expect(ctx.agentCreatedTabs.has(42)).toBe(false);
+      expect(isAgentControlledTab(ctx, 0)).toBe(true);
+    });
   });
 
   describe("findBorrowingSession", () => {
diff --git a/apps/extension/src/session-manager/manager.ts b/apps/extension/src/session-manager/manager.ts
index d721c41..4c0df05 100644
--- a/apps/extension/src/session-manager/manager.ts
+++ b/apps/extension/src/session-manager/manager.ts
@@ -117,6 +117,13 @@ export class SessionManager {
     return Array.from(this.sessions.values());
   }
 
+  /** Remove a closed tab from agent-created ownership tracking. */
+  forgetAgentCreatedTab(tabId: number): void {
+    for (const ctx of this.sessions.values()) {
+      ctx.agentCreatedTabs.delete(tabId);
+    }
+  }
+
   /**
    * Look up whether `tabId` is currently borrowed by some *other*
    * session than the one calling. Used by M8 `tab_borrow` to refuse
diff --git a/apps/extension/src/tools/__tests__/dispatcher.test.ts b/apps/extension/src/tools/__tests__/dispatcher.test.ts
index 6d7dceb..8509187 100644
--- a/apps/extension/src/tools/__tests__/dispatcher.test.ts
+++ b/apps/extension/src/tools/__tests__/dispatcher.test.ts
@@ -458,6 +458,16 @@ describe("ToolDispatcher", () => {
   });
 
   it("invokes onBrowserControlResumed for browser-control tools but not passive reads", async () => {
+    vi.stubGlobal("chrome", {
+      tabs: {
+        create: vi.fn(async (props: chrome.tabs.CreateProperties) => ({
+          id: 7,
+          windowId: props.windowId,
+          url: props.url,
+          active: props.active,
+        })),
+      },
+    });
     const { transport, sent, deliver } = fakeTransport();
     const sessions = new SessionManager({
       agentWindow: {
@@ -467,7 +477,13 @@ describe("ToolDispatcher", () => {
       },
     });
     const onBrowserControlResumed = vi.fn();
-    const dispatcher = new ToolDispatcher({ transport, sessions, onBrowserControlResumed });
+    const onAgentTabClaimed = vi.fn();
+    const dispatcher = new ToolDispatcher({
+      transport,
+      sessions,
+      onBrowserControlResumed,
+      onAgentTabClaimed,
+    });
     dispatcher.start();
 
     deliver(makeRequest("tool.session_start", { session_id: "aa11" }));
@@ -484,6 +500,7 @@ describe("ToolDispatcher", () => {
     });
     await flushMicrotasks();
     expect(onBrowserControlResumed).toHaveBeenCalledWith("aa11");
+    expect(onAgentTabClaimed).toHaveBeenCalledWith(7, 1);
   });
 
   it("reasserts remembered hover before follow-up work and releases only after actions", async () => {
diff --git a/apps/extension/src/tools/__tests__/emulate.test.ts b/apps/extension/src/tools/__tests__/emulate.test.ts
index 3782fdc..d8e27af 100644
--- a/apps/extension/src/tools/__tests__/emulate.test.ts
+++ b/apps/extension/src/tools/__tests__/emulate.test.ts
@@ -20,7 +20,7 @@ function fakeAgentWindow(ids: number[]) {
       return id;
     }),
     remove: vi.fn(async () => {}),
-    ensureActiveTab: vi.fn(async () => 7),
+    ensureActiveTab: vi.fn(async () => 1),
   };
 }
 
@@ -197,7 +197,6 @@ describe("handleEmulate", () => {
 
   it("targets an explicit tab_id", async () => {
     const sm = await makeManager();
-    sm.get("aa11")?.agentCreatedTabs.add(9);
     const deps = makeDeps();
     const result = await handleEmulate(
       sm,
@@ -277,7 +276,6 @@ describe("handleEmulate", () => {
 
   it("keeps emulation state per tab", async () => {
     const sm = await makeManager();
-    sm.get("aa11")?.agentCreatedTabs.add(9);
     const deps = makeDeps();
     await handleEmulate(sm, { session_id: "aa11", overrides: fullOverrides }, deps);
     await handleEmulate(
diff --git a/apps/extension/src/tools/__tests__/evaluate.test.ts b/apps/extension/src/tools/__tests__/evaluate.test.ts
index d9c352d..0065499 100644
--- a/apps/extension/src/tools/__tests__/evaluate.test.ts
+++ b/apps/extension/src/tools/__tests__/evaluate.test.ts
@@ -12,7 +12,7 @@ function fakeAgentWindow(ids: number[]) {
       return id;
     }),
     remove: vi.fn(async () => {}),
-    ensureActiveTab: vi.fn(async () => 4),
+    ensureActiveTab: vi.fn(async () => 1),
   };
 }
 
diff --git a/apps/extension/src/tools/__tests__/human-loop.test.ts b/apps/extension/src/tools/__tests__/human-loop.test.ts
index f5a0f18..a500735 100644
--- a/apps/extension/src/tools/__tests__/human-loop.test.ts
+++ b/apps/extension/src/tools/__tests__/human-loop.test.ts
@@ -52,16 +52,10 @@ function installHelpLifecycleChrome() {
 
 function fakeManager(sessionId: string, agentWindowId: number, tabId: number) {
   const refStore = new RefStore();
-  const session = {
-    sessionId,
-    agentWindowId,
-    refStore,
-    borrowedTabs: new Map(),
-    agentCreatedTabs: new Set([tabId]),
-  };
   const mgr = {
-    get: (id: string) => (id === sessionId ? session : null),
-    findByWindowId: (wid: number) => (wid === agentWindowId ? session : null),
+    get: (id: string) =>
+      id === sessionId ? { sessionId, agentWindowId, refStore, borrowedTabs: new Map() } : null,
+    findByWindowId: (wid: number) => (wid === agentWindowId ? { sessionId } : null),
   } as unknown as SessionManager;
   return mgr;
 }
@@ -363,7 +357,6 @@ describe("handleRequestHelp", () => {
               agentWindowId: 99,
               refStore,
               borrowedTabs: new Map(),
-              agentCreatedTabs: new Set([5]),
             }
           : null,
       findByWindowId: (wid: number) => (wid === 99 ? { sessionId: "abcd" } : null),
@@ -420,7 +413,6 @@ describe("handleRequestHelp", () => {
                 }),
               },
               borrowedTabs: new Map(),
-              agentCreatedTabs: new Set([5]),
             }
           : null,
       findByWindowId: (wid: number) => (wid === 99 ? { sessionId: "abcd" } : null),
@@ -506,7 +498,6 @@ describe("handleRequestHelp", () => {
               agentWindowId: 99,
               refStore,
               borrowedTabs: new Map(),
-              agentCreatedTabs: new Set([5]),
             }
           : null,
       findByWindowId: (wid: number) => (wid === 99 ? { sessionId: "abcd" } : null),
diff --git a/apps/extension/src/tools/__tests__/interaction.test.ts b/apps/extension/src/tools/__tests__/interaction.test.ts
index bf4313b..45ab74e 100644
--- a/apps/extension/src/tools/__tests__/interaction.test.ts
+++ b/apps/extension/src/tools/__tests__/interaction.test.ts
@@ -21,7 +21,7 @@ function fakeAgentWindow(ids: number[]) {
       return id;
     }),
     remove: vi.fn(async () => {}),
-    ensureActiveTab: vi.fn(async () => 4),
+    ensureActiveTab: vi.fn(async () => 1),
   };
 }
 
@@ -113,7 +113,6 @@ describe("handleClick", () => {
     const sm = new SessionManager({ agentWindow: fakeAgentWindow([100]) });
     const ctx = await sm.start("aa11");
     ctx.refStore.set("e3", 1234, { tabId: 4 });
-    ctx.agentCreatedTabs.add(5);
     const fake = makeFakeCdp({});
     const res = await handleClick(
       sm,
diff --git a/apps/extension/src/tools/__tests__/navigation.test.ts b/apps/extension/src/tools/__tests__/navigation.test.ts
index 5e08f55..921771e 100644
--- a/apps/extension/src/tools/__tests__/navigation.test.ts
+++ b/apps/extension/src/tools/__tests__/navigation.test.ts
@@ -21,7 +21,7 @@ function fakeAgentWindow(ids: number[]) {
       return id;
     }),
     remove: vi.fn(async () => {}),
-    ensureActiveTab: vi.fn(async () => 4),
+    ensureActiveTab: vi.fn(async () => 1),
   };
 }
 
diff --git a/apps/extension/src/tools/__tests__/observation.test.ts b/apps/extension/src/tools/__tests__/observation.test.ts
index d5a5855..0763262 100644
--- a/apps/extension/src/tools/__tests__/observation.test.ts
+++ b/apps/extension/src/tools/__tests__/observation.test.ts
@@ -28,7 +28,7 @@ function fakeAgentWindow(ids: number[]) {
       return id;
     }),
     remove: vi.fn(async () => {}),
-    ensureActiveTab: vi.fn(async () => 4),
+    ensureActiveTab: vi.fn(async () => 1),
   };
 }
 
diff --git a/apps/extension/src/tools/__tests__/record-steps.test.ts b/apps/extension/src/tools/__tests__/record-steps.test.ts
index e37a793..f933b49 100644
--- a/apps/extension/src/tools/__tests__/record-steps.test.ts
+++ b/apps/extension/src/tools/__tests__/record-steps.test.ts
@@ -84,7 +84,6 @@ function fakeManager() {
             agentWindowId: AGENT_WINDOW_ID,
             refStore: { resolve: () => null, replace: () => {} },
             borrowedTabs: new Map(),
-            agentCreatedTabs: new Set([TAB_ID]),
           }
         : null,
     findByWindowId: (windowId: number) =>
diff --git a/apps/extension/src/tools/__tests__/session.test.ts b/apps/extension/src/tools/__tests__/session.test.ts
index 4853939..d4cd548 100644
--- a/apps/extension/src/tools/__tests__/session.test.ts
+++ b/apps/extension/src/tools/__tests__/session.test.ts
@@ -301,8 +301,6 @@ describe("handleSessionStop window release (issue #57)", () => {
     const ctx = await sm.start("aa11");
     ctx.agentCreatedTabs.add(11);
     ctx.agentCreatedTabs.add(12);
-    // home tab id from ensureActiveTab override
-    ctx.agentCreatedTabs.add(10); // home tab
     // user tab 99 (created via Chrome UI, NOT in agentCreatedTabs)
     const state: FakeState = {
       tabs: new Map([
@@ -340,7 +338,6 @@ describe("handleSessionStop window release (issue #57)", () => {
     const sm = new SessionManager({ agentWindow: aw });
     const ctx = await sm.start("aa11");
     ctx.agentCreatedTabs.add(11);
-    ctx.agentCreatedTabs.add(10); // home tab
     const state: FakeState = {
       tabs: new Map([
         [10, { id: 10, windowId: agentWindowId } as chrome.tabs.Tab],
@@ -369,7 +366,6 @@ describe("handleSessionStop window release (issue #57)", () => {
     const sm = new SessionManager({ agentWindow: aw });
     const ctx = await sm.start("aa11");
     ctx.agentCreatedTabs.add(11); // no longer in state → remove throws
-    ctx.agentCreatedTabs.add(10); // home tab
     const state: FakeState = {
       tabs: new Map([[10, { id: 10, windowId: agentWindowId } as chrome.tabs.Tab]]),
       windowsClosed: new Set(),
@@ -399,7 +395,6 @@ describe("handleSessionStop window release (issue #57)", () => {
     const sm = new SessionManager({ agentWindow: aw });
     const ctx = await sm.start("aa11");
     ctx.agentCreatedTabs.add(11);
-    ctx.agentCreatedTabs.add(10); // home tab
     const state: FakeState = {
       tabs: new Map([
         [10, { id: 10, windowId: agentWindowId } as chrome.tabs.Tab],
@@ -427,7 +422,6 @@ describe("handleSessionStop window release (issue #57)", () => {
     const sm = new SessionManager({ agentWindow: aw });
     const ctx = await sm.start("aa11");
     ctx.agentCreatedTabs.add(11);
-    ctx.agentCreatedTabs.add(10); // home tab
     const state: FakeState = {
       tabs: new Map([[11, { id: 11, windowId: agentWindowId } as chrome.tabs.Tab]]),
       windowsClosed: new Set(),
@@ -473,7 +467,6 @@ describe("handleSessionStop window release (issue #57)", () => {
     const sm = new SessionManager({ agentWindow: aw });
     const ctx = await sm.start("aa11");
     ctx.agentCreatedTabs.add(11); // Step 4 remove() will throw for this tab
-    ctx.agentCreatedTabs.add(10); // home tab
     const state: FakeState = {
       tabs: new Map([
         [10, { id: 10, windowId: agentWindowId } as chrome.tabs.Tab],
diff --git a/apps/extension/src/tools/__tests__/tabs.test.ts b/apps/extension/src/tools/__tests__/tabs.test.ts
index 4a120a1..ffc6444 100644
--- a/apps/extension/src/tools/__tests__/tabs.test.ts
+++ b/apps/extension/src/tools/__tests__/tabs.test.ts
@@ -60,24 +60,19 @@ describe("handleTabList", () => {
     expect(res.tabs.map((t) => t.scope).sort()).toEqual(["agent", "user"]);
   });
 
-  it("scope=user includes unclaimed tabs inside the Agent Window", async () => {
+  it("scope=user excludes the requesting session's Agent Window", async () => {
     const sm = new SessionManager({ agentWindow: fakeAgentWindow([100]) });
     await sm.start("aa11");
     const tabs = fakeChromeTabsApi([
       { windowId: 100, url: "chrome://newtab/", title: "Agent New Tab", active: true },
-      { windowId: 100, url: "https://user.example/", title: "Free user tab", active: false },
       { windowId: 200, url: "https://example.com", title: "Example", active: true },
       { windowId: 200, url: "https://github.com", title: "GitHub", active: false },
     ]);
     const res = await handleTabList(sm, { session_id: "aa11", scope: "user" }, tabs);
     if ("code" in res) throw new Error(`unexpected error: ${JSON.stringify(res)}`);
-    expect(res.tabs).toHaveLength(3);
+    expect(res.tabs).toHaveLength(2);
     expect(res.tabs.every((t) => t.scope === "user")).toBe(true);
-    expect(res.tabs.map((t) => t.url)).toEqual([
-      "https://user.example/",
-      "https://example.com",
-      "https://github.com",
-    ]);
+    expect(res.tabs.map((t) => t.url)).toEqual(["https://example.com", "https://github.com"]);
   });
 
   it("scope=agent returns only the requesting session's Agent Window tabs", async () => {
@@ -302,8 +297,7 @@ describe("handleTabCreate", () => {
 describe("handleTabClose", () => {
   it("removes a tab inside the Agent Window", async () => {
     const sm = new SessionManager({ agentWindow: fakeAgentWindow([100]) });
-    const ctx = await sm.start("aa11");
-    ctx.agentCreatedTabs.add(5);
+    await sm.start("aa11");
     const state: FakeTabState = {
       tabs: new Map([[5, { id: 5, windowId: 100 } as chrome.tabs.Tab]]),
       nextTabId: 50,
@@ -316,23 +310,6 @@ describe("handleTabClose", () => {
     expect(spies.remove).toHaveBeenCalledWith(5);
   });
 
-  it("refuses to close a free tab in the Agent Window", async () => {
-    const sm = new SessionManager({ agentWindow: fakeAgentWindow([100]) });
-    await sm.start("aa11");
-    const state: FakeTabState = {
-      tabs: new Map([[5, { id: 5, windowId: 100 } as chrome.tabs.Tab]]),
-      nextTabId: 50,
-      windowsClosed: new Set(),
-    };
-    const { api, spies } = makeTabMutationApi(state);
-    const res = await handleTabClose(sm, { session_id: "aa11", tab_id: 5 }, { tabs: api });
-    expect(res).toMatchObject({
-      code: "permission_denied",
-      data: { reason: "agent_window_scope" },
-    });
-    expect(spies.remove).not.toHaveBeenCalled();
-  });
-
   it("rejects closing a borrowed tab — caller must tab_return first", async () => {
     const sm = new SessionManager({ agentWindow: fakeAgentWindow([100]) });
     const ctx = await sm.start("aa11");
@@ -383,8 +360,7 @@ describe("handleTabClose", () => {
 describe("handleTabSelect", () => {
   it("activates a tab inside the Agent Window", async () => {
     const sm = new SessionManager({ agentWindow: fakeAgentWindow([100]) });
-    const ctx = await sm.start("aa11");
-    ctx.agentCreatedTabs.add(3);
+    await sm.start("aa11");
     const state: FakeTabState = {
       tabs: new Map([[3, { id: 3, windowId: 100 } as chrome.tabs.Tab]]),
       nextTabId: 50,
diff --git a/apps/extension/src/tools/__tests__/waits.test.ts b/apps/extension/src/tools/__tests__/waits.test.ts
index 81346be..ce0f3aa 100644
--- a/apps/extension/src/tools/__tests__/waits.test.ts
+++ b/apps/extension/src/tools/__tests__/waits.test.ts
@@ -12,7 +12,7 @@ function fakeAgentWindow(ids: number[]) {
       return id;
     }),
     remove: vi.fn(async () => {}),
-    ensureActiveTab: vi.fn(async () => 4),
+    ensureActiveTab: vi.fn(async () => 1),
   };
 }
 
diff --git a/apps/extension/src/tools/dispatcher.ts b/apps/extension/src/tools/dispatcher.ts
index 8590478..2752791 100644
--- a/apps/extension/src/tools/dispatcher.ts
+++ b/apps/extension/src/tools/dispatcher.ts
@@ -115,6 +115,8 @@ export interface DispatcherDeps {
   onSessionsChanged?: () => void;
   /** Invoked before a tool that dispatches page input or mutates browser state is forwarded. */
   onBrowserControlResumed?: (sessionId: string) => void;
+  /** Invoked after a tab is explicitly claimed so its overlay can be refreshed immediately. */
+  onAgentTabClaimed?: (tabId: number, windowId: number) => void;
   /** User approval for `tool.tab_borrow` (overlay in content script). */
   approveBorrow?: BorrowConfirmationApprover;
   /** i18n notification copy for `tool.request_help` (resolved per-call). */
@@ -144,6 +146,7 @@ export class ToolDispatcher {
   private readonly recording?: RecordRuntimeDeps;
   private readonly onSessionsChanged?: () => void;
   private readonly onBrowserControlResumed?: (sessionId: string) => void;
+  private readonly onAgentTabClaimed?: (tabId: number, windowId: number) => void;
   private readonly approveBorrow?: BorrowConfirmationApprover;
   private readonly helpNotificationCopy?: () => { title: string; body: string };
   private subscription: { dispose(): void } | null = null;
@@ -164,6 +167,7 @@ export class ToolDispatcher {
     this.recording = deps.recording;
     this.onSessionsChanged = deps.onSessionsChanged;
     this.onBrowserControlResumed = deps.onBrowserControlResumed;
+    this.onAgentTabClaimed = deps.onAgentTabClaimed;
     this.approveBorrow = deps.approveBorrow;
     this.helpNotificationCopy = deps.helpNotificationCopy;
   }
@@ -309,8 +313,15 @@ export class ToolDispatcher {
       }
       case "tool.tab_list":
         return handleTabList(this.sessions, req.params as TabListParams, chromeTabsApi, signal);
-      case "tool.tab_create":
-        return handleTabCreate(this.sessions, req.params as TabCreateParams, { signal });
+      case "tool.tab_create": {
+        const result = await handleTabCreate(this.sessions, req.params as TabCreateParams, {
+          signal,
+        });
+        if (!isRpcError(result)) {
+          this.onAgentTabClaimed?.(result.tab_id, result.window_id);
+        }
+        return result;
+      }
       case "tool.tab_close":
         return this.withHoverReleaseForRequest(
           req.params as TabCloseParams,
@@ -319,11 +330,16 @@ export class ToolDispatcher {
         );
       case "tool.tab_select":
         return handleTabSelect(this.sessions, req.params as TabSelectParams, { signal });
-      case "tool.tab_borrow":
-        return handleTabBorrow(this.sessions, req.params as TabBorrowParams, {
+      case "tool.tab_borrow": {
+        const result = await handleTabBorrow(this.sessions, req.params as TabBorrowParams, {
           signal,
           approveBorrow: this.approveBorrow,
         });
+        if (!isRpcError(result)) {
+          this.onAgentTabClaimed?.(result.tab_id, result.agent_window_id);
+        }
+        return result;
+      }
       case "tool.tab_return":
         return handleTabReturn(this.sessions, req.params as TabReturnParams, { signal });
       case "tool.window_resize":
diff --git a/apps/extension/src/tools/shared.ts b/apps/extension/src/tools/shared.ts
index 829ba0f..3b2cb79 100644
--- a/apps/extension/src/tools/shared.ts
+++ b/apps/extension/src/tools/shared.ts
@@ -8,11 +8,7 @@
 
 import type { CdpDebuggee, DialogCursor } from "@/browser-driver/chromium-cdp";
 import type { CdpFrameGraph, CdpTarget } from "@/browser-driver/frame-graph";
-import {
-  isAgentControlledTab,
-  type SessionContext,
-  type SessionManager,
-} from "@/session-manager/manager";
+import type { SessionContext, SessionManager } from "@/session-manager/manager";
 import { normaliseRef } from "@/session-manager/ref-store";
 import type { ConsoleResult, JavaScriptDialogInfo, RpcError } from "@/transport/types";
 import { rpcError } from "./errors";
@@ -327,22 +323,23 @@ export async function resolveCdpAccessibleTargetTab(
 }
 
 /**
- * Sandbox guard: M7 write tools (click / fill / press / navigate*) must
- * refuse tabs the session has not explicitly claimed. A controlled tab must
- * also remain inside the session's Agent Window.
+ * Sandbox guard: M7 write tools (click / fill / press / navigate*)
+ * MUST refuse to touch a tab outside the session's Agent Window
+ * (§6 — borrowing brings the tab into the Agent Window first).
  *
- * Returns an `RpcError` for free tabs and tabs outside the Agent Window.
+ * Returns an `RpcError` when the resolved target sits in a user window;
+ * `null` on success.
  */
 export function enforceAgentWindow(
   ctx: SessionContext,
   target: { tabId: number; windowId: number },
   toolName: string,
 ): RpcError | null {
-  if (target.windowId !== ctx.agentWindowId || !isAgentControlledTab(ctx, target.tabId)) {
+  if (target.windowId !== ctx.agentWindowId) {
     return rpcError(
       "permission_denied",
       "agent_window_scope",
-      `${toolName} can only act on tabs claimed by this session inside the Agent Window (tab ${target.tabId} is free or outside window ${ctx.agentWindowId}; borrow it first)`,
+      `${toolName} can only act on tabs inside the Agent Window (tab ${target.tabId} is in window ${target.windowId}; borrow it first)`,
     );
   }
   return null;
diff --git a/apps/extension/src/tools/tabs.ts b/apps/extension/src/tools/tabs.ts
index 1192fa6..9057354 100644
--- a/apps/extension/src/tools/tabs.ts
+++ b/apps/extension/src/tools/tabs.ts
@@ -7,11 +7,7 @@ import {
   OVERLAY_AGENT_OVERLAY_RESET,
   type OverlayAgentOverlayResetMessage,
 } from "@/lib/overlay-bridge";
-import {
-  isAgentControlledTab,
-  type SessionContext,
-  type SessionManager,
-} from "@/session-manager/manager";
+import type { SessionContext, SessionManager } from "@/session-manager/manager";
 import type { RpcError } from "@/transport/types";
 import { rpcError } from "./errors";
 import { isRpcError, lookupSession } from "./shared";
@@ -29,9 +25,11 @@ export interface TabInfo {
   window_id?: number;
   active?: boolean;
   /**
-   * Ownership relative to the requesting session. Explicitly created or
-   * borrowed tabs in this session's Agent Window are `agent`; every other
-   * visible tab is `user`. Other sessions' Agent Windows are filtered out.
+   * Where the tab sits relative to the requesting session: tabs in any
+   * window other than an Agent Window are `user`; tabs in this
+   * session's own Agent Window are `agent`. Tabs in *other* sessions'
+   * Agent Windows are filtered out entirely (cross-session isolation,
+   * design §6).
    */
   scope?: "user" | "agent";
 }
@@ -261,8 +259,7 @@ export async function handleTabList(
     if (typeof t.id !== "number") continue;
     const winId = typeof t.windowId === "number" ? t.windowId : -1;
     if (otherAgentWindowIds.has(winId)) continue;
-    const tabScope: "user" | "agent" =
-      winId === myAgentWindowId && isAgentControlledTab(ctx, t.id) ? "agent" : "user";
+    const tabScope: "user" | "agent" = winId === myAgentWindowId ? "agent" : "user";
     if (scope === "user" && tabScope !== "user") continue;
     if (scope === "agent" && tabScope !== "agent") continue;
     tabs.push({
@@ -484,13 +481,13 @@ async function authoriseAgentTab(
       `${toolName}: tab ${tabId} is borrowed by session ${otherBorrower}`,
     );
   }
-  if (tab.windowId === ctx.agentWindowId && isAgentControlledTab(ctx, tab.id)) {
+  if (tab.windowId === ctx.agentWindowId) {
     return tab;
   }
   return rpcError(
     "permission_denied",
     "agent_window_scope",
-    `${toolName}: tab ${tabId} is not claimed by this session in Agent Window ${ctx.agentWindowId}`,
+    `${toolName}: tab ${tabId} is not in Agent Window ${ctx.agentWindowId}`,
   );
 }