diff --git a/apps/extension/src/entrypoints/background.ts b/apps/extension/src/entrypoints/background.ts
index ac8904a..d75aca1 100644
--- a/apps/extension/src/entrypoints/background.ts
+++ b/apps/extension/src/entrypoints/background.ts
@@ -25,7 +25,7 @@ import { recordFrameCoordinator } from "@/lib/recording/frame-coordinator";
 import { attachSessionsLiveFlag } from "@/lib/sessions-live-flag";
 import { createDisconnectCleanup } from "@/session-manager/disconnect-cleanup";
 import { attachSessionEventHandler } from "@/session-manager/event-handler";
-import { SessionManager } from "@/session-manager/manager";
+import { isAgentControlledTab, SessionManager } from "@/session-manager/manager";
 import {
   attachBorrowNotificationButtonHandler,
   attachBorrowNotificationClickHandler,
@@ -83,26 +83,21 @@ export default defineBackground(() => {
   }
 
   /**
-   * Authoritative overlay state for a *specific* tab. A user-created tab
-   * (`ctx.userTabs`) inside the Agent Window must never show the control
-   * mask — return hidden immediately. This is what lets a freshly-mounted
-   * content script receive the correct state on its first `overlay.ready`
-   * ping instead of flashing control and then hiding (design: decide before
-   * showing, never show then correct).
+   * Authoritative overlay state for a specific tab. Agent Window tabs are
+   * free by default; only tabs explicitly claimed through session startup,
+   * `tab_create`, or `tab_borrow` receive the control overlay.
    */
   function overlayStateForTab(tabId?: number, windowId?: number): OverlayAgentStateMessage {
     if (typeof tabId === "number" && typeof windowId === "number") {
       const ctx = sessions.findByWindowId(windowId);
-      if (ctx && ctx.userTabs.has(tabId)) {
-        return {
-          type: OVERLAY_AGENT_STATE,
-          sessionId: null,
-          mode: "hidden",
-          generation: overlayGeneration,
-        };
-      }
+      if (ctx && isAgentControlledTab(ctx, tabId)) return overlayStateForWindow(windowId);
     }
-    return overlayStateForWindow(windowId);
+    return {
+      type: OVERLAY_AGENT_STATE,
+      sessionId: null,
+      mode: "hidden",
+      generation: overlayGeneration,
+    };
   }
 
   async function pushOverlayStateToTab(
@@ -122,20 +117,11 @@ export default defineBackground(() => {
   }
 
   async function pushOverlayStateForWindow(windowId: number): Promise<void> {
-    const ctx = sessions.findByWindowId(windowId);
-    const baseState = overlayStateForWindow(windowId);
     const tabs = await chrome.tabs.query({ windowId });
     await Promise.all(
       tabs.map((tab) => {
         if (typeof tab.id !== "number") return Promise.resolve();
-        // A user-created tab in the Agent Window must stay free for the user
-        // to operate — never show the control mask over it. Override the
-        // window-level control state with a hidden state for those tabs.
-        const isUserTab = ctx ? ctx.userTabs.has(tab.id) : false;
-        const state: OverlayAgentStateMessage = isUserTab
-          ? { ...baseState, sessionId: null, mode: "hidden" }
-          : baseState;
-        return pushOverlayStateToTab(tab.id, state);
+        return pushOverlayStateForTab(tab.id, windowId);
       }),
     );
   }
@@ -175,27 +161,13 @@ export default defineBackground(() => {
     if (typeof tab.windowId !== "number") return;
     pushOverlayStateForAgentWindow(tab.windowId);
   });
-  // A new tab inside an Agent Window is agent-owned (the home tab, matched by
-  // homeTabId, or a tool.tab_create flagged by the pending count) or
-  // user-created (via Chrome UI). Classify it so user-opened tabs are kept
-  // free and can be pushed a hidden overlay immediately. See
-  // SessionManager.classifyNewTab.
+  // Newly observed tabs are free unless the agent's own creation path has
+  // already claimed their concrete id. This listener only renders that
+  // state; it never infers or mutates ownership from event ordering.
   chrome.tabs.onCreated.addListener((tab) => {
     if (typeof tab.windowId !== "number" || typeof tab.id !== "number") return;
-    const kind = sessions.classifyNewTab(tab.id, tab.windowId);
-    if (kind === "user") {
-      // User tab: explicitly free it from the agent control mask.
-      void pushOverlayStateToTab(tab.id, {
-        type: OVERLAY_AGENT_STATE,
-        sessionId: null,
-        mode: "hidden",
-        generation: overlayGeneration,
-      });
-    } else if (kind === "agent") {
-      // Agent tab (or home tab): make sure it reflects the session's
-      // current control mode.
-      void pushOverlayStateForAgentWindow(tab.windowId);
-    }
+    if (!sessions.findByWindowId(tab.windowId)) return;
+    void pushOverlayStateForTab(tab.id, tab.windowId);
   });
   // 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
@@ -369,15 +341,18 @@ 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;
-      sendResponse({ sessionId: ctx?.sessionId ?? null });
+      const sessionId =
+        ctx && typeof tabId === "number" && isAgentControlledTab(ctx, tabId) ? ctx.sessionId : null;
+      sendResponse({ sessionId });
       return false;
     }
 
     if (msg.kind === OVERLAY_MSG_READY) {
-      // Decide per-tab *before* sending: a user-created tab gets hidden
-      // immediately so the content script never flashes the control mask.
+      // Resolve from explicit tab ownership before sending so free tabs never
+      // flash the control mask.
       if (typeof sender.tab?.id === "number") {
         void pushOverlayStateForTab(sender.tab.id, sender.tab.windowId);
       }
diff --git a/apps/extension/src/session-manager/__tests__/manager.test.ts b/apps/extension/src/session-manager/__tests__/manager.test.ts
index 46d44be..fbbbf7e 100644
--- a/apps/extension/src/session-manager/__tests__/manager.test.ts
+++ b/apps/extension/src/session-manager/__tests__/manager.test.ts
@@ -1,6 +1,6 @@
 import { describe, expect, it, vi } from "vitest";
 import type { AgentWindowApi, AgentWindowCreateOptions } from "../agent-window";
-import { SessionManager } from "../manager";
+import { isAgentControlledTab, SessionManager } from "../manager";
 
 function fakeAgentWindow(): AgentWindowApi & {
   createMock: ReturnType<typeof vi.fn>;
@@ -151,50 +151,29 @@ describe("SessionManager", () => {
     expect(sm.list()).toEqual([]);
   });
 
-  describe("classifyNewTab (user vs agent tab freedom)", () => {
-    it("classifies the home tab as agent", async () => {
+  describe("explicit tab claims", () => {
+    it("claims the exact home tab returned by AgentWindowApi", async () => {
       const sm = new SessionManager({ agentWindow: fakeAgentWindow() });
       const ctx = await sm.start("aa11");
-      // The home tab is agent-owned and matched by home tab id. The prior
-      // windowInitializing flag was cleared before the home tab's onCreated
-      // arrived, so matching it by boot timing was never reliable.
-      const homeTabId = ctx.homeTabId as number;
-      const kind = sm.classifyNewTab(homeTabId, ctx.agentWindowId);
-      expect(kind).toBe("agent");
-      expect(ctx.agentCreatedTabs.has(homeTabId)).toBe(true);
-      expect(ctx.userTabs.has(homeTabId)).toBe(false);
+      expect(ctx.agentCreatedTabs).toEqual(new Set([0]));
+      expect(isAgentControlledTab(ctx, 0)).toBe(true);
     });
 
-    it("classifies a pending tab_create tab as agent", async () => {
+    it("keeps every other tab free without classification state", async () => {
       const sm = new SessionManager({ agentWindow: fakeAgentWindow() });
       const ctx = await sm.start("aa11");
-      sm.markAgentTabPending(ctx.agentWindowId);
-      const kind = sm.classifyNewTab(11, ctx.agentWindowId);
-      expect(kind).toBe("agent");
-      expect(ctx.agentCreatedTabs.has(11)).toBe(true);
-      expect(ctx.userTabs.has(11)).toBe(false);
-      expect(ctx.pendingAgentTabCount).toBe(0);
+      expect(isAgentControlledTab(ctx, 99)).toBe(false);
+      expect(ctx).not.toHaveProperty("userTabs");
+      expect(ctx).not.toHaveProperty("pendingAgentTabCount");
     });
 
-    it("classifies a user-opened tab (no pending) as user and keeps it free", async () => {
+    it("treats a committed borrow as an explicit claim", async () => {
       const sm = new SessionManager({ agentWindow: fakeAgentWindow() });
       const ctx = await sm.start("aa11");
-      const kind = sm.classifyNewTab(99, ctx.agentWindowId);
-      expect(kind).toBe("user");
-      expect(ctx.userTabs.has(99)).toBe(true);
-      expect(ctx.agentCreatedTabs.has(99)).toBe(false);
-    });
-
-    it("matches multiple pending agent tabs to multiple onCreated events", async () => {
-      const sm = new SessionManager({ agentWindow: fakeAgentWindow() });
-      const ctx = await sm.start("aa11");
-      sm.markAgentTabPending(ctx.agentWindowId);
-      sm.markAgentTabPending(ctx.agentWindowId);
-      expect(sm.classifyNewTab(11, ctx.agentWindowId)).toBe("agent");
-      // Second agent tab still pending → agent; not mistaken for user.
-      expect(sm.classifyNewTab(12, ctx.agentWindowId)).toBe("agent");
-      // No more pending → a later user tab is user.
-      expect(sm.classifyNewTab(99, ctx.agentWindowId)).toBe("user");
+      const reservation = sm.tryReserveBorrow(42, ctx.sessionId);
+      if ("borrowedBy" in reservation) throw new Error("unexpected borrow conflict");
+      reservation.commit({ tabId: 42, originalWindowId: 7, originalIndex: 3 });
+      expect(isAgentControlledTab(ctx, 42)).toBe(true);
     });
   });
 
diff --git a/apps/extension/src/session-manager/manager.ts b/apps/extension/src/session-manager/manager.ts
index 5c4f530..d721c41 100644
--- a/apps/extension/src/session-manager/manager.ts
+++ b/apps/extension/src/session-manager/manager.ts
@@ -7,40 +7,19 @@ export interface SessionContext {
   refStore: RefStore;
   borrowedTabs: Map<number, BorrowedTab>;
   /**
-   * Tabs created by the agent via `tool.tab_create` in this session's
-   * Agent Window. Tracked so `session_stop` can close them before
-   * releasing the window (design §3.1). User-created tabs (via Chrome UI)
-   * never enter this set.
+   * Tabs explicitly claimed by the agent because it created them. This
+   * includes the Agent Window's home tab and tabs created by `tool.tab_create`.
+   * Tabs opened through Chrome UI never enter this set.
    */
   agentCreatedTabs: Set<number>;
-  /**
-   * Tabs the user opened themselves inside the Agent Window via Chrome UI
-   * (new-tab button, Cmd+T, right-click → open in new tab, …). These must
-   * NOT be controlled by the agent overlay and must be left free for the
-   * user to operate. Distinguishing them from agent-created tabs is done
-   * by the `chrome.tabs.onCreated` listener in background.ts, which consults
-   * `pendingAgentTabCount` to tell "opened via tool.tab_create" apart from
-   * "opened by the user".
-   */
-  userTabs: Set<number>;
-  /**
-   * Number of agent tabs currently being created via `tool.tab_create` but
-   * whose `onCreated` event has not yet been observed. `handleTabCreate`
-   * increments this *before* calling `chrome.tabs.create`; the
-   * `onCreated` listener decrements it once per new tab in the Agent Window.
-   * This lets us tell agent-created tabs from user-created tabs even though
-   * both fire `onCreated` (design §3.1, user-tab freedom fix).
-   */
-  pendingAgentTabCount: number;
-  /**
-   * Id of the home tab created/activated when the session started
-   * (`ensureActiveTab`). Used to clean up the home tab precisely on
-   * stop instead of matching by URL (design §3.2).
-   */
-  homeTabId: number | null;
   createdAtMs: number;
 }
 
+/** Whether this session has explicitly claimed control of `tabId`. */
+export function isAgentControlledTab(ctx: SessionContext, tabId: number): boolean {
+  return ctx.agentCreatedTabs.has(tabId) || ctx.borrowedTabs.has(tabId);
+}
+
 export interface BorrowedTab {
   tabId: number;
   originalWindowId: number;
@@ -219,10 +198,10 @@ export class SessionManager {
         agentWindowId: windowId,
         refStore: new RefStore(),
         borrowedTabs: new Map(),
-        agentCreatedTabs: new Set(),
-        userTabs: new Set(),
-        pendingAgentTabCount: 0,
-        homeTabId,
+        // The home tab is the session's first explicit claim. Every other
+        // tab remains free until `tab_create` or `tab_borrow` identifies it
+        // by its concrete Chrome tab id.
+        agentCreatedTabs: new Set([homeTabId]),
         createdAtMs: this.now(),
       };
       this.sessions.set(sessionId, ctx);
@@ -240,50 +219,6 @@ export class SessionManager {
     }
   }
 
-  /**
-   * Record that a `tool.tab_create` is about to open a tab. Called *before*
-   * `chrome.tabs.create` so the pending count is visible to the
-   * `onCreated` listener that will fire for the new tab.
-   */
-  markAgentTabPending(windowId: number): void {
-    const ctx = this.findByWindowId(windowId);
-    if (!ctx) return;
-    ctx.pendingAgentTabCount += 1;
-  }
-
-  /**
-   * Release a pending `tool.tab_create` slot when the create itself failed
-   * and no `chrome.tabs.onCreated` event will arrive to consume it. Without
-   * this the counter leaks and the *next* tab the user opens is misclassified
-   * as agent-created — the exact confusion this counter exists to prevent.
-   */
-  releaseAgentTabPending(windowId: number): void {
-    const ctx = this.findByWindowId(windowId);
-    if (!ctx || ctx.pendingAgentTabCount === 0) return;
-    ctx.pendingAgentTabCount -= 1;
-  }
-
-  /**
-   * Called by the `onCreated` listener for each new tab in an Agent Window.
-   * Returns `"agent"` when the tab is the window's home tab or corresponds to
-   * a pending `tool.tab_create` (and registers it), `"user"` when the user
-   * opened it via Chrome UI.
-   */
-  classifyNewTab(tabId: number, windowId: number): "agent" | "user" | "unknown" {
-    const ctx = this.findByWindowId(windowId);
-    if (!ctx) return "unknown";
-    // The home tab is agent-owned. Its `onCreated` fires before any
-    // window-initialising flag would be observable, so we match it by home
-    // tab id rather than by boot timing (which left the branch unreachable).
-    if (tabId === ctx.homeTabId || ctx.pendingAgentTabCount > 0) {
-      if (ctx.pendingAgentTabCount > 0) ctx.pendingAgentTabCount -= 1;
-      ctx.agentCreatedTabs.add(tabId);
-      return "agent";
-    }
-    ctx.userTabs.add(tabId);
-    return "user";
-  }
-
   /**
    * Tear down a session: close its Agent Window and drop the context.
    *
diff --git a/apps/extension/src/tools/__tests__/dispatcher.test.ts b/apps/extension/src/tools/__tests__/dispatcher.test.ts
index dc5e421..6d7dceb 100644
--- a/apps/extension/src/tools/__tests__/dispatcher.test.ts
+++ b/apps/extension/src/tools/__tests__/dispatcher.test.ts
@@ -196,9 +196,8 @@ describe("ToolDispatcher", () => {
       },
     });
     await sessions.start("aa11");
-    // Register tab 7 as agent-created (pending counter + onCreated classify).
-    sessions.markAgentTabPending(4242);
-    sessions.classifyNewTab(7, 4242);
+    // Register the exact tab id returned by the agent's creation path.
+    sessions.get("aa11")?.agentCreatedTabs.add(7);
 
     const dispatcher = new ToolDispatcher({ transport, sessions });
     dispatcher.start();
diff --git a/apps/extension/src/tools/__tests__/emulate.test.ts b/apps/extension/src/tools/__tests__/emulate.test.ts
index d8e27af..3782fdc 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 () => 1),
+    ensureActiveTab: vi.fn(async () => 7),
   };
 }
 
@@ -197,6 +197,7 @@ 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,
@@ -276,6 +277,7 @@ 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 0065499..d9c352d 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 () => 1),
+    ensureActiveTab: vi.fn(async () => 4),
   };
 }
 
diff --git a/apps/extension/src/tools/__tests__/human-loop.test.ts b/apps/extension/src/tools/__tests__/human-loop.test.ts
index a500735..f5a0f18 100644
--- a/apps/extension/src/tools/__tests__/human-loop.test.ts
+++ b/apps/extension/src/tools/__tests__/human-loop.test.ts
@@ -52,10 +52,16 @@ 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 ? { sessionId, agentWindowId, refStore, borrowedTabs: new Map() } : null,
-    findByWindowId: (wid: number) => (wid === agentWindowId ? { sessionId } : null),
+    get: (id: string) => (id === sessionId ? session : null),
+    findByWindowId: (wid: number) => (wid === agentWindowId ? session : null),
   } as unknown as SessionManager;
   return mgr;
 }
@@ -357,6 +363,7 @@ describe("handleRequestHelp", () => {
               agentWindowId: 99,
               refStore,
               borrowedTabs: new Map(),
+              agentCreatedTabs: new Set([5]),
             }
           : null,
       findByWindowId: (wid: number) => (wid === 99 ? { sessionId: "abcd" } : null),
@@ -413,6 +420,7 @@ describe("handleRequestHelp", () => {
                 }),
               },
               borrowedTabs: new Map(),
+              agentCreatedTabs: new Set([5]),
             }
           : null,
       findByWindowId: (wid: number) => (wid === 99 ? { sessionId: "abcd" } : null),
@@ -498,6 +506,7 @@ 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 45ab74e..bf4313b 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 () => 1),
+    ensureActiveTab: vi.fn(async () => 4),
   };
 }
 
@@ -113,6 +113,7 @@ 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 921771e..5e08f55 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 () => 1),
+    ensureActiveTab: vi.fn(async () => 4),
   };
 }
 
diff --git a/apps/extension/src/tools/__tests__/observation.test.ts b/apps/extension/src/tools/__tests__/observation.test.ts
index 0763262..d5a5855 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 () => 1),
+    ensureActiveTab: vi.fn(async () => 4),
   };
 }
 
diff --git a/apps/extension/src/tools/__tests__/record-steps.test.ts b/apps/extension/src/tools/__tests__/record-steps.test.ts
index f933b49..e37a793 100644
--- a/apps/extension/src/tools/__tests__/record-steps.test.ts
+++ b/apps/extension/src/tools/__tests__/record-steps.test.ts
@@ -84,6 +84,7 @@ 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 57e53e3..4853939 100644
--- a/apps/extension/src/tools/__tests__/session.test.ts
+++ b/apps/extension/src/tools/__tests__/session.test.ts
@@ -302,7 +302,7 @@ describe("handleSessionStop window release (issue #57)", () => {
     ctx.agentCreatedTabs.add(11);
     ctx.agentCreatedTabs.add(12);
     // home tab id from ensureActiveTab override
-    ctx.homeTabId = 10;
+    ctx.agentCreatedTabs.add(10); // home tab
     // user tab 99 (created via Chrome UI, NOT in agentCreatedTabs)
     const state: FakeState = {
       tabs: new Map([
@@ -340,7 +340,7 @@ describe("handleSessionStop window release (issue #57)", () => {
     const sm = new SessionManager({ agentWindow: aw });
     const ctx = await sm.start("aa11");
     ctx.agentCreatedTabs.add(11);
-    ctx.homeTabId = 10;
+    ctx.agentCreatedTabs.add(10); // home tab
     const state: FakeState = {
       tabs: new Map([
         [10, { id: 10, windowId: agentWindowId } as chrome.tabs.Tab],
@@ -369,7 +369,7 @@ 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.homeTabId = 10;
+    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 +399,7 @@ describe("handleSessionStop window release (issue #57)", () => {
     const sm = new SessionManager({ agentWindow: aw });
     const ctx = await sm.start("aa11");
     ctx.agentCreatedTabs.add(11);
-    ctx.homeTabId = 10;
+    ctx.agentCreatedTabs.add(10); // home tab
     const state: FakeState = {
       tabs: new Map([
         [10, { id: 10, windowId: agentWindowId } as chrome.tabs.Tab],
@@ -427,7 +427,7 @@ describe("handleSessionStop window release (issue #57)", () => {
     const sm = new SessionManager({ agentWindow: aw });
     const ctx = await sm.start("aa11");
     ctx.agentCreatedTabs.add(11);
-    ctx.homeTabId = 10;
+    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 +473,7 @@ 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.homeTabId = 10;
+    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 a44fe47..4a120a1 100644
--- a/apps/extension/src/tools/__tests__/tabs.test.ts
+++ b/apps/extension/src/tools/__tests__/tabs.test.ts
@@ -60,19 +60,24 @@ describe("handleTabList", () => {
     expect(res.tabs.map((t) => t.scope).sort()).toEqual(["agent", "user"]);
   });
 
-  it("scope=user excludes the requesting session's Agent Window", async () => {
+  it("scope=user includes unclaimed tabs inside the 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(2);
+    expect(res.tabs).toHaveLength(3);
     expect(res.tabs.every((t) => t.scope === "user")).toBe(true);
-    expect(res.tabs.map((t) => t.url)).toEqual(["https://example.com", "https://github.com"]);
+    expect(res.tabs.map((t) => t.url)).toEqual([
+      "https://user.example/",
+      "https://example.com",
+      "https://github.com",
+    ]);
   });
 
   it("scope=agent returns only the requesting session's Agent Window tabs", async () => {
@@ -221,6 +226,7 @@ describe("handleTabCreate", () => {
       active: true,
     });
     expect(res).toMatchObject({ tab_id: 10, window_id: 100 });
+    expect(sm.get("aa11")?.agentCreatedTabs.has(10)).toBe(true);
   });
 
   it("forwards explicit url + index + active=false", async () => {
@@ -257,7 +263,7 @@ describe("handleTabCreate", () => {
     expect(res).toMatchObject({ code: "invalid_params" });
   });
 
-  it("releases the pending agent-tab slot when chrome.tabs.create fails", async () => {
+  it("does not claim any tab when chrome.tabs.create fails", async () => {
     const sm = new SessionManager({ agentWindow: fakeAgentWindow([100]) });
     await sm.start("aa11");
     const state: FakeTabState = { tabs: new Map(), nextTabId: 10, windowsClosed: new Set() };
@@ -267,18 +273,37 @@ describe("handleTabCreate", () => {
     const res = await handleTabCreate(sm, { session_id: "aa11" }, { tabs: api });
     expect(res).toMatchObject({ code: "protocol_error" });
 
-    // No tab was opened, so no `onCreated` event will consume the pending
-    // slot. Without the release the counter leaks and the next tab the user
-    // opens is misclassified as agent-created — the exact confusion the
-    // counter exists to prevent.
-    expect(sm.classifyNewTab(42, 100)).toBe("user");
+    expect(sm.get("aa11")?.agentCreatedTabs).toEqual(new Set([1]));
+  });
+
+  it("claims the returned id even when a user tab appears during tab creation", async () => {
+    const sm = new SessionManager({ agentWindow: fakeAgentWindow([100]) });
+    const ctx = await sm.start("aa11");
+    const state: FakeTabState = { tabs: new Map(), nextTabId: 10, windowsClosed: new Set() };
+    const { api, spies } = makeTabMutationApi(state);
+    let resolveCreate!: (tab: chrome.tabs.Tab) => void;
+    const createResult = new Promise<chrome.tabs.Tab>((resolve) => {
+      resolveCreate = resolve;
+    });
+    spies.create.mockImplementationOnce(() => createResult);
+
+    const pending = handleTabCreate(sm, { session_id: "aa11" }, { tabs: api });
+    // A user can create an unrelated same-window tab while Chrome is still
+    // resolving the agent request. There is no pending slot for it to steal.
+    expect(ctx.agentCreatedTabs.has(99)).toBe(false);
+    resolveCreate({ id: 10, windowId: 100, active: true } as chrome.tabs.Tab);
+
+    await expect(pending).resolves.toMatchObject({ tab_id: 10 });
+    expect(ctx.agentCreatedTabs.has(10)).toBe(true);
+    expect(ctx.agentCreatedTabs.has(99)).toBe(false);
   });
 });
 
 describe("handleTabClose", () => {
   it("removes a tab inside the Agent Window", async () => {
     const sm = new SessionManager({ agentWindow: fakeAgentWindow([100]) });
-    await sm.start("aa11");
+    const ctx = await sm.start("aa11");
+    ctx.agentCreatedTabs.add(5);
     const state: FakeTabState = {
       tabs: new Map([[5, { id: 5, windowId: 100 } as chrome.tabs.Tab]]),
       nextTabId: 50,
@@ -291,6 +316,23 @@ 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");
@@ -341,7 +383,8 @@ describe("handleTabClose", () => {
 describe("handleTabSelect", () => {
   it("activates a tab inside the Agent Window", async () => {
     const sm = new SessionManager({ agentWindow: fakeAgentWindow([100]) });
-    await sm.start("aa11");
+    const ctx = await sm.start("aa11");
+    ctx.agentCreatedTabs.add(3);
     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 ce0f3aa..81346be 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 () => 1),
+    ensureActiveTab: vi.fn(async () => 4),
   };
 }
 
diff --git a/apps/extension/src/tools/session.ts b/apps/extension/src/tools/session.ts
index 165f874..b3af268 100644
--- a/apps/extension/src/tools/session.ts
+++ b/apps/extension/src/tools/session.ts
@@ -269,7 +269,8 @@ export async function handleSessionStop(
     return { code: "cancelled", message: "session_stop aborted before window close" };
   }
 
-  // Step 4: close the agent-created tabs and the home tab. `tabsApi` is a
+  // Step 4: close every tab explicitly created by the agent, including the
+  // home tab. `tabsApi` is a
   // TabMutationApi (remove only); `queryApi` is a separate read-only
   // ChromeTabsApi. If neither is injected, we conservatively fall back to
   // closing the window (see Step 5).
@@ -277,7 +278,7 @@ export async function handleSessionStop(
   const queryApi = deps.tabsQuery;
 
   if (tabsApi) {
-    // 4a: close each agent-created tab that still exists.
+    // Close each agent-created tab that still exists.
     const agentCreatedTabIds = Array.from(ctx.agentCreatedTabs);
     for (const tabId of agentCreatedTabIds) {
       try {
@@ -288,16 +289,6 @@ export async function handleSessionStop(
         console.warn(`[bsk session_stop] failed to close agent tab ${tabId}`, err);
       }
     }
-
-    // 4b: close the home tab by id (not by URL — avoids deleting a user's
-    // `about:blank` tab). Non-fatal if it's already gone.
-    if (ctx.homeTabId != null) {
-      try {
-        await tabsApi.remove(ctx.homeTabId);
-      } catch (err) {
-        console.warn(`[bsk session_stop] failed to close home tab`, err);
-      }
-    }
   }
 
   // Step 5: decide whether to release (keep) the window or close it.
@@ -312,7 +303,6 @@ export async function handleSessionStop(
       // regression). Exclude any id still tracked in agentCreatedTabs.
       const userTabs = liveWindowTabs.filter((t) => {
         if (t.id === undefined) return false;
-        if (t.id === ctx.homeTabId) return false;
         return !ctx.agentCreatedTabs.has(t.id);
       });
       const leakedAgentTabs = liveWindowTabs.filter(
diff --git a/apps/extension/src/tools/shared.ts b/apps/extension/src/tools/shared.ts
index 3b2cb79..829ba0f 100644
--- a/apps/extension/src/tools/shared.ts
+++ b/apps/extension/src/tools/shared.ts
@@ -8,7 +8,11 @@
 
 import type { CdpDebuggee, DialogCursor } from "@/browser-driver/chromium-cdp";
 import type { CdpFrameGraph, CdpTarget } from "@/browser-driver/frame-graph";
-import type { SessionContext, SessionManager } from "@/session-manager/manager";
+import {
+  isAgentControlledTab,
+  type SessionContext,
+  type SessionManager,
+} from "@/session-manager/manager";
 import { normaliseRef } from "@/session-manager/ref-store";
 import type { ConsoleResult, JavaScriptDialogInfo, RpcError } from "@/transport/types";
 import { rpcError } from "./errors";
@@ -323,23 +327,22 @@ export async function resolveCdpAccessibleTargetTab(
 }
 
 /**
- * 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).
+ * 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.
  *
- * Returns an `RpcError` when the resolved target sits in a user window;
- * `null` on success.
+ * Returns an `RpcError` for free tabs and tabs outside the Agent Window.
  */
 export function enforceAgentWindow(
   ctx: SessionContext,
   target: { tabId: number; windowId: number },
   toolName: string,
 ): RpcError | null {
-  if (target.windowId !== ctx.agentWindowId) {
+  if (target.windowId !== ctx.agentWindowId || !isAgentControlledTab(ctx, target.tabId)) {
     return rpcError(
       "permission_denied",
       "agent_window_scope",
-      `${toolName} can only act on tabs inside the Agent Window (tab ${target.tabId} is in window ${target.windowId}; borrow it first)`,
+      `${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)`,
     );
   }
   return null;
diff --git a/apps/extension/src/tools/tabs.ts b/apps/extension/src/tools/tabs.ts
index c84c098..1192fa6 100644
--- a/apps/extension/src/tools/tabs.ts
+++ b/apps/extension/src/tools/tabs.ts
@@ -7,7 +7,11 @@ import {
   OVERLAY_AGENT_OVERLAY_RESET,
   type OverlayAgentOverlayResetMessage,
 } from "@/lib/overlay-bridge";
-import type { SessionContext, SessionManager } from "@/session-manager/manager";
+import {
+  isAgentControlledTab,
+  type SessionContext,
+  type SessionManager,
+} from "@/session-manager/manager";
 import type { RpcError } from "@/transport/types";
 import { rpcError } from "./errors";
 import { isRpcError, lookupSession } from "./shared";
@@ -25,11 +29,9 @@ export interface TabInfo {
   window_id?: number;
   active?: boolean;
   /**
-   * 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).
+   * 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.
    */
   scope?: "user" | "agent";
 }
@@ -259,7 +261,8 @@ 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 ? "agent" : "user";
+    const tabScope: "user" | "agent" =
+      winId === myAgentWindowId && isAgentControlledTab(ctx, t.id) ? "agent" : "user";
     if (scope === "user" && tabScope !== "user") continue;
     if (scope === "agent" && tabScope !== "agent") continue;
     tabs.push({
@@ -369,6 +372,7 @@ function buildCreateProps(
  * Returns the created tab on success, or an `RpcError` on failure.
  */
 async function createTabAndCleanup(
+  ctx: SessionContext,
   deps: TabManagementDeps,
   createProps: chrome.tabs.CreateProperties,
 ): Promise<CreatedChromeTab | RpcError> {
@@ -381,25 +385,28 @@ async function createTabAndCleanup(
       message: err instanceof Error ? err.message : String(err),
     };
   }
-  if (aborted(deps.signal, "tab_create")) {
-    // We already opened the tab; close it on abort so we don't leak.
-    if (typeof tab.id === "number") {
-      try {
-        await getTabsApi(deps).remove(tab.id);
-      } catch (cleanupErr) {
-        return rpcError(
-          "protocol_error",
-          "cleanup_failed",
-          `tab_create aborted but cleanup of tab ${tab.id} failed: ${describeError(cleanupErr)}`,
-          { resource_type: "tab", resource_id: tab.id },
-        );
-      }
-    }
-    return { code: "cancelled", message: "tab_create aborted" };
-  }
   if (typeof tab.id !== "number") {
     return { code: "protocol_error", message: "chrome.tabs.create returned no tab id" };
   }
+  // Claim the concrete id returned by Chrome. Ownership never depends on
+  // matching this request to an asynchronous onCreated event.
+  ctx.agentCreatedTabs.add(tab.id);
+  if (aborted(deps.signal, "tab_create")) {
+    try {
+      await getTabsApi(deps).remove(tab.id);
+      ctx.agentCreatedTabs.delete(tab.id);
+    } catch (cleanupErr) {
+      // Keep the claim when cleanup fails so session_stop can retry instead
+      // of releasing an agent-owned tab to the user.
+      return rpcError(
+        "protocol_error",
+        "cleanup_failed",
+        `tab_create aborted but cleanup of tab ${tab.id} failed: ${describeError(cleanupErr)}`,
+        { resource_type: "tab", resource_id: tab.id },
+      );
+    }
+    return { code: "cancelled", message: "tab_create aborted" };
+  }
   return { ...tab, id: tab.id };
 }
 
@@ -426,27 +433,8 @@ export async function handleTabCreate(
   const paramErr = validateTabCreateParams(params);
   if (paramErr) return paramErr;
 
-  // Flag the pending agent tab *before* create so the chrome.tabs.onCreated
-  // listener (background.ts) can tell this tab apart from a user-opened tab.
-  manager.markAgentTabPending(ctx.agentWindowId);
-
-  const tab = await createTabAndCleanup(deps, buildCreateProps(ctx, params));
-  if (isRpcError(tab)) {
-    // Release the pending slot when no tab was actually opened: with no
-    // `onCreated` event to consume it, the counter would leak and the next
-    // tab the user opens would be misclassified as agent-created. An abort
-    // (`cancelled`) is the exception — the tab did open (and was cleaned up),
-    // so its `onCreated` still consumes the slot.
-    if (tab.code !== "cancelled") {
-      manager.releaseAgentTabPending(ctx.agentWindowId);
-    }
-    return tab;
-  }
-
-  // Track agent-created tabs so session_stop can clean them up (design §3.1).
-  // The onCreated listener also adds this id; this is an idempotent fallback
-  // in case the listener has not fired yet.
-  ctx.agentCreatedTabs.add(tab.id);
+  const tab = await createTabAndCleanup(ctx, deps, buildCreateProps(ctx, params));
+  if (isRpcError(tab)) return tab;
 
   return {
     tab_id: tab.id,
@@ -496,13 +484,13 @@ async function authoriseAgentTab(
       `${toolName}: tab ${tabId} is borrowed by session ${otherBorrower}`,
     );
   }
-  if (tab.windowId === ctx.agentWindowId) {
+  if (tab.windowId === ctx.agentWindowId && isAgentControlledTab(ctx, tab.id)) {
     return tab;
   }
   return rpcError(
     "permission_denied",
     "agent_window_scope",
-    `${toolName}: tab ${tabId} is not in Agent Window ${ctx.agentWindowId}`,
+    `${toolName}: tab ${tabId} is not claimed by this session in Agent Window ${ctx.agentWindowId}`,
   );
 }
 
@@ -775,13 +763,6 @@ async function executeBorrowCore(
   );
   if (moveErr) return moveErr;
 
-  // Best-effort activation — failure is non-fatal.
-  try {
-    await p.tabsApi.update(p.tabId, { active: true });
-  } catch (err) {
-    console.debug("[bsk tab_borrow] activate after move failed", err);
-  }
-
   return { originalWindowId, originalIndex };
 }
 
@@ -845,6 +826,12 @@ export async function handleTabBorrow(
         message: err instanceof Error ? err.message : String(err),
       };
     }
+    // Activate after commit so overlay/event observers see the tab as claimed.
+    try {
+      await tabsApi.update(params.tab_id, { active: true });
+    } catch (err) {
+      console.debug("[bsk tab_borrow] activate after move failed", err);
+    }
     return {
       tab_id: params.tab_id,
       original_window_id: coreResult.originalWindowId,