diff --git a/apps/extension/src/session-manager/__tests__/agent-window.test.ts b/apps/extension/src/session-manager/__tests__/agent-window.test.ts
index 92585f1..e99118c 100644
--- a/apps/extension/src/session-manager/__tests__/agent-window.test.ts
+++ b/apps/extension/src/session-manager/__tests__/agent-window.test.ts
@@ -44,3 +44,53 @@ describe("chromeAgentWindowApi.ensureActiveTab", () => {
     expect(update).not.toHaveBeenCalled();
   });
 });
+
+describe("chromeAgentWindowApi.create", () => {
+  const create = vi.fn();
+
+  beforeEach(() => {
+    vi.stubGlobal("chrome", {
+      windows: { create },
+    });
+    create.mockReset();
+    create.mockResolvedValue({ id: 100 });
+  });
+
+  afterEach(() => {
+    vi.unstubAllGlobals();
+  });
+
+  it("focuses Agent Windows by default", async () => {
+    await chromeAgentWindowApi.create(AGENT_WINDOW_HOME);
+
+    expect(create).toHaveBeenCalledWith({
+      type: "normal",
+      focused: true,
+      url: AGENT_WINDOW_HOME,
+    });
+  });
+
+  it("can create an Agent Window without stealing focus", async () => {
+    await chromeAgentWindowApi.create(AGENT_WINDOW_HOME, { focused: false });
+
+    expect(create).toHaveBeenCalledWith({
+      type: "normal",
+      focused: false,
+      url: AGENT_WINDOW_HOME,
+    });
+  });
+
+  it("passes an optional window size through the options object", async () => {
+    await chromeAgentWindowApi.create(AGENT_WINDOW_HOME, {
+      size: { width: 1280, height: 800 },
+    });
+
+    expect(create).toHaveBeenCalledWith({
+      type: "normal",
+      focused: true,
+      url: AGENT_WINDOW_HOME,
+      width: 1280,
+      height: 800,
+    });
+  });
+});
diff --git a/apps/extension/src/session-manager/__tests__/manager.test.ts b/apps/extension/src/session-manager/__tests__/manager.test.ts
index 9196126..c58b5d1 100644
--- a/apps/extension/src/session-manager/__tests__/manager.test.ts
+++ b/apps/extension/src/session-manager/__tests__/manager.test.ts
@@ -1,5 +1,5 @@
 import { describe, expect, it, vi } from "vitest";
-import type { AgentWindowApi } from "../agent-window";
+import type { AgentWindowApi, AgentWindowCreateOptions } from "../agent-window";
 import { SessionManager } from "../manager";
 
 function fakeAgentWindow(): AgentWindowApi & {
@@ -8,7 +8,7 @@ function fakeAgentWindow(): AgentWindowApi & {
   ensureActiveTabMock: ReturnType<typeof vi.fn>;
 } {
   let nextId = 100;
-  const createMock = vi.fn(async (_url: string) => {
+  const createMock = vi.fn(async (_url: string, _opts?: AgentWindowCreateOptions) => {
     const id = nextId++;
     return id;
   });
@@ -30,7 +30,7 @@ describe("SessionManager", () => {
     const sm = new SessionManager({ agentWindow: aw, now: () => 1700000000000 });
     const ctx = await sm.start("aa11");
     expect(aw.createMock).toHaveBeenCalledOnce();
-    expect(aw.createMock).toHaveBeenCalledWith("about:blank");
+    expect(aw.createMock).toHaveBeenCalledWith("about:blank", {});
     expect(aw.ensureActiveTabMock).toHaveBeenCalledOnce();
     expect(aw.ensureActiveTabMock).toHaveBeenCalledWith(100, "about:blank");
     expect(ctx.sessionId).toBe("aa11");
@@ -42,11 +42,22 @@ describe("SessionManager", () => {
   it("forwards an optional window size when starting a session", async () => {
     const aw = fakeAgentWindow();
     const sm = new SessionManager({ agentWindow: aw });
-    const ctx = await sm.start("aa11", { width: 1280, height: 800 });
-    expect(aw.createMock).toHaveBeenCalledWith("about:blank", { width: 1280, height: 800 });
+    const ctx = await sm.start("aa11", { size: { width: 1280, height: 800 } });
+    expect(aw.createMock).toHaveBeenCalledWith("about:blank", {
+      size: { width: 1280, height: 800 },
+    });
     expect(ctx.agentWindowId).toBe(100);
   });
 
+  it("forwards an explicit unfocused start to the Agent Window", async () => {
+    const aw = fakeAgentWindow();
+    const sm = new SessionManager({ agentWindow: aw });
+
+    await sm.start("aa11", { focused: false });
+
+    expect(aw.createMock).toHaveBeenCalledWith("about:blank", { focused: false });
+  });
+
   it("indexes the session by sessionId and agent window id", async () => {
     const aw = fakeAgentWindow();
     const sm = new SessionManager({ agentWindow: aw });
diff --git a/apps/extension/src/session-manager/agent-window.ts b/apps/extension/src/session-manager/agent-window.ts
index d880767..f345b32 100644
--- a/apps/extension/src/session-manager/agent-window.ts
+++ b/apps/extension/src/session-manager/agent-window.ts
@@ -8,7 +8,7 @@
  */
 
 export interface AgentWindowApi {
-  create(url: string, size?: { width: number; height: number }): Promise<number>;
+  create(url: string, opts?: AgentWindowCreateOptions): Promise<number>;
   remove(windowId: number): Promise<void>;
   /**
    * Guarantee the Agent Window has an active, CDP-navigable tab.
@@ -18,16 +18,24 @@ export interface AgentWindowApi {
   ensureActiveTab(windowId: number, url: string): Promise<void>;
 }
 
+/** Creation hints for a new Agent Window. */
+export interface AgentWindowCreateOptions {
+  /** Optional outer size in CSS pixels. */
+  size?: { width: number; height: number };
+  /** Defaults to true so existing sessions keep visible Agent Windows. */
+  focused?: boolean;
+}
+
 /** Initial tab URL for every new session's Agent Window. */
 export const AGENT_WINDOW_HOME = "about:blank";
 
 export const chromeAgentWindowApi: AgentWindowApi = {
-  async create(url: string, size?: { width: number; height: number }): Promise<number> {
+  async create(url: string, opts: AgentWindowCreateOptions = {}): Promise<number> {
     const win = await chrome.windows.create({
       type: "normal",
-      focused: true,
+      focused: opts.focused ?? true,
       url,
-      ...(size ? { width: size.width, height: size.height } : {}),
+      ...(opts.size ? { width: opts.size.width, height: opts.size.height } : {}),
     });
     if (typeof win?.id !== "number") {
       throw new Error("[bh] chrome.windows.create returned no window id");
diff --git a/apps/extension/src/session-manager/manager.ts b/apps/extension/src/session-manager/manager.ts
index 6c65175..d2c1022 100644
--- a/apps/extension/src/session-manager/manager.ts
+++ b/apps/extension/src/session-manager/manager.ts
@@ -25,6 +25,14 @@ export interface SessionManagerOptions {
   now?: () => number;
 }
 
+/** Options for starting a session's Agent Window. */
+export interface SessionStartOptions {
+  /** Optional Agent Window outer size in CSS pixels. */
+  size?: { width: number; height: number };
+  /** Defaults to true so existing clients keep visible Agent Windows. */
+  focused?: boolean;
+}
+
 /**
  * Owner of all live agent sessions inside the extension.
  *
@@ -128,18 +136,11 @@ export class SessionManager {
    * Returns the created window id so callers can echo it back to the
    * daemon in the `tool.session_start` reply.
    */
-  async start(
-    sessionId: string,
-    size?: { width: number; height: number },
-  ): Promise<SessionContext> {
+  async start(sessionId: string, opts: SessionStartOptions = {}): Promise<SessionContext> {
     if (this.sessions.has(sessionId)) {
       throw new Error(`[bh] session ${sessionId} already exists`);
     }
-    // Only pass `size` when given so the no-size call shape (and its
-    // chrome.windows.create payload) stays exactly as before.
-    const windowId = size
-      ? await this.agentWindow.create(AGENT_WINDOW_HOME, size)
-      : await this.agentWindow.create(AGENT_WINDOW_HOME);
+    const windowId = await this.agentWindow.create(AGENT_WINDOW_HOME, opts);
     await this.agentWindow.ensureActiveTab(windowId, AGENT_WINDOW_HOME);
     const ctx: SessionContext = {
       sessionId,
diff --git a/apps/extension/src/tools/__tests__/dispatcher.test.ts b/apps/extension/src/tools/__tests__/dispatcher.test.ts
index cb4e5ea..2280b26 100644
--- a/apps/extension/src/tools/__tests__/dispatcher.test.ts
+++ b/apps/extension/src/tools/__tests__/dispatcher.test.ts
@@ -69,6 +69,25 @@ describe("ToolDispatcher", () => {
     });
   });
 
+  it("forwards an unfocused session start to the Agent Window", async () => {
+    const { transport, deliver } = fakeTransport();
+    const create = vi.fn(async () => 4242);
+    const sessions = new SessionManager({
+      agentWindow: {
+        create,
+        remove: vi.fn(),
+        ensureActiveTab: vi.fn(async () => {}),
+      },
+    });
+    const dispatcher = new ToolDispatcher({ transport, sessions });
+    dispatcher.start();
+
+    deliver(makeRequest("tool.session_start", { session_id: "aa11", focused: false }));
+    await flushMicrotasks();
+
+    expect(create).toHaveBeenCalledWith("about:blank", { focused: false });
+  });
+
   it("routes tool.session_stop and replies with empty result", async () => {
     const { transport, sent, deliver } = fakeTransport();
     const sessions = new SessionManager({
diff --git a/apps/extension/src/tools/__tests__/window.test.ts b/apps/extension/src/tools/__tests__/window.test.ts
index 55fc0fd..3d86504 100755
--- a/apps/extension/src/tools/__tests__/window.test.ts
+++ b/apps/extension/src/tools/__tests__/window.test.ts
@@ -107,7 +107,7 @@ describe("handleSessionStart window size", () => {
     const sm = new SessionManager({ agentWindow: aw });
     const result = await handleSessionStart(sm, { session_id: "aa11", width: 1280, height: 800 });
     expect(result).toEqual({ agent_window_id: 100 });
-    expect(aw.create).toHaveBeenCalledWith("about:blank", { width: 1280, height: 800 });
+    expect(aw.create).toHaveBeenCalledWith("about:blank", { size: { width: 1280, height: 800 } });
   });
 
   it("creates the window without size when width/height are omitted", async () => {
@@ -115,7 +115,15 @@ describe("handleSessionStart window size", () => {
     const sm = new SessionManager({ agentWindow: aw });
     const result = await handleSessionStart(sm, { session_id: "aa11" });
     expect(result).toEqual({ agent_window_id: 100 });
-    expect(aw.create).toHaveBeenCalledWith("about:blank");
+    expect(aw.create).toHaveBeenCalledWith("about:blank", {});
+  });
+
+  it("forwards focused: false to the Agent Window", async () => {
+    const aw = fakeAgentWindow([100]);
+    const sm = new SessionManager({ agentWindow: aw });
+    const result = await handleSessionStart(sm, { session_id: "aa11", focused: false });
+    expect(result).toEqual({ agent_window_id: 100 });
+    expect(aw.create).toHaveBeenCalledWith("about:blank", { focused: false });
   });
 
   it("rejects a lone width without height", async () => {
diff --git a/apps/extension/src/tools/session.ts b/apps/extension/src/tools/session.ts
index d3312af..edf564e 100644
--- a/apps/extension/src/tools/session.ts
+++ b/apps/extension/src/tools/session.ts
@@ -50,6 +50,8 @@ export interface SessionStartParams {
   width?: number;
   /** Optional Agent Window outer height in CSS pixels (100..=7680). */
   height?: number;
+  /** Defaults to true so existing clients preserve visible Agent Windows. */
+  focused?: boolean;
 }
 
 export interface SessionStartResult {
@@ -99,7 +101,10 @@ export async function handleSessionStart(
   const sizeOrErr = validateWindowSize(params.width, params.height);
   if (isRpcError(sizeOrErr)) return sizeOrErr;
   try {
-    const ctx = await manager.start(params.session_id, sizeOrErr);
+    const ctx = await manager.start(params.session_id, {
+      size: sizeOrErr,
+      focused: params.focused,
+    });
     return { agent_window_id: ctx.agentWindowId };
   } catch (err) {
     // chrome.windows.create / SessionManager failures are not CDP
diff --git a/crates/bsk-cli/skill/SKILL.md b/crates/bsk-cli/skill/SKILL.md
index b88fffa..31a7a09 100644
--- a/crates/bsk-cli/skill/SKILL.md
+++ b/crates/bsk-cli/skill/SKILL.md
@@ -42,7 +42,7 @@ Every automation task **must** follow this lifecycle. Do **not** rely on idle ti
 3. bsk session stop <id>          → REQUIRED when done (even on error paths)
 ```
 
-Optional: `bsk session start --browser <instance-id-or-label>` when multiple browsers are connected (`bsk browsers` / error output lists them).
+Optional: `bsk session start --browser <instance-id-or-label>` when multiple browsers are connected (`bsk browsers` / error output lists them). Add `--no-focus` to open the Agent Window in the background without stealing focus from the user's current window.
 
 Emergency cleanup: `bsk session stop --all` or the Agent Window overlay **Stop all**.
 
@@ -129,6 +129,7 @@ Details and flags: **`bsk <cmd> --help`**
 | Command | Summary |
 |---------|---------|
 | `bsk session start` | Open Agent Window (`--width`/`--height` for initial size); prints **4-letter session id** |
+| `bsk session start --no-focus` | Open Agent Window in the background without stealing focus |
 | `bsk session stop <id>` | End session, close Agent Window, auto-return borrowed tabs |
 | `bsk session stop --all` | Stop every active session |
 | `bsk session list` | List active sessions |
diff --git a/crates/bsk-cli/src/cli/record.rs b/crates/bsk-cli/src/cli/record.rs
index 84d3daa..ec72d9a 100644
--- a/crates/bsk-cli/src/cli/record.rs
+++ b/crates/bsk-cli/src/cli/record.rs
@@ -17,7 +17,7 @@ use crate::cli::business_rpc;
 use crate::cli::ensure_daemon::ensure_daemon;
 use crate::cli::error::{CliError, Format};
 use crate::cli::record_state;
-use crate::cli::session::{start_session, stop_session};
+use crate::cli::session::{SessionStartOptions, start_session, stop_session};
 
 /// Max wait for the user to click 结束 in the browser (24 hours).
 const RECORD_AWAIT_TIMEOUT_MS: u32 = 86_400_000;
@@ -85,7 +85,13 @@ fn dispatch_start(args: RecordStartArgs, format: Format) -> Result<(), CliError>
     }
 
     let info = ensure_daemon().context("ensure daemon is running")?;
-    let session = start_session(info.sock_path.clone(), args.browser, None, None)?;
+    let session = start_session(
+        info.sock_path.clone(),
+        SessionStartOptions {
+            browser: args.browser,
+            ..SessionStartOptions::default()
+        },
+    )?;
 
     let start_params = RecordStartParams {
         session_id: session.session_id.clone(),
diff --git a/crates/bsk-cli/src/cli/session.rs b/crates/bsk-cli/src/cli/session.rs
index 5d4a144..5365282 100644
--- a/crates/bsk-cli/src/cli/session.rs
+++ b/crates/bsk-cli/src/cli/session.rs
@@ -64,6 +64,10 @@ pub struct SessionStartArgs {
     /// `--width` and `--height` must be given to take effect.
     #[arg(long, value_parser = window_size)]
     pub height: Option<u32>,
+
+    /// Open the Agent Window in the background without stealing focus.
+    #[arg(long)]
+    pub no_focus: bool,
 }
 
 /// Parse a `--width` / `--height` Agent Window dimension (CSS pixels).
@@ -99,6 +103,8 @@ struct StartParams {
     width: Option<u32>,
     #[serde(skip_serializing_if = "Option::is_none")]
     height: Option<u32>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    focused: Option<bool>,
 }
 
 #[derive(Debug, Deserialize)]
@@ -173,7 +179,15 @@ fn run_start(sock: PathBuf, args: SessionStartArgs, format: Format) -> Result<()
             }
         });
     }
-    let result = start_session(sock, args.browser, args.width, args.height);
+    let result = start_session(
+        sock,
+        SessionStartOptions {
+            browser: args.browser,
+            width: args.width,
+            height: args.height,
+            focused: args.no_focus.then_some(false),
+        },
+    );
     waited.store(true, Ordering::SeqCst);
     match result {
         Ok(reply) => match format {
@@ -197,20 +211,27 @@ fn run_start(sock: PathBuf, args: SessionStartArgs, format: Format) -> Result<()
     Ok(())
 }
 
+/// Options for [`start_session`]: browser selection plus Agent Window
+/// creation hints. `None` fields keep the extension-side defaults
+/// (focused window, browser-chosen size).
+#[derive(Debug, Default, Clone)]
+pub struct SessionStartOptions {
+    pub browser: Option<String>,
+    pub width: Option<u32>,
+    pub height: Option<u32>,
+    pub focused: Option<bool>,
+}
+
 /// Start a session and open the Agent Window. Used by `session start` and `record start`.
-pub fn start_session(
-    sock: PathBuf,
-    browser: Option<String>,
-    width: Option<u32>,
-    height: Option<u32>,
-) -> Result<StartReply, CliError> {
+pub fn start_session(sock: PathBuf, opts: SessionStartOptions) -> Result<StartReply, CliError> {
     call(
         sock,
         Method::SessionStart,
         Some(StartParams {
-            browser_instance_id: browser,
-            width,
-            height,
+            browser_instance_id: opts.browser,
+            width: opts.width,
+            height: opts.height,
+            focused: opts.focused,
         }),
         SESSION_START_IPC_TIMEOUT,
     )
diff --git a/crates/bsk-cli/src/daemon/ipc.rs b/crates/bsk-cli/src/daemon/ipc.rs
index 6d0c9f3..b464416 100644
--- a/crates/bsk-cli/src/daemon/ipc.rs
+++ b/crates/bsk-cli/src/daemon/ipc.rs
@@ -42,8 +42,8 @@ use tracing::{debug, warn};
 use super::abort::AbortRegistry;
 use super::queue::{DEFAULT_TOOL_TIMEOUT, DispatchError};
 use super::sessions::{
-    SessionId, StartSessionError, StopSessionError, snapshot_status_entries, start_session,
-    stop_session,
+    AgentWindowOptions, SessionId, StartSessionError, StopSessionError, snapshot_status_entries,
+    start_session, stop_session,
 };
 use super::state::{DAEMON_VERSION, DaemonState, PROTOCOL_VERSION};
 
@@ -545,6 +545,8 @@ struct CliSessionStartParams {
     pub width: Option<u32>,
     #[serde(default)]
     pub height: Option<u32>,
+    #[serde(default)]
+    pub focused: Option<bool>,
 }
 
 #[derive(Debug, Clone, Serialize, Deserialize)]
@@ -638,6 +640,7 @@ async fn handle_session_start(state: &Arc<DaemonState>, params: Value) -> Result
             browser_instance_id: None,
             width: None,
             height: None,
+            focused: None,
         }
     } else {
         serde_json::from_value(params).map_err(|err| RpcError {
@@ -664,7 +667,10 @@ async fn handle_session_start(state: &Arc<DaemonState>, params: Value) -> Result
         &state.sessions,
         &state.tool_queues,
         params.browser_instance_id.as_deref(),
-        window_size,
+        AgentWindowOptions {
+            size: window_size,
+            focused: params.focused,
+        },
         state.config.extension_connect_wait,
         DEFAULT_RPC_TIMEOUT,
     )
diff --git a/crates/bsk-cli/src/daemon/sessions.rs b/crates/bsk-cli/src/daemon/sessions.rs
index 0e74e14..542f645 100644
--- a/crates/bsk-cli/src/daemon/sessions.rs
+++ b/crates/bsk-cli/src/daemon/sessions.rs
@@ -390,6 +390,17 @@ pub enum StopSessionError {
 /// of live sessions.
 const SESSION_ID_MAX_RESERVE_ATTEMPTS: u32 = 64;
 
+/// Agent Window creation hints forwarded to the extension on
+/// `tool.session_start`. `None` fields keep the extension-side defaults
+/// (focused window, browser-chosen size).
+#[derive(Debug, Default, Clone, Copy)]
+pub struct AgentWindowOptions {
+    /// Optional outer size as `(width, height)` CSS pixels.
+    pub size: Option<(u32, u32)>,
+    /// Optional focus hint (`None` = extension default: focused).
+    pub focused: Option<bool>,
+}
+
 /// Ask the chosen browser to create a fresh Agent Window for a brand-new
 /// session id, registering the result on success.
 ///
@@ -401,8 +412,7 @@ pub async fn start_session(
     sessions: &Arc<SessionRegistry>,
     queues: &Arc<ToolQueueRegistry>,
     requested: Option<&str>,
-    // Optional Agent Window outer size as `(width, height)` CSS pixels.
-    window_size: Option<(u32, u32)>,
+    window: AgentWindowOptions,
     connect_wait: Duration,
     timeout_dur: Duration,
 ) -> Result<Session, StartSessionError> {
@@ -429,8 +439,9 @@ pub async fn start_session(
     let params = SessionStartParams {
         session_id: session_id.0.clone(),
         browser_instance_id: Some(client.id.0.clone()),
-        width: window_size.map(|(width, _)| width),
-        height: window_size.map(|(_, height)| height),
+        width: window.size.map(|(width, _)| width),
+        height: window.size.map(|(_, height)| height),
+        focused: window.focused,
     };
     let rpc_id = next_rpc_id("sess-start");
     let request = RequestFrame {
diff --git a/crates/bsk-cli/tests/cli_parse.rs b/crates/bsk-cli/tests/cli_parse.rs
index 174ae40..f8cb0f2 100644
--- a/crates/bsk-cli/tests/cli_parse.rs
+++ b/crates/bsk-cli/tests/cli_parse.rs
@@ -5,6 +5,7 @@ use std::time::Duration;
 use bsk::cli::daemon::{DaemonCmd, parse_duration};
 use bsk::cli::navigate::NavigateCmd;
 use bsk::cli::record::{RecordCmd, RecordSub};
+use bsk::cli::session::{SessionCmd, SessionSub};
 use bsk::{Cli, Command};
 use clap::Parser;
 
@@ -500,3 +501,15 @@ fn rejects_invalid_emulate_values() {
         .is_err()
     );
 }
+
+#[test]
+fn parses_session_start_no_focus() {
+    let cli = parse(&["bsk", "session", "start", "--no-focus"]);
+    let Command::Session(SessionCmd {
+        sub: SessionSub::Start(args),
+    }) = cli.command
+    else {
+        panic!("expected session start subcommand");
+    };
+    assert!(args.no_focus);
+}
diff --git a/crates/bsk-cli/tests/sessions_ipc.rs b/crates/bsk-cli/tests/sessions_ipc.rs
index 9e6af21..69c87c9 100644
--- a/crates/bsk-cli/tests/sessions_ipc.rs
+++ b/crates/bsk-cli/tests/sessions_ipc.rs
@@ -144,8 +144,9 @@ async fn session_start_stop_round_trip_via_ipc() {
             if let Frame::Request(req) = frame {
                 let reply = match req.method {
                     Method::ToolSessionStart => {
-                        let _: SessionStartParams =
+                        let params: SessionStartParams =
                             serde_json::from_value(req.params.clone().unwrap()).unwrap();
+                        assert_eq!(params.focused, Some(false));
                         let result = SessionStartResult {
                             agent_window_id: Some(4242),
                         };
@@ -177,6 +178,7 @@ async fn session_start_stop_round_trip_via_ipc() {
     #[derive(serde::Serialize)]
     struct StartParams {
         browser_instance_id: Option<String>,
+        focused: Option<bool>,
     }
     #[derive(serde::Deserialize, Debug)]
     struct StartReply {
@@ -191,6 +193,7 @@ async fn session_start_stop_round_trip_via_ipc() {
             Method::SessionStart,
             Some(StartParams {
                 browser_instance_id: None,
+                focused: Some(false),
             }),
             Duration::from_secs(5),
         )
diff --git a/crates/bsk-protocol/schema/tool_session_start_params.json b/crates/bsk-protocol/schema/tool_session_start_params.json
index b8a2ae8..0b8eb21 100644
--- a/crates/bsk-protocol/schema/tool_session_start_params.json
+++ b/crates/bsk-protocol/schema/tool_session_start_params.json
@@ -12,6 +12,13 @@
         "null"
       ]
     },
+    "focused": {
+      "description": "Whether the new Agent Window should take focus. Omitted means the extension's default (`true`) for compatibility with older clients.",
+      "type": [
+        "boolean",
+        "null"
+      ]
+    },
     "height": {
       "description": "Optional Agent Window outer height in CSS pixels (100..=7680).",
       "type": [
diff --git a/crates/bsk-protocol/src/tools/session.rs b/crates/bsk-protocol/src/tools/session.rs
index 7d4ffdd..ac935b2 100644
--- a/crates/bsk-protocol/src/tools/session.rs
+++ b/crates/bsk-protocol/src/tools/session.rs
@@ -16,6 +16,10 @@ pub struct SessionStartParams {
     /// Optional Agent Window outer height in CSS pixels (100..=7680).
     #[serde(default, skip_serializing_if = "Option::is_none")]
     pub height: Option<u32>,
+    /// Whether the new Agent Window should take focus. Omitted means the
+    /// extension's default (`true`) for compatibility with older clients.
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub focused: Option<bool>,
 }
 
 #[derive(Debug, Clone, PartialEq, Serialize, Deserialize, JsonSchema)]
@@ -49,6 +53,23 @@ mod tests {
     use super::*;
     use serde_json::json;
 
+    #[test]
+    fn session_start_focus_is_optional_and_round_trips_false() {
+        let defaulted: SessionStartParams = serde_json::from_value(json!({
+            "session_id": "aa11"
+        }))
+        .unwrap();
+        assert_eq!(defaulted.focused, None);
+
+        let background: SessionStartParams = serde_json::from_value(json!({
+            "session_id": "aa11",
+            "focused": false
+        }))
+        .unwrap();
+        assert_eq!(background.focused, Some(false));
+        assert_eq!(serde_json::to_value(background).unwrap()["focused"], false);
+    }
+
     #[test]
     fn session_stop_result_round_trips_auto_return_payload() {
         let result: SessionStopResult = serde_json::from_value(json!({
diff --git a/skill/SKILL.md b/skill/SKILL.md
index b88fffa..31a7a09 100644
--- a/skill/SKILL.md
+++ b/skill/SKILL.md
@@ -42,7 +42,7 @@ Every automation task **must** follow this lifecycle. Do **not** rely on idle ti
 3. bsk session stop <id>          → REQUIRED when done (even on error paths)
 ```
 
-Optional: `bsk session start --browser <instance-id-or-label>` when multiple browsers are connected (`bsk browsers` / error output lists them).
+Optional: `bsk session start --browser <instance-id-or-label>` when multiple browsers are connected (`bsk browsers` / error output lists them). Add `--no-focus` to open the Agent Window in the background without stealing focus from the user's current window.
 
 Emergency cleanup: `bsk session stop --all` or the Agent Window overlay **Stop all**.
 
@@ -129,6 +129,7 @@ Details and flags: **`bsk <cmd> --help`**
 | Command | Summary |
 |---------|---------|
 | `bsk session start` | Open Agent Window (`--width`/`--height` for initial size); prints **4-letter session id** |
+| `bsk session start --no-focus` | Open Agent Window in the background without stealing focus |
 | `bsk session stop <id>` | End session, close Agent Window, auto-return borrowed tabs |
 | `bsk session stop --all` | Stop every active session |
 | `bsk session list` | List active sessions |