diff --git a/README.md b/README.md
index d085849..2c31661 100644
--- a/README.md
+++ b/README.md
@@ -1,6 +1,6 @@
 # GrokBoy
 
-Minimal local **GrokBot-like** CLI agent. Phase **P3**: optional Playwright DOM browser tools.
+Minimal local **GrokBot-like** CLI agent. Phase **P4**: human browser handoff (login/OTP/captcha).
 
 ## Status
 
@@ -10,8 +10,9 @@ Minimal local **GrokBot-like** CLI agent. Phase **P3**: optional Playwright DOM
 | P1 shell / files + ReAct | done |
 | P2 completion / loop guard / truncation | done |
 | P3 browser (Playwright DOM) | done (optional) |
+| P4 human handoff | done |
 
-No Docker desktop, no Codex/LazyBoy fork.
+No Docker desktop, no Codex/LazyBoy fork. Product notes: [`docs/PRODUCT.md`](docs/PRODUCT.md).
 
 ## Setup (macOS)
 
@@ -21,6 +22,7 @@ export GROKBOY_API_KEY=your_key   # or XAI_API_KEY
 # export GROKBOY_BASE_URL=https://api.x.ai/v1
 # export GROKBOY_MODEL=grok-4.6
 # export GROKBOY_CONTEXT_CHARS=100000
+# export GROKBOY_BROWSER_HEADED=1   # visible Chromium (recommended for handoff)
 
 cd ~/GrokBoy
 cargo run -p grokboy -- chat
@@ -36,29 +38,48 @@ npm install
 npx playwright install chromium
 ```
 
-Then the agent can use `browser_navigate`, `browser_snapshot`, `browser_click`, `browser_type`, `browser_eval` (DOM/selector/role — not screenshot-first).
+Then the agent can use `browser_navigate`, `browser_snapshot`, `browser_click`, `browser_type`, `browser_eval`, and **`browser_handoff`** (DOM/selector/role — not screenshot-first).
+
+### Human handoff (P4)
+
+When the agent hits a login / OTP / captcha wall it calls `browser_handoff`:
+
+1. Chromium opens **headed** (visible) — or relaunches headed if it was headless.
+2. Terminal prints why it paused (ZH-TW + English) and what to do.
+3. You complete the wall in the browser, then press **Enter** in that terminal (or type `abort`).
+4. Agent resumes with a fresh **DOM snapshot**.
+
+```bash
+# Prefer headed for runs that may need handoff:
+export GROKBOY_BROWSER_HEADED=1
+cargo run -p grokboy -- run "打開需要登入的頁面並完成任務"
+```
+
+Tests / CI: `GROKBOY_HANDOFF_AUTO=1` auto-resumes (no interactive Enter).
 
 ## Commands
 
 - `grokboy chat` — interactive streaming chat (no tools)
 - `grokboy run "<prompt>"` — one-shot agent with tools
 - `grokboy run --session <id> "<prompt>"` — continue a saved session
-- `grokboy smoke` — offline checks (no API key / no Chromium required)
+- `grokboy smoke` — offline checks (no API key / no interactive handoff)
 - `grokboy help`
 
 Tools: `shell`, `list_dir`, `read_file`, `write_file`, `report_done`, `report_blocked`,
-`browser_navigate`, `browser_snapshot`, `browser_click`, `browser_type`, `browser_eval`.
+`browser_navigate`, `browser_snapshot`, `browser_click`, `browser_type`, `browser_eval`,
+`browser_handoff`.
 
 Sessions are stored under `~/.grokboy/sessions/<id>.json` (may include `last_browser_url`).
 
 The agent stops on `report_done` / `report_blocked`, blocks identical tool rounds (×3), and truncates old context when over budget.
 
-## 繁體中文
+## Traditional Chinese
 
-本機終端機 coding assistant。P3 可選 Playwright DOM 瀏覽器工具（非截圖優先）。
+本機終端機 coding assistant。P4 支援瀏覽器人工接手（登入／OTP／驗證碼）：代理暫停 → 你在可見 Chromium 完成 → 終端機按 Enter 繼續。
 
 ```bash
 export GROKBOY_API_KEY=你的金鑰
+export GROKBOY_BROWSER_HEADED=1
 cd ~/GrokBoy
 cargo run -p grokboy -- smoke
 # 可選瀏覽器：
@@ -73,5 +94,6 @@ cargo run -p grokboy -- chat
 crates/grokboy-core/   # config, model, tools, browser, agent, session
 crates/grokboy/        # CLI binary
 tools/playwright/      # optional Node Playwright helper (JSONL)
+docs/PRODUCT.md
 docs/ACCEPTANCE.md
 ```
diff --git a/crates/grokboy-core/src/agent.rs b/crates/grokboy-core/src/agent.rs
index 2b04f1e..5fbb984 100644
--- a/crates/grokboy-core/src/agent.rs
+++ b/crates/grokboy-core/src/agent.rs
@@ -15,8 +15,9 @@ pub const AGENT_SYSTEM: &str = "\
 You are GrokBoy, a concise local coding assistant with tools.
 Use tools when they help solve the task; otherwise answer directly.
 Prefer short, clear answers. Traditional Chinese is welcome when the user writes in Chinese.
-Available tools: shell, list_dir, read_file, write_file, report_done, report_blocked, and optional browser_* (Playwright DOM: browser_navigate, browser_snapshot, browser_click, browser_type, browser_eval).
+Available tools: shell, list_dir, read_file, write_file, report_done, report_blocked, and optional browser_* (Playwright DOM: browser_navigate, browser_snapshot, browser_click, browser_type, browser_eval, browser_handoff).
 For web pages prefer DOM snapshot + selector/role click/type — not screenshots or pixel XY clicks.
+If you hit a login / OTP / captcha wall you cannot pass alone, call browser_handoff with a clear reason so the human can help in the visible browser, then continue from the returned snapshot.
 When the task is finished, call report_done with a short summary.
 If you are stuck or cannot proceed, call report_blocked with the reason — do not invent results or loop.
 Do not invent tool results — call the tools.";
@@ -314,6 +315,7 @@ mod tests {
         assert!(AGENT_SYSTEM.contains("report_done"));
         assert!(AGENT_SYSTEM.contains("report_blocked"));
         assert!(AGENT_SYSTEM.contains("browser_navigate") || AGENT_SYSTEM.contains("browser_"));
+        assert!(AGENT_SYSTEM.contains("browser_handoff"));
         assert!(AGENT_SYSTEM.contains("Traditional Chinese") || AGENT_SYSTEM.contains("Chinese"));
     }
 
diff --git a/crates/grokboy-core/src/browser.rs b/crates/grokboy-core/src/browser.rs
index 77623da..dcb8812 100644
--- a/crates/grokboy-core/src/browser.rs
+++ b/crates/grokboy-core/src/browser.rs
@@ -1,8 +1,12 @@
-//! Optional Playwright DOM browser tools (P3).
+//! Optional Playwright DOM browser tools (P3) + human handoff (P4).
 //!
 //! Spawns `tools/playwright/browser_helper.mjs` (JSONL over stdin/stdout).
 //! Fail closed with an install hint when Node / Playwright / Chromium is missing.
 //! Prefer selectors / roles — not screenshot-first control.
+//!
+//! Env:
+//! - `GROKBOY_BROWSER_HEADED=1` — launch Chromium headed (visible) by default
+//! - `GROKBOY_HANDOFF_AUTO=1` — auto-resume handoff (tests); `abort` to auto-abort
 
 use anyhow::{Context, Result, anyhow};
 use serde_json::{Value, json};
@@ -10,6 +14,7 @@ use std::io::{BufRead, BufReader, Write};
 use std::path::{Path, PathBuf};
 use std::process::{Child, ChildStdin, ChildStdout, Command, Stdio};
 use std::sync::{Arc, Mutex, OnceLock};
+use std::sync::mpsc;
 use std::time::{Duration, Instant};
 
 pub const INSTALL_HINT: &str = "Playwright browser tools unavailable. Install with: \
@@ -399,6 +404,27 @@ pub fn browser_tool_definitions() -> Vec<Value> {
                 }
             }
         }),
+        json!({
+            "type": "function",
+            "function": {
+                "name": "browser_handoff",
+                "description": "Pause for human help on auth walls (login, OTP, captcha). Shows a headed Chromium window, prints terminal instructions (ZH+EN), waits for Enter (or abort), then returns a DOM snapshot so the agent can continue. Prefer when automation cannot pass the wall.",
+                "parameters": {
+                    "type": "object",
+                    "properties": {
+                        "reason": {
+                            "type": "string",
+                            "description": "Why human help is needed (e.g. login page, OTP, captcha)"
+                        },
+                        "timeout_secs": {
+                            "type": "integer",
+                            "description": "Seconds to wait for user (default 300)"
+                        }
+                    },
+                    "required": ["reason"]
+                }
+            }
+        }),
     ]
 }
 
@@ -411,6 +437,7 @@ pub fn is_browser_tool(name: &str) -> bool {
             | "browser_click"
             | "browser_type"
             | "browser_eval"
+            | "browser_handoff"
     )
 }
 
@@ -420,6 +447,10 @@ pub async fn execute_browser_tool(
     name: &str,
     args: &Value,
 ) -> Result<Value> {
+    if name == "browser_handoff" {
+        return execute_browser_handoff(cwd, last_url, args).await;
+    }
+
     // Run blocking helper I/O off the async runtime.
     let cwd = cwd.to_path_buf();
     let name = name.to_string();
@@ -460,6 +491,185 @@ pub async fn execute_browser_tool(
     .map_err(|e| anyhow!("browser task join: {e}"))?
 }
 
+/// Outcome of waiting for the human during handoff.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub enum HandoffWait {
+    Resumed,
+    Aborted(String),
+    TimedOut,
+}
+
+const DEFAULT_HANDOFF_TIMEOUT_SECS: u64 = 300;
+
+/// Wait for stdin line (Enter to continue, `abort` to cancel) or timeout.
+///
+/// `GROKBOY_HANDOFF_AUTO=1|resume` skips the wait (tests).
+/// `GROKBOY_HANDOFF_AUTO=abort` auto-aborts.
+pub fn wait_for_handoff_resume(timeout_secs: u64) -> HandoffWait {
+    match std::env::var("GROKBOY_HANDOFF_AUTO") {
+        Ok(v) => {
+            let v = v.trim().to_ascii_lowercase();
+            if v == "1" || v == "true" || v == "yes" || v == "resume" || v == "continue"
+            {
+                return HandoffWait::Resumed;
+            }
+            if v == "abort" || v == "0" || v == "false" || v == "no" {
+                return HandoffWait::Aborted(format!("GROKBOY_HANDOFF_AUTO={v}"));
+            }
+        }
+        Err(_) => {}
+    }
+
+    let (tx, rx) = mpsc::channel::<Result<String, String>>();
+    std::thread::spawn(move || {
+        let stdin = std::io::stdin();
+        let mut line = String::new();
+        match stdin.lock().read_line(&mut line) {
+            Ok(0) => {
+                let _ = tx.send(Err("stdin closed (EOF)".into()));
+            }
+            Ok(_) => {
+                let _ = tx.send(Ok(line));
+            }
+            Err(e) => {
+                let _ = tx.send(Err(format!("stdin read error: {e}")));
+            }
+        }
+    });
+
+    match rx.recv_timeout(Duration::from_secs(timeout_secs.max(1))) {
+        Ok(Ok(line)) => {
+            let t = line.trim().to_ascii_lowercase();
+            if t == "abort" || t == "q" || t == "quit" || t == "cancel" {
+                HandoffWait::Aborted(format!("user typed {t}"))
+            } else {
+                // Empty line (Enter) or any other input → resume.
+                HandoffWait::Resumed
+            }
+        }
+        Ok(Err(msg)) => HandoffWait::Aborted(msg),
+        Err(mpsc::RecvTimeoutError::Timeout) => HandoffWait::TimedOut,
+        Err(mpsc::RecvTimeoutError::Disconnected) => {
+            HandoffWait::Aborted("handoff wait thread disconnected".into())
+        }
+    }
+}
+
+fn print_handoff_instructions(reason: &str, timeout_secs: u64) {
+    let banner = format!(
+        "
+╔══════════════════════════════════════════════════════════════╗
+║  GrokBoy P4 — Human handoff / 人工接手                       ║
+╚══════════════════════════════════════════════════════════════╝
+
+【為什麼暫停 / Why paused】
+  {reason}
+
+【請你做什麼 / What to do】
+  1. 看著已開啟的 Chromium 視窗（headed / visible）。
+     Look at the visible Chromium window.
+  2. 完成登入、OTP、驗證碼或其他真人操作。
+     Complete login / OTP / captcha (or whatever is blocking).
+  3. 完成後回到這個終端機，按 Enter 繼續。
+     When done, return here and press Enter to continue.
+  4. 若要放棄，輸入 abort 再按 Enter。
+     To give up, type abort then Enter.
+
+  Timeout / 逾時: {timeout_secs}s
+  (tests: GROKBOY_HANDOFF_AUTO=1 to auto-resume)
+
+─── waiting for Enter / 等待 Enter ───
+"
+    );
+    eprintln!("{banner}");
+    let _ = std::io::Write::flush(&mut std::io::stderr());
+}
+
+/// `browser_handoff`: prepare headed browser, wait for human, return snapshot.
+pub async fn execute_browser_handoff(
+    cwd: &Path,
+    last_url: &LastUrlSlot,
+    args: &Value,
+) -> Result<Value> {
+    let reason = args
+        .get("reason")
+        .and_then(|v| v.as_str())
+        .map(str::trim)
+        .filter(|s| !s.is_empty())
+        .ok_or_else(|| anyhow!("browser_handoff: missing 'reason'"))?
+        .to_string();
+
+    let timeout_secs = args
+        .get("timeout_secs")
+        .and_then(|v| v.as_u64())
+        .filter(|&n| n > 0)
+        .unwrap_or(DEFAULT_HANDOFF_TIMEOUT_SECS);
+
+    let cwd = cwd.to_path_buf();
+    let last_url = last_url.clone();
+
+    tokio::task::spawn_blocking(move || {
+        // 1) Ensure headed Chromium (JSONL daemon keeps page state when already headed).
+        let prep = browser_request(
+            &cwd,
+            json!({
+                "op": "handoff_prepare",
+                "reason": reason,
+            }),
+        )?;
+
+        if prep.get("ok") != Some(&json!(true)) {
+            return Ok(response_to_tool_json(prep));
+        }
+        update_last_url(&last_url, &prep);
+
+        // 2) Instruct the human (bilingual).
+        print_handoff_instructions(&reason, timeout_secs);
+
+        // 3) Block until Enter / abort / timeout.
+        let wait = wait_for_handoff_resume(timeout_secs);
+        match wait {
+            HandoffWait::Aborted(msg) => {
+                return Ok(json!({
+                    "status": "blocked",
+                    "blocked": true,
+                    "handoff": "aborted",
+                    "reason": format!("human handoff aborted: {msg}"),
+                    "original_reason": reason,
+                }));
+            }
+            HandoffWait::TimedOut => {
+                return Ok(json!({
+                    "status": "blocked",
+                    "blocked": true,
+                    "handoff": "timeout",
+                    "reason": format!(
+                        "human handoff timed out after {timeout_secs}s (fail-closed)"
+                    ),
+                    "original_reason": reason,
+                }));
+            }
+            HandoffWait::Resumed => {}
+        }
+
+        // 4) Snapshot so the model can continue from post-login DOM.
+        let snap = browser_request(&cwd, json!({ "op": "snapshot" }))?;
+        update_last_url(&last_url, &snap);
+        let mut out = response_to_tool_json(snap);
+        if let Some(obj) = out.as_object_mut() {
+            obj.insert("handoff".into(), json!("resumed"));
+            obj.insert("handoff_reason".into(), json!(reason));
+            obj.insert(
+                "message".into(),
+                json!("Human handoff resumed; DOM snapshot attached."),
+            );
+        }
+        Ok(out)
+    })
+    .await
+    .map_err(|e| anyhow!("browser handoff join: {e}"))?
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -524,12 +734,44 @@ mod tests {
 
     #[test]
     fn browser_defs_count() {
-        assert_eq!(browser_tool_definitions().len(), 5);
+        assert_eq!(browser_tool_definitions().len(), 6);
         assert!(is_browser_tool("browser_navigate"));
         assert!(is_browser_tool("browser_snapshot"));
+        assert!(is_browser_tool("browser_handoff"));
         assert!(!is_browser_tool("shell"));
     }
 
+    #[test]
+    fn handoff_auto_resume_and_abort() {
+        // SAFETY: tests run serially for this env in practice; restore after.
+        let prev = std::env::var("GROKBOY_HANDOFF_AUTO").ok();
+        unsafe { std::env::set_var("GROKBOY_HANDOFF_AUTO", "1") };
+        assert_eq!(wait_for_handoff_resume(1), HandoffWait::Resumed);
+        unsafe { std::env::set_var("GROKBOY_HANDOFF_AUTO", "abort") };
+        match wait_for_handoff_resume(1) {
+            HandoffWait::Aborted(msg) => assert!(msg.contains("abort"), "{msg}"),
+            other => panic!("expected aborted, got {other:?}"),
+        }
+        match prev {
+            Some(v) => unsafe { std::env::set_var("GROKBOY_HANDOFF_AUTO", v) },
+            None => unsafe { std::env::remove_var("GROKBOY_HANDOFF_AUTO") },
+        }
+    }
+
+    #[test]
+    fn oneshot_ping_lists_handoff_prepare() {
+        let root = repo_cwd();
+        if which_node().is_none() {
+            return;
+        }
+        let resp = browser_oneshot(&root, &json!({"op": "ping", "id": "h1"})).unwrap();
+        let ops = resp["ops"].as_array().expect("ops");
+        assert!(
+            ops.iter().any(|o| o.as_str() == Some("handoff_prepare")),
+            "{resp}"
+        );
+    }
+
     #[test]
     fn install_hint_constant() {
         assert!(INSTALL_HINT.contains("npx playwright install chromium"));
diff --git a/crates/grokboy-core/src/lib.rs b/crates/grokboy-core/src/lib.rs
index bbbf59a..f092825 100644
--- a/crates/grokboy-core/src/lib.rs
+++ b/crates/grokboy-core/src/lib.rs
@@ -12,7 +12,7 @@ pub use agent::{
     context_char_budget, message_char_len, messages_char_len, round_signature, run_agent,
     run_agent_with, tool_call_signature, truncate_messages,
 };
-pub use browser::{INSTALL_HINT as BROWSER_INSTALL_HINT, browser_oneshot, browser_self_test, find_helper_script};
+pub use browser::{INSTALL_HINT as BROWSER_INSTALL_HINT, HandoffWait, browser_oneshot, browser_self_test, find_helper_script, wait_for_handoff_resume};
 pub use config::Config;
 pub use model::{ChatMessage, FunctionCall, Role, ToolCall, chat_completion, stream_chat};
 pub use session::{Session, load_or_create, load_session, save_session, sessions_dir};
diff --git a/crates/grokboy-core/src/tools.rs b/crates/grokboy-core/src/tools.rs
index 713a49a..3600f15 100644
--- a/crates/grokboy-core/src/tools.rs
+++ b/crates/grokboy-core/src/tools.rs
@@ -1,4 +1,4 @@
-//! Built-in tools: shell, files, completion, and optional Playwright browser (P3).
+//! Built-in tools: shell, files, completion, Playwright browser (P3), human handoff (P4).
 
 use anyhow::{Context, Result, anyhow};
 use serde_json::{Value, json};
@@ -496,7 +496,7 @@ mod tests {
     fn tool_defs_include_core_and_browser() {
         let defs = tool_definitions();
         let arr = defs.as_array().unwrap();
-        assert_eq!(arr.len(), 11); // 6 core + 5 browser
+        assert_eq!(arr.len(), 12); // 6 core + 6 browser (incl. handoff)
         let names: Vec<&str> = arr
             .iter()
             .map(|t| t["function"]["name"].as_str().unwrap())
@@ -512,6 +512,38 @@ mod tests {
         assert!(names.contains(&"browser_click"));
         assert!(names.contains(&"browser_type"));
         assert!(names.contains(&"browser_eval"));
+        assert!(names.contains(&"browser_handoff"));
+    }
+
+    #[tokio::test]
+    async fn browser_handoff_auto_resume_protocol() {
+        // Offline: with GROKBOY_HANDOFF_AUTO=1, missing Chromium still fail-closes
+        // OR (if Chromium present) resumes and returns snapshot/error JSON — never hangs.
+        let prev = std::env::var("GROKBOY_HANDOFF_AUTO").ok();
+        unsafe { std::env::set_var("GROKBOY_HANDOFF_AUTO", "1") };
+        let root = PathBuf::from(env!("CARGO_MANIFEST_DIR")).join("../..");
+        let root = root.canonicalize().unwrap_or(root);
+        let ctx = ToolContext::new(root);
+        let out = execute_tool(
+            &ctx,
+            "browser_handoff",
+            &json!({"reason": "unit test OTP wall", "timeout_secs": 5}).to_string(),
+        )
+        .await;
+        let v: Value = serde_json::from_str(&out).unwrap();
+        // Either resumed snapshot, or fail-closed blocked/error (no Chromium) — must not panic.
+        let okish = v.get("handoff").and_then(|h| h.as_str()) == Some("resumed")
+            || v.get("error").is_some()
+            || v.get("blocked") == Some(&json!(true));
+        assert!(okish, "unexpected handoff result: {v}");
+        // missing reason fails closed
+        let bad = execute_tool(&ctx, "browser_handoff", &json!({}).to_string()).await;
+        let bad: Value = serde_json::from_str(&bad).unwrap();
+        assert!(bad.get("error").is_some(), "{bad}");
+        match prev {
+            Some(v) => unsafe { std::env::set_var("GROKBOY_HANDOFF_AUTO", v) },
+            None => unsafe { std::env::remove_var("GROKBOY_HANDOFF_AUTO") },
+        }
     }
 
     #[tokio::test]
diff --git a/crates/grokboy/src/main.rs b/crates/grokboy/src/main.rs
index aecca8f..b946004 100644
--- a/crates/grokboy/src/main.rs
+++ b/crates/grokboy/src/main.rs
@@ -50,7 +50,7 @@ async fn run() -> Result<()> {
 fn print_help() {
     println!(
         "\
-GrokBoy — minimal local CLI agent (P3: optional Playwright DOM browser)
+GrokBoy — minimal local CLI agent (P4: human browser handoff)
 
 USAGE:
   grokboy chat                      Interactive streaming chat (no tools)
@@ -65,11 +65,15 @@ ENV:
   GROKBOY_BASE_URL         default https://api.x.ai/v1
   GROKBOY_MODEL            default grok-4.6
   GROKBOY_CONTEXT_CHARS    context budget (default 100000)
+  GROKBOY_BROWSER_HEADED   1 = always launch Chromium headed (visible)
+  GROKBOY_HANDOFF_AUTO     1 = auto-resume handoff (tests); abort = auto-abort
 
 Tools: shell, list_dir, read_file, write_file, report_done, report_blocked,
-       browser_navigate, browser_snapshot, browser_click, browser_type, browser_eval
+       browser_navigate, browser_snapshot, browser_click, browser_type, browser_eval,
+       browser_handoff
 Sessions: ~/.grokboy/sessions/<id>.json
 Browser (optional): cd tools/playwright && npm i && npx playwright install chromium
+Handoff: agent pauses on login/OTP/captcha → you fix in headed Chromium → Enter
 "
     );
 }
@@ -182,7 +186,7 @@ async fn cmd_run(args: &[String]) -> Result<()> {
 }
 
 async fn cmd_smoke() -> Result<()> {
-    println!("GrokBoy smoke (offline P3)…");
+    println!("GrokBoy smoke (offline P4)…");
     let stamp = uuid_like();
     let dir = std::env::temp_dir().join(format!("grokboy-smoke-{stamp}"));
     std::fs::create_dir_all(&dir).context("temp dir")?;
@@ -281,11 +285,20 @@ async fn cmd_smoke() -> Result<()> {
     }
     println!("  completion     ok");
 
-    // tool definitions present (6 core + 5 browser)
+    // tool definitions present (6 core + 6 browser incl. handoff)
     let defs = tool_definitions();
     let n_tools = defs.as_array().map(|a| a.len()).unwrap_or(0);
-    if n_tools != 11 {
-        return Err(anyhow!("expected 11 tool defs, got {n_tools}"));
+    if n_tools != 12 {
+        return Err(anyhow!("expected 12 tool defs, got {n_tools}"));
+    }
+    let tool_names: Vec<&str> = defs
+        .as_array()
+        .unwrap()
+        .iter()
+        .filter_map(|t| t["function"]["name"].as_str())
+        .collect();
+    if !tool_names.contains(&"browser_handoff") {
+        return Err(anyhow!("browser_handoff missing from tool defs"));
     }
     println!("  tool defs      ok");
 
@@ -314,6 +327,37 @@ async fn cmd_smoke() -> Result<()> {
         println!("  browser fail   ok");
     }
 
+    // handoff protocol (auto-resume; no interactive stdin)
+    // SAFETY: smoke is single-threaded for this env toggle.
+    let prev_auto = std::env::var("GROKBOY_HANDOFF_AUTO").ok();
+    unsafe { std::env::set_var("GROKBOY_HANDOFF_AUTO", "1") };
+    let handoff = execute_tool(
+        &ToolContext::new(repo.clone()),
+        "browser_handoff",
+        &json!({"reason": "smoke handoff check", "timeout_secs": 3}).to_string(),
+    )
+    .await;
+    let handoff_v: serde_json::Value = serde_json::from_str(&handoff)?;
+    // Fail-closed without Chromium, or resumed with snapshot if browsers installed.
+    if handoff_v.get("error").is_none()
+        && handoff_v.get("blocked").is_none()
+        && handoff_v.get("handoff").and_then(|h| h.as_str()) != Some("resumed")
+    {
+        return Err(anyhow!("unexpected handoff smoke result: {handoff_v}"));
+    }
+    println!("  handoff auto   ok");
+    // abort path
+    unsafe { std::env::set_var("GROKBOY_HANDOFF_AUTO", "abort") };
+    let abort_wait = grokboy_core::wait_for_handoff_resume(1);
+    if !matches!(abort_wait, grokboy_core::HandoffWait::Aborted(_)) {
+        return Err(anyhow!("expected HandoffWait::Aborted, got {abort_wait:?}"));
+    }
+    println!("  handoff abort  ok");
+    match prev_auto {
+        Some(v) => unsafe { std::env::set_var("GROKBOY_HANDOFF_AUTO", v) },
+        None => unsafe { std::env::remove_var("GROKBOY_HANDOFF_AUTO") },
+    }
+
     // context truncation
     let mut msgs = vec![
         ChatMessage::system(AGENT_SYSTEM),
diff --git a/docs/ACCEPTANCE.md b/docs/ACCEPTANCE.md
index 7718529..80caf64 100644
--- a/docs/ACCEPTANCE.md
+++ b/docs/ACCEPTANCE.md
@@ -26,3 +26,14 @@
 - [x] Thin Node helper under `tools/playwright/` (JSONL / one-shot JSON)
 - [x] Fail closed with install hint when Node/Playwright/Chromium missing
 - [x] `cargo test` / `grokboy smoke` pass without Playwright browsers installed
+
+## P4 — human browser handoff
+- [x] Spec: `docs/PRODUCT.md` (north star, P0–P3, P4, non-goals)
+- [x] Tool `browser_handoff` (`reason` required, optional `timeout_secs`)
+- [x] Headed Chromium for handoff (`handoff_prepare`; relaunch if was headless)
+- [x] Bilingual (繁中 + EN) terminal instructions; Enter continue / `abort` / timeout → fail-closed
+- [x] After resume: DOM snapshot returned as tool result
+- [x] Env docs: `GROKBOY_BROWSER_HEADED`, `GROKBOY_HANDOFF_AUTO`
+- [x] Wired into tool defs + agent system prompt; fail-closed if no browser
+- [x] Offline tests / smoke without API key or interactive stdin (`GROKBOY_HANDOFF_AUTO`)
+- [x] README status table updated
diff --git a/docs/PRODUCT.md b/docs/PRODUCT.md
new file mode 100644
index 0000000..4048d3d
--- /dev/null
+++ b/docs/PRODUCT.md
@@ -0,0 +1,48 @@
+# GrokBoy product notes
+
+**North star:** a local **Grok Bot–like** agent — thin CLI core, tool-using ReAct loop, optional Playwright **DOM** browser (not screenshot-first). GrokBoy is the sole main line; LazyBoy is reference only (no fork).
+
+## Done (P0–P3)
+
+| Phase | What |
+|-------|------|
+| **P0** | Streaming chat CLI (`grokboy chat`), xAI/OpenAI-compatible API |
+| **P1** | Tools + ReAct (`shell`, files), sessions under `~/.grokboy/sessions/` |
+| **P2** | Completion contract (`report_done` / `report_blocked`), loop guard, context truncation |
+| **P3** | Optional Playwright DOM tools: navigate / snapshot / click / type / eval (fail-closed) |
+
+## P4 — Human handoff (this slice)
+
+Auth walls (login, OTP, captcha) often cannot be automated safely. P4 adds **`browser_handoff`**:
+
+1. Agent calls `browser_handoff` with a `reason` (optional `timeout_secs`).
+2. Helper ensures Chromium is **headed** (visible); may relaunch from headless and restore URL.
+3. Terminal prints bilingual (繁中 + English) instructions.
+4. Loop **blocks** until you press **Enter** (continue) or type **`abort`**, or timeout → fail-closed blocked.
+5. On resume, a **DOM snapshot** is returned so the model can continue.
+
+### Env
+
+- `GROKBOY_BROWSER_HEADED=1` — always launch Chromium headed (recommended when handoff is likely).
+- `GROKBOY_HANDOFF_AUTO=1` — auto-resume (tests / CI); `abort` to auto-abort.
+
+### Non-goals (this slice)
+
+- Multi-agent orchestration
+- Desktop accessibility / native UI automation
+- External connectors / SaaS integrations
+- Forking LazyBoy or Codex
+
+### Acceptance (summary)
+
+See `docs/ACCEPTANCE.md` section P4. Short list:
+
+- `browser_handoff` registered and wired into the agent loop
+- Headed Chromium for handoff; JSONL daemon keeps state within one run when already headed
+- Bilingual terminal prompt; Enter / abort / timeout fail-closed
+- Post-resume DOM snapshot as tool result
+- Offline `cargo test` / `grokboy smoke` without API key or interactive stdin
+
+## Roadmap hint (later)
+
+P5+ may deepen persistence, richer session UX, or more tools — still thin core, DOM-first browser.
diff --git a/tools/playwright/README.md b/tools/playwright/README.md
index 1214eb0..03e21e2 100644
--- a/tools/playwright/README.md
+++ b/tools/playwright/README.md
@@ -1,6 +1,6 @@
 # GrokBoy Playwright helper (optional)
 
-Thin Node script used by Rust `browser_*` tools.
+Thin Node script used by Rust `browser_*` tools (P3) and human handoff (P4).
 
 ## Install
 
@@ -16,6 +16,14 @@ npx playwright install chromium
 - **One-shot**: `node browser_helper.mjs --cmd '{"op":"ping"}'`
 - **Self-test** (no Chromium): `npm run self-test`
 
-Ops: `ping`, `navigate`, `snapshot`/`dom`, `click`, `type`, `eval`, `close`, `status`.
+Ops: `ping`, `navigate`, `snapshot`/`dom`, `click`, `type`, `eval`, `close`, `status`, **`handoff_prepare`**.
+
+### `handoff_prepare`
+
+Ensures a **headed** (visible) Chromium window and brings it to front. If the daemon was headless, closes and relaunches headed, then navigates back to the last URL when possible.
+
+Note: relaunch uses a new browser context (cookies from the prior headless session are not carried unless storage state is added later). Prefer `GROKBOY_BROWSER_HEADED=1` so the first launch is already headed and state survives across handoff within one run.
+
+Env: `GROKBOY_BROWSER_HEADED=1` launches headed for normal navigate as well.
 
 Prefer CSS selector or `role`+`name` over screenshots.
diff --git a/tools/playwright/browser_helper.mjs b/tools/playwright/browser_helper.mjs
index b7913a3..c466d68 100755
--- a/tools/playwright/browser_helper.mjs
+++ b/tools/playwright/browser_helper.mjs
@@ -7,7 +7,7 @@
  *   One-shot:  node browser_helper.mjs --cmd '<json>'
  *   Self-test: node browser_helper.mjs --self-test   (no Chromium required)
  *
- * Ops: ping, navigate, snapshot, click, type, eval, close, status
+ * Ops: ping, navigate, snapshot, click, type, eval, close, status, handoff_prepare
  * Prefer CSS selector or role+name. Screenshots are NOT the primary control surface.
  */
 
@@ -25,6 +25,20 @@ let browser = null;
 let context = null;
 let page = null;
 let lastUrl = null;
+/** Whether the current browser was launched headed (visible). */
+let browserHeaded = false;
+
+function envHeadedDefault() {
+  const v = (process.env.GROKBOY_BROWSER_HEADED || "").trim().toLowerCase();
+  if (v === "1" || v === "true" || v === "yes") return true;
+  if (v === "0" || v === "false" || v === "no") return false;
+  return false;
+}
+
+function wantHeaded(forceHeaded) {
+  if (forceHeaded === true) return true;
+  return envHeadedDefault();
+}
 
 function ok(id, extra = {}) {
   return { id: id ?? null, ok: true, ...extra };
@@ -50,11 +64,11 @@ async function loadPlaywright() {
   }
 }
 
-async function ensurePage() {
-  if (page) return page;
+async function launchBrowser(headed) {
   const { chromium } = await loadPlaywright();
   try {
-    browser = await chromium.launch({ headless: true });
+    browser = await chromium.launch({ headless: !headed });
+    browserHeaded = headed;
   } catch (e) {
     const err = new Error(
       `${INSTALL_HINT} (chromium launch failed: ${e.message})`
@@ -67,6 +81,78 @@ async function ensurePage() {
   return page;
 }
 
+async function ensurePage(opts = {}) {
+  const headed = wantHeaded(opts.headed === true);
+  if (page) {
+    // Already open; if caller needs headed and we are headless, relaunch below via handoff_prepare.
+    return page;
+  }
+  return launchBrowser(headed);
+}
+
+/** Close current browser and reopen headed, restoring lastUrl when possible. */
+async function ensureHeadedForHandoff() {
+  const restore = lastUrl || (page ? page.url() : null);
+  if (browser && browserHeaded && page) {
+    try {
+      await page.bringToFront();
+    } catch {
+      /* ignore */
+    }
+    return {
+      prepared: true,
+      headed: true,
+      relaunched: false,
+      url: page.url(),
+      restored: false,
+    };
+  }
+
+  if (browser) {
+    await browser.close().catch(() => {});
+    browser = null;
+    context = null;
+    page = null;
+    browserHeaded = false;
+  }
+
+  await launchBrowser(true);
+  let restored = false;
+  if (restore && restore !== "about:blank") {
+    try {
+      await page.goto(String(restore), {
+        waitUntil: "domcontentloaded",
+        timeout: 30000,
+      });
+      lastUrl = page.url();
+      restored = true;
+    } catch (e) {
+      // Keep headed page even if restore fails; caller still sees visible window.
+      lastUrl = page.url();
+      return {
+        prepared: true,
+        headed: true,
+        relaunched: true,
+        url: lastUrl,
+        restored: false,
+        restore_error: String(e.message || e),
+      };
+    }
+  }
+  try {
+    await page.bringToFront();
+  } catch {
+    /* ignore */
+  }
+  return {
+    prepared: true,
+    headed: true,
+    relaunched: true,
+    url: page ? page.url() : null,
+    restored,
+  };
+}
+
 /** Accessibility-ish text snapshot: roles, names, and useful selectors — not pixels. */
 async function buildSnapshot(p) {
   const nodes = await p.evaluate(() => {
@@ -242,14 +328,27 @@ async function handle(req) {
             "eval",
             "close",
             "status",
+            "handoff_prepare",
           ],
         });
 
       case "status": {
         return ok(id, {
           browser_open: !!browser,
+          headed: browserHeaded,
           last_url: lastUrl,
           page_url: page ? page.url() : null,
+          env_headed: envHeadedDefault(),
+        });
+      }
+
+      case "handoff_prepare": {
+        // Bring a visible Chromium window forward for human login/OTP/captcha.
+        const info = await ensureHeadedForHandoff();
+        return ok(id, {
+          ...info,
+          message:
+            "Browser is headed (visible). Complete login/OTP/captcha in the window, then resume in the terminal.",
         });
       }
 
@@ -342,6 +441,7 @@ async function handle(req) {
         browser = null;
         context = null;
         page = null;
+        browserHeaded = false;
         return ok(id, { closed: true, last_url: lastUrl });
       }
 
@@ -375,6 +475,10 @@ async function runSelfTest() {
   // Protocol: ping without playwright
   let r = await handle({ id: "t1", op: "ping" });
   cases.push(["ping", r.ok === true && r.pong === true]);
+  cases.push([
+    "ping_lists_handoff_prepare",
+    Array.isArray(r.ops) && r.ops.includes("handoff_prepare"),
+  ]);
 
   // Missing fields fail closed
   r = await handle({ id: "t2", op: "navigate" });