diff --git a/ceki_sdk/_browser.py b/ceki_sdk/_browser.py
index 41f7f63..1bc2d98 100644
--- a/ceki_sdk/_browser.py
+++ b/ceki_sdk/_browser.py
@@ -6,6 +6,7 @@
 import logging
 import mimetypes
 import os
+import random
 from datetime import datetime, timedelta, timezone
 from pathlib import Path
 from typing import TYPE_CHECKING, Any, Awaitable, Callable, Coroutine, Literal, cast
@@ -41,6 +42,17 @@
 
 _ERROR_TERMINAL = {-1011, -1012, -1015, -1018}
 
+# task 4109 — anti-detect branching for Browser.type().
+# When both gates pass, a long text-with-selector call routes through the
+# real system-clipboard Ctrl+V path (from task 4098) instead of the per-key
+# Ceki.typeText path. Perfect per-key rhythm on a long string is a classic
+# bot signal; a paste event with inputType=insertFromPaste looks like the
+# normal "user pasted from clipboard" behavior. Named constants live here
+# (not inside the method) so tests can pin them and future tuning does not
+# leave magic numbers in two places.
+TYPE_PASTE_MIN_CHARS = 500
+TYPE_PASTE_PROBABILITY = 0.625
+
 
 def _resolve_human(human) -> Humanizer | None:
     if os.environ.get("CEKI_HUMAN_DISABLE", "").lower() in ("1", "true", "yes"):
@@ -279,6 +291,33 @@ async def type(
         # because Chrome routed the bare CDP eval to the page's service-worker
         # execution context where `document` is undefined. chrome.scripting
         # always lands in a page frame.
+        #
+        # task 4109 — anti-detect branching. For LONG text delivered into a
+        # KNOWN selector, roll the dice against TYPE_PASTE_PROBABILITY and (if
+        # the gate opens) route through the real-clipboard Ctrl+V path from
+        # task 4098 instead of Ceki.typeText. Reasons this branch is gated on
+        # both `selector` and length:
+        #   - no selector => we don't know where to focus for the OS paste,
+        #     so the per-char path (which types into current focus) is the
+        #     only sane fallback;
+        #   - short text has no rhythm-signature problem to begin with, and
+        #     paste-events on short strings look weirder than per-key ones.
+        # Humanizer pre-click / after-hooks still run — the selector focus
+        # inside _hotkey_paste_into replaces the extension's focus step for
+        # this branch.
+        if (
+            selector is not None
+            and len(text) > TYPE_PASTE_MIN_CHARS
+            and random.random() < TYPE_PASTE_PROBABILITY
+        ):
+            h_pre = self._humanize_for_call(human)
+            if h_pre:
+                await h_pre.before("type")
+            await self._hotkey_paste_into(selector, text)
+            if h_pre:
+                await h_pre.after("type")
+            return
+
         h = self._humanize_for_call(human)
         if h:
             if self._last_pointer is not None and selector is None:
@@ -542,32 +581,25 @@ async def copy(self) -> str:
         await self._dispatch_hotkey("c", "KeyC")
         return selection
 
-    async def paste(self, selector: str, text: str) -> None:
-        """Put ``text`` into the OS clipboard, focus ``selector``, Ctrl+V it in.
+    async def _hotkey_paste_into(self, selector: str, text: str) -> None:
+        """Real system-clipboard paste of ``text`` into ``selector``.
 
-        Real system-clipboard paste: a temporary offscreen ``<textarea>`` is
-        created and selected, then a synthetic ``Ctrl+C`` flips the OS
-        clipboard to ``text``. The temp element is removed, the target element
-        is focused, and a synthetic ``Ctrl+V`` fires — which dispatches a real
-        ``ClipboardEvent`` (``paste`` handler + ``input`` event with
-        ``inputType='insertFromPaste'``). Verified against real headed
-        Chromium in contract task 4098.
+        Shared 6-CDP-call sequence (from task 4098):
 
-        Both ``selector`` and ``text`` are JSON-escaped when interpolated into
-        the ``Runtime.evaluate`` expression — quotes, backticks, backslashes,
-        newlines, and unicode are safe.
+          1. Runtime.evaluate — build offscreen ``<textarea>``, set value, focus+select
+          2. Input.dispatchKeyEvent keyDown ``c`` (Ctrl+C — flips OS clipboard)
+          3. Input.dispatchKeyEvent keyUp   ``c``
+          4. Runtime.evaluate — remove temp element, focus target selector
+          5. Input.dispatchKeyEvent keyDown ``v`` (Ctrl+V — fires paste event)
+          6. Input.dispatchKeyEvent keyUp   ``v``
 
-        Args:
-            selector: CSS selector for the target input / textarea /
-                contentEditable / any focusable element.
-            text: Arbitrary string to paste. Empty string is allowed; it
-                seeds an empty clipboard and Ctrl+V still fires the
-                ``paste`` event.
+        Both ``selector`` and ``text`` are JSON-escaped when interpolated —
+        quotes, backticks, backslashes, newlines, and unicode are safe.
 
-        Raises:
-            The underlying ``Runtime.evaluate`` will surface a JS TypeError if
-            ``querySelector`` returns ``null`` — the send() call will reject
-            with a CDP error rather than silently swallowing it.
+        Called by :meth:`paste` (public API) and :meth:`type` (task 4109
+        anti-detect branch for long text). Extracting this keeps the two
+        callers wire-identical and avoids the copy() logging noise inside a
+        type() call.
         """
         text_lit = json.dumps(text)
         seed_expr = (
@@ -601,6 +633,35 @@ async def paste(self, selector: str, text: str) -> None:
         })
         await self._dispatch_hotkey("v", "KeyV")
 
+    async def paste(self, selector: str, text: str) -> None:
+        """Put ``text`` into the OS clipboard, focus ``selector``, Ctrl+V it in.
+
+        Real system-clipboard paste: a temporary offscreen ``<textarea>`` is
+        created and selected, then a synthetic ``Ctrl+C`` flips the OS
+        clipboard to ``text``. The temp element is removed, the target element
+        is focused, and a synthetic ``Ctrl+V`` fires — which dispatches a real
+        ``ClipboardEvent`` (``paste`` handler + ``input`` event with
+        ``inputType='insertFromPaste'``). Verified against real headed
+        Chromium in contract task 4098.
+
+        Both ``selector`` and ``text`` are JSON-escaped when interpolated into
+        the ``Runtime.evaluate`` expression — quotes, backticks, backslashes,
+        newlines, and unicode are safe.
+
+        Args:
+            selector: CSS selector for the target input / textarea /
+                contentEditable / any focusable element.
+            text: Arbitrary string to paste. Empty string is allowed; it
+                seeds an empty clipboard and Ctrl+V still fires the
+                ``paste`` event.
+
+        Raises:
+            The underlying ``Runtime.evaluate`` will surface a JS TypeError if
+            ``querySelector`` returns ``null`` — the send() call will reject
+            with a CDP error rather than silently swallowing it.
+        """
+        await self._hotkey_paste_into(selector, text)
+
     def set_human(self, profile) -> "HumanProfile | None":
         prev = self._humanizer.profile if self._humanizer else None
         self._humanizer = _resolve_human(profile)
diff --git a/tests/test_type_paste_branch.py b/tests/test_type_paste_branch.py
new file mode 100644
index 0000000..e8b20f4
--- /dev/null
+++ b/tests/test_type_paste_branch.py
@@ -0,0 +1,305 @@
+"""Anti-detect probabilistic paste path in ``Browser.type()`` (task 4109).
+
+``type(text, selector=...)`` gets two gates before it routes through the
+real-clipboard Ctrl+V path from task 4098:
+
+  1. ``selector`` is not ``None`` (we need a focus target for the OS paste).
+  2. ``len(text) > TYPE_PASTE_MIN_CHARS`` (short text has no rhythm signature).
+  3. ``random.random() < TYPE_PASTE_PROBABILITY``.
+
+When any gate fails, the existing per-key ``Ceki.typeText`` path is used
+verbatim. These tests pin every gate, prove the shared hotkey helper is the
+same wire shape as :func:`Browser.paste`, and guard the module constants
+against future magic-number drift.
+"""
+
+from __future__ import annotations
+
+import json
+from unittest.mock import AsyncMock, patch
+
+from ceki_sdk import _browser
+from ceki_sdk._browser import Browser
+
+
+def _make_browser() -> Browser:
+    client = AsyncMock()
+    client._active_browsers = {}
+    client.chat_url = "https://test/chat"
+    client.api_key = "test"
+
+    match = AsyncMock()
+    match.session_id = "typepaste-1"
+    match.schedule_id = 1
+    match.chat_topic_id = "t1"
+    match.browser_info = {}
+    match.provider_user_id = None
+
+    # CEKI_HUMAN_DISABLE=1 so the humanizer does not fire click() calls that
+    # would add noise to send.await_count assertions.
+    with patch.dict("os.environ", {"CEKI_HUMAN_DISABLE": "1"}):
+        return Browser(client, match)
+
+
+def _assert_ctrl_hotkey(call: dict, *, typ: str, key: str, code: str) -> None:
+    assert call["method"] == "Input.dispatchKeyEvent"
+    params = call["params"]
+    assert params["type"] == typ
+    assert params["modifiers"] == 2
+    assert params["key"] == key
+    assert params["code"] == code
+    vk = ord(key.upper())
+    assert params["windowsVirtualKeyCode"] == vk
+    assert params["nativeVirtualKeyCode"] == vk
+
+
+# ──────────────────────────────────────────────────────────────────────────
+# Module constants
+# ──────────────────────────────────────────────────────────────────────────
+
+
+def test_constants_at_module_scope():
+    """The threshold and probability MUST live at module scope so tests can
+    pin them and future tuning does not leave magic numbers in two places
+    (``type()`` body + ``_hotkey_paste_into`` doc string, etc.)."""
+    assert _browser.TYPE_PASTE_MIN_CHARS == 500
+    assert _browser.TYPE_PASTE_PROBABILITY == 0.625
+
+
+# ──────────────────────────────────────────────────────────────────────────
+# Gate A — no selector → NEVER paste path, even for long text and forced dice
+# ──────────────────────────────────────────────────────────────────────────
+
+
+async def test_type_no_selector_never_paste_even_when_long_and_dice_low():
+    b = _make_browser()
+    b.send = AsyncMock(return_value={"result": {}})
+
+    long_text = "X" * 5000
+    with patch("ceki_sdk._browser.random.random", return_value=0.0):
+        await b.type(long_text)
+
+    # One Ceki.typeText call — nothing else.
+    assert b.send.await_count == 1
+    call = b.send.await_args_list[0][0][0]
+    assert call["method"] == "Ceki.typeText"
+    assert call["params"]["text"] == long_text
+    assert "selector" not in call["params"]
+
+
+# ──────────────────────────────────────────────────────────────────────────
+# Gate B — text length
+# ──────────────────────────────────────────────────────────────────────────
+
+
+async def test_type_short_text_with_selector_stays_per_key():
+    b = _make_browser()
+    b.send = AsyncMock(return_value={"result": {}})
+
+    with patch("ceki_sdk._browser.random.random", return_value=0.0):
+        await b.type("a" * 100, selector="#in")
+
+    assert b.send.await_count == 1
+    call = b.send.await_args_list[0][0][0]
+    assert call["method"] == "Ceki.typeText"
+    assert call["params"]["selector"] == "#in"
+
+
+async def test_type_at_threshold_exact_500_stays_per_key():
+    """Boundary: ``> 500`` means 500 exactly still goes per-key."""
+    b = _make_browser()
+    b.send = AsyncMock(return_value={"result": {}})
+
+    with patch("ceki_sdk._browser.random.random", return_value=0.0):
+        await b.type("a" * 500, selector="#in")
+
+    assert b.send.await_count == 1
+    assert b.send.await_args_list[0][0][0]["method"] == "Ceki.typeText"
+
+
+async def test_type_empty_string_with_selector_never_paste():
+    b = _make_browser()
+    b.send = AsyncMock(return_value={"result": {}})
+
+    with patch("ceki_sdk._browser.random.random", return_value=0.0):
+        await b.type("", selector="#in")
+
+    assert b.send.await_count == 1
+    assert b.send.await_args_list[0][0][0]["method"] == "Ceki.typeText"
+
+
+async def test_type_whitespace_only_and_single_char_never_paste():
+    b = _make_browser()
+    b.send = AsyncMock(return_value={"result": {}})
+
+    with patch("ceki_sdk._browser.random.random", return_value=0.0):
+        await b.type("   ", selector="#in")
+        await b.type("x", selector="#in")
+
+    # Two per-key calls, nothing else.
+    assert b.send.await_count == 2
+    for c in b.send.await_args_list:
+        assert c[0][0]["method"] == "Ceki.typeText"
+
+
+# ──────────────────────────────────────────────────────────────────────────
+# Gate C — probability
+# ──────────────────────────────────────────────────────────────────────────
+
+
+async def test_type_long_with_selector_dice_above_threshold_stays_per_key():
+    """random() >= 0.625 falls through to Ceki.typeText."""
+    b = _make_browser()
+    b.send = AsyncMock(return_value={"result": {}})
+
+    long_text = "X" * 600
+    with patch("ceki_sdk._browser.random.random", return_value=0.9):
+        await b.type(long_text, selector="#in")
+
+    assert b.send.await_count == 1
+    assert b.send.await_args_list[0][0][0]["method"] == "Ceki.typeText"
+
+
+async def test_type_long_with_selector_dice_at_boundary_stays_per_key():
+    """random() == 0.625 (not strictly less than) falls through."""
+    b = _make_browser()
+    b.send = AsyncMock(return_value={"result": {}})
+
+    with patch("ceki_sdk._browser.random.random", return_value=0.625):
+        await b.type("X" * 600, selector="#in")
+
+    assert b.send.await_count == 1
+    assert b.send.await_args_list[0][0][0]["method"] == "Ceki.typeText"
+
+
+# ──────────────────────────────────────────────────────────────────────────
+# Paste path fires: exact 6-CDP-call sequence, no Ceki.typeText
+# ──────────────────────────────────────────────────────────────────────────
+
+
+async def test_type_long_with_selector_dice_low_fires_paste_sequence():
+    b = _make_browser()
+    b.send = AsyncMock(return_value={"result": {}})
+
+    long_text = "X" * 600
+    with patch("ceki_sdk._browser.random.random", return_value=0.0):
+        await b.type(long_text, selector="#in")
+
+    # Exactly the 6-call clipboard-hotkey sequence from task 4098:
+    #   1 seed textarea, 2-3 Ctrl+C, 4 cleanup+focus, 5-6 Ctrl+V.
+    assert b.send.await_count == 6
+    calls = [c[0][0] for c in b.send.await_args_list]
+
+    # No Ceki.typeText anywhere — the paste path replaces it entirely.
+    for c in calls:
+        assert c["method"] != "Ceki.typeText"
+
+    assert calls[0]["method"] == "Runtime.evaluate"
+    seed_expr = calls[0]["params"]["expression"]
+    assert "document.createElement('textarea')" in seed_expr
+    # Text JSON-escaped into the seed.
+    assert json.dumps(long_text) in seed_expr
+
+    _assert_ctrl_hotkey(calls[1], typ="keyDown", key="c", code="KeyC")
+    _assert_ctrl_hotkey(calls[2], typ="keyUp", key="c", code="KeyC")
+
+    assert calls[3]["method"] == "Runtime.evaluate"
+    cf_expr = calls[3]["params"]["expression"]
+    assert "__ceki_paste_tmp__" in cf_expr
+    assert 'document.querySelector("#in")' in cf_expr
+    assert ".focus()" in cf_expr
+
+    _assert_ctrl_hotkey(calls[4], typ="keyDown", key="v", code="KeyV")
+    _assert_ctrl_hotkey(calls[5], typ="keyUp", key="v", code="KeyV")
+
+
+async def test_type_paste_path_json_escapes_weird_selector_and_text():
+    """Selector + text both go through json.dumps in the shared helper."""
+    b = _make_browser()
+    b.send = AsyncMock(return_value={"result": {}})
+
+    nasty_selector = " #in[data-x='\"a\\\"b\"'] "
+    nasty_text = (
+        'X' * 600
+        + '\n"quotes",\n`backticks`,\\backslash,ñü'
+    )
+
+    with patch("ceki_sdk._browser.random.random", return_value=0.0):
+        await b.type(nasty_text, selector=nasty_selector)
+
+    calls = [c[0][0] for c in b.send.await_args_list]
+
+    seed_expr = calls[0]["params"]["expression"]
+    # Raw text with un-escaped newline must NOT appear un-escaped.
+    assert nasty_text not in seed_expr
+    assert json.dumps(nasty_text) in seed_expr
+
+    cf_expr = calls[3]["params"]["expression"]
+    assert f"document.querySelector({json.dumps(nasty_selector)})" in cf_expr
+
+
+# ──────────────────────────────────────────────────────────────────────────
+# paste() still works — regression guard on the 4098 wire shape
+# ──────────────────────────────────────────────────────────────────────────
+
+
+async def test_paste_public_still_emits_the_same_6_call_sequence():
+    """After refactor into _hotkey_paste_into, paste() must still be
+    identical on the wire to task 4098 — same 6 CDP calls, same order.
+    """
+    b = _make_browser()
+    b.send = AsyncMock(return_value={"result": {}})
+
+    await b.paste("#foo", "bar")
+
+    assert b.send.await_count == 6
+    calls = [c[0][0] for c in b.send.await_args_list]
+
+    assert calls[0]["method"] == "Runtime.evaluate"
+    assert "document.createElement('textarea')" in calls[0]["params"]["expression"]
+    _assert_ctrl_hotkey(calls[1], typ="keyDown", key="c", code="KeyC")
+    _assert_ctrl_hotkey(calls[2], typ="keyUp", key="c", code="KeyC")
+    assert calls[3]["method"] == "Runtime.evaluate"
+    assert 'document.querySelector("#foo")' in calls[3]["params"]["expression"]
+    _assert_ctrl_hotkey(calls[4], typ="keyDown", key="v", code="KeyV")
+    _assert_ctrl_hotkey(calls[5], typ="keyUp", key="v", code="KeyV")
+
+
+# ──────────────────────────────────────────────────────────────────────────
+# Statistical sanity — dice distribution matches the constant
+# ──────────────────────────────────────────────────────────────────────────
+
+
+async def test_statistical_gate_matches_probability_constant():
+    """Feed a deterministic cycle of ``random()`` values through 200 calls,
+    count paste-path invocations, confirm they match the ``< 0.625`` count.
+
+    Belt-and-braces guard that the comparison is strict ``<`` (not ``<=``)
+    and that we did not accidentally invert the condition.
+    """
+    # 200 values in [0, 1). 125 are < 0.625, 75 are >= 0.625.
+    values = [i / 200 for i in range(200)]
+    expected_paste = sum(1 for v in values if v < _browser.TYPE_PASTE_PROBABILITY)
+    assert expected_paste == 125  # sanity check on the fixture itself
+
+    paste_hits = 0
+    per_key_hits = 0
+
+    long_text = "X" * 600
+
+    it = iter(values)
+    with patch("ceki_sdk._browser.random.random", side_effect=lambda: next(it)):
+        for _ in range(200):
+            b = _make_browser()
+            b.send = AsyncMock(return_value={"result": {}})
+            await b.type(long_text, selector="#in")
+            n_calls = b.send.await_count
+            if n_calls == 6:
+                paste_hits += 1
+            elif n_calls == 1:
+                per_key_hits += 1
+            else:
+                raise AssertionError(f"unexpected send count {n_calls}")
+
+    assert paste_hits == expected_paste
+    assert per_key_hits == 200 - expected_paste