diff --git a/docs/acpremote.md b/docs/acpremote.md
index 54d9b07..961134c 100644
--- a/docs/acpremote.md
+++ b/docs/acpremote.md
@@ -281,7 +281,7 @@ Current transport behavior is intentionally narrow:
 - one WebSocket text message carries one ACP JSON message
 - binary WebSocket frames are rejected
 - transport limits are configurable through `TransportOptions`
-- bearer token auth is optional
+- bearer token auth is optional; the token is read from the `Authorization: Bearer <token>` header or, for browser clients that cannot set headers, from a `bearer.<token>` entry in `Sec-WebSocket-Protocol` (the server only echoes `acp.v1`, never the token)
 - metadata and health endpoints are served alongside the WebSocket transport
 
 ## Documented Remote-Host Flows
diff --git a/packages/transports/acpremote/src/acpremote/auth.py b/packages/transports/acpremote/src/acpremote/auth.py
index 8ec2d05..f9fb6ac 100644
--- a/packages/transports/acpremote/src/acpremote/auth.py
+++ b/packages/transports/acpremote/src/acpremote/auth.py
@@ -1,6 +1,7 @@
 from __future__ import annotations as _annotations
 
 from collections.abc import Mapping
+from hmac import compare_digest
 
 __all__ = (
     "bearer_headers",
@@ -9,6 +10,9 @@
 )
 
 
+_BEARER_SUBPROTOCOL_PREFIX = "bearer."
+
+
 def bearer_headers(token: str | None) -> dict[str, str] | None:
     stripped = normalized_bearer_token(token)
     if stripped is None:
@@ -29,7 +33,35 @@ def is_bearer_authorized(
     headers: Mapping[str, str],
     token: str | None,
 ) -> bool:
+    """Check the ``Authorization`` header, then ``bearer.<token>`` WebSocket subprotocols.
+
+    Browser WebSocket clients cannot set custom headers, so they carry the token as an extra
+    ``Sec-WebSocket-Protocol`` entry next to ``acp.v1``.
+    """
     expected_token = normalized_bearer_token(token)
     if expected_token is None:
         return True
-    return headers.get("Authorization") == f"Bearer {expected_token}"
+    authorization = headers.get("Authorization")
+    if authorization is not None and compare_digest(
+        authorization.encode(),
+        f"Bearer {expected_token}".encode(),
+    ):
+        return True
+    return any(
+        compare_digest(candidate.encode(), expected_token.encode())
+        for candidate in _subprotocol_bearer_tokens(headers)
+    )
+
+
+def _subprotocol_bearer_tokens(headers: Mapping[str, str]) -> list[str]:
+    try:
+        raw_subprotocols = headers.get("Sec-WebSocket-Protocol")
+    except LookupError:
+        return []
+    if raw_subprotocols is None:
+        return []
+    return [
+        entry.removeprefix(_BEARER_SUBPROTOCOL_PREFIX)
+        for entry in (part.strip() for part in raw_subprotocols.split(","))
+        if entry.startswith(_BEARER_SUBPROTOCOL_PREFIX)
+    ]
diff --git a/tests/acpremote/test_helpers.py b/tests/acpremote/test_helpers.py
index 8c33fd7..2a68157 100644
--- a/tests/acpremote/test_helpers.py
+++ b/tests/acpremote/test_helpers.py
@@ -21,6 +21,7 @@
     SetSessionConfigOptionResponse,
     SetSessionModeResponse,
 )
+from acpremote import auth as auth_module
 from acpremote import client as client_module
 from acpremote import command as command_module
 from acpremote import proxy_agent as proxy_agent_module
@@ -28,6 +29,7 @@
 from acpremote.client import RemoteClientConnection
 from acpremote.config import TransportOptions
 from acpremote.metadata import ServerMetadata
+from websockets.datastructures import Headers
 from websockets.exceptions import ConnectionClosedOK
 
 
@@ -1198,3 +1200,11 @@ async def test_stream_helper_paths_cover_protocol_and_sender_edge_cases() -> Non
     )
     with pytest.raises(asyncio.CancelledError):
         await cancelled_transport._sender_loop()
+
+
+def test_bearer_authorization_rejects_duplicated_subprotocol_headers() -> None:
+    headers = Headers()
+    headers["Sec-WebSocket-Protocol"] = "bearer.secret-token"
+    headers["Sec-WebSocket-Protocol"] = "acp.v1"
+
+    assert auth_module.is_bearer_authorized(headers, "secret-token") is False
diff --git a/tests/acpremote/test_phase2.py b/tests/acpremote/test_phase2.py
index c2cc35a..f4b5e73 100644
--- a/tests/acpremote/test_phase2.py
+++ b/tests/acpremote/test_phase2.py
@@ -285,6 +285,45 @@ async def test_phase2_websocket_path_and_bearer_auth_are_enforced() -> None:
     assert client.updates[0].update.content.text == "phase2 auth success"
 
 
+@pytest.mark.asyncio
+@pytest.mark.parametrize(
+    ("subprotocols", "expected_status", "expected_subprotocol"),
+    [
+        ([Subprotocol("acp.v1"), Subprotocol("bearer.secret-token")], None, "acp.v1"),
+        ([Subprotocol("bearer.secret-token")], None, None),
+        ([Subprotocol("acp.v1"), Subprotocol("bearer.wrong-token")], 401, None),
+        ([Subprotocol("acp.v1")], 401, None),
+    ],
+    ids=["acp-and-bearer", "bearer-only", "wrong-token", "no-token"],
+)
+async def test_phase2_websocket_accepts_bearer_token_from_subprotocol(
+    subprotocols: list[Subprotocol],
+    expected_status: int | None,
+    expected_subprotocol: str | None,
+) -> None:
+    server = await serve_acp(cast("Agent", _EchoAgent()), bearer_token="secret-token")
+    assert server.sockets is not None
+    port = next(iter(server.sockets)).getsockname()[1]
+    try:
+        if expected_status is not None:
+            with pytest.raises(InvalidStatus) as rejected:
+                async with connect(
+                    f"ws://127.0.0.1:{port}/acp/ws",
+                    subprotocols=subprotocols,
+                ):
+                    raise AssertionError("unreachable")  # pragma: no cover
+            assert rejected.value.response.status_code == expected_status
+        else:
+            async with connect(
+                f"ws://127.0.0.1:{port}/acp/ws",
+                subprotocols=subprotocols,
+            ) as websocket:
+                assert websocket.subprotocol == expected_subprotocol
+    finally:
+        server.close()
+        await server.wait_closed()
+
+
 @pytest.mark.asyncio
 async def test_phase2_rejects_mixed_server_and_transport_options() -> None:
     with pytest.raises(ValueError, match="pass either options or server_options, not both"):