fix(acpremote): negotiate acp.v1 optionally

docs/acpremote.md

@@ -53,6 +53,9 @@ framework target still needs adapter dispatch:
 acpkit serve examples.pydantic.finance_agent:acp_agent --host 0.0.0.0 --port 8080
 ```
 
+ACP Kit servers negotiate the `acp.v1` WebSocket subprotocol when a client requests it. Clients
+that omit a subprotocol remain supported.
+
 Expose a stdio ACP command:
 
 ```bash

packages/transports/acpremote/src/acpremote/server.py

@@ -2,6 +2,7 @@
 
 import json
 import os
+from collections.abc import Sequence
 from http import HTTPStatus
 from typing import Literal
 from urllib.parse import urlsplit
@@ -11,6 +12,7 @@
 from websockets.asyncio.server import Server, ServerConnection, serve
 from websockets.datastructures import Headers
 from websockets.http11 import Request, Response
+from websockets.typing import Subprotocol
 
 from .auth import is_bearer_authorized
 from .command import CommandOptions, run_remote_command_connection
@@ -26,6 +28,16 @@
     "serve_stdio_command",
 )
 
+_ACP_SUBPROTOCOL = Subprotocol("acp.v1")
+
+
+def _select_acp_subprotocol(
+    connection: ServerConnection,
+    subprotocols: Sequence[Subprotocol],
+) -> Subprotocol | None:
+    del connection
+    return _ACP_SUBPROTOCOL if _ACP_SUBPROTOCOL in subprotocols else None
+
 
 async def run_remote_agent_connection(
     agent: Agent,
@@ -86,6 +98,8 @@ async def serve_remote_agent(
         close_timeout=resolved_transport.close_timeout,
         max_size=resolved_transport.max_size,
         max_queue=resolved_transport.max_queue,
+        subprotocols=[_ACP_SUBPROTOCOL],
+        select_subprotocol=_select_acp_subprotocol,
     )
 
 
@@ -185,6 +199,8 @@ async def serve_stdio_command(
         close_timeout=resolved_transport.close_timeout,
         max_size=resolved_transport.max_size,
         max_queue=resolved_transport.max_queue,
+        subprotocols=[_ACP_SUBPROTOCOL],
+        select_subprotocol=_select_acp_subprotocol,
     )
 
 

tests/acpremote/test_phase2.py

@@ -33,6 +33,7 @@
 )
 from websockets.asyncio.client import connect
 from websockets.exceptions import InvalidStatus
+from websockets.typing import Subprotocol
 
 
 @dataclass(slots=True)
@@ -212,6 +213,29 @@ async def test_phase2_http_metadata_and_health_routes_are_served() -> None:
     assert missing_body == b"not found\n"
 
 
+@pytest.mark.asyncio
+@pytest.mark.parametrize(
+    ("subprotocols", "expected_subprotocol"),
+    [([Subprotocol("acp.v1")], "acp.v1"), (None, None)],
+)
+async def test_phase2_websocket_negotiates_acp_v1_optionally(
+    subprotocols: list[Subprotocol] | None,
+    expected_subprotocol: str | None,
+) -> None:
+    server = await serve_acp(cast("Agent", _EchoAgent()))
+    assert server.sockets is not None
+    port = next(iter(server.sockets)).getsockname()[1]
+    try:
+        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_websocket_path_and_bearer_auth_are_enforced() -> None:
     server = await serve_acp(