wait_for_channel and signal_channel block the FastMCP event loop (sync subprocess.run) #18

Summary

wait_for_channel and signal_channel are registered as synchronous (def, not async def) tools that run blocking subprocess.run(..., timeout=N) calls on FastMCP's event-loop thread. For the duration of the timeout the server cannot service stdio reads/writes, respond to the MCP client's keepalive pings, or run other tool handlers. Clients whose stdio keepalive is shorter than the wait_for_channel timeout can disconnect mid-wait.

0.1.0a2 explicitly addressed the same class of bug for wait_for_text / wait_for_content_change — per CHANGES, "unblock FastMCP event loop via asyncio.to_thread around capture_pane" (commit 0a408fe) — but left the channel-wait pair untouched.

Evidence

src/libtmux_mcp/tools/wait_for_tools.py:99-154 — wait_for_channel is a plain def decorated with the sync @handle_tool_errors. The body is a synchronous subprocess.run with a user-supplied timeout of up to 30 s by default and no upper bound:

@handle_tool_errors
def wait_for_channel(
    channel: str,
    timeout: float = 30.0,
    socket_name: str | None = None,
) -> str:
    ...
    try:
        subprocess.run(argv, check=True, capture_output=True, timeout=timeout)
    except subprocess.TimeoutExpired as e:
        ...

src/libtmux_mcp/tools/wait_for_tools.py:157-196 — signal_channel has the same structure (sync def, sync subprocess.run bounded by _SIGNAL_TIMEOUT_SECONDS). Signalling is normally fast, but the pattern is still wrong.

Contrast — already-correct pattern: src/libtmux_mcp/tools/pane_tools/wait.py:100 declares async def wait_for_text(...) and offloads blocking work via asyncio.to_thread(...). That is the pattern the channel pair should match.

Reproduction

Environment: libtmux-mcp==0.1.0a2 via uvx libtmux-mcp==0.1.0a2, connected to any MCP client with stdio keepalive. The event-loop block is client-agnostic; the observed-disconnect behavior in step 4 was seen with Claude Code but any client whose stdio keepalive is shorter than the wait_for_channel timeout is at risk.

1. Create an isolated tmux server so this doesn't pollute default: tmux -L wfc_repro new-session -ds repro.
2. From the MCP client, call wait_for_channel on a channel that will never be signalled, with a timeout safely larger than the client's stdio keepalive: {"channel": "unsignalled", "timeout": 30, "socket_name": "wfc_repro"}.
3. While the call is pending, call any other libtmux-mcp tool (e.g. list_sessions) from the same client. Observe it is queued — the server cannot respond until wait_for_channel returns or times out.
4. After the timeout fires, ToolError is raised. On Claude Code specifically, the client-side MCP harness marks the server as disconnected (system-reminder: "The following deferred tools are no longer available (their MCP server disconnected)") even though the Python process is still alive per ps.
5. Tear down: tmux -L wfc_repro kill-server.

Observed during 0.1.0a2 smoke-testing with timeout=2.0 — the minimum timeout at which the disconnect was seen. The disconnect is not guaranteed on every client or every call, but the event-loop block itself is 100% reproducible.

Expected behavior

While wait_for_channel is pending, the server continues to service other tool calls, MCP pings, and cancellation signals — cooperative concurrency as the rest of the tool surface already provides.

Suggested fix

Port the asyncio.to_thread(...) pattern used for wait_for_text:

@handle_tool_errors_async
async def wait_for_channel(
    channel: str,
    timeout: float = 30.0,
    socket_name: str | None = None,
) -> str:
    server = _get_server(socket_name=socket_name)
    cname = _validate_channel_name(channel)
    argv = _tmux_argv(server, "wait-for", cname)
    try:
        await asyncio.to_thread(
            subprocess.run,
            argv, check=True, capture_output=True, timeout=timeout,
        )
    except subprocess.TimeoutExpired as e:
        ...

Same treatment for signal_channel. handle_tool_errors_async already exists in _utils.py (added in 0.1.0a2 per commit a13898f) — no new infrastructure required.

Tests

Add regression coverage in tests/test_wait_for_tools.py:

1. Assert both functions are asyncio.iscoroutinefunction(...).
2. Fire wait_for_channel with a 2 s unsignalled timeout on a dedicated event loop, and concurrently schedule an asyncio.sleep(0) coroutine via asyncio.gather. Assert the concurrent coroutine completes in ≪ 2 s — today it would block for the full timeout.
3. Consider mirroring the CancelledError propagation tests already present for wait_for_text in tests/test_pane_tools.py — the channel pair should respect the same contract.

Related

• Commit 0a408fe mcp(fix[pane_tools/wait]): unblock FastMCP event loop via asyncio.to_thread around capture_pane — the analogous fix for the pane-wait pair.
• Commit a13898f mcp(refactor[_utils]): add handle_tool_errors_async for Context-using tools — the decorator needed for the port.
• CHANGES 0.1.0a2 › What's new › New tools › Waits lists channel tools alongside pane-wait tools; only the latter got the event-loop fix.

Environment

• libtmux-mcp: 0.1.0a2 (PyPI + local HEAD)
• FastMCP: 3.2.4
• tmux: 3.6a
• Python: 3.13
• OS: Linux (WSL2)
• MCP client observed disconnecting: Claude Code