Merge PR #1728: harden MCP tool cancellation handling

Author: Re-bin

nanobot/agent/tools/mcp.py

@@ -36,6 +36,7 @@ def parameters(self) -> dict[str, Any]:
 
     async def execute(self, **kwargs: Any) -> str:
         from mcp import types
+
         try:
             result = await asyncio.wait_for(
                 self._session.call_tool(self._original_name, arguments=kwargs),
@@ -44,6 +45,23 @@ async def execute(self, **kwargs: Any) -> str:
         except asyncio.TimeoutError:
             logger.warning("MCP tool '{}' timed out after {}s", self._name, self._tool_timeout)
             return f"(MCP tool call timed out after {self._tool_timeout}s)"
+        except asyncio.CancelledError:
+            # MCP SDK's anyio cancel scopes can leak CancelledError on timeout/failure.
+            # Re-raise only if our task was externally cancelled (e.g. /stop).
+            task = asyncio.current_task()
+            if task is not None and task.cancelling() > 0:
+                raise
+            logger.warning("MCP tool '{}' was cancelled by server/SDK", self._name)
+            return "(MCP tool call was cancelled)"
+        except Exception as exc:
+            logger.exception(
+                "MCP tool '{}' failed: {}: {}",
+                self._name,
+                type(exc).__name__,
+                exc,
+            )
+            return f"(MCP tool call failed: {type(exc).__name__})"
+
         parts = []
         for block in result.content:
             if isinstance(block, types.TextContent):

tests/test_mcp_tool.py

@@ -0,0 +1,99 @@
+from __future__ import annotations
+
+import asyncio
+import sys
+from types import ModuleType, SimpleNamespace
+
+import pytest
+
+from nanobot.agent.tools.mcp import MCPToolWrapper
+
+
+class _FakeTextContent:
+    def __init__(self, text: str) -> None:
+        self.text = text
+
+
+@pytest.fixture(autouse=True)
+def _fake_mcp_module(monkeypatch: pytest.MonkeyPatch) -> None:
+    mod = ModuleType("mcp")
+    mod.types = SimpleNamespace(TextContent=_FakeTextContent)
+    monkeypatch.setitem(sys.modules, "mcp", mod)
+
+
+def _make_wrapper(session: object, *, timeout: float = 0.1) -> MCPToolWrapper:
+    tool_def = SimpleNamespace(
+        name="demo",
+        description="demo tool",
+        inputSchema={"type": "object", "properties": {}},
+    )
+    return MCPToolWrapper(session, "test", tool_def, tool_timeout=timeout)
+
+
+@pytest.mark.asyncio
+async def test_execute_returns_text_blocks() -> None:
+    async def call_tool(_name: str, arguments: dict) -> object:
+        assert arguments == {"value": 1}
+        return SimpleNamespace(content=[_FakeTextContent("hello"), 42])
+
+    wrapper = _make_wrapper(SimpleNamespace(call_tool=call_tool))
+
+    result = await wrapper.execute(value=1)
+
+    assert result == "hello\n42"
+
+
+@pytest.mark.asyncio
+async def test_execute_returns_timeout_message() -> None:
+    async def call_tool(_name: str, arguments: dict) -> object:
+        await asyncio.sleep(1)
+        return SimpleNamespace(content=[])
+
+    wrapper = _make_wrapper(SimpleNamespace(call_tool=call_tool), timeout=0.01)
+
+    result = await wrapper.execute()
+
+    assert result == "(MCP tool call timed out after 0.01s)"
+
+
+@pytest.mark.asyncio
+async def test_execute_handles_server_cancelled_error() -> None:
+    async def call_tool(_name: str, arguments: dict) -> object:
+        raise asyncio.CancelledError()
+
+    wrapper = _make_wrapper(SimpleNamespace(call_tool=call_tool))
+
+    result = await wrapper.execute()
+
+    assert result == "(MCP tool call was cancelled)"
+
+
+@pytest.mark.asyncio
+async def test_execute_re_raises_external_cancellation() -> None:
+    started = asyncio.Event()
+
+    async def call_tool(_name: str, arguments: dict) -> object:
+        started.set()
+        await asyncio.sleep(60)
+        return SimpleNamespace(content=[])
+
+    wrapper = _make_wrapper(SimpleNamespace(call_tool=call_tool), timeout=10)
+    task = asyncio.create_task(wrapper.execute())
+    await started.wait()
+
+    task.cancel()
+
+    with pytest.raises(asyncio.CancelledError):
+        await task
+
+
+@pytest.mark.asyncio
+async def test_execute_handles_generic_exception() -> None:
+    async def call_tool(_name: str, arguments: dict) -> object:
+        raise RuntimeError("boom")
+
+    wrapper = _make_wrapper(SimpleNamespace(call_tool=call_tool))
+
+    result = await wrapper.execute()
+
+    assert result == "(MCP tool call failed: RuntimeError)"