fix: cover MCP reconnect edge cases

maintainer edit: handle prompt sessions that report Connection closed outside McpError, and match reconnect registration prefixes with the same sanitization used by MCP wrapper names.

nanobot/agent/tools/mcp.py

@@ -514,6 +514,13 @@ class MCPPromptWrapper(_MCPWrapperBase):
                 )
                 return f"(MCP prompt call failed: {exc.error.message} [code {exc.error.code}])"
             except Exception as exc:
+                if await self._refresh_session_after_termination(
+                    exc,
+                    refreshed_session,
+                    "prompt",
+                ):
+                    refreshed_session = True
+                    continue
                 if _is_transient(exc):
                     if not retried_transient:
                         retried_transient = True
@@ -1066,10 +1073,7 @@ def _server_signature(cfg: Any) -> Any:
 
 
 def _tool_prefix(server_name: str) -> str:
-    safe_name = "".join(ch if ch.isalnum() or ch in {"_", "-"} else "_" for ch in server_name)
-    while "__" in safe_name:
-        safe_name = safe_name.replace("__", "_")
-    return f"mcp_{safe_name}_"
+    return _sanitize_name(f"mcp_{server_name}_")
 
 
 def _unregister_server_tools(state: Any, registry: ToolRegistry, server_name: str) -> int:

tests/agent/test_mcp_connection.py

@@ -288,6 +288,55 @@ async def test_mcp_tool_reconnects_after_session_terminated(
     assert loop.tools.get("mcp_remote_quote") is not old_tool
 
 
+@pytest.mark.asyncio
+async def test_mcp_reconnect_handler_uses_sanitized_server_prefix(
+    tmp_path,
+    monkeypatch: pytest.MonkeyPatch,
+):
+    loop = _make_loop(tmp_path, mcp_servers={"remote_": object()})
+    connect_count = 0
+
+    class _FakeSession:
+        def __init__(self, index: int) -> None:
+            self.index = index
+
+        async def call_tool(self, _name: str, arguments: dict[str, Any]) -> Any:
+            assert arguments == {}
+            if self.index == 1:
+                raise McpError(ErrorData(code=-32000, message="Session terminated"))
+            return SimpleNamespace(
+                content=[mcp_types.TextContent(type="text", text="recovered")]
+            )
+
+    async def _fake_connect(servers, registry):
+        nonlocal connect_count
+        stacks = {}
+        for name in servers:
+            connect_count += 1
+            tool_def = SimpleNamespace(
+                name="quote",
+                description="quote tool",
+                inputSchema={"type": "object", "properties": {}},
+            )
+            registry.register(MCPToolWrapper(_FakeSession(connect_count), name, tool_def))
+            stack = AsyncExitStack()
+            await stack.__aenter__()
+            stacks[name] = stack
+        return stacks
+
+    monkeypatch.setattr("nanobot.agent.tools.mcp.connect_mcp_servers", _fake_connect)
+
+    await loop._connect_mcp()
+    old_tool = loop.tools.get("mcp_remote_quote")
+    assert isinstance(old_tool, MCPToolWrapper)
+
+    output = await old_tool.execute()
+
+    assert output == "recovered"
+    assert connect_count == 2
+    assert loop.tools.get("mcp_remote_quote") is not old_tool
+
+
 @pytest.mark.asyncio
 async def test_concurrent_mcp_reconnect_reuses_fresh_session(
     tmp_path,

tests/agent/test_mcp_transient_retry.py

@@ -464,3 +464,29 @@ async def test_prompt_reconnects_on_session_terminated():
     assert output == "fresh prompt"
     assert old_session.get_prompt.call_count == 1
     assert new_session.get_prompt.call_count == 1
+
+
+@pytest.mark.asyncio
+async def test_prompt_reconnects_on_connection_closed_exception():
+    """Prompt should reconnect when the SDK reports a closed session as a generic exception."""
+    old_session = AsyncMock()
+    old_session.get_prompt = AsyncMock(side_effect=RuntimeError("Connection closed"))
+    new_session = AsyncMock()
+    new_session.get_prompt = AsyncMock(return_value=_make_prompt_result("fresh prompt"))
+
+    wrapper = MCPPromptWrapper(old_session, "test_server", _make_prompt_def())
+    replacement = MCPPromptWrapper(new_session, "test_server", _make_prompt_def())
+
+    async def reconnect(server_name: str, tool_name: str, stale_tool):
+        assert server_name == "test_server"
+        assert tool_name == "mcp_test_server_prompt_test_prompt"
+        assert stale_tool is wrapper
+        return replacement
+
+    wrapper.set_reconnect_handler(reconnect)
+
+    output = await wrapper.execute()
+
+    assert output == "fresh prompt"
+    assert old_session.get_prompt.call_count == 1
+    assert new_session.get_prompt.call_count == 1