fix(mcp): include exception type in error messages when str(exc) is empty

Some exception classes (e.g. anyio.ClosedResourceError) are raised without
a message argument, so str(exc) returns an empty string. The existing error
format f'{type(exc).__name__}: {exc}' would produce messages like
'MCP call failed: ClosedResourceError: ' with nothing after the colon.

Add _exc_str() helper that falls back to repr(exc) when str(exc) is empty,
and apply it to all 6 MCP error formatting sites (5 tool/prompt/resource
handlers + 1 sampling handler).

Fixes #19417

tests/tools/test_mcp_empty_error_message.py

@@ -0,0 +1,89 @@
+"""Regression tests for MCP error messages when str(exc) is empty.
+
+Issue #19417: ClosedResourceError (and similar exceptions raised without a
+message argument) produced ``MCP call failed: ClosedResourceError: `` with
+nothing after the colon, making debugging impossible.
+
+Fix: ``_exc_str()`` falls back to ``repr(exc)`` when ``str(exc)`` is empty.
+"""
+
+import json
+from types import SimpleNamespace
+from unittest.mock import MagicMock, patch
+
+import pytest
+
+from tools.mcp_tool import _exc_str, _sanitize_error
+
+
+# ---------------------------------------------------------------------------
+# _exc_str unit tests
+# ---------------------------------------------------------------------------
+
+
+class _EmptyMessageError(Exception):
+    """Exception whose __str__ returns empty string (like anyio.ClosedResourceError)."""
+
+    def __str__(self):
+        return ""
+
+
+class _NormalError(Exception):
+    pass
+
+
+def test_exc_str_returns_str_when_nonempty():
+    exc = _NormalError("something broke")
+    assert _exc_str(exc) == "something broke"
+
+
+def test_exc_str_falls_back_to_repr_when_str_empty():
+    exc = _EmptyMessageError()
+    result = _exc_str(exc)
+    assert result != ""
+    assert "_EmptyMessageError" in result
+
+
+def test_exc_str_falls_back_to_repr_for_whitespace_only():
+    """str(exc) that is only whitespace should also trigger the repr fallback."""
+    exc = Exception("   ")
+    result = _exc_str(exc)
+    # After strip(), the text is empty, so repr is used
+    assert result.strip() != ""
+
+
+def test_exc_str_handles_closedresource_like_exception():
+    """Simulate anyio.ClosedResourceError which has no message."""
+    # Replicate the real anyio.ClosedResourceError behavior
+    exc = type("ClosedResourceError", (Exception,), {"__str__": lambda self: ""})()
+    result = _exc_str(exc)
+    assert "ClosedResourceError" in result
+    assert result != ""
+
+
+# ---------------------------------------------------------------------------
+# Integration: error message format in _sanitize_error
+# ---------------------------------------------------------------------------
+
+
+def test_error_message_not_empty_when_exc_has_no_message():
+    """The formatted error string should always contain the exception class name."""
+    exc = _EmptyMessageError()
+    error_msg = _sanitize_error(
+        f"MCP call failed: {type(exc).__name__}: {_exc_str(exc)}"
+    )
+    assert "ClosedResourceError" not in error_msg or "_EmptyMessageError" in error_msg
+    # The key invariant: the message must not end with ": "
+    assert not error_msg.endswith(": ")
+    # And it must contain the exception type name
+    assert "_EmptyMessageError" in error_msg
+
+
+def test_error_message_preserves_normal_exception_text():
+    """Normal exceptions should still show their message text."""
+    exc = _NormalError("connection refused")
+    error_msg = _sanitize_error(
+        f"MCP call failed: {type(exc).__name__}: {_exc_str(exc)}"
+    )
+    assert "connection refused" in error_msg
+    assert "_NormalError" in error_msg