fix: make UrlElicitationRequiredError pickle-safe

Add __reduce__ method so pickle reconstructs with the correct
(elicitations, message) arguments instead of the (code, message, data)
tuple stored in Exception.args.

Closes #2431

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: Christian-Sidak

src/mcp/shared/exceptions.py

@@ -94,6 +94,10 @@ def elicitations(self) -> list[ElicitRequestURLParams]:
         """The list of URL elicitations required before the request can proceed."""
         return self._elicitations
 
+    def __reduce__(self) -> tuple[type, tuple[list[ElicitRequestURLParams], str]]:
+        """Support pickling by reconstructing with the original __init__ signature."""
+        return (self.__class__, (self._elicitations, self.error.message))
+
     @classmethod
     def from_error(cls, error: ErrorData) -> UrlElicitationRequiredError:
         """Reconstruct from an ErrorData received over the wire."""

tests/shared/test_exceptions.py

@@ -1,5 +1,7 @@
 """Tests for MCP exception classes."""
 
+import pickle
+
 import pytest
 
 from mcp.shared.exceptions import MCPError, UrlElicitationRequiredError
@@ -162,3 +164,61 @@ def test_url_elicitation_required_error_exception_message() -> None:
 
     # The exception's string representation should match the message
     assert str(error) == "URL elicitation required"
+
+
+def test_mcp_error_pickle_roundtrip() -> None:
+    """Test that MCPError survives a pickle round-trip."""
+    original = MCPError(code=-32600, message="Invalid request", data={"detail": "bad"})
+
+    restored = pickle.loads(pickle.dumps(original))
+
+    assert type(restored) is MCPError
+    assert restored.error.code == original.error.code
+    assert restored.error.message == original.error.message
+    assert restored.error.data == original.error.data
+
+
+def test_url_elicitation_required_error_pickle_roundtrip() -> None:
+    """Test that UrlElicitationRequiredError survives a pickle round-trip."""
+    elicitations = [
+        ElicitRequestURLParams(
+            mode="url",
+            message="Auth required",
+            url="https://example.com/auth",
+            elicitation_id="test-123",
+        ),
+    ]
+    original = UrlElicitationRequiredError(elicitations, message="Please authenticate")
+
+    restored = pickle.loads(pickle.dumps(original))
+
+    assert type(restored) is UrlElicitationRequiredError
+    assert restored.error.code == URL_ELICITATION_REQUIRED
+    assert restored.error.message == "Please authenticate"
+    assert len(restored.elicitations) == 1
+    assert restored.elicitations[0].elicitation_id == "test-123"
+    assert restored.elicitations[0].url == "https://example.com/auth"
+
+
+def test_url_elicitation_required_error_pickle_default_message() -> None:
+    """Test pickle round-trip preserves the auto-generated default message."""
+    elicitations = [
+        ElicitRequestURLParams(
+            mode="url",
+            message="Auth",
+            url="https://example.com/auth",
+            elicitation_id="e1",
+        ),
+        ElicitRequestURLParams(
+            mode="url",
+            message="Auth2",
+            url="https://example.com/auth2",
+            elicitation_id="e2",
+        ),
+    ]
+    original = UrlElicitationRequiredError(elicitations)
+
+    restored = pickle.loads(pickle.dumps(original))
+
+    assert restored.error.message == "URL elicitations required"
+    assert len(restored.elicitations) == 2