fix: make UrlElicitationRequiredError pickle-safe via __reduce__

Pickle reconstructs exceptions by calling cls(*args). Since
UrlElicitationRequiredError inherits MCPError's args=(code, message, data),
it fails on restore because its __init__ expects (elicitations, message).

Adding __reduce__ to return (from_error, (self.error,)) ensures pickle
uses the existing from_error classmethod to reconstruct the object correctly.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: Christian-Sidak

src/mcp/shared/exceptions.py

@@ -104,3 +104,6 @@ def from_error(cls, error: ErrorData) -> UrlElicitationRequiredError:
         raw_elicitations = cast(list[dict[str, Any]], data.get("elicitations", []))
         elicitations = [ElicitRequestURLParams.model_validate(e) for e in raw_elicitations]
         return cls(elicitations, error.message)
+
+    def __reduce__(self) -> tuple:
+        return (self.from_error, (self.error,))

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 +164,34 @@ 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="Authentication Required")
+    restored = pickle.loads(pickle.dumps(original))
+
+    assert type(restored) is MCPError
+    assert restored.code == -32600
+    assert restored.message == "Authentication Required"
+    assert restored.error.code == -32600
+    assert restored.error.message == "Authentication Required"
+
+
+def test_url_elicitation_required_error_pickle_roundtrip() -> None:
+    """Test that UrlElicitationRequiredError survives a pickle round-trip."""
+    elicitation = ElicitRequestURLParams(
+        mode="url",
+        message="Auth required",
+        url="https://example.com/auth",
+        elicitation_id="test-123",
+    )
+    original = UrlElicitationRequiredError([elicitation])
+    restored = pickle.loads(pickle.dumps(original))
+
+    assert type(restored) is UrlElicitationRequiredError
+    assert restored.message == "URL elicitation required"
+    assert restored.error.code == URL_ELICITATION_REQUIRED
+    assert len(restored.elicitations) == 1
+    assert restored.elicitations[0].elicitation_id == "test-123"
+    assert restored.elicitations[0].url == "https://example.com/auth"