fix(shared): make UrlElicitationRequiredError pickle-safe

src/mcp/shared/exceptions.py

@@ -51,6 +51,16 @@ def from_error_data(cls, error: ErrorData) -> MCPError:
     def __str__(self) -> str:
         return self.message
 
+    def __reduce__(self) -> tuple[Any, tuple[type[MCPError], tuple[Any, ...]], dict[str, Any]]:
+        return (_restore_mcp_error, (type(self), self.args), self.__dict__)
+
+
+def _restore_mcp_error(error_type: type[MCPError], args: tuple[Any, ...]) -> MCPError:
+    """Reconstruct an MCPError without invoking a subclass constructor."""
+    restored = MCPError.__new__(error_type)
+    Exception.__init__(restored, *args)
+    return restored
+
 
 class NoBackChannelError(MCPError):
     """Raised when a server-initiated request has no channel that can deliver it.

tests/shared/test_exceptions.py

@@ -1,9 +1,11 @@
 """Tests for MCP exception classes."""
 
+import pickle
+
 import pytest
 from mcp_types import URL_ELICITATION_REQUIRED, ElicitRequestURLParams, ErrorData, JSONRPCError
 
-from mcp.shared.exceptions import MCPError, UrlElicitationRequiredError
+from mcp.shared.exceptions import MCPError, NoBackChannelError, UrlElicitationRequiredError
 
 
 def test_url_elicitation_required_error_create_with_single_elicitation() -> None:
@@ -173,3 +175,53 @@ def test_from_jsonrpc_error_preserves_code_message_and_data() -> None:
     )
     error = MCPError.from_jsonrpc_error(wire)
     assert error.error == ErrorData(code=URL_ELICITATION_REQUIRED, message="go elsewhere", data={"hint": "y"})
+
+
+def test_mcp_error_pickle_roundtrip() -> None:
+    """MCPError preserves its structured payload across 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.args == original.args
+    assert restored.error == original.error
+    assert str(restored) == str(original)
+
+
+def test_no_back_channel_error_pickle_roundtrip_preserves_method() -> None:
+    """NoBackChannelError preserves its method and structured payload when pickled."""
+    original = NoBackChannelError("sampling/createMessage")
+
+    restored = pickle.loads(pickle.dumps(original))
+
+    assert isinstance(restored, NoBackChannelError)
+    assert restored.args == original.args
+    assert restored.error == original.error
+    assert restored.method == original.method
+
+
+def test_url_elicitation_required_error_pickle_roundtrip() -> None:
+    """UrlElicitationRequiredError preserves its typed state when pickled."""
+    elicitations = [
+        ElicitRequestURLParams(
+            mode="url",
+            message="First authorization",
+            url="https://example.com/auth/first",
+            elicitation_id="auth-1",
+        ),
+        ElicitRequestURLParams(
+            mode="url",
+            message="Second authorization",
+            url="https://example.com/auth/second",
+            elicitation_id="auth-2",
+        ),
+    ]
+    original = UrlElicitationRequiredError(elicitations, message="Authorization required")
+
+    restored = pickle.loads(pickle.dumps(original))
+
+    assert type(restored) is UrlElicitationRequiredError
+    assert restored.args == original.args
+    assert restored.error == original.error
+    assert restored.elicitations == original.elicitations