fix(shared): make MCPError subclasses pickle-safe

Signed-off-by: ulofiai <monsterking@tutamail.com>

Author: ulofiai

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.
@@ -107,10 +117,6 @@ 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 constructor signature."""
-        return (self.__class__, (self._elicitations, self.message))
-
     @classmethod
     def from_error(cls, error: ErrorData) -> UrlElicitationRequiredError:
         """Reconstruct from an ErrorData received over the wire."""

tests/shared/test_exceptions.py

@@ -5,7 +5,7 @@
 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:
@@ -184,7 +184,21 @@ def test_mcp_error_pickle_roundtrip() -> None:
     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:
@@ -208,5 +222,6 @@ def test_url_elicitation_required_error_pickle_roundtrip() -> None:
     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