McpError is not pickle-safe and fails to unpickle

Initial Checks

• I confirm that I'm using the latest version of MCP Python SDK
• I confirm that I searched for my issue in https://github.com/modelcontextprotocol/python-sdk/issues before opening this issue

Description

Summary

mcp.shared.exceptions.McpError does not survive a normal cloudpickle.dumps() / cloudpickle.loads() round-trip.

The failure appears to come from McpError.__init__ expecting an ErrorData object, while exception unpickling reconstructs it with a plain string from Exception.args.

This is surfacing for us through background task execution, but the bug reproduces without Docket/FastMCP task machinery.

Actual behavior

Unpickling fails with:

AttributeError: 'str' object has no attribute 'message'

Traceback points at McpError.__init__:

class McpError(Exception):
    error: ErrorData

    def __init__(self, error: ErrorData):
        super().__init__(error.message)
        self.error = error

Expected behavior

McpError(ErrorData(...)) should round-trip through pickle/cloudpickle without crashing.

At minimum, this should work:

• serialize McpError
• deserialize McpError
• preserve the message
• preserve the error payload, or at least degrade safely without raising during unpickle

Suspected root cause

McpError stores error.message in Exception.args via super().__init__(error.message).

On unpickle, exception reconstruction uses args, so McpError is effectively reconstructed as:

McpError("Authentication Required")

But McpError.__init__ assumes error is always an ErrorData, so it does:

error.message

which crashes for str.

Suggested fix

McpError likely needs to be pickle-safe by design. Any of these would probably fix it:

1. Make __init__ accept both ErrorData and str, normalizing str into an ErrorData.
2. Implement __reduce__ so pickle reconstructs using the full ErrorData.
3. Ensure constructor args and exception state are aligned with standard exception pickling behavior.

A robust version would probably do both __reduce__ and tolerant initialization.

Notes

This bug is easy to misattribute to cloudpickle or task runners, but the reproducer above shows it is local to McpError itself.

Example Code

from importlib.metadata import version

import cloudpickle
from mcp.shared.exceptions import McpError
from mcp.types import ErrorData


print("Versions:")
print(f"  mcp={version('mcp')}")
print(f"  cloudpickle={version('cloudpickle')}")

original = McpError(ErrorData(code=-32600, message="Authentication Required"))

print("\nOriginal exception:")
print(f"  type={type(original).__name__}")
print(f"  str={str(original)!r}")
print(f"  error_type={type(original.error).__name__}")
print(f"  error_message={original.error.message!r}")

payload = cloudpickle.dumps(original)

print("\nUnpickling:")
restored = cloudpickle.loads(payload)
print(f"  restored_type={type(restored).__name__}")
print(f"  restored_args={restored.args!r}")
print(f"  restored_error={getattr(restored, 'error', None)!r}")

Python & MCP Python SDK

- `mcp==1.26.0`
- `fastmcp==3.2.3`
- `cloudpickle==3.1.2`
- Python 3.13

mcp.shared.exceptions の McpError から始め、提供された cloudpickle の例で失敗を再現します。アンピック解除中に Exception.args がどのように例外を再構築するかを確認し、その後、McError のラウンドトリップでメッセージとエラーペイロードが保持されるか、または例外を発生させずに劣化することを検証します。