fix: make MCPError subclasses pickle-safe - #3407
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UrlElicitationRequiredError and NoBackChannelError inherited MCPError's (code, message, data) Exception.args, but their __init__ signatures do not match that shape. This caused TypeError during unpickling and lost subclass-specific state. Add __reduce__ to each subclass so pickle reconstructs them correctly: - UrlElicitationRequiredError uses the existing from_error classmethod. - NoBackChannelError uses its (method) constructor. Add pickle round-trip tests for MCPError, UrlElicitationRequiredError, and NoBackChannelError. Fixes modelcontextprotocol#2431
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Summary
UrlElicitationRequiredError and NoBackChannelError failed to unpickle
because their inherited Exception.args from MCPError are
(code, message, data), which does not match their own __init__
signatures. This caused TypeError on restore and lost subclass-specific
state.
The fix adds __reduce__ to each subclass:
classmethod.
Pickle round-trip tests are added for MCPError,
UrlElicitationRequiredError, and NoBackChannelError.
Fixes #2431
This PR was prepared with the assistance of AI coding tools. The analysis,
patch, and tests were reviewed by a human contributor before submission.
Happy to iterate on feedback.