[koic]

Motivation and Context

MCP::Client::Stdio answers a server's ping inline while it waits for a response, and it already skips a ping whose id is not a JSON-RPC id. A String id holding bytes that are not valid UTF-8 passed that check, and the pong echoing it failed in JSON.generate with JSON::GeneratorError. The pong's own rescue covers only a pong that cannot be written, so the error escaped and failed the unrelated request whose response the loop was reading.

Such a ping is now skipped like one with a malformed id, since a pong could not carry the id back.

A number such as 1e400 fails the same way: the parser reads it as an infinite Float, which JSON.generate refuses, so a ping carrying one is skipped as well. A String id that is not valid UTF-8 can arrive in a plain ASCII frame too: the json gem's 2.x releases decode a lone surrogate escape into one.

How Has This Been Tested?

test/mcp/client/stdio_test.rb: a ping whose id is not valid UTF-8 is not answerable, and one with a valid id still is. Against the previous library the first is answerable. A ping whose id is 1e400, sent ahead of the awaited response, is not answered and the request completes; against the previous library it raises JSON::GeneratorError.

Breaking Changes

None.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed