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A common pattern in MCP tool handlers is to attach telemetry — session id, trace id, request id — to the args object before parsing, so the schema "knows about" the metadata. This is wrong: the schema is a contract with the CLIENT, and the client does not know about your server-side metadata. Mixing the two means the schema either has to permit fields the client cannot send, or rejects fields the client never sent.
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A common pattern in MCP tool handlers is to attach telemetry — session id, trace id, request id — to the args object before parsing, so the schema "knows about" the metadata. This is wrong: the schema is a contract with the CLIENT, and the client does not know about your server-side metadata. Mixing the two means the schema either has to permit fields the client cannot send, or rejects fields the client never sent.