Wymcp.
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Message validation: checks the parsed body against the era's compiled
protocol schema (Wymcp.JsonRpc.validate_mcp_request/2) and answers a
non-conforming message with HTTP 400 plus -32600, carrying the validator's
distilled error — the one-line summary as the envelope's message, one
entry per instance location under data.errors. This body's reader is the
human debugging a client rather than an LLM, since no LLM authors a JSON-RPC
message, so it keeps per-location detail where the tool-arguments path is
tuned for a next action. The message a client sends back is not reflected:
the client already holds it, and a body of junk answered with a copy of
itself doubled the bytes on the wire for nothing.
Two message kinds bypass validation, each because validating it could only produce an answer the protocol forbids:
- a JSON-RPC response — a client's answer to a server-initiated request.
It is not a protocol request at all, and
Wymcp.Plugs.Dispatchdelivers it to the session that minted its id. That bypass is legacy-only: a client's answer to a server-initiated request reaches wymcp only on the legacy lane. The contract survives via MRTR on the modern lane, and the bypass goes at the legacy decommission. - a modern-lane notification —
Wymcp.Plugs.Dispatchanswers it with the acceptance whatever its body, and JSON-RPC forbids an error response to a notification.
What the legacy schema checks is framing, not vocabulary: a legacy body
naming a method outside the legacy method table passes it, and so does one
naming a notification's method beside an id, or a request's method
without one. Which bodies reach a method at all, and which methods wymcp
acts on, is Wymcp.Plugs.Dispatch's to decide, there — by
message classification
first, then its table.
An unset era assign reads :legacy: era classification exists only on the
POST pipeline, and the default lane for a signal-less request is the legacy
one.
The default is legacy-only: it exists only because two eras do, and it
goes at the legacy decommission.