Wymcp.
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Message validation: checks the parsed body against the compiled protocol
schema (Wymcp.JsonRpc.validate_mcp_request/1) 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.
One message kind bypasses validation, because validating it could only
produce an answer the protocol forbids: a notification —
Wymcp.Plugs.Dispatch answers it with the
acceptance whatever its body, and JSON-RPC
forbids an error response to a notification.
One more is refused after the schema admits it: a body the schema names
besides a request and a notification is a JSON-RPC response, and the
spec forbids a client to send one — this server initiates no request of
its own, so nothing here could deliver it. It answers the same -32600
under a fixed sentence and its own rejection reason, :not_a_request,
with a null id: on such a body a request and a response are
indistinguishable, which is Wymcp.Response.rejection_id/1's rule. What
reaches Wymcp.Plugs.Dispatch is therefore a request or a notification
and nothing else.
Which kind a body is comes from the classification
Wymcp.Plugs.Classify set, never re-read here. Called ahead of that plug
— outside Wymcp.Plugs.Pipeline's chain — this one validates against the
schema alone and lets a conforming body through whatever its kind.
What the schema checks is framing, not vocabulary: a body naming a method
outside the method table passes it. 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.