JSON-RPC 2.0 envelopes and MCP protocol schema validation — the wire-format floor both eras share.

success_response/2 and the three error_response arities build the envelope every answer is sent as; Wymcp.Response does the sending. The error map is the one home for wymcp's JSON-RPC error codes: each error_type/0 atom pairs with its {code, message} tuple, so a call site names the condition and never the number. The four-arity form overrides that message and nothing else — the code still comes from the table — for a condition whose wording has to name what it refused. This module owns the envelope's shape; it owns no decision about which condition applies — that is each plug's and each method module's.

Message and output-schema validation

One self-contained MCP protocol schema per era — priv/schema-2025-11-25.json and priv/schema-2026-07-28.json, JSON Schema 2020-12 — is compiled to a JSV.Root at build time through a module attribute, so an inbound request is validated against the official protocol definition with no runtime schema parsing. Both schemas name the same entry point, JSONRPCMessage; validate_mcp_request/2 selects the era's root from the era Wymcp.Plugs.Validate passes it. @external_resource on both paths makes an edit to either schema file recompile this module.

A rejection is answered as a distilled map — a one-line message plus an errors list of %{at:, problems:} entries, one per instance location, locations sorted, each location's problems deduplicated — never JSV.normalize_error/1's full tree. It is a shape, not one function's return value. This module distills it on two paths: validate_mcp_request/2 on the -32600 message path, whose reader is the human debugging a client, and validate_schema/2 on a tool's output-schema failure, which is logged and never reaches the wire. Wymcp.Methods.ToolsCall builds a conforming one by hand for the -32602 arguments path, whose reader is the calling LLM. The full tree is JSON-encodable and was what previously reached the wire; it is simply too verbose for either reader — 2353 bytes against 435 for one garbage message, measured 2026-08-15.