The POST plug chain: every check and stage a POST passes through, in the one order that makes each stage's precondition true.

Wymcp.Plugs.OriginCheck        # before anything is parsed
parse_body                     # Plug.Parsers, JSON
Wymcp.Plugs.Classify           # message kind
Wymcp.Plugs.Auth               # authentication
Wymcp.Plugs.SingletonHeaders   # singleton-header cardinality
Wymcp.Plugs.Era                # era classification
Wymcp.Plugs.ProtocolFields     # modern-lane enforcement
Wymcp.Plugs.Session            # legacy-lane session resolution
Wymcp.Plugs.Validate           # MCP schema
Wymcp.Plugs.Dispatch           # method → its answering module

Why that order. The origin check runs before anything is parsed because nothing has validated Origin when it runs — Wymcp.Router's wire-check invariant, which is also where the wire checks and their order are stated. Parsing and classification sit between the origin check and the remaining wire checks so those checks' rejections can carry the body's id and know the message kind. Wymcp.RouterTest pins that placement end to end — its authentication describe proves the tag is present when the auth check, the first consumer, reads it, and its singleton-header check describe proves the same at the next site; Wymcp.Plugs.AuthTest and Wymcp.Plugs.SingletonHeadersTest call their plug directly rather than routing a request, so neither sees the order. Era classification runs after the singleton-header check, which guarantees at most one Mcp-Session-Id for it to read. Validation runs last before dispatch, so the wire and session checks answer first.

The chain is gated, not derived. The wire checks cannot be spliced in as one list — parsing and classification interleave them — so this module writes the order by hand and exposes it through chain/0, and Wymcp.WireCheckInvariantTest holds it to the router's list, in two cells that both read the chain only as far as Wymcp.Plugs.Session — the point past which a request has touched session state: every wire check is in that prefix, in list order, and every other plug in it is declared body-bound in body_bound_plugs/0, with the reason it runs on POST only. A plug added ahead of Wymcp.Plugs.Session therefore joins the wire-check list, and so runs on GET and DELETE, or declares why it cannot; a wire check moved past Wymcp.Plugs.Session leaves that prefix, and fails the first of the two cells. chain/0 reads Plug.Builder's accumulated @plugs, which is stored newest-first as {plug, opts, guards} tuples, so the accessor reverses it and keeps the first element; a function plug appears as its bare atom, exactly as it is declared.

Body parsing is an inline function rather than a plug so that a malformed body answers in wymcp's own dialect: Plug.Parsers.ParseError is rescued and answered HTTP 400 plus -32700 with data.reason, instead of propagating as Plug's own error.

GET and DELETE do not run this pipeline. Wymcp.Router runs the wire checks in those route bodies, straight from its list, speaking the plain-JSON dialect.