ADR-034: Final-revision modern response projection
Status: Proposed
Decision
Keep MCP 2026-07-28 response encoding in one transport-neutral codec, applied once at the stateless service's result boundary. The legacy serializer and initialize behavior are unchanged. Business handlers continue returning their existing content, structured content, and vendor metadata.
DiscoverResult is the single canonical discovery type. The historical ServerDiscoverResult export is an alias, not a second schema. Rust callers constructing the former RC shape must migrate protocol_version to supported_versions and move server_info into result metadata. The final wire result carries supportedVersions, resultType, ttlMs, cacheScope, and optional _meta; it does not emit RC protocolVersion or serverInfo. Discovery uses SUPPORTED_MODERN_PROTOCOL_VERSIONS, containing only compiled modern revisions, never legacy initialize versions. This follows the official SDK era contract.
The modern codec stamps resultType: complete and the server identity under _meta["io.modelcontextprotocol/serverInfo"]. Valid handler-authored identity and vendor metadata are retained. Missing or malformed identity falls back to the configured server identity without losing a completed business result. A different result kind fails closed; this change does not implement multi-round-trip input requests. JSON-RPC error responses are not success results and bypass the codec.
Only discovery, tool/prompt/resource lists, resource-template lists, and resource reads receive default cache fields (ttlMs: 0, cacheScope: private). Valid authored hints take precedence; invalid hints fall back conservatively. No positive TTL or shared authorization-context cache is enabled by default.
Validation and boundaries
Pure codec tests pin the closed method set, final discovery fixture, metadata preservation, malformed-result rejection, and unchanged legacy serialization. HTTP and opt-in official SDK 2.0.0 smoke tests cover discovery, list, and call with auto and pinned modern negotiation.
This addresses the response portion of #2436, not full protocol conformance. The request-routing decision in ADR-033, envelope/header validation, standard errors, and parameter-header support remain separate follow-up work. Capability truthfulness and resource/prompt provider wiring are separate changes. No adapter execution, thread-affinity, or public Skill authoring contract changes are required; no agent-plugins Skill update is needed for this response-only correction.