ADR-0001 — Serve both protocol eras

type: adr status: Accepted tags: [adr]

Context

MCP 2026-07-28 removed the initialize handshake: each request carries its version and client capabilities in _meta, and server/discover is mandatory. The agents this server is for still open with initialize at 2025-11-25 or earlier.

Decision

The server is dual-era. A request whose params._meta names io.modelcontextprotocol/protocolVersion is served statelessly under the modern rules. An initialize request selects legacy semantics for the rest of the process, at the requested version when supported and otherwise at 2025-11-25. Before either, any other request is refused with -32602, naming the supported versions, as the modern rules require for a request missing its metadata.

Consequences

• The only process state is [[domain/ProtocolEra|the session]]: era and legacy version.
• Modern requests are validated on every call; a legacy session never rechecks.
• Result shapes are shared. Modern fields (resultType, ttlMs, cacheScope, _meta) are harmless extra members for a legacy client.

Rejected

• Modern only: every current agent would fail at initialize.
• Legacy only: fails modern clients deterministically at server/discover.

Referenced by

[[decisions/_MOC]] · [[architecture/_MOC]] · [[domain/ProtocolEra]]