0009 Local MCP protocol types

Context

Spike S5 asks whether the Worker can serve MCP over Streamable HTTP using the protocol types of rmcp 3.5.1 without a tokio runtime. Its pass criterion is “using rmcp 3.5.1 protocol types (no tokio)”.

The crates.io index entry for rmcp 3.5.1 (read 2026-10-09 from index.crates.io) lists tokio (^1, features sync, macros, rt, time) and tokio-util (^0.7) as normal dependencies with optional: false. Any build that depends on rmcp therefore compiles tokio into the Worker, which AGENTS.md forbids. The pass criterion cannot be met as written, whatever the spike measures.

Taking a spike’s fallback needs an ADR (Decision records).

Decision

1. The S5 fallback is taken now, before M1: the Worker must implement the JSON-RPC envelope and the MCP messages it serves as its own serde types in crates/worker/src/mcp/schemas.rs, following schema.ts of the 2026-07-28 and 2025-11-25 revisions.
2. rmcp must not be a dependency of the Worker build. It is a native dev-dependency of crates/worker (pinned =3.4.1; amended 2026-10-10, see Amendments), used by a round-trip test that serialises every local type and reads it back with rmcp::model, and by the live MCP client test.
3. M1 still runs S5 against the local types and records the result: MCP Inspector and Claude Code connect, list tools and call one, and the bundle stays inside the S4 budget.

Consequences

• The Worker carries a few hundred lines of protocol types. The round-trip test against rmcp keeps them from drifting from the official SDK.
• A new MCP revision needs the local types updated by hand; the round-trip test fails until they are.
• If a later rmcp release makes tokio optional, a new ADR may switch the Worker to its types.

Alternatives considered

• Depend on rmcp and never start a runtime. Tokio’s sync, macros, rt and time features compile for wasm, but timers panic where the platform has none, and the dependency itself breaks the “No tokio” rule and adds to the bundle. Not chosen.
• Fork rmcp with tokio made optional. A fork is a maintained dependency with no upstream. Not chosen.

Amendments

• 2026-10-10. The pin in decision 2 is =3.4.1, not =3.5.1. 3.5.1 was published on 2026-10-05, inside the two-week age rule for dependencies (Rust workspace §3). 3.4.1, published on 2026-09-23, lists the same features and the same non-optional tokio dependency (crates.io sparse index, read 2026-10-10), so the context and the decision are unchanged.