docs(rfd): audit MCP-over-ACP for stateless MCP

[benbrandt]

Status / target

Draft audit, not a finished replacement wire schema. Target MCP 2026-07-28 only. This transport supplies new tools to agents; there is no requirement to support older MCP versions, initialization handshakes, session headers, or a legacy fallback mode.

Changes

• Record a complete modernization audit against the published 2026-07-28 specification, with primary-source links, implementation workstreams, decisions, security requirements, and conformance tests.
• Recommend direct serverId routing and request-scoped notifications/cancellation rather than stabilizing the old mcp/connect / connectionId / mcp/disconnect model.
• Cover per-request metadata/discovery, MRTR, subscriptions, cancellation and ID remapping, modern tool/result/cache schemas, optional extensions, HTTP header/stream/security requirements, and the currently insufficient rmcp dependency.
• Align existing examples with shared-schema serverId naming and v1/v2 capability shapes, and correct stale automatic-conductor-bridging claims in both RFDs.
• Clearly retain the earlier connection-oriented text only as an implementation checkpoint. It is not the intended stabilized design or a backwards-compatibility commitment.

Decisions still needed

• Exact ACP request/notification envelope and logical MCP request-ID mapping through proxies.
• Request-scoped cancellation, subscription completion, and server-declaration lifetime.
• Land the verified rmcp 3.4 dependency migration first: feat(rmcp)!: upgrade integration to rmcp 3.4 rust-sdk#372. Modern callers still need explicit 2026-07-28 metadata because the dependency defaults to 2025-11-25.

Validation

• npm run check passed (Clippy, formatting, spellcheck, schema/generator tests, doctests).
• All 11 JSON examples parse.
• No wire-schema changes or generated artifacts in this PR.

Implementation checkpoint

Companion draft: agentclientprotocol/rust-sdk#375. Its old connection/session APIs are explicitly not a compatibility commitment for the latest-only transport.