Serve both lifecycle eras over stdio with an era lock per SEP-2575 - #478
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koic wants to merge 1 commit into
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Serve both lifecycle eras over stdio with an era lock per SEP-2575#478koic wants to merge 1 commit into
koic wants to merge 1 commit into
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## Motivation and Context Third step of the stateless lifecycle (SEP-2575, modelcontextprotocol/modelcontextprotocol#2575) for the 2026-07-28 MCP spec release. `StdioTransport` now serves the legacy handshake lifecycle and the modern per-request-envelope lifecycle on one connection, with the same era-lock semantics as the TypeScript and Python SDKs: the first era-distinctive message to SUCCEED locks the connection era for its lifetime. - Each stdin frame is parsed once (`symbolize_names: true`) so era classification can inspect its method and `_meta`. Frames that are not JSON objects fall back to `ServerSession#handle_json`, keeping protocol-level error responses byte-identical. - A successful `initialize` locks `:legacy` (already a side effect of `ServerSession#mark_initialized!`). A successful `server/discover` or a successful request carrying the full modern `_meta` triple locks `:modern`. Failed era-distinctive messages (for example an unsupported envelope version) leave the connection unlocked, so a client probe can still fall back to the other era. - Era violations are rejected in-band by `Server#lift_request_envelope`, which now also covers the legacy side: a modern envelope arriving on a legacy-locked session is an invalid request (`-32600`), mirroring the existing modern-side rules (`initialize` after a modern lock is `-32022`; a missing envelope after a modern lock is `-32600`). - `StdioTransport#send_request` raises on a modern-locked session: the modern lifecycle forbids server-initiated JSON-RPC requests, which multi round-trip `input_required` results (SEP-2322) replace. The inline read loop inside `send_request` dispatches through the same era-aware path as the main loop. Refs modelcontextprotocol#389. ## How Has This Been Tested? New tests in `test/mcp/server/transports/stdio_transport_test.rb` drive full stdin/stdout round trips: legacy lock via `initialize` then rejection of a modern envelope, modern lock via `server/discover` then `-32022` (with `data.supported`) for a late `initialize`, modern lock via an envelope-carrying request, no lock when the probe fails with an unsupported version (a legacy `initialize` still succeeds afterwards), the envelope requirement after a modern lock, and the `send_request` prohibition on modern sessions. `bundle exec rake` (tests, RuboCop, and conformance baseline, including the stdio conformance scenarios) passes. ## Breaking Changes None. Legacy clients send `initialize` first and take the same code path as before; the era lock only constrains message sequences that mix lifecycles on one connection, which no legacy client produces.
atesgoral
approved these changes
Aug 2, 2026
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Motivation and Context
Third step of the stateless lifecycle (SEP-2575, modelcontextprotocol/modelcontextprotocol#2575) for the 2026-07-28 MCP spec release.
StdioTransportnow serves the legacy handshake lifecycle and the modern per-request-envelope lifecycle on one connection, with the same era-lock semantics as the TypeScript and Python SDKs: the first era-distinctive message to SUCCEED locks the connection era for its lifetime.symbolize_names: true) so era classification can inspect its method and_meta. Frames that are not JSON objects fall back toServerSession#handle_json, keeping protocol-level error responses byte-identical.initializelocks:legacy(already a side effect ofServerSession#mark_initialized!). A successfulserver/discoveror a successful request carrying the full modern_metatriple locks:modern. Failed era-distinctive messages (for example an unsupported envelope version) leave the connection unlocked, so a client probe can still fall back to the other era.Server#lift_request_envelope, which now also covers the legacy side: a modern envelope arriving on a legacy-locked session is an invalid request (-32600), mirroring the existing modern-side rules (initializeafter a modern lock is-32022; a missing envelope after a modern lock is-32600).StdioTransport#send_requestraises on a modern-locked session: the modern lifecycle forbids server-initiated JSON-RPC requests, which multi round-tripinput_requiredresults (SEP-2322) replace. The inline read loop insidesend_requestdispatches through the same era-aware path as the main loop.Refs #389.
How Has This Been Tested?
New tests in
test/mcp/server/transports/stdio_transport_test.rbdrive full stdin/stdout round trips: legacy lock viainitializethen rejection of a modern envelope, modern lock viaserver/discoverthen-32022(withdata.supported) for a lateinitialize, modern lock via an envelope-carrying request, no lock when the probe fails with an unsupported version (a legacyinitializestill succeeds afterwards), the envelope requirement after a modern lock, and thesend_requestprohibition on modern sessions.bundle exec rake(tests, RuboCop, and conformance baseline, including the stdio conformance scenarios) passes.Breaking Changes
None. Legacy clients send
initializefirst and take the same code path as before; the era lock only constrains message sequences that mix lifecycles on one connection, which no legacy client produces.Types of changes
Checklist