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Raise the rmcp floor to 3.5.1 and pin the listener's in-band refusal - #484
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rmcp 3.5.1 (#483) changed one thing a client of `--listen` can see: a JSON-RPC error a handler returns on the 2026-07-28 route — this server's surface refusal, the SDK's own `tool not found`, an argument that does not parse — is sent as HTTP 200 with the error in the body, and only a request missing a `_meta` key the revision requires is still a 400 (upstream #1322). Every 3.x before it sent each of those as 400, which on that route is the signal the revision's backward-compatibility rule gives a client for a server that predates it, so a `--tools crash` listener refusing `debug_batch` could demote a sessionless client to the handshake era mid-conversation. Nothing in the code depended on the fix: the lockfile held it and the manifest's floor was still 3.2.0, so a resolver picking 3.5.0 built and passed. The floor moves to 3.5.1 and the smoke tier pins both arms against a real listener, which is what makes it a floor: pinned to 3.5.0 the new test fails at its first assertion with a 400 (checked here, then the lock restored byte for byte). The session-bearing route and stdio always carried errors in band and are unchanged. The three places that carry the floor's rationale say so: the manifest comment, `docs/architecture.md`'s revision paragraphs, and the protocol tier's list in `docs/smoke-test.md`, whose counts move by one. Co-Authored-By: Claude Mythos 5.1 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_011Gsd7tFcMV8ozSUCFWrusT
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📝 Walkthrough
Merge Risk: 🔵 Low · up to Clients or operators could misinterpret other stateless error responses. Narrow the wording; the documented test cases themselves remain covered. Pre-merge checks |
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The title clearly summarises the two main changes: raising the rmcp minimum version to 3.5.1 and pinning the listener refusal behaviour. |
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HTTP 200 as errors pass by.
Missing _meta earns 400,
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| not found`, an argument that does not parse — is answered `200` with the error in the body, and only | ||
| a request missing a `_meta` key the revision requires is an HTTP `400` (upstream #1322). Every 3.x |
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The claim that only a missing _meta key produces HTTP 400 contradicts the existing protocol test: a stateless request with complete _meta but no Mcp-Method header is also asserted to return 400 in tests/mcp_smoke.rs:5892-5910. This overstates the new dependency behavior and documents an incorrect client-visible contract; describe the retained 400 class as malformed stateless transport metadata rather than limiting it to _meta keys.
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Inline comments:
Review comments at @Cargo.toml:
- Around line 21-22: Narrow the `rmcp` 3.5.1 status guarantee to
handler-generated `-32602` errors, avoiding wording that implies all other
responses are in-band. In `Cargo.toml` lines 21–22, replace “answers the rest in
band” with that specific guarantee; in `docs/architecture.md` lines 75–76,
remove “only” and distinguish the other protocol statuses; in
`tests/mcp_smoke.rs` lines 5928–5929, clarify that the test covers one
malformed-request HTTP 400 case.
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CHANGELOG.mdCargo.tomldocs/architecture.mddocs/smoke-test.mdtests/mcp_smoke.rs
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The previous commit said only a request short a `_meta` key is still an HTTP 400 under rmcp 3.5.1. Both review bots read that against the tree and found it wrong, and the counterexample was already pinned beside the new test: a request with complete `_meta` and no `Mcp-Method` header is asserted as 400 in `an_under_specified_stateless_request_is_told_which_ part_it_is_missing`. The 3.5.1 mapper keeps 400 for a header mismatch, an unsupported revision and a missing required capability as well, and 404 for an unknown method; what #1322 moved is a handler's -32602 and nothing else. So every copy of the claim now says what moved rather than what is left: the manifest comment, the architecture page, the changelog entry and the test's doc comment. The test itself is unchanged — its control arm was always one of the transport's 400s, and its comment now says so instead of calling it the only one. Co-Authored-By: Claude Mythos 5.1 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_011Gsd7tFcMV8ozSUCFWrusT
Claude Code 2.1.296 opens a stdio server with `server/discover` carrying full `_meta`, and three seconds without an answer later sends `initialize` naming 2025-11-25 on the same connection (measured against a recorder server on 2026-10-12). Every rmcp before 3.5.1 let that probe commit the connection to the sessionless lifecycle for good: the handshake was answered and every request after it refused with -32602 for the `_meta` the handshake era never carries. 3.5.1 answers a discover without committing (upstream #1248), so the next request still chooses. Driven by hand at two builds, the release on 3.4.1 refused the `tools/list` and the build on 3.5.1 served it; the new protocol-tier test fails against 3.5.0 at that assertion, checked by lowering the manifest for the duration and pinning it. This server answers a warm discover in 60 ms, so the fallback needs a start slower than the probe's three seconds; the sequence is pinned regardless, as the floor's fourth reason and the one that reaches the client this server is registered with. The changelog entry from #484 described the release's other change as a stdio `serve` that keeps waiting after a malformed opener, and counted one where there were four; it now says what #1248 is, and why #1308, #1318 and #1285 reach nothing here. The manifest comment, the architecture note and the smoke-test doc carry the fourth reason. Co-Authored-By: Claude Mythos 5.1 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_011Gsd7tFcMV8ozSUCFWrusT
What
Cargo.toml: thermcpfloor moves from3.2.0to3.5.1. The lock has pinned 3.5.1 since Bump the cargo-minor-and-patch group with 3 updates #483, so nothing else in the graph moves.tests/mcp_smoke.rs:a_refusal_on_the_stateless_route_is_in_band_and_a_malformed_request_is_not, protocol tier. A--tools crashlistener refusingdebug_batchon2026-07-28answers HTTP 200 with-32602and this server's surface refusal in the body; the same call one_metakey short answers 400 naming the key.docs/architecture.md,docs/smoke-test.md,CHANGELOG.md: the floor's third reason, the test's entry in the protocol-tier list (its counts move by one), and the changelog entry.Why
Follows from the assessment of #483. rmcp 3.5.1's one change a
--listenclient can see is upstream #1322: before it, every handler-32602on the sessionless route went out as HTTP 400, which the revision's backward-compatibility rule has a client read as a server that predates2026-07-28and fall back toinitializeon. Nothing here required the fix, so a build resolving 3.5.0 was green and served the old status. The floor and the test make the behaviour a property of this server rather than of the lockfile.Verification
On the x64 bench, dev profile, 2026-10-12:
cargo metadata --lockedresolves with the new floor andgit diff Cargo.lockis empty, so the lock is untouched.cargo fmt --all --checkandcargo clippy --all-targets -- -D warnings: clean.cargo test: 1389 unit tests and 148mcp_smoketests pass (21 ignored, the gated tiers; the dump tier was not turned on, and nothing here needs it).cargo update -p rmcp --precise 3.5.0and the new test fails at its first assertion,left: 400, right: 200, with the body carrying this server's surface refusal, which is exactly the demotion case.--precise 3.5.1restored the lock byte for byte.npx markdownlint-cli2@0.23.2 README.md CHANGELOG.md "docs/**/*.md" "skills/**/*.md": 0 issues in 73 files.Not covered: 3.5.1's stdio-side change (#1248,
servecontinuing to wait after a malformed opener rather than exiting) has no test here; nothing in this server depends on it.🤖 Generated with Claude Code
https://claude.ai/code/session_011Gsd7tFcMV8ozSUCFWrusT
Summary by CodeRabbit
_metakey continue to return HTTP 400.rmcpversion is now 3.5.1. Session-bearing routes and stdio behaviour are unchanged.