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Answer a declined discover probe ahead of rmcp - #38
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Claude Code now opens a stdio session with server/discover at 2026-07-28 and falls back to initialize on the same session when the server declines that revision, which this one always does. rmcp marks a session as needing per-request _meta the moment its first message is not initialize, before the handler rejects the probe, and never clears the mark. The fallback handshake succeeded and then tools/list was refused for missing _meta, so "/mcp" showed the server connected with no tools. rmcp 3.5.0 behaves the same. A thin transport wrapper answers a discover for a declined revision with the same unsupported-version error rmcp sends, until initialize passes through, so rmcp sees initialize first and keeps the session on the lifecycle it negotiated. A discover naming a revision this server speaks still goes to rmcp. A failed write of that reply is logged before the session ends. The 2026-07-28 exclusion is unchanged. The new lifecycle test replays the probe, the fallback and tools/list. Starting the wrapper as already initialized, which hands the probe to rmcp, fails it with the reported error.
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Claude Code 2.1.285 now opens a stdio session with
server/discoverat 2026-07-28, and when the server declines that revision (this one always does, seeEXCLUDED_PROTOCOL_VERSIONS) it falls back toinitializeon the same session. rmcp marks the session as requiring per-request_metaas soon as the first message is notinitialize, before the probe is rejected, and never clears it. The fallback handshake succeeds, thentools/listis refused with-32602 request _meta is missing or has malformed required fields, so/mcpshows the server connected with no tools. rmcp 3.5.0 has the same behavior.This adds a small transport wrapper in
src/main.rs. Untilinitializearrives, it answers a discover for a declined revision itself, with the same-32022unsupported-version error rmcp sends, so rmcp seesinitializefirst. A discover that names a revision this server supports still goes to rmcp. The 2026-07-28 exclusion is unchanged.Verification: I captured Claude Code's actual exchange through a logging wrapper and replayed it against the new binary, and
tools/listnow answers. The new testa_declined_discover_probe_falls_back_to_initialize(test-process-lifecycle) replays the probe, the fallback andtools/list. As a negative control, starting the wrapper as already initialized (so the probe reaches rmcp) makes that test fail with the reported error.scripts/run-gates.shpassed all 18 gates on macOS with Frama-C 33.0 (unit 651, lifecycle 40, stdio 157).The underlying problem is in rmcp, and the wrapper can go once rmcp stops setting the flag for a probe it rejects.
Summary by cubic
Fixes a Claude Code compatibility issue where a
server/discoverprobe caused the fallbackinitializesession to show no tools. Declined probes are now answered before rmcp sees them, so the fallback handshake remains usable andtools/listsucceeds.tools/list, and negotiated protocol state.Written for commit d37c0a6. Summary will update on new commits.