fix(client): fall back from discover for any non-modern error - #1133
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`ClientLifecycleMode::Auto` only fell back to the legacy `initialize` handshake when `server/discover` failed with JSON-RPC `-32601` (`METHOD_NOT_FOUND`). Deployed legacy servers commonly reject an unknown pre-`initialize` request with other implementation-defined errors (`-32600`, `-32602`) or with session-middleware errors before method dispatch, so Auto broke against servers that previously worked. Per the 2026-07-28 backward-compatibility guidance, the fallback MUST NOT be keyed to one specific error code: the client falls back to `initialize` for every discover failure that does not positively identify a modern server. The recognized modern failures — a negotiated version mismatch, a missing-required-capability error, or a header validation failure — are surfaced instead of triggering a fallback. Fixes modelcontextprotocol#1040.
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| // Fall back to the legacy `initialize` handshake. The 2026-07-28 | ||
| // backward-compatibility guidance is explicit that this MUST NOT | ||
| // be keyed to one specific error code. | ||
| Err(_) => { |
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Err(_) also catches 401 and 403, as well as client-side failures like NoPreferredProtocolVersion. Neither indicates that the peer is legacy, but Auto now attempts initialize and may hide the original actionable error.
| // must not trigger a legacy fallback. Surface the error. | ||
| Err(error) if is_modern_server_error(&error) => return Err(error), | ||
| // Any other outcome — `-32601`, `-32602`, `-32600`, another | ||
| // implementation-defined error, or no response at all — means |
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discover_startup waits in expect_response until a message comes in or the transport closes. So, if a legacy server is open but silently ignores server/discover, it never reaches this branch.
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ClientLifecycleMode::Autoonly fell back to the legacyinitializehandshake whenserver/discoverfailed with JSON-RPC-32601(METHOD_NOT_FOUND). Deployed legacy servers commonly reject an unknown pre-initializerequest with other implementation-defined errors (-32600,-32602) or with session-middleware errors before method dispatch, so Auto broke against servers that previously worked.Per the 2026-07-28 backward-compatibility guidance, the fallback MUST NOT be keyed to one specific error code: the client falls back to
initializefor every discover failure that does not positively identify a modern server. The recognized modern failures — a negotiated version mismatch, a missing-required-capability error, or a header validation failure — are surfaced instead of triggering a fallback.Fixes #1040.
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