Summary

• make ClientLifecycleMode::Auto own conservative modern-to-legacy lifecycle negotiation
• recognize correlated METHOD_NOT_FOUND, INVALID_REQUEST, and INVALID_PARAMS responses
• recognize unsupported-version failures that explicitly reject the attempted modern version, including known initial HTTP 400 responses with id: null and no supported-version list
• fall back after initial HTTP 404/405 while rejecting 401/403, unrelated IDs, established-session failures, arbitrary messages, and unknown, mixed, or future protocol versions
• expose typed HTTP status and response body data consistently for reqwest, custom HTTP adapters, and Unix-socket transports
• keep the HTTP worker available for a real legacy initialize retry without fabricating a JSON-RPC response

Downgrade policy

Auto mode retries legacy initialization only when the discovery probe provides explicit legacy evidence:

• a correlated -32601, -32600, or -32602 response
• an explicit rejection of the attempted modern protocol version, with any advertised versions exclusively historical
• a recognized initial null-ID prevalidation response
• an initial HTTP 404 or 405

Authentication failures, unrelated response IDs, established-session errors, malformed or arbitrary errors, and unknown/mixed/future version evidence fail without downgrade.

Transport behavior

HttpStatusError carries the original status and body independently of the HTTP backend. Valid JSON-RPC errors in an initial HTTP 400 continue through normal modern-version negotiation; raw prevalidation failures remain typed transport errors that Auto mode can inspect. When a downgrade is safe, Auto sends a real legacy initialize request through the still-live transport worker.

Custom StreamableHttpClient implementations can participate by returning UnexpectedHttpStatus(HttpStatusError) for non-success discovery responses.

Validation

• full rmcp suite with the documented non-local feature set
• focused lifecycle, reqwest, custom-adapter, and Unix-socket integration suites
• draft 2026-07-28 conformance client suite: 376 passed, 0 failed, 0 warnings
• cargo clippy -p rmcp --all-features --all-targets -- -D warnings
• minimal client-only non-HTTP build
• formatting and whitespace checks

This is intended to let Codex remove its complete legacy_discovery_fallback_response workaround while preserving deployed-server interoperability and downgrade safety.

Fixes #1040