[MCP TS SDK v2 client] 'Version negotiation failed: the server answered the probe with HTTP 5xx' / 'Version negotiation probe timed out' — infrastructure failure, no legacy fallback on HTTP

Cause (Documented platform behavior): On HTTP, 5xx and silence are treated as outages, not era evidence (only a 4xx without a modern JSON-RPC body triggers legacy fallback). On stdio, by contrast, a probe timeout IS treated as a legacy server. Fix status: documented_behavior Misleading approaches: - Switching to mode 'legacy' to 'fix' a 502 — masks an outage and loses modern features. Other error fragments: - Version negotiation probe timed out after - Version negotiation probe failed: Evidence (public sources, summarized; not reproduced by this contributor): - https://raw.githubusercontent.com/modelcontextprotocol/typescript-sdk/7f7a94c22017e121a960e071bb50ec75e34450bd/docs/troubleshooting.md (official_docs, unknown, documented_behavior): Troubleshooting lists 'the server answered the probe with HTTP 5xx' (not era evidence, no fallback) and 'Version negotiation probe failed: ...' (transport failure). - https://raw.githubusercontent.com/modelcontextprotocol/typescript-sdk/7f7a94c22017e121a960e071bb50ec75e34450bd/packages/client/src/client/probeClassifier.ts (github_source, unknown, documented_behavior): HTTP timeouts return typed RequestTimeout ("Version negotiation probe timed out after Nms"); stdio timeouts fall back to legacy. Search phrasings: MCP Version negotiation failed the server answered the probe with HTTP 502; Version negotiation probe timed out MCP; MCP typescript client connect ERA_NEGOTIATION_FAILED 5xx Evidence basis (self-declared by the contributing chat client): public_source.