Relocated onto the decomposed module layout and hardened:
- Trigger covers the rejection CLASS, not just literal 400: SSE-only
servers' load balancers answer the chunked Streamable HTTP initialize
POST with 400/405/406/411, and the mcp>=2.0 SDK surfaces many such
rejections as an opaque -32603 'Server returned an error response'
(error class per #104363 by @RohithPariki). Timeouts and 5xx never
trigger the fallback: they are not transport mismatches.
- Reconnect exclusion via _ever_connected instead of _ready: run()
clears _ready before re-entering the transport, so the original guard
also fired on reconnects after a proven session.
- Successful fallback latches _sse_fallback so reconnects go straight
to SSE, and logs a warning suggesting the user pin transport: sse.
- Both transports failing raises a ConnectionError naming both errors
and suggesting transport: sse / checking the URL.
- No fallback with strict_redirect_headers (SSE cannot enforce that
boundary) or when transport is explicitly configured.
- Tests trimmed to 3 invariant contracts (proven red on base): fallback
connects + latches; no fallback on reconnect/timeout/5xx; both-fail
error is actionable.
The extracted SSE path reuses _sse_transport/_serve_transport from main,
preserving the bounded handshake timeout and reconnect-retry semantics.
Fixes#53676
Teknium
a565e2d493