The preflight probe in mcp-health-check only ever sent a bare GET to the
server URL. Some Streamable HTTP MCP servers route POST exclusively and
answer any GET with 404 — api.telnyx.com/v2/mcp is one — so the probe
failed permanently against a perfectly healthy server.
404 is not in HEALTHY_HTTP_CODES, so every probe failed, the backoff
compounded to the 10-minute ceiling, and the hook blocked every tool call
for that server before it left the machine while `claude mcp list` still
reported it Connected.
Replay a failed GET as a real JSON-RPC initialize POST and accept that as
proof of life. Whitelisting 404 was the alternative, but it would mask
genuine outages on every other server.
Adds a regression test with a POST-only server that 404s all GETs and
validates the initialize body; it fails without this change.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
