MCP servers that reject the modern initialize request now get retried over legacy HTTP+SSE instead of failing, recorded in telemetry
What
- When a Streamable-HTTP MCP server's initialize POST request fails (for example, a 405 Method Not Allowed or similar transport error), Claude Code now retries the connection using the legacy HTTP+SSE transport instead of failing immediately — closing the failed transport and re-dialing with
mode: "legacy". - This fallback is gated behind
tengu_mcp_legacy_sse_fallback, dials with an abort controller, and uses a capped retry budget. - The
mcp_connectsuccess telemetry event gained a newnegotiatedLegacySsefield recording whether a connection ended up using the legacy SSE transport.
Why Some MCP servers only support the older HTTP+SSE transport and reject the newer Streamable-HTTP handshake. This fallback lets Claude Code still connect to them automatically instead of the connection simply failing, and the telemetry field makes it possible to see how often the older transport is still needed.
tengu_mcp_legacy_sse_fallback Not enough to sayNothing here resolved what this flag was doing on this version, so nothing here should be read as on or off.
This account: no value returned · anonymous baseline: no value returned · compiled default in v2.1.265: on
Read once, for one account on one subscription tier, against v2.1.265. It isn't a statement about your account. What a flag value here can and cannot tell you
Gate state for `tengu_flag` and `tengu_mcp_legacy_sse_fallback` is unread, so whether this fallback is active for any given account is…
New in this build: tengu_mcp_legacy_sse_fallback