Initiate the transport's backward-compatibility fallback after a modern
request was rejected in a way that signals an older server (HTTP: a
400/404/405 POST -> open the legacy HTTP+SSE GET stream and switch to the
two-endpoint transport). A no-op on transports without a fallback path
(stdio), symmetric with startServerStream/setBearerToken. The client
follows this with the legacy initialize handshake.
Initiate the transport's backward-compatibility fallback after a modern request was rejected in a way that signals an older server (HTTP: a 400/404/405 POST -> open the legacy HTTP+SSE GET stream and switch to the two-endpoint transport). A no-op on transports without a fallback path (stdio), symmetric with startServerStream/setBearerToken. The client follows this with the legacy initialize handshake.