Opt-in backwards compatibility with the deprecated 2024-11-05 HTTP+SSE
two-endpoint transport (basic/transports §HTTP with SSE; and the
2025-06-18 / 2025-11-25 / draft §Backwards Compatibility guidance:
"Servers wanting to support older clients should: Continue to host both the
SSE and POST endpoints of the old transport, alongside the new MCP
endpoint"). This is a SHOULD, so it is off by default. When enabled,
mountMcp ALSO mounts the two legacy endpoints alongside the modern MCP
endpoint:
- GET legacySsePath: opens a text/event-stream, immediately emits an
endpoint event whose data is legacyMessagePath (the URI the client
must POST to), then holds the stream open delivering server messages as
SSE message events;
- POST legacyMessagePath: accepts a single JSON-RPC message, processes
it, replies 202 Accepted with no body, and pushes any JSON-RPC
response back onto the open GET stream as a message event.
A 2024-11-05-only client can then negotiate the legacy transport against a D
MCP server. The modern Streamable HTTP endpoint is unchanged.
Opt-in backwards compatibility with the deprecated 2024-11-05 HTTP+SSE two-endpoint transport (basic/transports §HTTP with SSE; and the 2025-06-18 / 2025-11-25 / draft §Backwards Compatibility guidance: "Servers wanting to support older clients should: Continue to host both the SSE and POST endpoints of the old transport, alongside the new MCP endpoint"). This is a SHOULD, so it is off by default. When enabled, mountMcp ALSO mounts the two legacy endpoints alongside the modern MCP endpoint: - GET legacySsePath: opens a text/event-stream, immediately emits an endpoint event whose data is legacyMessagePath (the URI the client must POST to), then holds the stream open delivering server messages as SSE message events; - POST legacyMessagePath: accepts a single JSON-RPC message, processes it, replies 202 Accepted with no body, and pushes any JSON-RPC response back onto the open GET stream as a message event. A 2024-11-05-only client can then negotiate the legacy transport against a D MCP server. The modern Streamable HTTP endpoint is unchanged.