docs: note HTTP+SSE transport deprecation per the feature lifecycle policy #3045

Where was this agreed?

Requested by maintainer David Soria Parra as part of preparing the modelcontextprotocol.io documentation for the 2026-07-28 specification release.

Description

The 2026-07-28 spec release adds a feature lifecycle policy and deprecated-features registry, reclassifying the HTTP+SSE transport and includeContext as deprecated. Several pages still recommend SSE without a caveat.

Files to update:

• docs/registry/remote-servers.mdx — note the "sse" transport is Deprecated (link /specification/draft/deprecated); recommend Streamable HTTP
• docs/docs/develop/build-client.mdx — caveat the WebFlux SSE "for production deployments" recommendation (~line 1035)
• docs/docs/develop/build-server.mdx — add a deprecation caveat to the SSE-transport starter link (~line 1120)

Part of the 2026-07-28 release documentation tracking (milestone: 2026-07-28 Specification Release).