Why was the HTTP+SSE transport deprecated as an MCP transport in 2025?

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Quick reference

TL;DR

HTTP+SSE split the two directions across a long-lived SSE stream and separate POSTs; that asymmetry broke on serverless and stateless infra, so Streamable HTTP replaced it with one endpoint.

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Easy to grasp

Imagine ordering at a drive-through where you shout your order through one window but the food comes out a totally different window that has to stay open the whole time. If that second window ever closes, the kitchen loses track of your whole order. The old MCP remote transport worked like that: replies streamed back on a long-open server channel while your requests went through a separate door. Cloud platforms that spin servers up and down hated keeping that channel open. The new design uses a single window for both ordering and receiving. The reply can still stream when needed, but nothing has to stay propped open, so it works on platforms that start fresh for every request.

Key concepts

Streamable HTTP transport HTTP+SSE transport Stateless / serverless transport

Quick scan · 2–4 min

Everything important, quickly.

4 min: name the asymmetry, explain the lifecycle pain, contrast the unified single-endpoint design, then tie it to stateless serverless deployment and optional streaming.

Side-by-side

Aspect	HTTP+SSE (deprecated)	Streamable HTTP (current)
Endpoints	Separate SSE stream + POST endpoint	Single unified endpoint
Server-to-client	Long-lived SSE GET stream	Optional SSE on the POST response
Connection model	Persistent open socket required	Per-request, no held socket
Statefulness	Stateful, sticky routing needed	Stateless-friendly
Serverless fit	Poor, cannot hold stream open	Clean, maps to Workers and Lambda
Spec revision	2024-11-05	2025-03-26 onward

Real-world examples

Real products, models, and research that use this idea.

Cloudflare's remote MCP server guides target Streamable HTTP so Workers can serve MCP statelessly without holding an SSE socket open.
Anthropic's MCP spec revision 2025-03-26 introduced Streamable HTTP and deprecated the standalone HTTP+SSE transport from the 2024-11-05 revision.
The official MCP TypeScript and Python SDKs ship a StreamableHTTPServerTransport, with the older SSE transport kept only for backward compatibility.

Interviewer follow-ups

What an interviewer would ask next. Try answering before peeking at the approach.

QHow does Streamable HTTP let a stateless server still support multi-message sessions?

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QIf streaming is optional in Streamable HTTP, when should a server actually stream the response?

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QWhat backward-compatibility path exists for clients that only speak the old HTTP+SSE transport?

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60-second cheat sheet

The night-before-the-interview bullets. Scan these on the way to the call.

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References

Primary sources. Skim if you want the original framing.