Why was HTTP+SSE deprecated as an MCP transport in 2025 and what does Streamable HTTP improve?

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Explain the technical reasons the HTTP+SSE transport was deprecated in MCP during 2025 and describe the specific improvements Streamable HTTP provides.

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Quick reference

TL;DR

HTTP+SSE used two endpoints with mismatched connection models, awkward for stateless and serverless deploys; Streamable HTTP uses one endpoint with optional server streaming and resumable sessions.

Memory aid

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Easy to grasp

Imagine ordering at a restaurant where you shout your order across the room to the kitchen, but the kitchen only replies through a separate intercom line you have to keep open the whole meal. If either the intercom or your shouting drops out, the staff lose track of who you are and what you ordered. That two-line setup is HTTP+SSE: client requests went one way over plain web calls, and server messages came back over a separate always-on stream. Streamable HTTP collapses both into one counter. You speak and listen at the same window, the kitchen can still stream your food out in courses, and if you step away it can pick up where it left off. One channel, far less to keep in sync, and it fits how cheap modern web hosting actually works.

Key concepts

Streamable HTTP transport HTTP+SSE transport Session resumption Serverless deployment

Quick scan · 2–4 min

Everything important, quickly.

4 min: the two channel SSE design, why it leaked complexity, the serverless and load balancer failures, then Streamable HTTP's single endpoint, optional streaming, and session resumption, ending with migration.

Side-by-side

Concern	HTTP+SSE (deprecated)	Streamable HTTP (current)
Endpoints	Two: SSE GET stream plus POST	One: single POST endpoint
Server to client	Persistent SSE stream	Optional chunked body on the POST reply
Client to server	HTTP POST on a separate route	HTTP POST on the same route
Serverless fit	Poor: long-lived stream per client	Good: stateless request and reply
Resumability	Reconnect plus manual correlation	Session ID header plus event-ID resume

Real-world examples

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

Cloudflare Workers and AWS Lambda host remote MCP servers on Streamable HTTP because stateless POSTs fit the per-request execution model.
Anthropic's MCP spec revision in 2025 marked HTTP+SSE as the legacy transport and made Streamable HTTP the standard for remote servers.
Hosts like Claude Code and Cursor connect to hosted MCP servers such as GitHub, Sentry, and Linear over Streamable HTTP rather than the old dual-endpoint SSE setup.

Interviewer follow-ups

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

QHow does Streamable HTTP support resuming a session after a network drop?

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QWhy does a long-lived SSE stream specifically break on a load-balanced deployment?

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QWhen migrating a server, how do you keep older SSE-only clients working?

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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.