Glossary

Streamable HTTP

How an agent talks to an MCP server that lives at a URL instead of on your laptop.

By Roger Campos · Last updated: September 2026

TL;DR

Streamable HTTP is the transport the Model Context Protocol uses for remote servers. The client sends each JSON-RPC message as an HTTP POST to one endpoint; the server answers with a single JSON response or, when it has several messages to send, a Server-Sent Events stream. It replaced MCP's earlier HTTP+SSE transport in 2025.

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How it works

MCP's first remote transport, HTTP+SSE, needed two endpoints and a long-lived Server-Sent Events connection for every client — awkward behind load balancers and for serverless hosting. The 2025-03-26 revision of the spec replaced it with streamable HTTP:

1
The server has one endpoint, such as https://example.com/mcp.
2
The client POSTs each JSON-RPC message to it, accepting both application/json and text/event-stream.
3
The server answers with one JSON response — or, if it wants to send progress or several messages, opens an SSE stream on that response.
4
Optionally, the server issues an Mcp-Session-Id header for stateful sessions, and the client may GET the endpoint for server-initiated messages.

Listing a server's tools

POST /mcp HTTP/1.1
Host: urlpipe.dev
Authorization: Bearer YOUR_TOKEN
Content-Type: application/json
Accept: application/json, text/event-stream

{"jsonrpc": "2.0", "id": 1, "method": "tools/list"}

In practice

Why it matters

A server that answers every message with plain JSON and keeps no session is an ordinary HTTP API: it scales horizontally, sits behind any proxy and authenticates with a normal Authorization header. Streaming is there for the servers that need it — long-running tools reporting progress — without forcing it on the rest. Clients that only speak the older transport, or only stdio, can reach a streamable HTTP server through a small local bridge.

URLpipe

URLpipe's transport

URLpipe's MCP server at https://urlpipe.dev/mcp speaks streamable HTTP in its simplest form: every message gets a single JSON response, it opens no SSE stream and keeps no session. Authentication is a bearer token in the header, with no OAuth flow to complete. Long jobs don't need a stream: tools are async by default and return a token that get_result redeems, or pass sync: true to wait. See the MCP docs.

Try it

Streamable HTTP, on a page you choose

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FAQ

Frequently asked questions

What is the difference between SSE and streamable HTTP in MCP?

The older HTTP+SSE transport needed a separate SSE connection held open for every client. Streamable HTTP uses one endpoint and plain POSTs, and only streams when a response needs it.

Does streamable HTTP require sessions?

No. Sessions are optional; a server can be stateless and answer every message on its own.

Can a client that only speaks stdio use a streamable HTTP server?

Yes, through a small local bridge: the client starts the bridge as if it were a local server, and the bridge forwards each message to the remote endpoint.

Keep reading

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