This comparison is part of our MCP comparisons, within The MCP Guide. Facts are checked against each vendor's own documentation and pricing pages on the date shown.
Use Streamable HTTP for any new remote MCP server. The older HTTP+SSE transport has been deprecated since protocol version 2025-03-26, and the 2026-07-28 spec lists it in the official deprecated features registry with Streamable HTTP as the migration path. Keep an SSE endpoint only if you still have clients that cannot speak the newer transport.
Checked against: the Streamable HTTP page, the deprecated features registry and the original 2024-11-05 transports page of the MCP specification, read on 28 October 2026.
One naming note before we start. "SSE" here means the legacy HTTP+SSE transport. Server-Sent Events (SSE), the web standard for streaming text from a server, still exists inside Streamable HTTP as an optional reply format. The comparison is between two transports, not between SSE and no SSE.
Key Takeaways
- HTTP+SSE was the first remote MCP transport, defined in the 2024-11-05 spec, and needed two endpoints plus a long-lived connection.
- Streamable HTTP replaced it in 2025-03-26 with one endpoint that accepts a POST for every message.
- The 2026-07-28 spec removed sessions, the GET stream and stream resumability from Streamable HTTP.
- HTTP+SSE is listed as Deprecated, and the registry says its earliest removal is "three months after SEP-2596 reaches Final."
- Servers can host the old SSE and POST endpoints next to the new MCP endpoint during a migration.
At a glance
| HTTP+SSE (legacy) | Streamable HTTP (2026-07-28) | |
|---|---|---|
| Introduced | 2024-11-05 | 2025-03-26 |
| Status | Deprecated (since 2025-03-26, reclassified under SEP-2596) | Standard |
| Endpoints | Two: an SSE endpoint and a POST endpoint | One MCP endpoint |
| Connection model | Client keeps an SSE connection open for all server messages | Each message is its own POST |
| How replies arrive | SSE message events on the shared stream | JSON object or an SSE stream scoped to one request |
| Sessions | Implied by the open connection | None (removed in 2026-07-28) |
| Server-to-client notifications | On the shared SSE stream | On the request's stream, or a subscriptions/listen stream |
| Load balancing | Needs sticky routing to the open connection | "Any request can land on any instance" (release post) |
| Security baseline | Validate Origin, bind to localhost locally, authenticate | Same, plus HTTP 403 on an invalid Origin |
How HTTP+SSE worked
The 2024-11-05 transports page says the server "MUST provide two endpoints." One is "an SSE endpoint, for clients to establish a connection and receive messages from the server." The other is "a regular HTTP POST endpoint for clients to send messages to the server."
When a client connected, the server sent an endpoint event with the URL to POST to. Every server message then came back as an SSE message event on that open connection. So the client sent on one channel and listened on another. Our legacy HTTP+SSE reference walks through the full flow.
How Streamable HTTP works now
The Streamable HTTP page says "the server MUST provide a single HTTP endpoint path." Each client message is a new POST. For a request, the server returns either one JSON object or an SSE stream for that request only, and the final response "SHOULD terminate the stream."
The 2026-07-28 version went further than the 2025 versions. The page says earlier revisions let servers assign a session with Mcp-Session-Id, let clients open a standalone GET stream, and allowed resumable streams via Last-Event-ID. "None of these mechanisms are part of this revision." Long-lived change notifications now use subscriptions/listen, which "replaces the former resources/subscribe RPC and the HTTP GET endpoint."
Where HTTP+SSE is stronger
There is little reason to choose it for new work, but it has two narrow advantages.
- Compatibility with very old clients. A client built only for 2024-11-05 may know nothing else. If you must support such a client, an SSE endpoint keeps it working.
- One open stream for everything. Server messages arrive on one connection without the client asking for each one. In 2026-07-28, you get similar behavior only by opening a
subscriptions/listenstream and choosing which notification types to receive.
Both advantages shrink over time. The deprecated features registry says new implementations "SHOULD NOT adopt" a deprecated feature and existing ones "SHOULD migrate."
Where Streamable HTTP is stronger
- Simpler infrastructure. One endpoint, standard POST requests, and no connection that must stay pinned to one server instance.
- Horizontal scaling. The 2026-07-28 release post says every request is self-describing, "so any request can land on any instance behind a plain round-robin load balancer."
- Gateway friendly. Required headers such as
Mcp-MethodandMcp-Namelet gateways route and authorize on headers. See what MCP gateways do. - Clear cancellation. Closing a request's response stream "is itself the cancellation signal," per the spec.
- Active support. The release post says all four Tier 1 SDKs (TypeScript, Python, Go and C#) speak 2026-07-28.
Pricing
Both are parts of an open protocol, so neither has a license fee. Hosting cost is where they differ. A long-lived SSE connection holds server resources for as long as each client stays connected. Streamable HTTP requests are short by default, which suits serverless platforms. Check your host's own pricing page for how it bills long connections.
Security and admin controls
The security rules look alike on paper. The 2024-11-05 page already said servers "MUST validate the Origin header on all incoming connections to prevent DNS rebinding attacks," should bind only to localhost when local, and should authenticate all connections. The 2026-07-28 Streamable HTTP page keeps those rules and adds that servers must return HTTP 403 when the Origin header is present and invalid.
The bigger admin difference is authorization. The 2025-03-26 changelog added an OAuth 2.1 authorization framework in the same release that replaced HTTP+SSE with Streamable HTTP. If your SSE server still relies on a shared key in the URL or a header, migration is a good moment to move to the MCP authorization spec. Also review DNS rebinding risks for any server reachable from a browser.
How to migrate
The spec gives a compatibility recipe. Servers that want to support older clients should "continue to host both the SSE and POST endpoints of the old transport, alongside the new 'MCP endpoint'." Clients that want to support older servers should:
- Accept a server URL from the user.
- POST a request to it with the standard
Acceptheader. - If that fails with 400, 404 or 405 and the body is not a recognized modern JSON-RPC error, send a GET and wait for an
endpointevent. - If the event arrives, treat the server as legacy HTTP+SSE.
A server on 2026-07-28 that receives traffic from a 2025-era Streamable HTTP client "SHOULD" answer GET or DELETE with 405 Method Not Allowed, ignore Mcp-Session-Id and ignore Last-Event-ID. Plan for that if you upgrade a server from 2025-11-25.
Which to choose
Keep HTTP+SSE if:
- You have a known client that only speaks 2024-11-05 and cannot be updated yet.
- You run it next to Streamable HTTP, with a removal date on your roadmap.
Pick Streamable HTTP if:
- You are building any new remote MCP server.
- You want serverless hosting, round-robin load balancing or a gateway.
- You want current SDK support and the OAuth-based authorization flow.
Next step
If you run an older SSE server, list the clients that still use it and set a removal date. eSEOspace's MCP readiness audit checks transport, auth and spec version for existing servers. See all MCP comparisons, or compare stdio vs Streamable HTTP next.
Frequently asked questions
Is SSE removed from MCP?
Not yet. As of October 2026 the HTTP+SSE transport is Deprecated, not Removed. The registry lists its earliest removal as "three months after SEP-2596 reaches Final," and says actual removal is a Core Maintainer decision. The registry's Removed section says no features have been removed under the policy yet.
Does Streamable HTTP still use Server-Sent Events?
Yes, as a reply format. For a request, a server can answer with Content-Type: text/event-stream, a stream scoped to that one request. The client must support both JSON and SSE replies.
What happened to sessions and Mcp-Session-Id?
The 2026-07-28 changelog removed protocol-level sessions and the Mcp-Session-Id header. Servers that need state across calls use explicit handles passed as ordinary tool arguments. See the stateless protocol.
Can one server support both transports?
Yes. The spec suggests hosting the old SSE and POST endpoints alongside the new MCP endpoint. It notes you can merge the old POST endpoint with the new one, "but this may introduce unneeded complexity."
