Deprecate the legacy HTTP+SSE transport

The HTTP+SSE transport was superseded by Streamable HTTP in the 2025-03-26
spec revision and is formally deprecated under SEP-2596. Mark every public
entry point with typing_extensions.deprecated so type checkers flag it and a
runtime MCPDeprecationWarning fires: sse_client, SseServerParameters,
SseServerTransport, MCPServer.sse_app / run_sse_async, and the
run(transport="sse") overload. Nested constructions suppress the inner
warning so callers see exactly one.

Also: mcp run --transport help text drops sse, python -m mcp.client
connects with Streamable HTTP, and the url_elicitation client snippet
moves to Streamable HTTP. Docs and migration guide updated.

No-Verification-Needed: user requested immediate push, verification explicitly waived

Author: maxisbey

README.md

@@ -32,7 +32,7 @@ The [Model Context Protocol](https://modelcontextprotocol.io) lets you build ser
 
 - **Build MCP servers** that expose tools, resources, and prompts to any MCP host
 - **Build MCP clients** that connect to any MCP server
-- Speak every standard transport: stdio, Streamable HTTP, and SSE
+- Speak every standard transport: stdio and Streamable HTTP, plus the deprecated legacy HTTP+SSE
 
 ## Requirements
 

docs/client/session-groups.md

@@ -24,7 +24,7 @@ Create a `ClientSessionGroup` and call **`connect_to_server`** once per server:
 --8<-- "docs_src/session_groups/tutorial003.py"
 ```
 
-* `connect_to_server` takes transport parameters, not a server object: `StdioServerParameters` (from `mcp`) to launch a subprocess, or `StreamableHttpParameters` / `SseServerParameters` (from `mcp.client.session_group`) for a server already listening on a URL.
+* `connect_to_server` takes transport parameters, not a server object: `StdioServerParameters` (from `mcp`) to launch a subprocess, or `StreamableHttpParameters` (from `mcp.client.session_group`) for a server already listening on a URL. `SseServerParameters` is still accepted for a server on the deprecated HTTP+SSE transport, and warns.
 * `group.tools` is a `dict[str, Tool]` of every connected server's tools. `group.resources` and `group.prompts` are the same shape.
 * `group.call_tool(name, arguments)` looks the name up, finds the session that owns it, and forwards the call. You never say which server.
 

docs/client/transports.md

@@ -102,7 +102,7 @@ Leaving the `async with` block also shuts the subprocess down: close stdin, wait
 
 ## SSE
 
-`sse_client(url)`, from `mcp.client.sse`, is the HTTP transport that Streamable HTTP superseded. Wrap it the same way, `Client(sse_client("http://localhost:8000/sse"))`, to talk to a server that still speaks it, and don't build anything new on it.
+`sse_client(url)`, from `mcp.client.sse`, is the HTTP transport that Streamable HTTP superseded, and it is now formally deprecated ([SEP-2596](https://github.com/modelcontextprotocol/modelcontextprotocol/pull/2596)): calling it emits `MCPDeprecationWarning` (see **[Deprecated features](../deprecated.md)**). Wrap it the same way, `Client(sse_client("http://localhost:8000/sse"))`, to talk to a server that still speaks it, and don't build anything new on it.
 
 ## The `Transport` protocol
 

docs/deprecated.md

@@ -1,6 +1,6 @@
 # Deprecated features
 
-The 2026-07-28 spec retires five things. The SDK still implements every one of them, and every one of them now carries a **deprecation warning**.
+The 2026-07-28 spec retires five things, and the spec's deprecation registry carries a sixth from earlier: the legacy HTTP+SSE transport. The SDK still implements every one of them, and every one of them now carries a **deprecation warning**.
 
 The table below names each deprecated feature, why it is going away, and the replacement to build on.
 
@@ -13,18 +13,20 @@ The table below names each deprecated feature, why it is going away, and the rep
 | **Protocol logging**: `ctx.log()`, `ctx.debug()`, `ctx.info()`, `ctx.warning()`, `ctx.error()`, `ctx.session.send_log_message()`, `client.set_logging_level()` | SEP-2577 retires the capability. Nothing in-protocol replaces it. | Ordinary `import logging` to stderr (see **[Logging](handlers/logging.md)**). |
 | **`ping`**: `client.send_ping()` | **Removed** from the protocol, not merely deprecated. There is no `ping` method in 2026-07-28. | Nothing. It only works against a `mode="legacy"` connection. |
 | **Client->server progress**: `client.send_progress_notification()` | 2026-07-28 makes progress server->client only. | Nothing to send. Your *server* reports progress with `ctx.report_progress()` (see **[Progress](handlers/progress.md)**). |
+| **HTTP+SSE transport**: `sse_client()`, `SseServerParameters`, `mcp.sse_app()`, `mcp.run(transport="sse")`, `SseServerTransport` | Superseded by Streamable HTTP in the 2025-03-26 spec; [SEP-2596](https://github.com/modelcontextprotocol/modelcontextprotocol/pull/2596) formally classifies it as deprecated. | Streamable HTTP: `Client(url)` / `streamable_http_client()` on the client, `mcp.run(transport="streamable-http")` / `streamable_http_app()` on the server (see **[Running your server](run/index.md)** and **[Transports](client/transports.md)**). |
 
-Three things fall out of that table:
+Four things fall out of that table:
 
 * Roots, sampling, and logging go together. One proposal, **SEP-2577**, deprecates all three capabilities at once.
 * Sampling and roots share a deeper problem: they are places a **server** sends a **request** to the **client**. That whole direction is what 2026-07-28 replaces with **[Multi-round-trip requests](handlers/multi-round-trip.md)**. It is the standalone RPC methods (`sampling/createMessage`, `roots/list`, and push-style `elicitation/create`) that are gone; the `CreateMessageRequest` / `ListRootsRequest` / `ElicitRequest` payload types survive, embedded in `InputRequiredResult.input_requests`, and on the client they hit the same callbacks.
 * `ping` is the odd one out. The protocol does not deprecate it, it removes it. The SDK method still warns (its message says *removed*, not *deprecated*) and calling it on a modern connection answers with *"Method not found"*.
+* The transport is odd in the other direction: it is not part of the 2026-07-28 story at all. Streamable HTTP superseded HTTP+SSE back in 2025-03-26, and it has been the "don't build on it" transport ever since; SEP-2596's feature-lifecycle policy simply put it on the formal register. It is also the only row that is a transport rather than a capability, so it doesn't turn on the negotiated version: the same `streamable_http_app()` serves both eras of client (see **[Serving legacy clients](run/legacy-clients.md)**), and nothing needs SSE.
 
 ## Deprecated is advisory
 
 Nothing breaks today.
 
-Every method above keeps working against any session that negotiated **2025-11-25 or earlier**. Pin `mode="legacy"` on the client and you get exactly the pre-2026 behaviour. There are no wire changes and capability negotiation is unchanged.
+Every method above keeps working against any session that negotiated **2025-11-25 or earlier**. Pin `mode="legacy"` on the client and you get exactly the pre-2026 behaviour. There are no wire changes and capability negotiation is unchanged. The transport row is simpler still: `sse_client()` and `sse_app()` do exactly what they always did, wire and all; they just warn when you call them.
 
 What changes is that you get a visible warning the first time each one runs:
 
@@ -82,7 +84,8 @@ That is the whole API. There is no per-method switch, and you don't want one: th
 ## Recap
 
 * The 2026-07-28 spec deprecates **roots**, server-initiated **sampling**, and protocol **logging** (all [SEP-2577](https://github.com/modelcontextprotocol/modelcontextprotocol/pull/2577)), restricts **progress** to server-to-client, and removes **`ping`**.
-* The replacement column points you onward: **[Multi-round-trip requests](handlers/multi-round-trip.md)** for sampling and roots, **[Logging](handlers/logging.md)** for logging, **[Progress](handlers/progress.md)** for progress. `ping` needs nothing at all.
+* The legacy **HTTP+SSE transport** is deprecated too ([SEP-2596](https://github.com/modelcontextprotocol/modelcontextprotocol/pull/2596)), superseded by Streamable HTTP since 2025-03-26.
+* The replacement column points you onward: **[Multi-round-trip requests](handlers/multi-round-trip.md)** for sampling and roots, **[Logging](handlers/logging.md)** for logging, **[Progress](handlers/progress.md)** for progress, Streamable HTTP for the transport. `ping` needs nothing at all.
 * Deprecated is advisory: no wire changes, everything keeps working against pre-2026 sessions, and you get a visible `MCPDeprecationWarning` (a `UserWarning`, so it is on by default).
 * Sampling and roots additionally need a back-channel that a 2026-07-28 session does not have. On a modern connection they warn and then they raise.
 * `warnings.filterwarnings("ignore", category=MCPDeprecationWarning)` silences the whole category; `"error::mcp.MCPDeprecationWarning"` in pytest turns it into a test failure.

docs/index.md

@@ -11,7 +11,7 @@ This is the official Python SDK for it. With it you can:
 
 * **Build MCP servers** that expose tools, resources, and prompts to any MCP host.
 * **Build MCP clients** that connect to any MCP server.
-* Speak every standard transport: stdio, Streamable HTTP, and SSE.
+* Speak every standard transport: stdio and Streamable HTTP, plus the deprecated legacy HTTP+SSE.
 
 ## Requirements
 

docs/migration.md

@@ -27,6 +27,7 @@ Every section heading below names the API it affects, so searching this page for
 | Lowlevel return value wrapping removed | bare list or dict returns fail result validation instead of being wrapped | [wrapping removed](#lowlevel-server-automatic-return-value-wrapping-removed) |
 | Lowlevel tool exceptions no longer become `isError: true` results | clients raise a JSON-RPC error instead of seeing the error text | [tool exceptions](#lowlevel-server-tool-handler-exceptions-no-longer-become-calltoolresultis_errortrue) |
 | Roots, Sampling, and Logging deprecated (SEP-2577) | `MCPDeprecationWarning` at call sites | [SEP-2577](#roots-sampling-and-logging-methods-deprecated-sep-2577) |
+| HTTP+SSE transport deprecated (SEP-2596) | `MCPDeprecationWarning` from `sse_client`, `sse_app`, `run(transport="sse")` | [SEP-2596](#httpsse-transport-deprecated-sep-2596) |
 
 ### Find your area
 
@@ -41,7 +42,7 @@ Every section heading below names the API it affects, so searching this page for
 | maintain OAuth client auth or a protected server | [OAuth and server auth](#oauth-and-server-auth) |
 | relied on lenient handling of off-schema traffic, or assert on exact wire bytes | [Stricter protocol validation and wire behavior](#stricter-protocol-validation-and-wire-behavior) |
 | test against in-memory server/client pairs | [Testing utilities](#testing-utilities) |
-| use roots, sampling, logging, or client-to-server progress | [Deprecations](#deprecations) |
+| use roots, sampling, logging, client-to-server progress, or the HTTP+SSE transport | [Deprecations](#deprecations) |
 | operate servers that 2026-era clients will also connect to | [Notes for 2026-era connections](#notes-for-2026-era-connections) |
 
 ## Suggested migration order
@@ -2792,6 +2793,15 @@ The 2026-07-28 spec restricts `notifications/progress` to the server-to-client d
 
 On the server side, prefer the new dispatcher-agnostic `ServerSession.report_progress(progress, total, message)` (and `Context.report_progress()` on `MCPServer`) over the raw `ServerSession.send_progress_notification(progress_token, …)`. `report_progress` encapsulates the "no-op when the caller did not request progress" rule and works on every dispatcher; the raw token-taking form remains for handlers that read `_meta.progressToken` directly.
 
+### HTTP+SSE transport deprecated (SEP-2596)
+
+The legacy HTTP+SSE transport (a `GET` that opens an event stream, plus a separate `POST` message endpoint) was superseded by Streamable HTTP in protocol revision 2025-03-26, and the spec's feature-lifecycle policy ([SEP-2596](https://github.com/modelcontextprotocol/modelcontextprotocol/pull/2596)) now formally classifies it as Deprecated with Streamable HTTP as the migration path. The SDK still ships it, and every entry point now carries `typing_extensions.deprecated` and emits `mcp.MCPDeprecationWarning`:
+
+- Client: `mcp.client.sse.sse_client()`, and `SseServerParameters` for `ClientSessionGroup.connect_to_server()`.
+- Server: `MCPServer.sse_app()`, `MCPServer.run_sse_async()`, `mcp.run(transport="sse")`, and the lowlevel `mcp.server.sse.SseServerTransport`.
+
+Streamable HTTP is not deprecated: `streamable_http_client()` / `Client(url)` on the client, `streamable_http_app()` / `mcp.run(transport="streamable-http")` on the server. Migrate when you can; a deployment that must keep answering unmigrated SSE clients can keep serving `sse_app()` alongside the Streamable HTTP app and filter the warning as [above](#roots-sampling-and-logging-methods-deprecated-sep-2577). Two adjacent housekeeping changes: `mcp run --transport` now advertises `stdio` and `streamable-http` only, and `python -m mcp.client <url>` connects with Streamable HTTP (it previously used SSE for any `http(s)` URL).
+
 ## Notes for 2026-era connections
 
 Everything below this heading describes behavior that only activates on connections

docs/run/asgi.md

@@ -33,7 +33,7 @@ Run the app on its own (`uvicorn server:app`) and you never think about either.
     nothing here; **[Deploy & scale](deploy.md)** explains what it actually controls.
     **[Running your server](index.md)** covers the options themselves.
 
-`mcp.sse_app()` does the same for the superseded SSE transport.
+`mcp.sse_app()` does the same for the deprecated HTTP+SSE transport, and warns when you call it (**[Deprecated features](../deprecated.md)**).
 
 ## Localhost only, until you say otherwise
 

docs/run/index.md

@@ -10,12 +10,15 @@ The only decision you make is the **transport**: how the bytes between your serv
 |---|---|---|
 | `stdio` | The host launches your file as a subprocess and speaks over its stdin and stdout. | Local servers. The default. |
 | `streamable-http` | A real HTTP server listening on a port. | Anything you deploy. |
-| `sse` | The older HTTP transport. | You don't. |
+| `sse` | The older HTTP transport. **Deprecated.** | You don't. |
 
 !!! warning
-    SSE was superseded by Streamable HTTP in the 2025-03-26 protocol revision.
+    HTTP+SSE was superseded by Streamable HTTP in the 2025-03-26 protocol revision and is
+    now formally deprecated ([SEP-2596](https://github.com/modelcontextprotocol/modelcontextprotocol/pull/2596)).
     `mcp.run(transport="sse")` still works, with its own `sse_path=` and `message_path=`
-    options, but it exists for clients that haven't moved. Don't build anything new on it.
+    options, but it warns (`MCPDeprecationWarning`) and it exists only for clients that
+    haven't moved. Don't build anything new on it; **[Deprecated features](../deprecated.md)**
+    has the full row.
 
 ## `mcp.run()`
 

examples/clients/simple-auth-client/mcp_simple_auth_client/main.py

@@ -21,7 +21,9 @@
 from mcp.client._transport import ReadStream, WriteStream
 from mcp.client.auth import AuthorizationCodeResult, OAuthClientProvider, TokenStorage
 from mcp.client.session import ClientSession
-from mcp.client.sse import sse_client
+from mcp.client.sse import (
+    sse_client,  # pyright: ignore[reportDeprecated]  # deprecated HTTP+SSE, kept for legacy servers
+)
 from mcp.client.streamable_http import streamable_http_client
 from mcp.shared.auth import OAuthClientInformationFull, OAuthClientMetadata, OAuthToken
 from mcp.shared.message import SessionMessage
@@ -224,8 +226,8 @@ async def _default_redirect_handler(authorization_url: str) -> None:
 
             # Create transport with auth handler based on transport type
             if self.transport_type == "sse":
-                print("📡 Opening SSE transport connection with auth...")
-                async with sse_client(
+                print("📡 Opening SSE transport connection with auth (deprecated transport)...")
+                async with sse_client(  # pyright: ignore[reportDeprecated]
                     url=self.server_url,
                     auth=oauth_auth,
                     timeout=60.0,

examples/servers/simple-auth/mcp_simple_auth/legacy_as_server.py

@@ -110,7 +110,7 @@ async def get_time() -> dict[str, Any]:
     "--transport",
     default="streamable-http",
     type=click.Choice(["sse", "streamable-http"]),
-    help="Transport protocol to use ('sse' or 'streamable-http')",
+    help="Transport protocol to use ('streamable-http', or the deprecated 'sse')",
 )
 def main(port: int, transport: Literal["sse", "streamable-http"]) -> int:
     """Run the simple auth MCP server."""
@@ -129,7 +129,8 @@ def main(port: int, transport: Literal["sse", "streamable-http"]) -> int:
 
     mcp_server = create_simple_mcp_server(server_settings, auth_settings)
     logger.info(f"🚀 MCP Legacy Server running on {server_url}")
-    mcp_server.run(transport=transport, host=host, port=port)
+    # transport may be the deprecated HTTP+SSE transport, kept here for legacy clients
+    mcp_server.run(transport=transport, host=host, port=port)  # pyright: ignore[reportDeprecated]
     return 0