deprecate: Mark SSE transport as deprecated

The HTTP+SSE transport was replaced by Streamable HTTP in protocol
revision 2025-03-26. This change adds DeprecationWarning and docstring
markers to:

- sse_client in mcp/client/sse.py
- SseServerTransport in mcp/server/sse.py
- run_sse_async in MCPServer
- sse_app in MCPServer

The warnings direct users to use StreamableHTTPTransport instead.

Fixes #2278

Author: PR-Contributor

src/mcp/client/sse.py

@@ -1,4 +1,5 @@
 import logging
+import warnings
 from collections.abc import Callable
 from contextlib import asynccontextmanager
 from typing import Any
@@ -37,7 +38,11 @@ async def sse_client(
     auth: httpx.Auth | None = None,
     on_session_created: Callable[[str], None] | None = None,
 ):
-    """Client transport for SSE.
+    """Client transport for SSE (Deprecated).
+
+    .. deprecated::
+        The HTTP+SSE transport is deprecated. Use `streamable_http_client` with
+        `StreamableHTTPTransport` instead for connecting to MCP servers.
 
     `sse_read_timeout` determines how long (in seconds) the client will wait for a new
     event before disconnecting. All other HTTP operations are controlled by `timeout`.
@@ -154,4 +159,10 @@ async def post_writer(endpoint_url: str):
                 tg.start_soon(post_writer, endpoint_url)
 
                 yield read_stream, write_stream
-                tg.cancel_scope.cancel()
+    tg.cancel_scope.cancel()
+
+    warnings.warn(
+        "sse_client is deprecated. Use streamable_http_client with StreamableHTTPTransport instead.",
+        DeprecationWarning,
+        stacklevel=2,
+    )

src/mcp/server/mcpserver/server.py

@@ -854,7 +854,18 @@ async def run_sse_async(  # pragma: no cover
         message_path: str = "/messages/",
         transport_security: TransportSecuritySettings | None = None,
     ) -> None:
-        """Run the server using SSE transport."""
+        """Run the server using SSE transport (Deprecated).
+
+        .. deprecated::
+            The HTTP+SSE transport is deprecated. Use `run_streamable_http_async` instead.
+        """
+        import warnings
+        warnings.warn(
+            "run_sse_async is deprecated. Use run_streamable_http_async instead.",
+            DeprecationWarning,
+            stacklevel=2,
+        )
+
         import uvicorn
 
         starlette_app = self.sse_app(
@@ -915,7 +926,17 @@ def sse_app(
         transport_security: TransportSecuritySettings | None = None,
         host: str = "127.0.0.1",
     ) -> Starlette:
-        """Return an instance of the SSE server app."""
+        """Return an instance of the SSE server app (Deprecated).
+
+        .. deprecated::
+            The HTTP+SSE transport is deprecated. Use `streamable_http_app` instead.
+        """
+        import warnings
+        warnings.warn(
+            "sse_app is deprecated. Use streamable_http_app instead.",
+            DeprecationWarning,
+            stacklevel=2,
+        )
         # Auto-enable DNS rebinding protection for localhost (IPv4 and IPv6)
         if transport_security is None and host in ("127.0.0.1", "localhost", "::1"):
             transport_security = TransportSecuritySettings(

src/mcp/server/sse.py

@@ -1,4 +1,7 @@
-"""SSE Server Transport Module
+"""SSE Server Transport Module (Deprecated)
+
+.. deprecated::
+    The HTTP+SSE transport is deprecated. Use `StreamableHTTPTransport` instead.
 
 This module implements a Server-Sent Events (SSE) transport layer for MCP servers.
 
@@ -37,6 +40,7 @@ async def handle_sse(request):
 """
 
 import logging
+import warnings
 from contextlib import asynccontextmanager
 from typing import Any
 from urllib.parse import quote
@@ -61,7 +65,13 @@ async def handle_sse(request):
 
 
 class SseServerTransport:
-    """SSE server transport for MCP. This class provides two ASGI applications,
+    """SSE server transport for MCP (Deprecated).
+
+    .. deprecated::
+        The HTTP+SSE transport is deprecated. Use `StreamableHTTPTransport` instead
+        for new MCP server implementations.
+
+    This class provides two ASGI applications,
     suitable for use with a framework like Starlette and a server like Hypercorn:
 
         1. connect_sse() is an ASGI application which receives incoming GET requests,
@@ -79,6 +89,9 @@ def __init__(self, endpoint: str, security_settings: TransportSecuritySettings |
         """Creates a new SSE server transport, which will direct the client to POST
         messages to the relative path given.
 
+        .. deprecated::
+            The HTTP+SSE transport is deprecated. Use `StreamableHTTPTransport` instead.
+
         Args:
             endpoint: A relative path where messages should be posted
                     (e.g., "/messages/").
@@ -99,6 +112,12 @@ def __init__(self, endpoint: str, security_settings: TransportSecuritySettings |
 
         super().__init__()
 
+        warnings.warn(
+            "SseServerTransport is deprecated. Use StreamableHTTPTransport instead.",
+            DeprecationWarning,
+            stacklevel=2,
+        )
+
         # Validate that endpoint is a relative path and not a full URL
         if "://" in endpoint or endpoint.startswith("//") or "?" in endpoint or "#" in endpoint:
             raise ValueError(