Deprecate the SSE transport

docs/migration.md

@@ -149,6 +149,10 @@ named `mcp.os.win32.utilities` (was `client.stdio.win32`).
 
 The WebSocket transport has been removed: `mcp.client.websocket.websocket_client`, `mcp.server.websocket.websocket_server`, and the `ws` optional dependency extra (`mcp[ws]`) no longer exist. WebSocket was never part of the MCP specification. Use the streamable HTTP transport instead (`mcp.client.streamable_http.streamable_http_client` on the client, `streamable_http_app()` on the server), which supports bidirectional communication with server-to-client streaming over standard HTTP.
 
+### SSE transport deprecated
+
+The HTTP+SSE transport is now deprecated and emits a `DeprecationWarning`. The client transport `mcp.client.sse.sse_client` and the server transport `mcp.server.sse.SseServerTransport` (including the high-level `MCPServer.sse_app()` / `MCPServer.run(transport="sse")`, which build on it) are affected. HTTP+SSE was superseded by Streamable HTTP in protocol revision 2025-03-26 and the specification now documents it only for backwards compatibility. The transport still works and will be removed in a future version; migrate to the streamable HTTP transport (`streamable_http_client` on the client, `streamable_http_app()` on the server).
+
 ### Removed type aliases and classes
 
 The following deprecated type aliases and classes have been removed from `mcp.types`:

pyproject.toml

@@ -210,6 +210,12 @@ filterwarnings = [
     "error",
     # pywin32 internal deprecation warning
     "ignore:getargs.*The 'u' format is deprecated:DeprecationWarning",
+    # The SSE transport is intentionally deprecated (see sse_client / SseServerTransport). It is still
+    # exercised across the suite (directly and via the transport-parametrized interaction tests), so the
+    # SDK's own deprecation notice must not be promoted to an error here. tests/shared/test_sse.py asserts
+    # the warning is emitted.
+    "ignore:The SSE client transport is deprecated:DeprecationWarning",
+    "ignore:The SSE server transport is deprecated:DeprecationWarning",
 ]
 
 [tool.markdown.lint]

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
@@ -39,6 +40,10 @@ async def sse_client(
 ):
     """Client transport for SSE.
 
+    Deprecated: this transport will be removed in a future version. The HTTP+SSE
+    transport was superseded by Streamable HTTP in protocol revision 2025-03-26;
+    use the streamable HTTP transport (`streamable_http_client`) instead.
+
     `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`.
 
@@ -51,6 +56,13 @@ async def sse_client(
         auth: Optional HTTPX authentication handler.
         on_session_created: Optional callback invoked with the session ID when received.
     """
+    warnings.warn(
+        "The SSE client transport is deprecated and will be removed in a future version. The HTTP+SSE transport was"
+        " superseded by Streamable HTTP in protocol revision 2025-03-26; use the streamable HTTP transport"
+        " (`streamable_http_client`) instead.",
+        DeprecationWarning,
+        stacklevel=2,
+    )
     logger.debug(f"Connecting to SSE endpoint: {remove_request_params(url)}")
     async with httpx_client_factory(
         headers=headers, auth=auth, timeout=httpx.Timeout(timeout, read=sse_read_timeout)

src/mcp/server/sse.py

@@ -37,6 +37,7 @@ async def handle_sse(request):
 """
 
 import logging
+import warnings
 from contextlib import asynccontextmanager
 from typing import Any
 from urllib.parse import quote
@@ -70,6 +71,10 @@ class SseServerTransport:
         2. handle_post_message() is an ASGI application which receives incoming POST
            requests, which should contain client messages that link to a
            previously-established SSE session.
+
+    Deprecated: this transport will be removed in a future version. The HTTP+SSE
+    transport was superseded by Streamable HTTP in protocol revision 2025-03-26;
+    use the streamable HTTP transport instead.
     """
 
     _endpoint: str
@@ -100,6 +105,13 @@ def __init__(self, endpoint: str, security_settings: TransportSecuritySettings |
         Raises:
             ValueError: If the endpoint is a full URL instead of a relative path
         """
+        warnings.warn(
+            "The SSE server transport is deprecated and will be removed in a future version. The HTTP+SSE transport"
+            " was superseded by Streamable HTTP in protocol revision 2025-03-26; use the streamable HTTP transport"
+            " instead.",
+            DeprecationWarning,
+            stacklevel=2,
+        )
 
         super().__init__()
 

tests/shared/test_sse.py

@@ -108,6 +108,24 @@ def make_server_app() -> Starlette:
     return make_app(Server(SERVER_NAME, on_read_resource=_handle_read_resource))
 
 
+def test_sse_server_transport_emits_deprecation_warning() -> None:
+    """Constructing SseServerTransport warns that the SSE server transport is deprecated."""
+    with pytest.warns(DeprecationWarning, match="The SSE server transport is deprecated"):
+        SseServerTransport(
+            "/messages/", security_settings=TransportSecuritySettings(enable_dns_rebinding_protection=False)
+        )
+
+
+@pytest.mark.anyio
+async def test_sse_client_emits_deprecation_warning() -> None:
+    """Entering sse_client warns that the SSE client transport is deprecated."""
+    factory = in_process_client_factory(make_server_app())
+    with anyio.fail_after(5):
+        with pytest.warns(DeprecationWarning, match="The SSE client transport is deprecated"):
+            async with sse_client(f"{BASE_URL}/sse", httpx_client_factory=factory):  # pragma: no branch
+                pass
+
+
 @pytest.mark.anyio
 async def test_raw_sse_connection() -> None:
     """The SSE GET responds 200 with an event-stream content type, announcing the session