Deprecate the SSE transport

HTTP+SSE was superseded by Streamable HTTP in protocol revision
2025-03-26, and the spec now documents it only for backwards
compatibility. Emit a DeprecationWarning from both SSE transport
primitives — `sse_client` (client) and `SseServerTransport` (server) —
and note the deprecation in their docstrings. The high-level
`MCPServer.sse_app()` / `run(transport="sse")` build on
`SseServerTransport`, so they surface the warning too.

Unlike the WebSocket deprecation, SSE is used internally (the high-level
server app, `ClientSessionGroup`, the `python -m mcp.client` CLI) and
across the transport-parametrized interaction test suite. A
`typing_extensions.deprecated` decorator would therefore force a
`# pyright: ignore[reportDeprecated]` at every internal call site, which
AGENTS.md discourages. A plain `warnings.warn` gives the same runtime
signal with no static-analysis fallout.

`filterwarnings = ["error"]` would otherwise promote the SDK's own
deprecation notice to a test error wherever SSE is exercised, so add two
scoped `ignore` entries (mirroring the existing pywin32 one) and assert
the warnings are emitted in tests/shared/test_sse.py.

Closes #2278

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: agaonker

.serena/.gitignore

@@ -0,0 +1,2 @@
+/cache
+/project.local.yml

.serena/project.yml

@@ -0,0 +1,133 @@
+# the name by which the project can be referenced within Serena
+project_name: "python-sdk"
+
+
+# list of languages for which language servers are started; choose from:
+#   al                  angular             ansible             bash                clojure
+#   cpp                 cpp_ccls            crystal             csharp              csharp_omnisharp
+#   dart                elixir              elm                 erlang              fortran
+#   fsharp              go                  groovy              haskell             haxe
+#   hlsl                html                java                json                julia
+#   kotlin              lean4               lua                 luau                markdown
+#   matlab              msl                 nix                 ocaml               pascal
+#   perl                php                 php_phpactor        powershell          python
+#   python_jedi         python_ty           r                   rego                ruby
+#   ruby_solargraph     rust                scala               scss                solidity
+#   svelte              swift               systemverilog       terraform           toml
+#   typescript          typescript_vts      vue                 yaml                zig
+#   (This list may be outdated. For the current list, see values of Language enum here:
+#   https://github.com/oraios/serena/blob/main/src/solidlsp/ls_config.py
+#   For some languages, there are alternative language servers, e.g. csharp_omnisharp, ruby_solargraph.)
+# Note:
+#   - For C, use cpp
+#   - For JavaScript, use typescript
+#   - For Angular projects, use angular (subsumes typescript+html; requires `npm install` in the project root)
+#   - For Svelte projects, use svelte (subsumes typescript/javascript for .svelte projects; requires npm)
+#   - For SCSS / Sass / plain CSS, use scss (some-sass-language-server handles all three)
+#   - For Free Pascal/Lazarus, use pascal
+# Special requirements:
+#   Some languages require additional setup/installations.
+#   See here for details: https://oraios.github.io/serena/01-about/020_programming-languages.html#language-servers
+# When using multiple languages, the first language server that supports a given file will be used for that file.
+# The first language is the default language and the respective language server will be used as a fallback.
+# Note that when using the JetBrains backend, language servers are not used and this list is correspondingly ignored.
+languages:
+- python
+
+# the encoding used by text files in the project
+# For a list of possible encodings, see https://docs.python.org/3.11/library/codecs.html#standard-encodings
+encoding: "utf-8"
+
+# line ending convention to use when writing source files.
+# Possible values: unset (use global setting), "lf", "crlf", or "native" (platform default)
+# This does not affect Serena's own files (e.g. memories and configuration files), which always use native line endings.
+line_ending:
+
+# The language backend to use for this project.
+# If not set, the global setting from serena_config.yml is used.
+# Valid values: LSP, JetBrains
+# Note: the backend is fixed at startup. If a project with a different backend
+# is activated post-init, an error will be returned.
+language_backend:
+
+# whether to use project's .gitignore files to ignore files
+ignore_all_files_in_gitignore: true
+
+# advanced configuration option allowing to configure language server-specific options.
+# Maps the language key to the options.
+# Have a look at the docstring of the constructors of the LS implementations within solidlsp (e.g., for C# or PHP) to see which options are available.
+# No documentation on options means no options are available.
+ls_specific_settings: {}
+
+# list of additional workspace folder paths for cross-package reference support (e.g. in monorepos).
+# Paths can be absolute or relative to the project root.
+# Each folder is registered as an LSP workspace folder, enabling language servers to discover
+# symbols and references across package boundaries.
+# Currently supported for: TypeScript.
+# Example:
+#   additional_workspace_folders:
+#     - ../sibling-package
+#     - ../shared-lib
+additional_workspace_folders: []
+
+# list of additional paths to ignore in this project.
+# Same syntax as gitignore, so you can use * and **.
+# Note: global ignored_paths from serena_config.yml are also applied additively.
+ignored_paths: []
+
+# whether the project is in read-only mode
+# If set to true, all editing tools will be disabled and attempts to use them will result in an error
+# Added on 2025-04-18
+read_only: false
+
+# list of tool names to exclude.
+# This extends the existing exclusions (e.g. from the global configuration)
+# Find the list of tools here: https://oraios.github.io/serena/01-about/035_tools.html
+excluded_tools: []
+
+# list of tools to include that would otherwise be disabled (particularly optional tools that are disabled by default).
+# This extends the existing inclusions (e.g. from the global configuration).
+# Find the list of tools here: https://oraios.github.io/serena/01-about/035_tools.html
+included_optional_tools: []
+
+# fixed set of tools to use as the base tool set (if non-empty), replacing Serena's default set of tools.
+# This cannot be combined with non-empty excluded_tools or included_optional_tools.
+# Find the list of tools here: https://oraios.github.io/serena/01-about/035_tools.html
+fixed_tools: []
+
+# list of mode names that are to be activated by default, overriding the setting in the global configuration.
+# The full set of modes to be activated is base_modes (from global config) + default_modes + added_modes.
+# If the setting is undefined/empty, the default_modes from the global configuration (serena_config.yml) apply.
+# Otherwise, this overrides the setting from the global configuration (serena_config.yml).
+# Therefore, you can set this to [] if you do not want the default modes defined in the global config to apply
+# for this project.
+# This setting can, in turn, be overridden by CLI parameters (--mode).
+# See https://oraios.github.io/serena/02-usage/050_configuration.html#modes
+default_modes:
+
+# list of mode names to be activated additionally for this project, e.g. ["query-projects"]
+# The full set of modes to be activated is base_modes (from global config) + default_modes + added_modes.
+# See https://oraios.github.io/serena/02-usage/050_configuration.html#modes
+added_modes:
+
+# initial prompt for the project. It will always be given to the LLM upon activating the project
+# (contrary to the memories, which are loaded on demand).
+initial_prompt: ""
+
+# time budget (seconds) per tool call for the retrieval of additional symbol information
+# such as docstrings or parameter information.
+# This overrides the corresponding setting in the global configuration; see the documentation there.
+# If null or missing, use the setting from the global configuration.
+symbol_info_budget:
+
+# list of regex patterns which, when matched, mark a memory entry as read‑only.
+# Extends the list from the global configuration, merging the two lists.
+read_only_memory_patterns: []
+
+# list of regex patterns for memories to completely ignore.
+# Matching memories will not appear in list_memories or activate_project output
+# and cannot be accessed via read_memory or write_memory.
+# To access ignored memory files, use the read_file tool on the raw file path.
+# Extends the list from the global configuration, merging the two lists.
+# Example: ["_archive/.*", "_episodes/.*"]
+ignored_memory_patterns: []

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):
+                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