feat: expose CORS configuration (sparfenyuk#31)

Allow specifying CORS configuration so that a client can call the proxy from a different host/port.

Co-authored-by: Fedir Zadniprovskyi <[email protected]>
Co-authored-by: Sergey Parfenyuk <[email protected]>

Author: 3 people

README.md

@@ -51,10 +51,10 @@ This mode requires passing the URL to the MCP Server SSE endpoint as the first a
 
 Arguments
 
-| Name                 | Required | Description                                                           | Example                                       |
-| -------------------- | -------- | --------------------------------------------------------------------- | ----------------------------------------------|
-| `command_or_url`     | Yes      | The MCP server SSE endpoint to connect to                             | http://example.io/sse                         |
-| `--headers`          | No       | Headers to use for the MCP server SSE connection                      | Authorization 'Bearer my-secret-access-token' |
+| Name             | Required | Description                                      | Example                                       |
+| ---------------- | -------- | ------------------------------------------------ | --------------------------------------------- |
+| `command_or_url` | Yes      | The MCP server SSE endpoint to connect to        | http://example.io/sse                         |
+| `--headers`      | No       | Headers to use for the MCP server SSE connection | Authorization 'Bearer my-secret-access-token' |
 
 Environment Variables
 
@@ -104,13 +104,14 @@ This mode requires the `--sse-port` argument to be set. The `--sse-host` argumen
 
 Arguments
 
-| Name                 | Required                   | Description                                                      | Example              |
-| -------------------- | -------------------------- | ---------------------------------------------------------------- | -------------------- |
-| `command_or_url`     | Yes                        | The command to spawn the MCP stdio server                        | uvx mcp-server-fetch |
-| `--sse-port`         | No, random available       | The SSE server port to listen on                                 | 8080                 |
-| `--sse-host`         | No, `127.0.0.1` by default | The host IP address that the SSE server will listen on           | 0.0.0.0              |
-| `--env`              | No                         | Additional environment variables to pass to the MCP stdio server | FOO=BAR              |
-| `--pass-environment` | No                         | Pass through all  environment variables when spawning the server | --no-pass-environment |
+| Name                 | Required                   | Description                                                      | Example               |
+| -------------------- | -------------------------- | ---------------------------------------------------------------- | --------------------- |
+| `command_or_url`     | Yes                        | The command to spawn the MCP stdio server                        | uvx mcp-server-fetch  |
+| `--sse-port`         | No, random available       | The SSE server port to listen on                                 | 8080                  |
+| `--sse-host`         | No, `127.0.0.1` by default | The host IP address that the SSE server will listen on           | 0.0.0.0               |
+| `--env`              | No                         | Additional environment variables to pass to the MCP stdio server | FOO=BAR               |
+| `--pass-environment` | No                         | Pass through all environment variables when spawning the server  | --no-pass-environment |
+| `--allow-origin`     | No                         | Pass through all environment variables when spawning the server  | --allow-cors "\*"     |
 
 ### 2.2 Example usage
 
@@ -182,7 +183,9 @@ docker run -t ghcr.io/sparfenyuk/mcp-proxy:v0.3.2-alpine --help
 ## Command line arguments
 
 ```bash
-usage: mcp-proxy [-h] [-H KEY VALUE] [-e KEY VALUE] [--sse-port SSE_PORT] [--sse-host SSE_HOST] [--pass-environment] [command_or_url] [args ...]
+usage: mcp-proxy [-h] [-H KEY VALUE] [-e KEY VALUE] [--pass-environment | --no-pass-environment] [--sse-port SSE_PORT] [--sse-host SSE_HOST]
+                 [--allow-origin ALLOW_ORIGIN [ALLOW_ORIGIN ...]]
+                 [command_or_url] [args ...]
 
 Start the MCP proxy in one of two possible modes: as an SSE or stdio client.
 
@@ -206,12 +209,15 @@ stdio client options:
 SSE server options:
   --sse-port SSE_PORT   Port to expose an SSE server on. Default is a random port
   --sse-host SSE_HOST   Host to expose an SSE server on. Default is 127.0.0.1
+  --allow-origin ALLOW_ORIGIN [ALLOW_ORIGIN ...]
+                        Allowed origins for the SSE server. Can be used multiple times. Default is no CORS allowed.
 
 Examples:
   mcp-proxy http://localhost:8080/sse
   mcp-proxy --headers Authorization 'Bearer YOUR_TOKEN' http://localhost:8080/sse
   mcp-proxy --sse-port 8080 -- your-command --arg1 value1 --arg2 value2
   mcp-proxy your-command --sse-port 8080 -e KEY VALUE -e ANOTHER_KEY ANOTHER_VALUE
+  mcp-proxy your-command --sse-port 8080 --allow-origin='*'
 ```
 
 ## Testing

codecov.yml

@@ -0,0 +1,29 @@
+codecov:
+  require_ci_to_pass: true
+
+coverage:
+  precision: 2
+  round: down
+  range: "70...100"
+
+  status:
+    project:
+      default:
+        # Maintain overall project coverage
+        target: auto
+        threshold: 1%
+
+    patch:
+      default:
+        # Require 80% coverage on new/modified code
+        target: 80%
+        threshold: 5%
+
+    changes: false
+
+comment:
+  layout: "reach,diff,flags,files,footer"
+  behavior: default
+  require_changes: false
+  require_base: false
+  require_head: true

src/mcp_proxy/__main__.py

@@ -37,6 +37,7 @@ def main() -> None:
             "  mcp-proxy --headers Authorization 'Bearer YOUR_TOKEN' http://localhost:8080/sse\n"
             "  mcp-proxy --sse-port 8080 -- your-command --arg1 value1 --arg2 value2\n"
             "  mcp-proxy your-command --sse-port 8080 -e KEY VALUE -e ANOTHER_KEY ANOTHER_VALUE\n"
+            "  mcp-proxy your-command --sse-port 8080 --allow-origin='*'\n"
         ),
         formatter_class=argparse.RawTextHelpFormatter,
     )
@@ -96,6 +97,12 @@ def main() -> None:
         default="127.0.0.1",
         help="Host to expose an SSE server on. Default is 127.0.0.1",
     )
+    sse_server_group.add_argument(
+        "--allow-origin",
+        nargs="+",
+        default=[],
+        help="Allowed origins for the SSE server. Can be used multiple times. Default is no CORS allowed.",  # noqa: E501
+    )
 
     args = parser.parse_args()
 
@@ -135,6 +142,7 @@ def main() -> None:
     sse_settings = SseServerSettings(
         bind_host=args.sse_host,
         port=args.sse_port,
+        allow_origins=args.allow_origin if len(args.allow_origin) > 0 else None,
     )
     asyncio.run(run_sse_server(stdio_params, sse_settings))
 

src/mcp_proxy/sse_server.py

@@ -9,6 +9,8 @@
 from mcp.server import Server
 from mcp.server.sse import SseServerTransport
 from starlette.applications import Starlette
+from starlette.middleware import Middleware
+from starlette.middleware.cors import CORSMiddleware
 from starlette.requests import Request
 from starlette.routing import Mount, Route
 
@@ -21,10 +23,16 @@ class SseServerSettings:
 
     bind_host: str
     port: int
+    allow_origins: list[str] | None = None
     log_level: Literal["DEBUG", "INFO", "WARNING", "ERROR", "CRITICAL"] = "INFO"
 
 
-def create_starlette_app(mcp_server: Server, *, debug: bool = False) -> Starlette:
+def create_starlette_app(
+    mcp_server: Server,
+    *,
+    allow_origins: list[str] | None = None,
+    debug: bool = False,
+) -> Starlette:
     """Create a Starlette application that can server the provied mcp server with SSE."""
     sse = SseServerTransport("/messages/")
 
@@ -40,8 +48,20 @@ async def handle_sse(request: Request) -> None:
                 mcp_server.create_initialization_options(),
             )
 
+    middleware: list[Middleware] = []
+    if allow_origins is not None:
+        middleware.append(
+            Middleware(
+                CORSMiddleware,
+                allow_origins=allow_origins,
+                allow_methods=["*"],
+                allow_headers=["*"],
+            ),
+        )
+
     return Starlette(
         debug=debug,
+        middleware=middleware,
         routes=[
             Route("/sse", endpoint=handle_sse),
             Mount("/messages/", app=sse.handle_post_message),
@@ -64,7 +84,11 @@ async def run_sse_server(
         mcp_server = await create_proxy_server(session)
 
         # Bind SSE request handling to MCP server
-        starlette_app = create_starlette_app(mcp_server, debug=(sse_settings.log_level == "DEBUG"))
+        starlette_app = create_starlette_app(
+            mcp_server,
+            allow_origins=sse_settings.allow_origins,
+            debug=(sse_settings.log_level == "DEBUG"),
+        )
 
         # Configure HTTP server
         config = uvicorn.Config(

tests/test_sse_server.py

@@ -27,8 +27,8 @@ async def run_in_background(self) -> None:
                 await asyncio.sleep(1e-3)
             yield
         finally:
-            task.cancel()
-            self.shutdown()
+            self.should_exit = self.force_exit = True
+            await task
 
     @property
     def url(self) -> str:
@@ -47,7 +47,7 @@ async def test_create_starlette_app() -> None:
     async def list_prompts() -> list[types.Prompt]:
         return [types.Prompt(name="prompt1")]
 
-    app = create_starlette_app(server)
+    app = create_starlette_app(server, allow_origins=["*"])
 
     config = uvicorn.Config(app, port=0, log_level="info")
     server = BackgroundServer(config)