feat: Allow passing through all environment variables to server (sparfenyuk#27)

This PR adds the ability to pass through all environment variables to
the server, using the flag `--pass-environment` (default off).

This change allows a compose spec like this to access all environment
variables from an env_file (as well as PATH):

```yaml
server-brave-search:
  image: mcp-proxy:latest
  env_file: .brave.env
  # ENTRYPOINT from custom SSE-only image, for context
  entrypoint: ["mcp-proxy", "--sse-host=0.0.0.0", "--sse-port=8080", "--pass-environment", "--"]
  command: ["npx", "--yes", "@modelcontextprotocol/server-brave-search"]
```

The motivation behind this is I am developing in Docker Compose, with a
custom client and multiple servers. I made a debian-based mcp-proxy
image, with an entrypoint that set some SSE settings. When it came to
adding brave search, I spent a bit of time struggling against var
interpolation and an overridden entrypoint (cause: you can't access env
vars in entrypoints, I think). I figured passing through all environment
variables would be easier (and safe in a container).

I started adding tests for this, but found myself refactoring
\_\_main\_\_.py quite a bit to make it more testable. I've left that for
now in the aim of an easy-to-review PR, but am happy to continue that
effort if you'd like? Essentially breaking out the argument parser and
the SSE client/server parts into functions and validating their config.

Thanks again for this tool - it's made adding support for stdio servers
in our client much easier!

---------

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

Author: rmasters

README.md

@@ -104,12 +104,13 @@ 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              |
+| 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 |
 
 ### 2.2 Example usage
 
@@ -181,7 +182,7 @@ 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] [command_or_url] [args ...]
+usage: mcp-proxy [-h] [-H KEY VALUE] [-e KEY VALUE] [--sse-port SSE_PORT] [--sse-host SSE_HOST] [--pass-environment] [command_or_url] [args ...]
 
 Start the MCP proxy in one of two possible modes: as an SSE or stdio client.
 
@@ -199,6 +200,8 @@ stdio client options:
   args                  Any extra arguments to the command to spawn the server
   -e KEY VALUE, --env KEY VALUE
                         Environment variables used when spawning the server. Can be used multiple times.
+  --pass-environment, --no-pass-environment
+                        Pass through all environment variables when spawning the server.
 
 SSE server options:
   --sse-port SSE_PORT   Port to expose an SSE server on. Default is a random port

src/mcp_proxy/__main__.py

@@ -77,6 +77,12 @@ def main() -> None:
         help="Environment variables used when spawning the server. Can be used multiple times.",
         default=[],
     )
+    stdio_client_options.add_argument(
+        "--pass-environment",
+        action=argparse.BooleanOptionalAction,
+        help="Pass through all environment variables when spawning the server.",
+        default=False,
+    )
 
     sse_server_group = parser.add_argument_group("SSE server options")
     sse_server_group.add_argument(
@@ -112,10 +118,19 @@ def main() -> None:
 
     # Start a client connected to the given command, and expose as an SSE server
     logging.debug("Starting stdio client and SSE server")
+
+    # The environment variables passed to the server process
+    env: dict[str, str] = {}
+    # Pass through current environment variables if configured
+    if args.pass_environment:
+        env.update(os.environ)
+    # Pass in and override any environment variables with those passed on the command line
+    env.update(dict(args.env))
+
     stdio_params = StdioServerParameters(
         command=args.command_or_url,
         args=args.args,
-        env=dict(args.env),
+        env=env,
     )
     sse_settings = SseServerSettings(
         bind_host=args.sse_host,