fix: Pass args as string from client to server

[alex210501]

The arguments are passed a string between the client and the server.

See #649

Motivation and Context

When the first argument started with -- (e.g. --my-arg), the following error was triggered:

~ » npx @modelcontextprotocol/inspector --config ~/mcp.json --server my-server
Starting MCP inspector...
node:internal/util/parse_args/parse_args:87
    throw new ERR_PARSE_ARGS_INVALID_OPTION_VALUE(errorMessage);
          ^

TypeError [ERR_PARSE_ARGS_INVALID_OPTION_VALUE]: Option '--args' argument is ambiguous.
Did you forget to specify the option argument for '--args'?
To specify an option argument starting with a dash use '--args=-XYZ'.
    at checkOptionLikeValue (node:internal/util/parse_args/parse_args:87:11)
    at node:internal/util/parse_args/parse_args:382:9
    at Array.forEach (<anonymous>)
    at parseArgs (node:internal/util/parse_args/parse_args:378:3)
    at file:///Users/alejandro.borbolla/.npm/_npx/5a9d879542beca3a/node_modules/@modelcontextprotocol/inspector/server/build/index.js:25:20
    at ModuleJob.run (node:internal/modules/esm/module_job:370:25)
    at async onImport.tracePromise.__proto__ (node:internal/modules/esm/loader:665:26)
    at async asyncRunEntryPointWithESMLoader (node:internal/modules/run_main:99:5) {
  code: 'ERR_PARSE_ARGS_INVALID_OPTION_VALUE'
}

The reason is that the client passes the following argument to the server:

node <server-path> --command <command> --args <args>

So if the first argument of the MCP server started with --, it gives:

node <server-path> --command mcp-server-command --args --first-arg

This is an invalid syntax. The solution is to send the args as a string, and surrounded by ". Giving the following format:

node <server-path> --command <command> --args "<args>"

The " are simply removed by the server.

How Has This Been Tested?

It has been tested by:

• Sending parameters directly on the command line: npx @modelcontextprotocol/inspector -- myserver first-arg
• Using the config file: npx @modelcontextprotocol/inspector --config mcp.json --server myserver

Breaking Changes

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

[richardkmichael]

👋 Thank you for taking the time to investigate and send this PR!

The approach works, but I don't think it's ideal because:

1. It is "working around" the parser. It's not the quotes per se which permit parsing -- it's that a non-dash character (in this case, a quote) has been added to the beginning of the argument value. An equivalent fix could prefix any non-dash character to the args value, and then remove it; no need even for a terminating character (the closing quote). Sorry if you got that, I just wanted to clarify this because there can be confusion around quotes, shells, ARGV, etc.

2. The error message indicates the fix: use --args=-foo -- the "equal" notation. (This is what one would do, if invoking server directly on the command line.) Doing this "works with" the parser, which is also a simpler change (one file, at the caller; not two files).

I opened #664 for 2.

[alex210501]

Thank you for the explanation @richardkmichael, really appreciate your time.

Should I just close this PR, or do you want me to implement the change to fix #664? Happy to help on this if I can :) (also sorry for the solution, I've struggled a bit to understand how the CLI, client and server were working together, not very use to work in TS)

[richardkmichael]

Thank you for the explanation @richardkmichael, really appreciate your time.

Should I just close this PR, or do you want me to implement the change to fix #664?

I think you can close it. 664 is a change to use --args= notation.

Happy to help on this if I can :) (also sorry for the solution, I've struggled a bit to understand how the CLI, client and server were working together, not very use to work in TS)

Don't worry about the solution. I agree the flow of config through all the components is definitely not obvious. It's quite a tricky aspect to investigate. Nice job, and a good write-up too 👍

The Inspector moves quickly, and there are many small issues and improvements possible. So hopefully your learning can help contribution on another aspect of it.

[alex210501]

So hopefully your learning can help contribution on another aspect of it.

Sure, I'd be very glad to contribute to this project (I'm using it quite extensively 🙂)