Inspector CLI fails with `ERR_PARSE_ARGS_INVALID_OPTION_VALUE` due to unexpected --env

[HaiyiMei]

Describe the bug
Running the @modelcontextprotocol/inspector in CLI mode causes a crash due to an ambiguous --env argument, even though --env is not explicitly passed.

To Reproduce
Steps to reproduce the behavior:

1. Run the following command:

   npx @modelcontextprotocol/inspector --cli node build/index.js --method tools/list

Expected behavior
The inspector should run successfully and list the tools, without requiring or referencing the --env argument unless explicitly provided.

Logs

> npx @modelcontextprotocol/inspector --cli node build/index.js --method tools/list
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 '--env' argument is ambiguous.
Did you forget to specify the option argument for '--env'?
To specify an option argument starting with a dash use '--env=-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:///workspace/node_modules/@modelcontextprotocol/inspector/server/build/index.js:16:20 {
  code: 'ERR_PARSE_ARGS_INVALID_OPTION_VALUE'
}

Node.js v22.14.0

Additional context

• No --env option was manually specified in the CLI.
• Environment: Node.js v22.14.0
• This may be related to internal parseArgs() behavior incorrectly detecting an argument that wasn't passed.

[asionius]

How to suppress the error and keep moving temporarily?

[HaiyiMei]

As a follow-up, I noticed that the package published to npm appears to differ from the source code in this repository:
https://registry.npmjs.org/@modelcontextprotocol/inspector

For example, the published package does not seem to include the inspector-cli, nor is it mentioned in the description.

[JHSeo-git]

wrong...

It seems that the env key=value configuration is not being passed correctly.
Modifying the installed bin/cli.js as shown below allows it to work as expected.

# node_modules/@modelcontextprotocol/inspector/bin/cli.js:78:93
server = spawnPromise(
      "node",
      [
        inspectorServerPath,
-        ...(command ? [`--env`, command] : []),
+        ...(command ? [`--env`, `command=${command}`] : []),
        ...(mcpServerArgs ? [`--args=${mcpServerArgs.join(" ")}`] : []),
      ],
      {
        env: {
          ...process.env,
          PORT: SERVER_PORT,
          MCP_ENV_VARS: JSON.stringify(envVars),
        },
        signal: abort.signal,
        echoOutput: true,
      },
    );

This change should also be reflected in the code.

• inspector/cli/src/cli.ts

  Line 90 in cd28370

  ...(args.command ? [`--env`, args.command] : []),

maybe

 ...(args.command ? [`--env`, `command=${args.command}`] : []), 

but, only the server starts, and --cli --method tools/call does not execute as expected.

Based on the current code, after cloning and building locally, the CLI works as expected.
It seems that the changes have not yet been deployed or released.
The response below only addressed the program error, but the actual functionality is still missing.

[HaiyiMei]

@JHSeo-git Thanks for the reply. The error is indeed resolved, but the behavior still doesn’t match the expected outcome. Instead of running in CLI mode, it seems to inject the arguments on the website.

[JHSeo-git]

@HaiyiMei There are no issues when cloning the current version of the repository, building it, and running it.

To run npx @modelcontextprotocol/inspector --cli http://localhost:8181 --method=tools/list, it seems we'll need to wait for the current code to be deployed.

[HaiyiMei]

@JHSeo-git You are absolutely right, this PR is merged two days ago, and it is not released yet #177

[JHSeo-git]

@HaiyiMei It looks like the inspector has been deployed v0.10.x

we can call npx -y @modelcontextprotocol/inspector --cli http://localhost:8181 --method tools/call --tool-name echo --tool-arg message=123

[olaservo]

Confirming that as of 10.0.2 you should be able to use the CLI mode from the npm/npx installed version.

[HaiyiMei]

@JHSeo-git @olaservo Amazing, thanks guys!