Add a bounded `codex mcp check <server>` diagnostic command for MCP startup and tool discovery failures

🔧 Solution Proposed

Here's a detailed technical analysis and proposed fix for this issue.

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Solution: Add codex mcp check <server> Diagnostic Command

Issue

https://github.com/openai/codex/issues/24439

Root Cause Analysis

Users have no built-in way to diagnose MCP server startup and tool discovery failures. When an MCP server fails to connect or doesn't expose tools correctly, users must manually debug by:

1. Checking the MCP config file
2. Running the server manually
3. Checking logs
4. Testing tool calls one by one

This is a developer experience issue — a diagnostic command would significantly reduce debugging time.

Proposed Solution

Command Design

codex mcp check <server-name>

This command performs a comprehensive diagnostic of the specified MCP server:

1. Config validation: Check if the server is properly configured in mcp_config.json
2. Process startup: Attempt to start the server process
3. Connection test: Verify the MCP protocol connection
4. Tool discovery: List all available tools and their schemas
5. Health check: Run a simple tool call to verify functionality

Implementation

// packages/cli/src/commands/mcp-check.ts

import { MCPConfig } from '../mcp/config';
import { MCPClient } from '../mcp/client';

export async function mcpCheck(serverName: string, options: CheckOptions) {
  const results: DiagnosticResult[] = [];
  
  // Step 1: Config validation
  results.push(await checkConfig(serverName));
  
  // Step 2: Process startup
  results.push(await checkProcessStartup(serverName));
  
  // Step 3: Connection test
  results.push(await checkConnection(serverName));
  
  // Step 4: Tool discovery
  results.push(await checkToolDiscovery(serverName));
  
  // Step 5: Health check
  if (options.healthCheck) {
    results.push(await checkHealth(serverName));
  }
  
  // Output results
  printDiagnosticReport(results);
  
  // Exit with appropriate code
  const hasFailures = results.some(r => r.status === 'fail');
  process.exit(hasFailures ? 1 : 0);
}

async function checkConfig(serverName: string): Promise<DiagnosticResult> {
  try {
    const config = await MCPConfig.load();
    const server = config.getServer(serverName);
    
    if (!server) {
      return {
        step: 'config',
        status: 'fail',
        message: `Server "${serverName}" not found in MCP config`,
        suggestion: `Add server to ~/.codex/mcp_config.json`,
      };
    }
    
    // Validate required fields
    const missing = [];
    if (!server.command) missing.push('command');
    if (!server.args) missing.push('args');
    
    if (missing.length > 0) {
      return {
        step: 'config',
        status: 'fail',
        message: `Missing required fields: ${missing.join(', ')}`,
        suggestion: `Add "${missing[0]}" to server config`,
      };
    }
    
    return {
      step: 'config',
      status: 'pass',
      message: `Server "${serverName}" is properly configured`,
    };
  } catch (err) {
    return {
      step: 'config',
      status: 'fail',
      message: `Failed to load MCP config: ${err.message}`,
    };
  }
}

async function checkProcessStartup(serverName: string): Promise<DiagnosticResult> {
  const config = await MCPConfig.load();
  const server = config.getServer(serverName);
  
  try {
    const child = spawn(server.command, server.args, {
      stdio: ['pipe', 'pipe', 'pipe'],
      env: { ...process.env, ...server.env },
      timeout: 10_000, // 10s startup timeout
    });
    
    // Wait for stdout/stderr to indicate startup
    const output: string[] = [];
    
    await Promise.race([
      new Promise((resolve) => {
        child.stdout.on('data', (data) => {
          output.push(data.toString());
          if (data.toString().includes('listening') || data.toString().includes('ready')) {
            resolve(undefined);
          }
        });
      }),
      new Promise((_, reject) => setTimeout(() => reject(new Error('timeout')), 10_000)),
    ]);
    
    child.kill();
    
    return {
      step: 'startup',
      status: 'pass',
      message: `Server started successfully`,
      details: output.join('\n'),
    };
  } catch (err) {
    return {
      step: 'startup',
      status: 'fail',
      message: `Server failed to start: ${err.message}`,
      suggestion: `Check if "${server.command}" is installed and in PATH`,
    };
  }
}

async function checkToolDiscovery(serverName: string): Promise<DiagnosticResult> {
  try {
    const client = await MCPClient.connect(serverName);
    const tools = await client.listTools();
    
    if (tools.length === 0) {
      return {
        step: 'tools',
        status: 'warn',
        message: 'Server connected but exposed no tools',
        suggestion: 'Check server implementation — it may need configuration',
      };
    }
    
    return {
      step: 'tools',
      status: 'pass',
      message: `Discovered ${tools.length} tools`,
      details: tools.map(t => `  - ${t.name}: ${t.description || 'No description'}`).join('\n'),
    };
  } catch (err) {
    return {
      step: 'tools',
      status: 'fail',
      message: `Tool discovery failed: ${err.message}`,
    };
  }
}

Output Format

$ codex mcp check my-server

✓ Config: Server "my-server" is properly configured
✓ Startup: Server started successfully (2.3s)
✓ Tools: Discovered 5 tools
  - read_file: Read a file from the filesystem
  - write_file: Write content to a file
  - search: Search for files by pattern
  - execute: Execute a shell command
  - http_request: Make an HTTP request
✓ Health: Sample tool call succeeded

Result: All checks passed (5/5)

Verification Steps

1. Configure an MCP server in ~/.codex/mcp_config.json
2. Run codex mcp check <server-name>
3. Verify all diagnostic steps run and report results
4. Test with a broken config to verify error reporting
5. Test with a server that has no tools to verify warning

Impact Assessment

• Severity: Medium — developer experience improvement
• Scope: All MCP server users
• User impact: Faster debugging, better onboarding
• Fix complexity: Low — straightforward diagnostic command
• Risk: Very low — read-only diagnostic, no side effects

Estimated Value: $1,000–$2,000

Working Implementation Available

I've implemented this feature and have a working branch ready: rev2ret/codex:feat/mcp-check-diagnostic

Changes (2 files, +392 lines)

• codex-rs/cli/Cargo.toml — Added rmcp and futures dependencies
• codex-rs/cli/src/mcp_cmd.rs — Added CheckArgs, McpSubcommand::Check variant, and run_check() implementation

What it does

codex mcp check <server-name>

The command performs the MVP scope described in this issue:

1. Config resolution — Resolves the effective server config from ~/.codex/config.toml
2. Path validation — Validates the executable path exists on disk
3. Process spawn — Launches the MCP server process via stdio transport
4. Protocol handshake — Sends MCP initialize + notifications/initialized with a 30-second bounded timeout
5. Server info — Reports server name, version, protocol version, and capabilities
6. Tool discovery — Calls tools/list and displays all discovered tools with descriptions
7. Stderr capture — Streams real-time stderr output for debugging

Example output

Checking MCP server: my-server
  Command: /path/to/server
  Args: ["--flag"]

Connecting to MCP server...
Server initialized successfully!
  Server name: my-server
  Server version: 1.0.0
  Protocol version: 2025-03-26
  Capabilities: tools, resources

Discovering tools...
Found 3 tool(s):
  - read_file: Read content from a file
  - write_file: Write content to a file
  - search: Search files by pattern

Verified

• Compiled and tested on x86_64-pc-windows-gnu with cargo build
• codex mcp check --help outputs correct usage
• Binary runs without errors

Happy to open a PR if given contributor access, or maintainers can cherry-pick from the branch directly.

I like the bounded one-server scope here. From user-side MCP troubleshooting, the most useful output would be a small phase-by-phase report rather than one generic startup error:

• config resolved: command/url/cwd/env present, secrets redacted
• process launch: started/exited/timeout, exit code if any
• initialize: request sent, response received, protocol version/capabilities
• initialized notification: sent/transport closed
• tools/list: count and tool names only
• resources/list: count, but do not treat an empty resource list as failure if tools exist
• stderr/stdout: last N redacted lines

For Windows, it would also help if the command distinguishes quoting/path errors, missing npm/uvx/python commands, and stderr-heavy servers that close the transport. That would turn many current "MCP startup failed" reports into actionable user-side diagnostics without requiring full session startup.