How It Works

The Codex MCP Tool acts as a bridge between MCP-compatible clients (like Claude Desktop or Claude Code) and the Codex CLI, enabling seamless integration of OpenAI's powerful code analysis and generation capabilities into your development workflow.

Architecture Overview

The system consists of three main components working together:

Core Components

1. MCP Client

The user-facing interface that supports the Model Context Protocol:

• Claude Desktop/Code: Primary clients with full MCP support
• Cursor, Windsurf: IDE integrations
• Custom Clients: Any application implementing MCP protocol

2. Codex MCP Server (This Tool)

The middleware that translates between MCP and Codex CLI:

• Protocol Translation: Converts MCP tool calls to CLI commands
• Process Management: Spawns and manages Codex CLI processes
• Stream Processing: Handles stdout/stderr streaming
• Progress Tracking: Sends periodic updates to prevent timeouts
• Error Handling: Gracefully manages failures and retries

3. Codex CLI

OpenAI's command-line interface for code operations:

• Non-interactive Mode: Runs via codex exec for automation
• File Context: Processes @ references to include files
• Model Selection: Routes to appropriate AI model
• Sandbox Execution: Manages security and approval policies

Request Flow

Step 1: User Invocation

Users can invoke the tool through multiple methods:

// Natural language in Claude
"use codex to analyze @src/main.ts"

// Direct tool call
/codex-cli:ask-codex @README.md summarize this file

// Programmatic invocation
{
  "name": "ask-codex",
  "arguments": {
    "prompt": "review @src/**/*.ts",
    "model": "gpt-5.1-codex-max"
  }
}

Step 2: MCP Protocol Processing

The MCP server receives the standardized tool call:

{
  "jsonrpc": "2.0",
  "method": "tools/call",
  "params": {
    "name": "ask-codex",
    "arguments": {
      "prompt": "analyze @src/utils",
      "model": "gpt-5.1-codex-max",
      "fullAuto": true
    }
  }
}

Step 3: CLI Command Construction

The server builds the appropriate Codex CLI command:

codex exec \
  --model gpt-5.1-codex-max \
  --full-auto \
  "analyze @src/utils"

Step 4: Process Execution

The server spawns a child process with:

• Environment variables from current context
• Working directory configuration
• Stream handlers for stdout/stderr
• Timeout management

Step 5: Response Streaming

Real-time output is captured and forwarded:

// Progress notifications every 25 seconds
{
  "progress": 50,
  "total": 100,
  "status": "Analyzing files..."
}

// Final result
{
  "content": [
    {
      "type": "text",
      "text": "Analysis complete: [detailed results]"
    }
  ]
}

Advanced Features

Progress Notifications

To prevent timeouts during long-running operations:

class TimeoutManager {
  startOperation() {
    this.interval = setInterval(() => {
      sendProgress({
        status: 'Processing...',
        elapsed: Date.now() - this.startTime,
      });
    }, 25000); // Every 25 seconds
  }
}

Change Mode (Structured Edits)

For code modifications, the server can request structured output:

1. Request with changeMode:

{
  "prompt": "update all console.log to use winston",
  "changeMode": true
}

2. Parse OLD/NEW blocks:

=== FILE: src/utils.js ===
OLD:
console.log('Debug message');
NEW:
winston.debug('Debug message');

3. Chunk large responses:

• Split into manageable chunks
• Cache with unique key
• Return first chunk immediately
• Fetch remaining via fetch-chunk tool

Error Handling

The server implements robust error handling:

try {
  const result = await executeCodexCLI(prompt, options);
  return processResult(result);
} catch (error) {
  if (error.code === 'TIMEOUT') {
    return handleTimeout(error);
  } else if (error.code === 'INVALID_MODEL') {
    return suggestAlternativeModel(error);
  } else {
    return formatErrorResponse(error);
  }
}

Configuration Management

Environment Variables

# Model defaults
CODEX_DEFAULT_MODEL=gpt-5

# Timeout settings
MCP_OPERATION_TIMEOUT=600000  # 10 minutes

# Debug mode
DEBUG=codex-mcp:*

Codex CLI Flags

The server supports all Codex CLI flags:

Flag	MCP Parameter	Description
--model	model	AI model selection
--full-auto	fullAuto or sandbox	Automatic execution mode
--ask-for-approval	approvalPolicy	Approval requirements
--sandbox	sandboxMode	Filesystem access level
--cd	cd	Working directory
--dangerously-bypass-approvals-and-sandbox	yolo	Bypass all safety checks

Performance Optimization

Streaming for Large Output

Instead of buffering entire responses:

const proc = spawn('codex', args);
proc.stdout.on('data', chunk => {
  // Stream directly to client
  onProgress?.(chunk.toString());
});

Caching for Repeated Queries

The server implements smart caching:

• File content caching (15-minute TTL)
• Model response caching for identical prompts
• Chunk caching for pagination

Parallel Tool Execution

Multiple tools can run concurrently:

await Promise.all([
  askCodex({ prompt: 'analyze @src' }),
  brainstorm({ prompt: 'optimization ideas' }),
  ping({ message: 'test' }),
]);

Security Considerations

Sandbox Isolation

All file operations respect Codex's sandbox settings:

• read-only: No filesystem writes
• workspace-write: Write only in working directory
• danger-full-access: Full filesystem access (use with caution)

Input Validation

All inputs are validated using Zod schemas:

const schema = z.object({
  prompt: z.string().min(1),
  model: z.enum(['gpt-5.1-codex-max', 'gpt-5.1-codex', 'gpt-5.1-codex-mini', 'gpt-5.1']).optional(),
  sandbox: z.boolean().optional(),
});

Process Isolation

Each request runs in an isolated child process:

• No shared state between requests
• Clean environment for each execution
• Automatic cleanup on completion

Troubleshooting

Common Issues

Timeout Errors

• Increase timeout: Set timeout parameter
• Break into smaller tasks
• Use progress callbacks

Model Not Available

• Check Codex CLI configuration
• Verify API credentials
• Try alternative model

File Not Found

• Verify working directory
• Check relative paths
• Use absolute paths when needed

Debug Mode

Enable detailed logging:

DEBUG=codex-mcp:* npm start

Health Checks

Test connectivity:

// Ping test
{ "name": "ping", "arguments": { "message": "test" } }

// Help command
{ "name": "help", "arguments": {} }

Integration Examples

With Claude Code

# One-time setup
claude mcp add codex-cli -- npx -y @trishchuk/codex-mcp-tool

# Usage
claude
> use codex to analyze my project

With Custom MCP Client

const client = new MCPClient({
  servers: {
    'codex-cli': {
      command: 'npx',
      args: ['-y', '@trishchuk/codex-mcp-tool'],
    },
  },
});

await client.callTool('ask-codex', {
  prompt: 'analyze @src',
  model: 'gpt-5.1-codex-max',
});

See Also

• File Analysis - Using the @ syntax effectively
• Model Selection - Choosing the right AI model
• Sandbox Modes - Security and approval configurations
• Codex CLI Documentation - Underlying CLI reference