How to Use Codex from Claude Code

Codex is an AI-powered CLI tool that can help with complex debugging, code analysis, and technical questions. When you encounter difficult problems that would benefit from a second perspective or deep analysis, use Codex.

When to Use Codex

• •Debugging subtle bugs (e.g., bitstream alignment issues, off-by-one errors)
• •Analyzing complex algorithms against specifications
• •Getting a detailed code review with specific bug identification
• •Understanding obscure file formats or protocols
• •When you've tried multiple approaches and are stuck

The File-Based Pattern

Codex works best with a file-based input/output pattern:

Step 1: Create a Question File

Write your question and all relevant context to /tmp/question.txt:

Write to /tmp/question.txt:
- Clear problem statement
- The specific error or symptom
- The relevant code (full functions, not snippets)
- What you've already tried
- Specific questions you want answered

Example structure:

I have a [component] that fails with [specific error].

Here is the full function:
```c
[paste complete code]

Key observations:

1. •[What works]
2. •[What fails]
3. •[When it fails]

Can you identify:

1. •[Specific question 1]
2. •[Specific question 2]

Please write a detailed analysis to /tmp/reply.txt

### Step 2: Invoke Codex

Use this command pattern:

```bash
cat /tmp/question.txt | codex exec -o /tmp/reply.txt --full-auto

Flags:

• •exec: Non-interactive execution mode (required for CLI use)
• •-o /tmp/reply.txt: Write output to this file
• •--full-auto: Run autonomously without prompts

Step 3: Read the Reply

Read /tmp/reply.txt

Codex will provide detailed analysis. Evaluate its suggestions critically - it may identify real bugs but can occasionally misinterpret specifications.

Example Session

# 1. Create the question
Write /tmp/question.txt with:
- Problem: "Progressive JPEG decoder fails at block 1477 with Huffman error"
- Code: [full AC refinement function]
- Questions: "Identify bugs in EOB handling, ZRL handling, run counting"

# 2. Invoke Codex
cat /tmp/question.txt | codex exec -o /tmp/reply.txt --full-auto

# 3. Read and apply
Read /tmp/reply.txt
# Codex identified 12 potential bugs with detailed explanations
# Evaluate each, verify against spec, apply fixes

Tips

1. •

   Provide complete code: Don't truncate functions. Codex needs full context.

2. •

   Be specific: "Why does this fail?" is worse than "Why does Huffman decoding fail after processing 1477 blocks in AC refinement scan?"

3. •

   Include the spec: If debugging against a standard (JPEG, PNG, etc.), mention the relevant spec sections.

4. •

   Verify suggestions: Codex is helpful but not infallible. In one session, it incorrectly identified the EOB run formula as buggy when it was actually correct. Always verify against authoritative sources.

5. •

   Iterate if needed: If the first response doesn't solve the problem, create a new question.txt with additional context from what you learned.

Common Issues

"stdin is not a terminal": Use codex exec not bare codex

No output: Check that -o flag has a valid path

Timeout: For very complex questions, Codex may take time. The --full-auto flag helps avoid interactive prompts that would block.

Alternative: Direct Piping

For shorter questions:

echo "Explain the JPEG progressive AC refinement algorithm" | codex exec --full-auto

But for debugging, the file-based pattern is better because you can refine the question and keep a record.