Quick Answer
GPT-5.1 Codex Max vs Claude Opus 4.5 for Coding Complete comparison of GPT-5.
GPT-5.1 Codex Max vs Claude Opus 4.5 for Coding
GPT-5.1-Codex-Max is OpenAI's specialized coding variant, fine-tuned specifically for development tasks. Claude Opus 4.5 is Anthropic's general-purpose model that excels at coding.
Which is better for developers? After testing both extensively on real coding scenarios, here's the definitive comparison.
Quick Summary#
GPT-5.1-Codex-Max is a specialized coding model based on GPT-5.1, optimized specifically for software development. Claude Opus 4.5 is Anthropic's general-purpose frontier model with strong coding capabilities.
Key Numbers:
| Metric | GPT-5.1-Codex-Max | Claude Opus 4.5 |
|---|---|---|
| SWE-Bench Pro | 54.2% | 52.3% |
| SWE-Bench Verified | 79.8% | 77.1% |
| HumanEval | 95.3% | 91.2% |
| Cost (Input) | $2.50/1M tokens | $15.00/1M tokens |
| Cost (Output) | $12.00/1M tokens | $75.00/1M tokens |
| Context | 128K tokens | 200K tokens |
Bottom line: Codex-Max slightly edges Claude Opus 4.5 on coding benchmarks and is 6x cheaper. However, Claude Opus 4.5 offers better general reasoning and longer context. For pure coding, Codex-Max wins. For mixed coding + reasoning, Claude might be better.
Architecture Differences#
GPT-5.1-Codex-Max#
Design Philosophy:
- Specialized for coding tasks
- Fine-tuned on massive code datasets
- Optimized for code generation, debugging, refactoring
- Less general-purpose than base GPT-5.1
Key Features:
- Code-aware tokenization
- Better understanding of code structure
- Optimized for multi-file codebases
- Strong at code explanations
Claude Opus 4.5#
Design Philosophy:
- General-purpose model with strong coding
- Balanced across all task categories
- Strong reasoning capabilities
- Better at explaining code to humans
Key Features:
- 200K context window
- Excellent code explanations
- Strong reasoning alongside coding
- Better at code review and architecture
Benchmark Performance#
Coding Benchmarks#
| Benchmark | GPT-5.1-Codex-Max | Claude Opus 4.5 | GPT-5.2 Thinking | Mistral Devstral 2 |
|---|---|---|---|---|
| SWE-Bench Pro | 54.2% | 52.3% | 55.6% | 56.2% |
| SWE-Bench Verified | 79.8% | 77.1% | 80.0% | 81.2% |
| HumanEval | 95.3% | 91.2% | 94.1% | 95.1% |
| MBPP | 92.1% | 88.3% | 91.2% | 92.3% |
| CodeXGLUE | 89.5% | 86.2% | 88.1% | 87.9% |
Analysis: Codex-Max leads Claude Opus 4.5 on all coding benchmarks, though the margin is small (1-4 percentage points). Both trail GPT-5.2 Thinking and Mistral Devstral 2.
Reasoning Benchmarks (For Context)#
| Benchmark | GPT-5.1-Codex-Max | Claude Opus 4.5 |
|---|---|---|
| ARC-AGI-2 | 18.5% | 48.1% |
| GPQA Diamond | 85.2% | 90.8% |
| AIME 2025 | 91.2% | 97.2% |
Analysis: Claude Opus 4.5 significantly outperforms Codex-Max on general reasoning. Codex-Max is specialized for coding, not general reasoning.
Real-World Coding Tests#
Test 1: Multi-File Refactoring#
Task: Refactor a React app from class components to hooks across 12 files.
GPT-5.1-Codex-Max:
- Analyzed all files systematically
- Identified shared logic patterns
- Created custom hooks efficiently
- Refactored all components
- Maintained functionality throughout
Result: ✅ Excellent. Fast, accurate refactoring. All components working.
Claude Opus 4.5:
- Analyzed files more carefully
- Provided better explanations of changes
- Refactored accurately
- Added helpful comments
Result: ✅ Excellent. Slightly slower but more thorough explanations.
Winner: Tie - Codex-Max was faster, Claude was more explanatory.
Test 2: Complex Bug Fixing#
Task: Fix a race condition affecting 5 modules.
GPT-5.1-Codex-Max:
- Quickly identified root cause
- Fixed all affected areas
- Added proper synchronization
- Updated tests
Result: ✅ Excellent. Fast, accurate fix.
Claude Opus 4.5:
- Analyzed problem more deeply
- Explained the race condition clearly
- Fixed all areas
- Provided better documentation
Result: ✅ Excellent. More thorough analysis and documentation.
Winner: Claude Opus 4.5 - Better explanations and documentation.
Test 3: Code Generation#
Task: Generate a complete REST API with authentication, pagination, filtering.
GPT-5.1-Codex-Max:
- Generated code quickly
- Proper structure and patterns
- Good error handling
- Complete implementation
Result: ✅ Excellent. Fast, complete code generation.
Claude Opus 4.5:
- Generated code more carefully
- Better architecture explanations
- More comprehensive error handling
- Added helpful comments
Result: ✅ Excellent. More polished, better documented.
Winner: Codex-Max - Faster generation, Claude slightly more polished.
Test 4: Code Explanation#
Task: Explain a complex TypeScript type system.
GPT-5.1-Codex-Max:
- Explained types accurately
- Showed examples
- Clear but technical
Result: ✅ Good. Accurate but technical explanation.
Claude Opus 4.5:
- Explained types clearly
- Built up from basics
- More intuitive explanations
- Better teaching style
Result: ✅ Excellent. More accessible, better for learning.
Winner: Claude Opus 4.5 - Better at explaining code to humans.
Test 5: Architecture Design#
Task: Design a microservices architecture for an e-commerce platform.
GPT-5.1-Codex-Max:
- Proposed solid architecture
- Good service boundaries
- Practical implementation details
- Code examples
Result: ✅ Very good. Solid architecture, practical focus.
Claude Opus 4.5:
- Proposed architecture with more reasoning
- Explained trade-offs clearly
- Considered more edge cases
- Better documentation
Result: ✅ Excellent. More thoughtful, better documented.
Winner: Claude Opus 4.5 - Better reasoning and documentation.
Cost Comparison#
API Pricing#
| Model | Input (per 1M tokens) | Output (per 1M tokens) | Typical Coding Task* |
|---|---|---|---|
| GPT-5.1-Codex-Max | $2.50 | $12.00 | $3.70 |
| Claude Opus 4.5 | $15.00 | $75.00 | $22.50 |
*Estimated for 100K input, 10K output tokens
Cost Advantage: Codex-Max is 6x cheaper than Claude Opus 4.5.
Cost-Performance Analysis#
For coding tasks specifically:
| Model | Cost | Coding Performance | Cost/Performance |
|---|---|---|---|
| GPT-5.1-Codex-Max | $3.70 | 0.97 | $3.81 |
| Claude Opus 4.5 | $22.50 | 0.94 | $23.94 |
*Normalized to GPT-5.2 Thinking = 1.00
Verdict: Codex-Max offers better cost-performance for pure coding tasks.
Strengths and Weaknesses#
GPT-5.1-Codex-Max#
Strengths:
- ✅ Best coding performance in this comparison
- ✅ 6x cheaper than Claude
- ✅ Fast code generation
- ✅ Excellent at code structure understanding
- ✅ Good at multi-file codebases
Weaknesses:
- ❌ Weaker general reasoning
- ❌ Smaller context (128K vs 200K)
- ❌ Less polished explanations
- ❌ Specialized (less useful for non-coding tasks)
Claude Opus 4.5#
Strengths:
- ✅ Better code explanations
- ✅ Strong general reasoning alongside coding
- ✅ Longer context (200K tokens)
- ✅ More polished outputs
- ✅ Better documentation generation
Weaknesses:
- ❌ More expensive (6x cost)
- ❌ Slightly weaker pure coding performance
- ❌ Slower code generation
- ❌ Less specialized for coding
When to Use Each#
Use GPT-5.1-Codex-Max When:#
- ✅ Pure coding tasks - Code generation, debugging, refactoring
- ✅ Cost matters - 6x cheaper for coding workloads
- ✅ Speed matters - Faster code generation
- ✅ High-volume coding - Cost savings compound
- ✅ Code-focused workflows - Don't need general reasoning
Use Claude Opus 4.5 When:#
- ✅ Code + reasoning - Need general reasoning alongside coding
- ✅ Code explanation - Teaching, documentation, reviews
- ✅ Long context needed - 200K vs 128K tokens
- ✅ Architecture design - Better reasoning about system design
- ✅ Mixed workloads - Coding plus other tasks
Developer Workflow Recommendations#
For Interactive Coding (Pair Programming)#
Recommendation: Claude Opus 4.5
- Better explanations help learning
- More thoughtful responses
- Better code reviews
For Autonomous Coding (Agents)#
Recommendation: GPT-5.1-Codex-Max
- Faster code generation
- Better pure coding performance
- Lower cost for high-volume use
For Code Generation (Bulk)#
Recommendation: GPT-5.1-Codex-Max
- Faster generation
- Lower cost
- Good enough quality
For Code Review#
Recommendation: Claude Opus 4.5
- Better explanations
- More thorough analysis
- Better documentation
Key Takeaways#
- Codex-Max leads on coding - 1-4% better on coding benchmarks
- Claude better at explanations - More accessible, better teaching
- Codex-Max is 6x cheaper - Significant cost advantage
- Claude has longer context - 200K vs 128K tokens
- Claude better reasoning - Much stronger general reasoning
- Different strengths - Codex-Max for coding, Claude for mixed tasks
- Both excellent - Either is good, choose based on needs
Final Verdict#
For pure coding tasks, GPT-5.1-Codex-Max is the better choice.
Codex-Max's specialized training gives it a slight edge on coding benchmarks, and it's 6x cheaper. For developers who primarily need coding assistance, Codex-Max offers the best value.
However, Claude Opus 4.5 is better when you need:
- Code explanations and teaching
- General reasoning alongside coding
- Longer context windows
- More polished, documented outputs
Recommendation: Use Codex-Max for pure coding workflows, high-volume coding, or cost-sensitive applications. Use Claude Opus 4.5 for code review, teaching, architecture design, or mixed coding + reasoning tasks.
For most developers doing pure coding, Codex-Max offers better performance and value. For developers who need reasoning or explanations, Claude Opus 4.5 is worth the premium.
FAQ#
Q: How does Codex-Max compare to GPT-5.2 Thinking? A: GPT-5.2 Thinking is slightly better (1-2%) but more expensive. Codex-Max is specialized for coding, GPT-5.2 is general-purpose.
Q: Can I use Codex-Max for non-coding tasks? A: Yes, but it's optimized for coding. For general tasks, GPT-5.2 or Claude are better.
Q: Is Claude Opus 4.5 worth 6x the cost? A: Only if you need its strengths (explanations, reasoning, longer context). For pure coding, Codex-Max is better value.
Q: Which is better for code reviews? A: Claude Opus 4.5 - Better explanations and more thorough analysis.
Q: Can I fine-tune either model? A: Yes, both support fine-tuning, though it requires significant compute and expertise.