ChatGPT 5.4 Pro Extended vs Claude Opus 4.6 Extended Thinking: Which One Should You Use?

Mar 26, 2026

I spent the last month trying to decide between ChatGPT 5.4 Pro Extended and Claude Opus 4.6 Extended Thinking for my deep reasoning workflows. Both promise “extended thinking” but after extensive testing, I found they serve fundamentally different purposes.

Here’s what I learned.

The Problem: Choosing Between Two “Extended” Offerings

I had a complex architectural decision to make: design a multi-agent system for content processing. I needed an AI that could reason through multiple constraints, consider edge cases, and provide thorough analysis.

Both ChatGPT 5.4 Pro Extended and Claude Opus 4.6 Extended Thinking claim to offer extended reasoning capabilities. But after using both extensively, I discovered they’re optimized for completely different workflows.

First Attempt: ChatGPT 5.4 Pro Extended

I started with ChatGPT 5.4 Pro Extended. The promise was compelling: maximum reasoning depth for complex problems.

Query submitted: "Design a multi-agent system for content processing..."
Response time: 23 minutes
Result: Extremely thorough analysis with detailed reasoning chain

The output was impressive. The reasoning chain showed step-by-step thinking through each architectural decision. But there was a problem.

When I needed to iterate on the design, I was stuck. Each follow-up question would require another 10-30 minute wait. The workflow became:

Submit question
Wait 20+ minutes
Review comprehensive response
Submit follow-up
Wait another 20+ minutes

This doesn’t work for iterative development. By the time I got the second response, I had often lost context of what I was thinking.

Second Attempt: Claude Opus 4.6 Extended Thinking

I then tried Claude Opus 4.6 Extended Thinking. The experience was dramatically different.

Query submitted: "Design a multi-agent system for content processing..."
Response time: 1-2 minutes
Result: High-quality reasoning with visible thinking process

The response came in 1-2 minutes instead of 20+ minutes. More importantly, the reasoning quality was about 95% as good as 5.4 Pro Extended. And the writing quality was actually better.

But here’s what made the real difference: iteration.

1. Submit initial question (2 min wait)
2. Review response, identify gaps
3. Submit clarification (2 min wait)
4. Continue conversation naturally
5. Use checkpoint files to extend context

The conversational flow actually works. I could maintain context and iterate rapidly.

The Tool Integration Gap

There’s another critical factor that isn’t obvious at first: tool integration.

I assumed that ChatGPT 5.4 Pro Extended would integrate with Codex for coding workflows. It doesn’t. Codex runs on base 5.4, not Pro mode.

ChatGPT 5.4 Pro Extended: Standalone reasoning mode only
Claude Opus 4.6 Extended: Integrated with tool-use workflows

This means if you’re doing iterative coding sessions, Opus 4.6 Extended Thinking is your only real option for extended reasoning with tool integration.

The Checkpoint File Technique

One technique I discovered with Opus 4.6: checkpoint files extend its limits significantly.

# Current State
- Decided on LangGraph for orchestration
- Using context7 MCP for documentation retrieval
- Need to resolve: rate limiting strategy

# Questions for Next Session
1. How should we handle API rate limits?
2. What's the retry strategy for failed jobs?

By saving state between sessions, I could continue complex reasoning tasks across multiple shorter runs. This approach stretched Opus 4.6’s capabilities further than I initially expected.

When to Choose Each

After extensive testing, here’s my decision framework:

Choose Claude Opus 4.6 Extended Thinking when:

You need iterative back-and-forth reasoning
Writing quality matters (Opus writes better prose)
You want tool integration (MCP servers, code execution)
Response time is a factor (minutes vs tens of minutes)
You’re doing coding workflows

Choose ChatGPT 5.4 Pro Extended when:

You have a single, complex question that needs maximum depth
30-minute wait times are acceptable
You don’t need follow-up iterations
Standalone analysis is sufficient
You’re doing one-off deep research

The 95% Insight

The most important thing I learned: Opus 4.6 Extended Thinking provides about 95% of the reasoning capability of 5.4 Pro Extended, but with dramatically better workflow ergonomics.

That last 5% of reasoning depth rarely matters in practice. But the 20x difference in response time (1-2 min vs 20-30 min) matters enormously for productivity.

Common Mistakes to Avoid

I made these mistakes initially. Learn from them:

Assuming “Extended” means the same thing - They’re fundamentally different paradigms
Using 5.4 Pro Extended for iterative work - The workflow is painful
Ignoring tool integration - You’ll hit walls with 5.4 Pro Extended
Over-valuing raw reasoning depth - Convenience and workflow fit matter more
Not using checkpoint files - You can extend Opus 4.6’s capabilities significantly

Practical Recommendation

For most users doing technical work (coding, architecture, problem-solving), Claude Opus 4.6 Extended Thinking is the better choice. The reasoning quality is nearly as good, the writing is better, and the workflow is vastly superior.

Save ChatGPT 5.4 Pro Extended for those rare occasions when you have a single question that demands maximum depth and you’re willing to wait 30 minutes for the answer.

Final Words + More Resources

My intention with this article was to help others share my knowledge and experience. If you want to contact me, you can contact by email: Email me

Here are also the most important links from this article along with some further resources that will help you in this scope:

Oh, and if you found these resources useful, don’t forget to support me by starring the repo on GitHub!