GPT 5.5 vs Opus 4.7: Best Reasoning Level for AI Coding

I built the same web app six times to answer a simple question: when you're vibe coding with AI, which reasoning level actually gives you the best result?

The test used GPT-5.5 and Opus 4.7. Each model got the same prompt, and each model built the same sponsor portal at low, medium, and high reasoning or effort.

The surprise was not which model won. The surprise was that medium won twice.

Why I Used a Simple Prompt Instead of a PRD

I intentionally did not write a long product requirements document for this test.

If I gave both tools a detailed spec with exact colors, typography, components, database choices, and layout instructions, I would mostly be testing how well they follow instructions.

That is useful, but it was not the point of this test.

I wanted to know how much judgment each model could bring to the table when the prompt was clear, but not fully specced out.

The app brief was simple: build a sponsor and partner portal for the Clearmud YouTube channel. It needed a landing page, a demo login screen, and a dashboard with a sponsorship rate sheet.

The most important line was this:

Use your judgment on everything not specified.

That line forced the models to make decisions.

What GPT-5.5 Built at Low, Medium, and High Reasoning

Codex low did what I asked it to do. It built the pages, the login flow, and the dashboard. But it did not feel like Clearmud. It looked basic and off-brand.

Codex medium was stronger. The layout felt more polished, the login experience was better, and the dashboard had a more useful direction.

Codex high was interesting because it pulled some assets from the live site. That was exactly the kind of judgment I wanted to see. But even with that extra effort, the final output did not clearly beat medium.

My pick from the GPT-5.5 side was medium reasoning.

What Opus 4.7 Built at Low, Medium, and High Effort

Opus was a little harder to test because the effort setting carried across terminals, so I had to run the versions one at a time.

Opus low surprised me. It felt more designed than the Codex low output and gave the app a stronger first-pass structure.

Opus medium was the clear winner inside the Opus group. The login felt more immersive. The dashboard felt more sponsor-friendly. It started suggesting useful product directions, like bookings, video access, and a stronger sponsor workflow.

Opus high went in a direction I liked less. It had a polished editorial feel, almost like a magazine layout, but it did not feel like the sponsor portal I wanted.

So again, medium won.

The Bonus Round: Brand Matching and Product Direction

After picking the medium outputs from both tools, I gave each one a second prompt.

This time, I asked them to inspect clearmud.ai, analyze the tech stack, colors, light mode, dark mode, typography, and navigation, then rebuild the portal with shadcn components.

Codex did a better job pulling brand signals. It picked up the primary blue, font pairings, logo treatment, and light and dark mode styling.

But the output felt a bit too bubbly for a sponsor portal.

Opus missed some brand pieces, but it built the better product experience. The dashboard had a vertical menu, video access, and an asset upload section. Those are the kinds of details a sponsor or partner would actually care about.

So for the creative product pass, I picked Opus.

The Practical AI Coding Workflow I Would Use

The lesson is not that one model is always better than the other.

The better takeaway is workflow-specific:

• Use medium reasoning when you're exploring product direction.
• Use a simple prompt when you want to see how a model thinks.
• Use a PRD when you care about exact execution.
• Use Opus when you want stronger product direction from messy input.
• Use Codex as the final pass before production.

For anything revenue-generating, client-facing, or tied to user data, Codex is still my final review step. Security audits, codebase reviews, authentication checks, and exploit risk matter too much to skip.

But for early exploration, Opus gave me the sponsor portal direction I wanted to keep building from.

Final Takeaway

Medium reasoning was the sweet spot in this test.

High reasoning did not automatically mean better output. Low reasoning could get the job done, but it often missed the product feel. Medium gave me the best balance of quality, judgment, and direction.

My next step would be to take the Opus medium output, turn it into a proper PRD, specify the brand system, add Convex as the backend, and then send the final codebase through Codex for review.

Vibe first. Spec second. Ship after the safety pass.

Watch the full walkthrough here:

🎥 Full video: https://youtube.com/watch?v=Z1eyIp7jqyo

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