Gemini 3.1 Pro vs Claude Opus 4.6: Same App Build Comparison

Gemini 3.1 Pro just beat Claude Opus 4.6 on one of the hardest AI benchmarks ever created. But benchmarks are one thing. Actually building with these models is something else entirely.

I built the exact same web app with both models, side by side, using the same PRD and the same tech stack. Here's the full breakdown of what happened, which model performed better, and what actually matters when you're vibe coding.

The Setup: Same Everything

I prepared a detailed PRD for a web app based on Simon Sinek's Golden Circle theory. If you haven't heard of it, the basic idea is that the best brands don't lead with what they sell. They start with why they exist, then explain how they do it, and the actual product comes last. Apple is the classic example.

The app lets you enter a business idea and get it evaluated through the Why, How, and What framework. A brand strategy gut-check in under 30 seconds.

The PRD included styling specs, functionality requirements, and model-specific sections for each AI tool. I could paste the exact same document into both Claude Code and Gemini CLI without changing a word.

Split terminal. Ghosty with Claude on the right, Gemini on the left. Empty project folders. Same prompt: "Build me this web app." Go.

Speed: Claude Wins

Claude finished first. Noticeably first. Gemini followed within about a minute. Both were surprisingly fast for generating a complete web application from a multi-page PRD, but Claude had a clear speed advantage.

First Impressions: Gemini Looks Better, Claude Works Better

This is where the comparison got interesting.

Claude's app functioned perfectly on the first try. I entered a test business idea about healthy meal prep, and the Golden Circle analysis ran cleanly. Why, How, What. All there. But the styling was rough. No padding, basic hover effects, minimal visual polish. It looked more like raw HTML with some CSS sprinkled on top.

Gemini's app looked polished right away. Clean layout, nice module cards, proper hover effects, centered content. But when I tested the core feature, it crashed. "Failed to analyze the idea."

The bug? Gemini had referenced the wrong model name in its own API call. A Google model couldn't correctly identify which Google model to use. You'd think that would be the one thing it would get right.

Debugging: Both Fixed Quickly

Both issues got resolved with simple, conversational prompts.

For Claude, I took a screenshot of the rough UI and asked "Does this look right to you?" I also attached a screenshot of Gemini's cleaner output for comparison. Claude identified the styling issues and fixed them in one pass. After the update, the app looked significantly better.

For Gemini, I pasted the full error from the browser. It found the model name mismatch immediately and corrected it. Feature started working right away.

Both fixes took under a minute. No complex debugging sessions. Just normal conversation with screenshots and error messages.

Final Result: Nearly Identical

After one round of iteration each, both apps looked and worked almost the same. Same layout, same functionality, same visual treatment of the Golden Circle framework. I tested light mode, dark mode, multiple business ideas. Both performed well.

The outputs converged because the PRD was specific enough to give both models a clear target. That's the real story here.

The Verdict: Claude Opus 4.6 Gets the Edge

For this comparison, Claude Opus 4.6 wins. It was faster, its core functionality worked out of the box, and styling issues were fixed with a single screenshot prompt. Gemini 3.1 Pro has stronger visual design instincts initially, but a functional bug on launch costs it points.

That said, both models get the job done. The bigger lesson from this test: your PRD matters more than your model choice. When your instructions are specific enough, both models converge on the same output.

4 Takeaways for Your Next Vibe Coding Session

1. Write a PRD, not just a prompt. Include styling specs, functionality requirements, and expected behavior. Watch how much the output quality improves when you get specific.
2. Add model-specific sections. Include separate instruction blocks for each AI tool you use. Paste the same doc into Claude, Gemini, or anything else without rewriting.
3. Don't expect one-shot results. Even with a detailed PRD, both models needed iteration. That's normal. Plan for it.
4. Debug with screenshots. Instead of describing visual issues in text, screenshot the problem and ask the AI "does this look right to you?" It works surprisingly well.

What I Use Daily

I use both models every day. Claude Opus 4.6 for builds where functionality matters most. Gemini 3.1 Pro when I need a quick prototype that looks polished for a demo. The best approach: pick the tool that fits the task and write instructions so clear that either one can nail it.

Watch the Full Side-by-Side Build