Do I need two API keys?

You need a key for each provider you select. If both sides use the same provider (for example two OpenAI models), one key covers both. If side A is OpenAI and side B is Anthropic, you supply one key for each.

They are used only for the direct browser-to-provider requests and are never stored or sent to us. You pay each provider directly for the calls you make.

Are both models given exactly the same prompt?

Yes. Both sides receive the identical system and user prompt and the same temperature and max-token settings, so the response difference reflects the model, not the inputs.

Why does one side show a latency number?

The tool times each request from send to first complete response so you can weigh speed alongside quality. Latency varies with load, so run a few times before drawing conclusions.

Can one side fail while the other succeeds?

Yes. The two requests are independent, so a bad key, rate limit, or unsupported model on one side shows an error there while the other side still returns its result.

Side-by-Side Model Response Tester (BYO Key)

Side-by-side model response tester

Choosing between models is a lot easier when you can see them answer the same prompt at the same time. This split-screen tool runs one prompt against two models — any mix of OpenAI, Anthropic, and Google — using your own API keys, and shows both responses next to each other along with latency and token usage. It turns model selection from guesswork into a direct, repeatable comparison.

How it works

You configure two sides independently: a provider and model for A and for B, plus the matching API key for each provider. You write a single system and user prompt; both sides receive it identically, with the same temperature and max-token settings, so the only variable is the model. When you run, the tool fires both requests in parallel, times each one, and renders the two responses side by side with their token counts. The two calls are independent — if one side errors on a bad key or rate limit, the other still returns. Nothing is stored; your keys live only in the page until you refresh.

Tips and notes

Hold everything else constant. Same prompt, temperature, and max tokens — that is the whole point of a fair comparison.
Run more than once. Latency fluctuates and higher-temperature outputs vary; a few runs give a truer picture than one.
Compare cost, not just quality. A model that is marginally better but twice the tokens may lose on price at scale — watch the token counts.
Mix providers freely. Pit GPT-4o against Claude or Gemini directly; just supply the key for each provider you pick.