GPT-5 Codex Playground for Prompt Testing

A GPT-5 Codex coding playground is a UI to test coding prompts and run quick eval checks on code outputs from GPT-5 Codex—without wiring SDKs, IDE tooling, or API keys.

Prompt testing and evaluations for coding tasks: correctness checks, constraint adherence, refactoring quality, and repeatable regression tests across models.

No. You sign up, but API keys aren’t required. LangFast routes requests through our proxy.

To prevent abuse, apply fair-use limits, and let you save runs, reuse prompt sets, and share results with your team.

Correctness, edge-case handling, constraint compliance, readability, testability, and hallucinations (invented APIs, wrong imports, fake functions).

Use a fixed task set with expected outputs or acceptance criteria, then compare pass rates across runs and across models.

Yes. Save a prompt set and rerun it after edits to detect quality regressions and unintended behavior changes.

Yes—run the same tasks side-by-side to compare accuracy, style, and latency across providers.

Yes. Use eval prompts that enforce a format contract, like “unified diff only” or “JSON only,” and measure violations.

Yes. Evaluate refactor quality (simplicity, performance, readability) and review quality (issues spotted, actionable suggestions).

Yes. Inject snippets, stack traces, requirements, and project conventions to test prompts with production-like context.

Yes—export to cURL/JS/JSON so engineers can reproduce the exact call and parameters.

Yes. Share links for review, and keep a record of which prompt/version produced the output.

LangFast is free to use with some basic features. You need to provide your own API keys to run models and use the app. When you add your API keys, you pay the model provider (e.g., OpenAI) for the credits/tokens you use. LangFast premium features can be unlocked with a one-time purchase.

Wait for the reset or add paid usage to continue running coding evals.

We stream responses through a lightweight proxy. Latency varies by GPT-5 Codex and load; compare first-token time across models directly.

No. We don’t train on your prompts or code. Sharing is opt-in and retention is configurable.

Requests route to model providers. See the Data & Privacy page for processing regions and details.

LangChain is for building coding agents and workflows. LangFast is for testing and evaluating coding prompts before you build automation.

Those tools manage tracing, datasets, and evals inside pipelines. LangFast is an interactive test bench for quick coding prompt iteration and model comparison.