GPT-5 Chat Playground for Prompt Testing

A GPT-5 Chat chat playground is a UI to test multi-turn conversations on GPT-5 Chat—system prompts, user turns, and conversation memory—plus quick eval-style checks for consistency.

Prompt testing and evaluations for chat flows: instruction-following, tone control, tool-like behavior, and “does this break on turn 3?” regressions.

Yes. Bring your API keys. LangFast routes requests through our proxy.

To prevent abuse, apply fair-use limits, and let you save transcripts, rerun scripts, and share results with your team.

System prompt adherence, tone consistency, refusal behavior, safety boundaries, multi-turn instruction following, and formatting compliance (e.g., JSON or markdown).

Create a repeatable conversation script (same turns every time) and rerun it after prompt changes or model changes to spot regressions.

Yes. Swap system prompts while keeping the same conversation script, then compare results side-by-side.

Yes—run the same chat script across models/providers and pick the best behavior per cost and latency.

Yes. Use eval prompts that enforce schemas or formatting rules and measure how often the model drifts.

Yes. Inject real inputs (customer profile, ticket text, product specs) into the chat to test production-like scenarios.

Yes. Save transcripts, replay the same chat later, and share links for review or approval.

Yes—export to cURL/JS/JSON so engineering can reproduce the exact conversation setup.

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 keep running chat evaluations.

We stream responses through a lightweight proxy. Latency depends on GPT-5 Chat and load; you can compare first-token time across models.

It depends on GPT-5 Chat. We show the context window and key limits in the model picker.

No. We don’t train on your prompts or chats. 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 chat apps and agents. LangFast is for testing chat prompts and evaluating behavior before you build.

Those tools focus on tracing, datasets, and evals inside workflows. LangFast is an interactive chat test bench for fast prompt iteration and comparisons.