GPT-4o Mini playground for Prompt Testing
Run GPT-4o mini for cheaper, faster iterations. Compare quality vs full models and pick the right balance.
Bring your API keys. Pay once, use forever.
Best GPT-4o Mini Playground
Fast iteration
Use Mini for quick loops and cheaper testing.
Compare outputs
See if Mini meets your quality bar.
Test with variables
Repeatable runs with real inputs.
Share & replay
Links, transcripts, export.
Private by default
We don’t train on your prompts and data.
Instant access
Bring your API keys. Start testing immediately.
Why Us over other LLM Playgrounds
Other playgroundsFrom VC-baked companies
GPT-4o Mini playgroundPowered byLangFast
Explore All Features
Supported AI Models
- GPT-5
- GPT-5 Mini
- GPT-5 Nano
- GPT-5 Nano
- GPT-4.1
- GPT-4.1 Mini
- GPT-4.1 Nano
- GPT-4o
- GPT-4o Mini
- O1
- O1 Mini
- O3
- O3 Mini
- O4 Mini
- GPT-4 Turbo
- GPT-3.5 Turbo
- Claude AI Models (soon)
- Gemini AI Models (soon)
- Model Fine-tuning (soon)
Model configuration
- Custom System Instructions
- Reasoning Effort Control
- Stream Response Control
- Temperature Control
- Presence & Frequency Penalty
User Interface
- Customizable Workspace
- Wide Screen Support
- Hotkey & Shortcuts
- Voice Input (soon)
- Text-to-Speech (soon)
Playground Experience
- Prompt Library
- Prompt Templates & Variables
- Jinja2 Templates Support
- Upload Documents (soon)
- Language Output Control
- Parallel Chat Support
Prompt Management
- Prompt Folders
- Edit & Fork Prompts
- Prompt Versioning
- Upload Documents (soon)
- Share Prompts
Cost & Performance
- Cost estimation
- Token usage tracking
- Context length indicator
- Max token settings
Security and Privacy
- Private by Default
- API Tokens Cost Estimation
- No chats used for training
- Web Search & Live Data (soon)
Integrations
Plugins
- Custom Plugins (soon)
- Image search plugin (soon)
- Dall-E 3 (soon)
- Web page reader (soon)
Meet LangFast users
LangFast empowers hundreds of people to test and iterate on their prompts faster.
Frequently Asked Questions
What is a GPT-4o Mini playground?
A GPT-4o Mini playground is a browser UI to test prompts and run quick evals on GPT-4o Mini—typically used when you want speed, low latency, or cheap iteration.
What is this page optimized for?
Fast prompt iteration: short cycles, lots of runs, lightweight checks for consistency and formatting, and quick comparisons against bigger models.
Do I need an API key?
Yes. Bring your API keys. LangFast routes requests through our proxy.
Why require signup if it’s “fast”?
To keep latency stable (rate limiting + abuse prevention) and to let you save runs, compare results, and share them with your team.
What should I evaluate with speed-first models?
Whether the model is “good enough” on your tasks: format adherence, factual discipline (where relevant), and how often it fails on edge cases.
How do I test the speed vs quality trade-off?
Run the same prompt set on GPT-4o Mini and a larger model, then compare pass rates and output quality against your rubric.
Can I run regression tests?
Yes. Save a prompt set and rerun it after prompt edits or model changes to catch quality drift quickly.
Can I test strict formatting (JSON, tables, templates)?
Yes. Speed-first models are often where formatting breaks first—use eval prompts that enforce your output contract.
Can I use variables / templates for realistic inputs?
Yes. Inject real data (support tickets, product specs, CRM fields) to test prompts the way they’ll be used in production.
Can I compare GPT-4o Mini with other fast models?
Yes—side-by-side comparisons across providers make it easy to find the best cheap+fast option.
Is it free?
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.
What happens when I hit the free cap?
Wait for reset or add paid usage to continue running high-volume tests.
How fast is it in practice?
We stream responses via a lightweight proxy. Actual speed varies by model and load; use comparisons to measure first-token time and total latency.
What’s the context window?
It depends on GPT-4o Mini. We show the context window and key limits in the model picker.
Can I export calls for engineering?
Yes—export to cURL/JS/JSON so engineers can reproduce the exact request and parameters.
Can I share results?
Yes. Share links for review and keep a record of the prompt version that produced the result.
Do you train on my prompts?
No. We don’t train on your prompts or data. Sharing is opt-in and retention is configurable.
Where is my data processed?
Requests route to model providers. See the Data & Privacy page for regions and processing details.
How does this compare to LangChain?
LangChain is for building and orchestrating apps. LangFast is for rapid prompt testing and evals before you build pipelines.
How does this compare to Langfuse, Basalt, or PromptLayer?
Those tools manage tracing, datasets, and evals inside workflows. LangFast is the fastest interactive test bench—ideal before you invest in tooling.