GPT-5.1 Codex Max Playground for Prompt Testing
Stress-test GPT-5.1 Codex Max for complex coding. Compare correctness, reasoning depth, and runtime cost.
Bring your API keys. Pay once, use forever.
Best GPT-5.1 Codex Max Playground
Stress-test hard tasks
Use Max on the toughest coding, refactors, and heavy changes.
Run eval-style checks
Validate format, constraints, and edge-case behavior.
Compare outputs
Max vs cheaper models—prove the uplift before paying for it.
Share & export
Links, transcripts, and cURL/JS export.
Private by default
We don’t train on your code, prompts, or data.
Instant access
Bring your API keys. Start testing immediately.
Why Us over other LLM Playgrounds
Other playgroundsFrom VC-baked companies
GPT-5.1 Codex Max 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-5.1 Codex Max playground?
A GPT-5.1 Codex Max playground is a browser UI for prompt testing and evals on GPT-5.1 Codex Max—typically used when you care most about output quality, reliability, or hard edge cases.
What is this page optimized for?
Evaluating whether GPT-5.1 Codex Max delivers enough quality uplift to justify higher cost/latency—using repeatable prompt sets and side-by-side comparisons.
Do I need an API key?
Yes. Bring your API keys. LangFast routes requests through our proxy.
Why is signup required?
It keeps the system abuse-resistant and lets you save runs, manage retention, and share results cleanly with your team.
How do I determine if GPT-5.1 Codex Max is “worth it”?
Test your hardest prompts (edge cases, strict formatting, nuanced reasoning) against cheaper alternatives. If GPT-5.1 Codex Max consistently passes where others fail, it’s worth paying for.
What types of evals should I run for premium models?
Regression tests, rubric scoring, consistency checks, instruction-following tests, and “must-pass” prompts that represent real production risk.
Can I compare GPT-5.1 Codex Max against cheaper models?
Yes. Run the same prompt set side-by-side to quantify quality uplift versus cost and latency.
Can I evaluate consistency and stability?
Yes. Repeat runs on the same prompt set to see variance, formatting drift, and failure modes.
Can I test structured outputs like JSON or schemas?
Yes. Use eval prompts that enforce schema/format compliance and check how often the model deviates.
Can I use variables or templates for realistic tests?
Yes. Inject real inputs (tickets, policies, product data) to validate prompts on production-like content.
Can I export the request to ship it?
Yes—export to cURL/JS/JSON so engineering can reproduce the exact call and parameters.
Can I share results with stakeholders?
Yes. Share links for review and align on what “good” means before you commit to GPT-5.1 Codex Max in production.
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 if the premium model is too expensive for my workload?
Use GPT-5.1 Codex Max only for hard cases and route everything else to a cheaper model. The playground helps you design that split.
How fast is GPT-5.1 Codex Max?
We stream responses through a lightweight proxy. Speed depends on model/load; you can compare latency across models directly.
What’s the context window?
It depends on GPT-5.1 Codex Max. We show context limits and key capabilities next to the model in the picker.
Do you train on my prompts and data?
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 processing regions and details.
How does this compare to LangChain?
LangChain is for building production apps and orchestration. LangFast is for evaluating prompts/models first, before you build anything.
How does this compare to Langfuse, Basalt, or PromptLayer?
Those tools help manage evals, datasets, and tracing in pipelines. LangFast is the quickest way to run interactive prompt tests and decide which premium model to use.