LangFast
Instant LLM playground to test and compare prompts across 50+ models
About LangFast
LangFast is a lightweight prompt testing platform designed for product teams, prompt engineers, and developers working with LLMs. It provides instant access to a customizable prompt playground with no signup required, allowing users to build, test, and share prompt templates using Jinja2 syntax with real-time raw outputs directly from the LLM.
Pros & Cons
Pros
- No signup or API key setup required to start testing prompts immediately
- Side-by-side comparison across 50+ models makes it easy to find the best model-prompt combination
- Jinja2 template syntax enables reusable and dynamic prompt engineering
- JSON schema enforcement eliminates parsing errors in structured outputs
- Pay-as-you-go pricing keeps costs predictable for teams of all sizes
Cons
- Limited detailed documentation and community resources compared to larger platforms
- One-time pricing model may not suit teams needing ongoing enterprise support
- Relatively new tool with smaller user base than established alternatives like Langfuse or PromptFoo
- No self-hosted option for teams with strict data sovereignty requirements
Key Features
Side-by-Side Prompt Comparison
Test multiple prompt variations at once against different models to quickly identify the most effective combination of wording and parameters
50+ LLM Access
Immediate access to models from OpenAI, Anthropic, Google, and more without needing to manage your own API keys
No Signup Required
Start testing prompts instantly without creating an account or configuring API credentials
Jinja2 Template Syntax
Build reusable prompt templates using Jinja2 syntax for dynamic variable injection and prompt composition
Vision & Function Calling
Support for image inputs via Vision APIs and function definitions that enable LLMs to interact with internal systems
JSON Schema Enforcement
Enforce structured output by defining JSON schemas, ensuring LLM responses consistently match expected data structures
Test-Driven Prompt Engineering
Build comprehensive test cases and datasets with AI-powered evaluations and pre-defined success criteria
Shareable Workspaces
Share prompt templates and results with team members for collaborative prompt development and iteration
Pricing
- Limited prompt testing
- Access to select models
- Basic prompt playground
- 50+ LLM models
- Side-by-side comparison
- Jinja2 templates
- Vision & function calling
- JSON schema enforcement
- Shareable workspaces
Best For
Prompt Engineering for Product Teams
Product teams can quickly prototype and test prompt variations across multiple LLMs to find the best approach for their AI features
LLM Model Comparison
Developers can compare responses from OpenAI, Anthropic, Google, and other providers side by side to select the optimal model for their use case
Structured Output Validation
Teams building AI-powered APIs can enforce JSON schema output to ensure consistent, parseable responses from LLMs
Collaborative Prompt Development
Engineering teams can share and iterate on prompt templates together, building a library of tested prompts for production use
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