LangFast
Go to Playground

The Ultimate
  • LLM
  • AI API
  • OpenAI
  • LLM
Playground

Test, compare, and version prompts instantly in a shared workspace.

Start prompting – it's free

No signup, no API keys required

Variables

Parametrize your prompts with dynamic data inputs

Structured outputs

Ensure prompt outputs match expected JSON Schemas

Side-by-side editing

Compare and iterate on many prompts at once

Function calling

Enrich your prompts with capabilities of your internal systems

Image inputs

Harness the power of Vision API to analyze and process image content

All model settings

Take the full control of all settings that given AI model has
Test-driven prompt engineering

Evaluate and measure across large data sets

Build a collection of test cases to find the best prompt/model combination across different scenarios
Get Started For Free

Assertions & Metrics

Create test suites with AI-powered evals and pre-defined success criteria

Performance comparison

Create test runs on multiple prompts at once to find the top performer

Data Sets

Organize your data sets in spreadsheet-like experience

Sharable reports

Create test run reports that you can share with broader team

Comments & Feedback

Incorporate the team's feedback to verify expected quality

Token & Cost stats

Get access to real-time token usage and cost stats.

Work with the AI provider of your choice
I built LangFast out of frustration while leading AI development at Yola.
Prompt testing should be quick and painless. But every tool I tried had a clunky UI, required logins and API keys, took forever to set up, and came with a price tag of thousands per year for tiny product teams. That’s just ridiculous!
Eventually I gave up looking for solutions and built it by myself.
I genuinely believe it should be this: a lightweight prompt playground with no sign-up, no API keys, instant collaboration — just open a browser URL and start working. And mainly, it shouldn’t cost a fortune, especially when your team isn’t using it every day.

Eugene Gusarov

Maker of LangFa.st

Squeeze more out of that LLM

Unlock advanced tools with a lifetime Pro plan and pay as you go for what you use. No subscription required.

Hobby
$0
Free forever

  • All platform features (with limits)

  • Instant access to 6 LLMs
    skip the setup — no API keys required

  • 50 free requests daily
    or up to 1,500 requests/month in total

  • Unlimited private prompts

  • Unlimited public prompts

  • 7 days of logs and prompt history

  • For solo use only

Start for Free

Pro
$60
One-time payment. No subscription

  • Everything in Hobby

  • Instant access to 50+ LLMs
    skip the setup — no API keys required

  • 50,000 extra requests included
    then $20 per 20,000 requests

  • Full access to evaluations, workflows, and more coming soon

  • Logs and history never expire

  • Collaborate with your team members

  • Ad-free

 Start with Pro

Team
Custom
Need LangFast for your business?

  • Everything in Pro

  • 3 seats included
    then $120 per extra seat per year

  • Centralized team billing

  • Enforce privacy mode org-wide

  • Admin dashboard with usage stats

  • Custom service & data agreements

  • SAML/OIDC SSO

 Let's talk

Frequently Asked Questions

What is included in the free plan?

The free plan gets you access to essential LangFast features, 6 LLMs and 1,500 free LLM requests per month. You can have as many private and public prompts as you want.

What is a request?

A request is a call to an LLM provider such as OpenAI or Anthropic. From the playground, evalutaiotns, via the API, or from any forms you build.

Do I need my own LLM API Key(s)?

No. Everything is already preconfigured and available for instant testing for you out of the box. The LLM costs are already included in the pricing plans so that you can have predictable costs to you.

Can I use my own LLM API keys if I want to?

Yes, you can if you're on a Team plan. Just reach out to us, and I'll make it happen

Can I bring my own custom model?

Yes, we offer custom model support. Reach out and I'll make it happen.

Can I export my data?

Yes, you can export your data at anytime, just reach out to support@langfa.st

Who's behind this project?

LangFast is a portfolio company at Gusarov Startup Studio led by Eugene Gusarov, the author of $100M Startup Podcast and creator of AI-powered website builder at Yola.com that helps 15M+ users worldwide to establish their online presence.

How are you different from A, B, C, competitors?

We built LangFast because A, B, and C are slow, overcomplicated, and locked behind logins, API keys, and expensive subscriptions. LangFast is a no-bullshit, instant playground for prompt testing — no signup, no setup, 50+ models ready to go. It’s built for speed, not upsells.