LangFast

LangFast

Langfa.st
OpenUI

OpenUI

Thesys
+
+

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About

LangFast is a lightweight prompt testing platform designed for product teams, prompt engineers, and developers working with LLMs. It offers instant access to a customizable prompt playground—no signup required. Users can build, test, and share prompt templates using Jinja2 syntax with real-time raw outputs directly from the LLM, without any API abstractions. LangFast eliminates the friction of manual testing by letting teams validate prompts, iterate faster, and collaborate more effectively. Built by a team with experience scaling AI SaaS to 15M+ users, LangFast gives you full control over the prompt development process—while keeping costs predictable through a simple pay-as-you-go model.

About

OpenUI is an open-source (MIT) generative UI framework for AI agents. Instead of plain text, your agent writes OpenUI Lang, a compact, streaming-first language, and OpenUI renders it progressively as real UI: charts, tables, forms, cards and layouts. Rendering is safe by default: the model can only use components you register and no generated code is executed, so your design system stays in control. A framework-agnostic core powers first-party runtimes for React, Vue, Svelte and Angular, with ready-made chat interfaces and integrations for LangChain/LangGraph, the Vercel AI SDK, Mastra, Google ADK, assistant-ui, CopilotKit, AG-UI and A2UI. In its published benchmark, OpenUI Lang uses about half the tokens of equivalent JSON UI formats, so interfaces stream faster and cost less.

Platforms Supported

Windows Not Supported
Mac Not Supported
Linux Not Supported
Cloud Supported
On-Premises Not Supported
iPhone Not Supported
iPad Not Supported
Android Not Supported
Chromebook Not Supported

Platforms Supported

Windows Supported
Mac Supported
Linux Supported
Cloud Supported
On-Premises Supported
iPhone Supported
iPad Supported
Android Supported
Chromebook Supported

Audience

Product teams, software engineers, prompt engineers, AI developers, ML researchers, startup founders, and technical writers working on AI-powered features or language model integrations.

Audience

Developers building AI agents, copilots and chat apps who want agents to answer with interactive UI instead of text

Support

Phone Support Not Supported
24/7 Live Support Not Supported
Online Not Supported

Support

Phone Support Not Supported
24/7 Live Support Not Supported
Online Supported

API

Offers API Not Supported

API

Offers API Not Supported

Screenshots and Videos

No images available

Screenshots and Videos

No images available

Pricing

$60 one time
Free Version Not Supported
Free Trial Supported

Pricing

Free
Free Version Supported
Free Trial Not Supported

Reviews/Ratings

Overall 0.0 / 5
ease 0.0 / 5
features 0.0 / 5
design 0.0 / 5
support 0.0 / 5

This software hasn't been reviewed yet. Be the first to provide a review:

Review this Software

Reviews/Ratings

Overall 0.0 / 5
ease 0.0 / 5
features 0.0 / 5
design 0.0 / 5
support 0.0 / 5

This software hasn't been reviewed yet. Be the first to provide a review:

Review this Software

Training

Documentation Not Supported
Webinars Not Supported
Live Online Not Supported
In Person Not Supported

Training

Documentation Supported
Webinars Not Supported
Live Online Not Supported
In Person Not Supported

Company Information

Langfa.st
Founded: 2025
United States
langfa.st/new

Company Information

Thesys
Founded: 2024
United States
www.openui.com

Alternatives

Alternatives

LangGraph

LangGraph

LangChain

Categories

Prompt Engineering Supported

Categories

Integrations

A2UI Not Supported
AG-UI Not Supported
Agent Development Kit (ADK) Not Supported
CopilotKit Not Supported
LangChain Not Supported
LangGraph Not Supported
Mastra AI Not Supported
Vercel AI SDK Not Supported

Integrations

A2UI Supported
AG-UI Supported
Agent Development Kit (ADK) Supported
CopilotKit Supported
LangChain Supported
LangGraph Supported
Mastra AI Supported
Vercel AI SDK Supported
Claim LangFast and update features and information
Claim LangFast and update features and information
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Claim OpenUI and update features and information