LangFast

LangFast

Langfa.st
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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

Introducing Narrow AI: Take the Engineer out of Prompt Engineering Narrow AI autonomously writes, monitors, and optimizes prompts for any model - so you can ship AI features 10x faster at a fraction of the cost. Maximize quality while minimizing costs - Reduce AI spend by 95% with cheaper models - Improve accuracy through Automated Prompt Optimization - Achieve faster responses with lower latency models Test new models in minutes, not weeks - Easily compare prompt performance across LLMs - Get cost and latency benchmarks for each model - Deploy on the optimal model for your use case Ship LLM features 10x faster - Automatically generate expert-level prompts - Adapt prompts to new models as they are released - Optimize prompts for quality, cost and speed

Platforms Supported

Windows
Mac
Linux
Cloud
On-Premises
iPhone
iPad
Android
Chromebook

Platforms Supported

Windows
Mac
Linux
Cloud
On-Premises
iPhone
iPad
Android
Chromebook

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

AI Startups, AI Developers, AI Founders, CTOs

Support

Phone Support
24/7 Live Support
Online

Support

Phone Support
24/7 Live Support
Online

API

Offers API

API

Offers API

Screenshots and Videos

No images available

Screenshots and Videos

No images available

Pricing

$60 one time
Free Version
Free Trial

Pricing

$500/month/team
Free Version
Free Trial

Reviews/Ratings

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

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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
Webinars
Live Online
In Person

Training

Documentation
Webinars
Live Online
In Person

Company Information

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

Company Information

Narrow AI
Founded: 2024
United States
www.getnarrow.ai/

Alternatives

Alternatives

Vellum AI

Vellum AI

Vellum

Categories

Categories

Integrations

GPT-4
GPT-4o
Llama 3.1

Integrations

GPT-4
GPT-4o
Llama 3.1
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