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 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 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

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 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

$500/month/team
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

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

Alternatives

Alternatives

Categories

Prompt Engineering Supported

Categories

Integrations

GPT-4 Not Supported
GPT-4o Not Supported
Llama 3.1 Not Supported

Integrations

GPT-4 Supported
GPT-4o Supported
Llama 3.1 Supported
Claim LangFast and update features and information
Claim LangFast and update features and information
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Claim Narrow AI and update features and information