LangFast

LangFast

Langfa.st
Vellum AI

Vellum AI

Vellum
+
+

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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

Bring LLM-powered features to production with tools for prompt engineering, semantic search, version control, quantitative testing, and performance monitoring. Compatible across all major LLM providers. Quickly develop an MVP by experimenting with different prompts, parameters, and even LLM providers to quickly arrive at the best configuration for your use case. Vellum acts as a low-latency, highly reliable proxy to LLM providers, allowing you to make version-controlled changes to your prompts – no code changes needed. Vellum collects model inputs, outputs, and user feedback. This data is used to build up valuable testing datasets that can be used to validate future changes before they go live. Dynamically include company-specific context in your prompts without managing your own semantic search infra.

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

Developers wanting a powerful AI Development platform

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

Pricing

$60 one time
Free Version
Free Trial

Pricing

No information available.
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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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

Vellum
vellumai.net

Alternatives

Alternatives

Portkey

Portkey

Portkey.ai
Vellum AI

Vellum AI

Vellum

Categories

Categories

Integrations

No info available.

Integrations

No info available.
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