LangFast

LangFast

LangFast is a browser-based LLM playground and prompt evaluation platform. It lets product teams stream prompts against GPT models, compare outputs side-by-side across tabs, and iterate using Jinja2 templates, variables, structured outputs and function calling — all with your own API keys (BYOK), so you pay providers directly instead of a subscription markup. Beyond prototyping, it handles the boring-but-critical part: datasets, assertions and metrics, token/cost tracking, prompt versioning and folders, plus shareable links for cross-team review. API keys are stored encrypted server-side and your chats aren't used for training. Built for prompt engineers, AI product teams and developers shipping LLM features who want to catch regressions before production. Caveat: model coverage is currently OpenAI/GPT-only (Claude and Gemini are 'soon'), and you must bring your own API keys.

Productivity
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Added on
Nov 4, 2026
GPT-5 & O-seriesBYOKPrompt evalsSide-by-side compareJinja2 templatesShare linksToken & cost statsOne-time pricing150+ models
Product information

Everything worth knowing about LangFast

The complete picture — from everyday features to the technical detail builders and IT folks dig for.

Core features

What it actually does

Multi-Model Prompt Playground
Run and compare prompts across 150+ AI models (BYOK) with side-by-side chat tabs, streaming responses, and full model settings control.
Prompt Evaluations
Iteratively validate prompts with assertions, metrics, datasets, token and cost stats, and performance comparison before deployment.
Prompt Management & Versioning
Organize prompts in folders, edit and fork versions, use Jinja2 templates with dynamic variables, and share prompts across teams.
Collaboration & Share Links
Generate public share URLs or grant access by email with role-based permissions for cross-team prompt collaboration.
Bring Your Own Keys (BYOK)
Use your own API keys to run models — encrypted server-side, never exposed to the browser, no subscription required.
Technical capabilities
Low-Latency Streaming Proxy
Streams tokens through a tiny proxy layer for sub-second first-token times, with API keys stored encrypted server-side.
Cost & Token Tracking
Tracks token usage, estimates API costs, and shows context length and max-token settings per model.
Must watch videos
Jinjiang Inn Langfang High-Speed Railway Station Yinhe Nan Road | Langfang, China | Hotel Review 🏩
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Jinjiang Inn Langfang High-Speed Railway Station Yinhe Nan Road | Langfang, China | Hotel Review 🏩
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Theo Rose - Ceai lângă tine | Official Video
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Theo Rose - Ceai lângă tine | Official Video
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Is Jinjiang Inn Langfang Worth It? Honest Verdict⭐
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Is Jinjiang Inn Langfang Worth It? Honest Verdict⭐
Why travelers pick it Jinjiang Inn Langfang New Chaoyang Pedestrian Street (⭐⭐) Booking link: https://www.agoda.com/partners/partnersearch.aspx?pcs=1&cid=1973974&hid=522419&tag=InnspireStays Are you planning a trip to Langfang and looking for a convenient place to stay? The Jinjiang Inn Langfang New Chaoyang Pedestrian Street offers a comfortable and affordable experience near the city's main walking streets. Located 3km from Langfang city center Close to Langfang Century Square and New Century Walking Street Free breakfast buffet available Children aged 3-12 stay for free 24-hour reception and security service Includes free public Wi-Fi and on-site parking Features meeting and banquet facilities 👨‍👩‍👧‍👦 Family note: a practical option for trips with children Location: 📍 No,225,Xinkai Road, Guangyang, Langfang, China, 065099 Audio Credit: Artist: Casa Rosa Track Title: Pienso Viento Website: https://music.youtube.com Not a sponsored video. All hotel photos are the property of their owners; InnspireStays shows them solely to illustrate this review. New hotel walkthroughs from InnspireStays every week. 📺 Watch another hotel guide: 🏖️ APARTAMENTOS DIMAX: https://youtu.be/gZsoNQjlffo 🌆 Hotel Louis II: https://youtu.be/mhsjytO_QpA 🌟 The Zentral Suites & Apartments: https://youtu.be/5C9VGXpXQkU 🏢 La Quinta Inn & Suites by Wyndha...: https://youtu.be/NUyiHoSV2jY 🏩 Shark's bay oasis aqua park: https://youtu.be/ThgSXcXRyx8 🌟 St George's Hotel - Wembley: https://youtu.be/OF2vyuEkGtI 🏨 R Hotel Namba: https://youtu.be/igeYfI-MkYE ##InnspireStays #travelguide #fitness center #Jinjiang #hotelreview #hotelguide #place to stay #hotelbooking

Who uses it

Real use cases, no hype
Prompt A/B Testing
Run the same prompt across models and diff the outputs before you commit.
Prompt Version Control
Fork, version, and roll back prompts instead of losing them in Slack threads.
Structured Output Debugging
Nail down JSON schemas and function-calling payloads before they break prod.
Model Selection
Pick the cheapest model that still passes your eval, with data instead of vibes.
Token & Cost Budgeting
Track token spend per prompt and per model to kill expensive regressions.
Team Prompt Collaboration
Let subject-matter experts edit prompts without touching code or keys.
Pre-Production Validation
Build a test set of cases and assert on outputs before shipping the feature.
What's great
  • One-time payment, no subscription — $60 for lifetime access
  • BYOK model means you only pay raw provider API costs
  • Side-by-side model comparison in unlimited parallel tabs
  • Shareable prompt links with role-based permissions for teams
Technical strengths
  • Jinja2 templates + variables for dynamic, reusable prompts
  • Per-response token, cost, and context-window tracking
  • Prompt versioning, forking, and folder-based organization
Where it falls short
  • Advertises 150+ models but FAQ admits OpenAI/GPT only
  • Ticket price is not the real cost — you pay API bills on top
  • No free tier; you pay before you can properly try it
  • A dozen advertised features are still 'coming soon'
Technical limitations
  • Eval layer is thin — assertions and datasets, no CI-native eval harness
  • Prompts and keys route through their proxy backend layer
  • Solo-maintainer beta with broken docs pages signals reliability risk
For technical folks

The deep-dive specs

Model Coverage
OpenAI GPT-5, GPT-4.5, GPT-4.1, GPT-4o, O1, O3, O4 and GPT-3.5 family (Claude and Gemini coming soon)
Context Window
8K–200K tokens depending on the selected model
Templating Engine
Jinja2 templates with dynamic variables
API Key Handling
Sent over secure transport, encrypted server-side, plaintext used only for provider calls
Streaming
Token streaming through a low-latency proxy layer
Output Controls
Structured outputs, function calling, multimodality support
Model Settings
Temperature, presence/frequency penalty, reasoning effort, max tokens, system instructions
Storage
1 GB included per account

FAQ

Includes technical Q&A
Yes. You bring your own provider keys — that's why the subscription is a one-time fee. Your token spend goes straight to OpenAI, not to LangFast.

Traffic Insights

Monthly Visits

—

November 2026

Growth Rate

0%

vs last month

Dominance

0%

in Productivity

Top Country

—

Top Countries Breakdown

—

Growth Trend

Stable

Traffic has remained stable.

Honest pricing

No sneaky tiers, no “contact sales”

Here's who each plan is actually for — and where the hidden charges might hit.

1-Year Pass
$49/mo

Individuals

  • Access valid for 12 months
  • 150+ AI models (BYOK)
  • AI Playground & Chats
  • Prompt management
  • Prompt evaluations
  • Variables & Templates
  • Share links & Collaboration
View Full Plans & Pricing on LangFast

Official pricing, checkout, and plan upgrades are handled directly on langfa.st.

Reviews

What the internet actually thinks

Data refreshed weekly. No paid placements.

Trustpilot
Not reviewed yet
G2
G2
Not reviewed yet
C
Capterra
Not reviewed yet
Product Hunt
Not reviewed yet
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