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fastapi.online — OpenAPI Spec Generator
x402
fastapi.online — OpenAPI Spec Generator is a paid API for AI agents from api.fastapi.online, paid per call via x402, $0.005/call, status unknown (last checked 2026-09-15).
Generates an OpenAPI specification document from a plain-text description of an API
Price / call$0.005
Success—
StatusUnknown
How this service works
Chat, embeddings, OCR, speech-to-text, image editing, image understanding (captioning/Q&A), text-to-speech, translation, writing tools, classification tools, developer tools, local utilities, format/checksum validators, encoding & crypto utilities, date/time calculators, geo/coordinate math, math & finance calculators, novelty tools, and business/social writing generators — 179 endpoints in all, across multiple specialized self-hosted models. Pay per call with crypto — no signup — or get an API key.
Output
An OpenAPI specification document (likely JSON or YAML) describing the endpoints, request/response schemas, parameters, and structure of the API based on the plain-text description provided.
Example use cases
Scaffold API spec from idea
“I have an idea for a simple weather API that takes a city name and returns current temperature, humidity, and a 5-day forecast — can you generate a proper OpenAPI spec for it?”
Document internal microservice
“Generate an OpenAPI specification for our user authentication service — it has endpoints for signup, login, logout, password reset, and token refresh, all using JWT.”
Prototype API for a hackathon
“I'm building a recipe recommendation API for a hackathon — it takes dietary preferences and a list of ingredients and returns matching recipes with instructions. Can you produce an OpenAPI 3.0 spec for it?”
Technical details
Endpoint
POSThttps://api.fastapi.online/generate-openapiWhen to prefer
Choose this endpoint when you need a quick, pay-per-call OpenAPI spec generation without signing up for a full documentation platform. Ideal for prototyping, hackathons, or automating API documentation workflows where you have a natural language description of an API and need a structured spec output at low cost.
Side effects
- Description too short (below 2 character minimum) returns a validation error
- Description exceeds 2000 character maximum and is rejected
- Vague or ambiguous descriptions may produce incomplete or generic specs
- Payment failure (insufficient USDC) results in 402 response
- Server unavailability results in timeout or 5xx error
Input
Schema · Request body
| Field | Type | Description |
|---|---|---|
| description | string | the API to document |
Tags
x402
Reviews
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Zero review prompt (fires after every successful call)
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