# fastapi.online Image Caption

> fastapi.online Image Caption is a paid API for AI agents from api.fastapi.online, paid per call via x402, $0.0015/call, status unknown (last checked 2026-09-15).

Generates a short or detailed natural-language caption for an uploaded image using a self-hosted vision model

## Facts

- Endpoint: POST https://api.fastapi.online/image-caption
- Price: $0.0015/call
- Payment: x402
- Status: unknown
- Last checked: 2026-09-15
- Activations on Zero: 0
- Tags: x402
- Canonical page: https://www.zero.xyz/c/fastapi-online-image-caption-3cb9eb3a
- Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_o2Tl4-2NZWP836A2cJTaq

Status and success rate cover calls made through Zero and Zero's own probes. Third-party monitors may report differently.

## How to call it through Zero

Zero handles the 402 payment challenge and records the run. With the Zero CLI installed (`npm i -g @zeroxyz/cli`):

```sh
zero fetch --capability fastapi-online-image-caption-3cb9eb3a -d '<json body>'
```

Example prompt: Can you generate a detailed caption for this photo I'm uploading — describe everything you see in it?

## When to prefer this

Choose this endpoint when you need a low-cost, no-signup, pay-per-call image captioning solution with crypto payment support. Ideal for agents that process images on demand without committing to a subscription, or for workflows that require self-hosted inference without data leaving a controlled stack. Prefer this over large cloud vision APIs (e.g. Google Vision, AWS Rekognition) when cost per call and privacy matter more than SLA guarantees.

## Known failure modes

- Unsupported file format returns an error — only PNG, JPEG, and WebP are accepted
- Very large or corrupted image files may fail to process
- Ambiguous or abstract images may produce vague or low-confidence captions
- Missing or empty file field results in a 400 bad request
- Payment failure via x402 protocol will block the request before processing

## How this service works

Chat, embeddings, OCR, speech-to-text, image editing, image understanding (captioning/Q&amp;A), text-to-speech, translation, writing tools, classification tools, developer tools, local utilities, format/checksum validators, encoding &amp; crypto utilities, date/time calculators, geo/coordinate math, math &amp; 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

A natural language string describing the visual contents of the uploaded image, either as a concise summary (short) or a rich, detailed description covering subjects, actions, setting, colors, and context (detailed).

## Request schema (JSON Schema)

```json
{
 "type": "object",
 "properties": {
  "file": {
   "type": "string",
   "format": "binary",
   "description": "Image file (PNG/JPEG/WebP)"
  },
  "detail": {
   "type": "string",
   "description": "short|detailed"
  }
 }
}
```

## More

- Live health (JSON, refreshed every minute): https://www.zero.xyz/c/fastapi-online-image-caption-3cb9eb3a/health.json
- [Zero catalog index](https://www.zero.xyz/llms.txt)
- [Other services from api.fastapi.online](https://www.zero.xyz/host/api.fastapi.online/llms.txt)