Generate Image Alt Text

Use POST /image/alt or mynth.image.alt() to generate concise alt text for a single existing image URL. The API creates an async task and returns a taskId. The SDK waits for completion by default; use altAsync() when you want to poll yourself. This endpoint is for images you already have (including ones not generated by Mynth). For content classification of existing images, use Rate Image Content (POST /image/rate).

Pricing

/image/alt costs $0.0004 per image. You are charged only when the task completes successfully. The cost is reserved up front. If the task fails, the reserve is released. If your balance cannot cover the reserve, the request returns 422 INSUFFICIENT_BALANCE.

Generate alt text with the SDK

import Mynth from "@mynthio/sdk";

const mynth = new Mynth();

const result = await mynth.image.alt({
  url: "https://example.com/photo.webp",
});

console.log(result.alt); // "A ceramic mug on a wooden table."
console.log(result.url);
console.log(result.cost);

Local files

Pass file instead of url when the image is local. The SDK uploads first, then calls /image/alt with the resulting URL. See Upload input images.

const result = await mynth.image.alt({
  file, // File, Blob, Buffer, or path — same inputs as upload
});

Task failures

If alt text cannot be generated, the task status is failed. The SDK’s wait() throws TaskAsyncTaskFailedError.

Code	Meaning
`FETCH_FAILED`	The image could not be downloaded from the URL.
`ALT_GENERATION_FAILED`	The image was fetched but alt text generation failed.
`UNKNOWN_ERROR`	An unexpected error. Retry if it persists.

Constraints

Constraint	Limit
URLs per request	1
Alt text length	1 to 160 characters

Alt text describes visible content only. The model is prompted for concise, objective labels (typically under ~100 characters); the API allows up to 160.

Generate alt text with REST

curl https://api.mynth.io/image/alt \
  -X POST \
  -H "Authorization: Bearer $MYNTH_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "url": "https://example.com/photo.webp"
  }'

Response (201):

{
  "data": {
    "taskId": "tsk_01KE7XWWEQ4MCGWKBQKJ1G47RP",
    "estimatedCost": "0.0004"
  }
}

Poll /tasks/:id for the completed result. Full fields live in Image Alt Request.

Start now, wait later

Use altAsync() when you want to create the task now and wait later:

const task = await mynth.image.altAsync({
  url: "https://example.com/photo.webp",
});

console.log(task.id);

const result = await task.wait();
console.log(result.alt);

Poll /tasks/:id/status and /tasks/:id, or listen for task.image.alt.completed / task.image.alt.failed webhooks. See Async and polling and Use webhooks.

How alt text fits

You submit one image URL (or upload a file via the SDK).
Mynth fetches the image and generates short alt text with a vision model.
On success the result is { url, alt }. On failure the task is failed.
Billing charges only successful tasks; reserved balance for failures is released.

Getting started

Generation

Control and delivery

Image analysis

Integrations

Pricing

Generate alt text with the SDK

Local files

Task failures

Constraints

Generate alt text with REST

Start now, wait later

How alt text fits

​Pricing

​Generate alt text with the SDK

​Local files

​Task failures

​Constraints

​Generate alt text with REST

​Start now, wait later

​How alt text fits

Pricing

Generate alt text with the SDK

Local files

Task failures

Constraints

Generate alt text with REST

Start now, wait later

How alt text fits