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The AltTextLab API requires a publicly accessible image URL. Images stored in a private S3 bucket cannot be accessed directly — but you can grant temporary, scoped access using an S3 presigned URL. A presigned URL embeds your credentials and an expiry time into the URL itself. You generate it server-side, pass it to the API, and it becomes invalid once the signature expires — no permanent exposure, no bucket policy changes required.

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How it works

  1. Your server generates a presigned URL for the S3 object, valid for a short window (30–60 seconds is usually enough).
  2. You send the presigned URL to the AltTextLab API as imageUrl.
  3. The API fetches the image, generates alt text, and returns the result.
  4. The presigned URL expires automatically.
Your bucket stays private throughout.

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Step 1: Generate a presigned URL

Use the AWS SDK for your language to sign a GetObject request. Set the expiry short enough that the URL cannot be reused — 60 seconds works for most cases. 
import { S3Client, GetObjectCommand } from '@aws-sdk/client-s3';
import { getSignedUrl } from '@aws-sdk/s3-request-presigner';

const client = new S3Client({ region: 'us-east-1' });

const command = new GetObjectCommand({
  Bucket: 'your-private-bucket',
  Key: 'path/to/image.jpg',
});

const signedUrl = await getSignedUrl(client, command, { expiresIn: 60 });
Your AWS credentials need s3:GetObject permission on the bucket or key. The credentials used to sign the URL are never exposed — only the signature is embedded in the URL.

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Step 2: Pass the presigned URL to the API

Use the signed URL as imageUrl in your generation request. Everything else works the same as with a public URL. 
const response = await fetch('https://app.alttextlab.com/api/v1/alt-text/generate', {
  method: 'POST',
  headers: {
    'Content-Type': 'application/json',
    'x-api-key': 'YOUR_API_KEY',
  },
  body: JSON.stringify({
    imageUrl: signedUrl,
    lang: 'en',
    style: 'neutral',
  }),
});

const data = await response.json();
console.log(data.result);

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Complete example

import { S3Client, GetObjectCommand } from '@aws-sdk/client-s3';
import { getSignedUrl } from '@aws-sdk/s3-request-presigner';

async function generateAltText(bucket, key) {
  const client = new S3Client({ region: 'us-east-1' });
  const command = new GetObjectCommand({ Bucket: bucket, Key: key });
  const signedUrl = await getSignedUrl(client, command, { expiresIn: 60 });

  const response = await fetch('https://app.alttextlab.com/api/v1/alt-text/generate', {
    method: 'POST',
    headers: {
      'Content-Type': 'application/json',
      'x-api-key': process.env.ALTTEXTLAB_API_KEY,
    },
    body: JSON.stringify({ imageUrl: signedUrl, lang: 'en' }),
  });

  const data = await response.json();
  return data.result;
}

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AWS documentation