netlify-image-cdn

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• SKILL.md
• user-uploads.md · references/

SKILL.md (first lines)

# Netlify Image CDN

Transform images by requesting the endpoint with a `url` query parameter. This is the current and only documented surface — there is no legacy form.

```
GET /.netlify/images?url=<source>[&w=][&h=][&fit=][&position=][&fm=][&q=]
```

```bash
# resize a deployed image to 50px wide
curl -vs 'https://mysitename.netlify.app/.netlify/images?url=/owl.jpeg&w=50'
```

`url` is required; all other parameters are optional.

## Query parameters

| Parameter | Purpose | Values | Default |
|-----------|---------|--------|---------|
| `url` | Source asset (required) | Relative path or remote URL | — |
| `w` | Width in pixels | Integer | — |
| `h` | Height in pixels | Integer | — |
| `fit` | Resize behavior | `contain`, `cover`, `fill` | `contain` |
| `position` | Crop anchor when `fit=cover` | `top`, `bottom`, `left`, `right`, `center` | `center` |
| `fm` | Output format | `avif`, `jpg`, `png`, `webp`, `gif`, `blurhash` | content-negotiated |
| `q` | Quality for lossy output | Integer `1`–`100` | `75` |

## Common transformations

```bash
# resize + crop to a 50px square, retaining the left side
curl -vs 'https://mysitename.netlify.app/.netlify/images?url=/owl.jpeg&fit=cover&w=50&h=50&position=left'

# convert JPEG to PNG (response carries content-type: image/png)
curl -vs 'https://mysitename.netlify.app/.netlify/images?url=/owl.jpeg&fm=png'

# convert JPEG to AVIF at medium quality
curl -vs 'https://mysitename.netlify.app/.netlify/images?url=/owl.jpeg&fm=avif&q=50'
```

### fit behavior

- **`contain` (default):** maintains aspect ratio; one dimension may come back smaller than requested. Supply one dimension and the other is computed.
- **`cover`:** fills exactly, cropping excess. **Requires BOTH `w` and `h`** — omitting either is invalid. Use `position` to choose what's retained.
- **`fill`:** fills exactly, stretching/squishing if aspect ratios differ.

### Format notes

- `q` applies only when output is `avif`, `jpg`, `gif`, or `webp`.
- `webp` and `gif` can be static or animated.
- If `fm` is omitted, format is content-negotiated from the `Accept` header: `webp` if accepted, else `avif` if accepted, else the original format. A source-only request (no other params) still converts to `webp`/`avif` but keeps size and shape.

## Remote source images

Remote sources must be allowlisted in `netlify.toml` before transformation, or the request fails.

```toml
[images]
  remote_images = ['https://my-images\.com/.*', 'https://animals.more-images.com/[bcr]at/.*']
```

Percent-encode remote source URLs before placing them in the `url` parameter with `encodeURIComponent` — a URL containing `?` or `&` breaks otherwise.

```js
const src = `/.netlify/images?url=${encodeURIComponent('https://my-images.com/owl.jpeg?v=2')}&w=400`;
```

Constraints:
- Remote sources must be **publicly accessible**.
- Credential-bearing headers (`Authorization`, `Cookie`) are **NOT forwarded** when fetching a remote source. For authenticated sources, use URLs that carry their own authorization (e.g. S3 presigned URLs) and make sure your `remote_images` patterns match those full URLs.

### remote_images regex escaping

The only meaningful escape is the literal dot (`\.`). Forward slashes are NOT metacharacters — never write `https:\/\/`. In `netlify.toml`, use single-quoted literal strings (`'https://example\.com/.*'`) or double the backslash in double-quoted strings (`"https://example\\.com/.*"`). A bare `\.` inside double quotes is invalid TOML.

## Response codes

- Invalid transformation parameter values → `404`.
- Valid new transformation → `200` with content and matching `content-type`.
- Previously transformed (cached) image → `304`.

## Reusing parameters across images

Map a friendly path to the endpoint with a redirect/rewrite.

`_redirects`:
```
/transform-small/* /.netlify/images?url=/:splat&w=50&h=50 200
```

`netlify.toml`:
```toml
[[redirects]]
  from = "/transform-small/*"
  to = "/.netlify/images?url=/:splat&w=50&h=50"
  status = 200
```

Then `GET /transform-small/owl.jpeg` returns the transformed image. **Cross-site redirects for transformations are NOT recommended** — they can degrade site performance.

## Caching headers

Apply custom headers to source images on the site's own domain; they carry through to the transformed output.

`netlify.toml`:
```toml
[[headers]]
  for = "/source-images/*"
  [headers.values]
    Cache-Control = "public, max-age=604800, must-revalidate"
```

- Custom headers can be applied to source images on the site's domain only — NOT to remote source images (Netlify does respect cache headers the external domain sends).
- `Cache-Control` on source images applies only to browsers and CDNs in front of Netlify, NOT the Netlify Cache itself.

## Blur placeholders (fm=blurhash)

`fm=blurhash` returns a BlurHash **text string**, not image bytes. Pointing an `<img src>` (or CSS background) at it renders nothing. Fetch the string ahead of time, decode it client-side with a BlurHash library (https://blurha.sh), and load the real image as a separate request without `fm=blurhash`.

From netlify/context-and-tools, MIT.

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