title: Human Guide description: Human-maintained guidance for @lastshotlabs/slingshot-image

@lastshotlabs/slingshot-image handles on-the-fly image optimization behind a single Snapshot route. It fetches an approved source image, validates transform parameters, optionally transforms with sharp, and caches generated variants.

When To Use It

Use this package when your app needs:

• responsive image resizing behind a stable app-owned endpoint
• format conversion such as AVIF, WebP, JPEG, or PNG
• origin allow-list enforcement for remote image fetches
• a default cache so repeat image variants are cheap

Do not use it as a full media pipeline. It is request-time optimization, not offline asset processing.

Minimum Setup

The plugin is mostly zero-config. Defaults are:

• routePrefix: '/_snapshot/image'
• maxWidth: 4096
• maxHeight: 4096
• allowedOrigins: []
• an in-memory LRU cache with 500 entries when no custom cache is provided

If allowedOrigins is empty, only relative URLs are effectively useful for remote safety. Add explicit hostnames before allowing absolute upstream image URLs.

What You Get

The plugin mounts:

• GET {routePrefix}

The route supports query parameters for:

• url
• w
• optional h
• optional f
• optional q

It also:

• validates width and height against hard upper bounds
• rejects disallowed source URLs before any fetch happens
• fetches relative paths through the current app host
• returns long-lived immutable cache headers on generated variants

When sharp is available, the plugin performs actual transforms. When sharp is absent, the package falls back to serving originals with a warning instead of crashing startup.

Common Customization

The main extension points are:

• allowedOrigins for remote-source policy
• maxWidth and maxHeight for transform limits
• routePrefix if your app wants a different public image URL
• cache if you need a custom cache adapter instead of the default in-memory LRU

If you need to change behavior, start in:

• src/plugin.ts for defaults and cache wiring
• src/routes.ts for request validation and response behavior
• src/transform.ts for format conversion logic
• src/cache.ts for the default memory cache

Gotchas

• Width is required. Missing or invalid w returns 400.
• Relative URLs are always allowed and are fetched from the current app host. Be deliberate about what local routes can return image bytes.
• The route validates requested dimensions against absolute limits first, then applies your plugin config limits during transform.
• The default cache is process-local. Multi-instance deployments need a custom shared cache if you want cross-instance reuse.

Key Files

• src/index.ts
• src/plugin.ts
• src/config.schema.ts
• src/routes.ts
• src/transform.ts
• src/cache.ts