imgix

Promptfree, for everyone, and the only version there is

Copy the prompt for Claude CodeCopy the prompt for CodexCopy the prompt for Cursor

Copy

Build me the image transformation service that replaces imgix.

The split is the same for every product in this category: **sharp does the transformations for free; the delivery network is what you should rent.**

STACK
- Node 20+ with Fastify as the origin
- sharp for every operation
- SQLite through better-sqlite3 for the derivative index
- Object storage for originals and derivatives
- Bunny or Cloudflare in front — buy this part
- Caddy in front of the origin

THE URL CONTRACT
- /i/<signature>/<ops>/<path>
- **Sign the operations. An unsigned transformation endpoint is a free rendering farm on your bill**, and it is discovered by scanners within days of going live
- An allow-list of widths and a small set of named presets rather than arbitrary parameters. A dozen presets cover a whole site and cap the derivative count

THE OPERATIONS WORTH IMPLEMENTING
- Resize with a fit mode, crop with a gravity, rotate honouring EXIF, quality, format, blur, and a solid background for transparency flattening. That is the ninety-percent set
- Smart crop: sharp's attention strategy is good enough and free. **Do not build face detection** unless you have measured that you need it
- Refuse anything above a megapixel cap on input — a crafted image is a memory exhaustion attack and sharp will try

FORMAT SELECTION
- AVIF, then WebP, then the original format, chosen from the Accept header, **with `Vary: Accept` set.** Miss the Vary and a shared cache serves AVIF to a browser that cannot display it, which looks like a broken site to a fraction of visitors you will struggle to reproduce
- AVIF encoding is slow. Encode it once on a background job and serve WebP until it is ready, rather than blocking the first request
- Quality per format, tuned once by looking at real images, not by a number somebody read

CACHING PROPERLY
- Immutable, long cache headers, because the signature changes when the operations do
- Derivatives written to storage on first render and never regenerated
- **A stampede lock per derivative**: a popular page going live must produce one render, not four hundred
- Purge by changing the signature, not by trying to clear somebody else's cache

OPERATIONS
- A render queue with a concurrency cap tied to core count
- Bytes served, renders per day and cache hit ratio recorded. **The hit ratio is the number that tells you whether this is cheap or expensive**, and it is invisible without recording it

WHAT MATTERS MOST
Signed URLs, the Vary header and the stampede lock. Each of the three has a specific failure that is expensive and hard to diagnose.