# Fix env.ASSETS binding missing on custom domain route

## TL;DR

`env.ASSETS` only exists when the worker is configured with static assets. Add an `[assets]` section to wrangler.toml pointing at your build output directory, serve static paths through `env.ASSETS.fetch(request)` with an index.html fallback for SPA routes, and make sure the custom domain route is attached to the worker.

## Verbatim error

```text
workerd error: env.ASSETS binding missing on custom domain route
```

## Steps

1. Open wrangler.toml and add `[assets]` with `directory` set to your build output, e.g. `directory = "./dist"`. Expected: the section is present and the path exists.
2. Deploy with `npx wrangler deploy`. Expected: the deploy output mentions the assets upload.
3. In the worker, serve static paths with `env.ASSETS.fetch(request)`. Expected: asset requests return 200.
4. Add an SPA fallback: if the ASSETS fetch returns 404, fetch `index.html` from ASSETS instead. Expected: client-side routes render.
5. Attach the custom domain route to the worker (dashboard custom domains tab or a wrangler route). Expected: the domain serves this worker.
6. Verify on the custom domain: fetch a static file and an SPA route directly. Expected: both return 200 with no binding errors.

## Use this when

- `env.ASSETS` is undefined on a custom domain
- Assets work on workers.dev but not on the custom domain
- You migrated from Pages to Workers static assets and the binding vanished

## Not for this skill when

- The assets directory path is wrong (files not found is a different error)
- You serve assets from R2 with your own code (no ASSETS binding involved)
- You are on the legacy Workers Sites setup (different mechanism)

## Variant phrasings

- ASSETS binding undefined custom domain
- workers static assets not served
- env.ASSETS.fetch 404
- workers assets binding missing

## Why it happens

The ASSETS binding is synthesized from the `[assets]` config at deploy time. Without that section, `env.ASSETS` is undefined, and custom-domain routes - which do not get the default workers.dev asset handling - expose the gap that workers.dev hides.

## Edge cases

- The `[assets]` directory must exist at deploy time or the deploy fails.
- `_headers` and `_redirects` files inside the assets directory are honored by the asset serving.
- Large asset counts slow down deploys; keep build artifacts like source maps out of the directory if you do not serve them.

## Provenance

Resolved from the public thread: https://vectle.com/posts/pst_EAekhAvUlUqM4M-csLhw_w