# Fix "binding is not defined" for KV namespace in local dev

## TL;DR

Local dev binds the preview KV namespace, so your `[[kv_namespaces]]` entry needs a valid `preview_id`. Run `wrangler kv namespace list`, put the real preview namespace ID in the binding config, and fully restart `wrangler dev`. Without a `preview_id` there is nothing to attach locally, so `env.MY_KV` stays undefined.

## Verbatim error

```text
workerd: "binding is not defined" for KV namespace in local dev
```

## Steps

1. Run `npx wrangler kv namespace list`. Expected: a JSON list with an `id` and `title` for each namespace in your account.
2. In wrangler.toml, find your `[[kv_namespaces]]` entry and set both `id` (production) and `preview_id` (preview) to real namespace IDs from that list. Expected: both fields present, no placeholder values.
3. Stop `wrangler dev` completely and start it again (a full restart, not hot reload). Expected: dev boots with no binding errors.
4. In the handler, reference `env.MY_KV` exactly matching the `binding` name in wrangler.toml. Expected: no ReferenceError when the code runs.
5. Smoke-test: `await env.MY_KV.put("ping", "1")` then `await env.MY_KV.get("ping")`. Expected: the round-trip returns "1" locally.

## Use this when

- "binding is not defined" appears only in `wrangler dev`, never after deploy
- KV works in production but `env.MY_KV` is undefined locally
- You recently added the KV binding and never set a `preview_id`

## Not for this skill when

- The binding is invalid in production too (the production `id` is wrong or stale)
- The namespace was deleted (recreate it and update both IDs)
- D1 or R2 bindings are missing (same shape of problem, different wrangler commands)

## Variant phrasings

- kv namespace undefined wrangler dev
- env KV binding not found local
- miniflare kv binding missing
- ReferenceError: MY_KV is not defined workers dev

## Why it happens

`wrangler dev` attaches preview resources, not production ones. A binding entry with only an `id` has nothing to bind locally, so the runtime leaves `env.MY_KV` undefined and throws a ReferenceError the moment your code touches it.

## Edge cases

- A `preview_id` pointing at a deleted namespace produces a different error at dev boot; re-list and fix it.
- Local KV data does not persist across restarts unless you pass a persist directory to `wrangler dev`.
- Binding names are case-sensitive; `env.my_kv` and `env.MY_KV` are different bindings.

## Provenance

Resolved from the public thread: https://vectle.com/posts/pst_xTK_CmsRgiuDvlVyBCfMFQ