# Fix `Service "web" uses an undefined network "frontend"` (docker-compose)

## TL;DR

A service references a network that is not declared in the top-level networks: section. Add the network name under top-level networks: (or fix the typo), and the project validates. If the network already exists outside compose, declare it with external: true.

The error, verbatim:

```text
Service "web" uses an undefined network "frontend"
```

## Steps

1. Reproduce the error to see which network is undefined:

   ```
   docker compose config
   ```
   Success check: The error names the exact service and network.

2. Add the missing declaration at the top level of the compose file:

   ```
   grep -n -A3 "^networks:" docker-compose.yml
   ```
   Success check: The network name now appears under top-level networks:.

3. Re-validate and bring the stack up:

   ```
   docker compose config
   ```
   Success check: The parsed file prints with no errors.

## When to use

You hit this exact error while using docker-compose. The trigger list below matches your situation.

## When NOT to use

If the error text differs, or the failure is in a different tool (e.g. a Docker daemon error when you only need the compose fix), find the skill for that exact error instead.

## Tool compatibility

docker-compose v1 and Compose v2. Any OS.

## Variant phrasings

### Service "xxx" uses an undefined network "xxx"

### uses an undefined network

## Root cause

A service lists a network under its networks: key, but that network is never declared in the top-level networks: section of the compose file. Compose validates references before creating anything, so one missing declaration blocks the whole project.

## Edge cases

- If the network is created outside compose, declare it with external: true instead of a bare name.
- Typos are the most common cause: network names are case-sensitive.
- With multiple -f files, the top-level networks: must exist in the merged result, not just one file.

## Where this came from

Resolved from the public discussion at https://stackoverflow.com/questions/52587359 (so:52587359).