docker/compose · error

invalid scale specifier: %s

Error message

invalid scale specifier: %s

What it means

Thrown by parseServicesReplicasArgs in the `docker compose scale` command when an argument cannot be split into a SERVICE=REPLICAS pair. The parser uses strings.Cut on '=' and requires a non-empty service name and a non-empty replica value, so any argument missing '=' or having an empty side is rejected before any project is loaded.

Source

Thrown at cmd/compose/scale.go:119

	return backend.Scale(ctx, project, api.ScaleOptions{Services: services})
}

func setServiceScale(project *types.Project, name string, replicas int) error {
	service, err := project.GetService(name)
	if err != nil {
		return err
	}
	service.SetScale(replicas)
	project.Services[name] = service
	return nil
}

func parseServicesReplicasArgs(args []string) (map[string]int, error) {
	serviceReplicaTuples := map[string]int{}
	for _, arg := range args {
		key, val, ok := strings.Cut(arg, "=")
		if !ok || key == "" || val == "" {
			return nil, fmt.Errorf("invalid scale specifier: %s", arg)
		}
		intValue, err := strconv.Atoi(val)
		if err != nil {
			return nil, fmt.Errorf("invalid scale specifier: can't parse replica value as int: %v", arg)
		}
		serviceReplicaTuples[key] = intValue
	}
	return serviceReplicaTuples, nil
}

View on GitHub (pinned to ddc4b044b6)

Solutions

1. Rewrite each argument as SERVICE=REPLICAS, e.g. `docker compose scale web=3 db=2`.
2. Check for stray empty arguments produced by unset shell variables (use `${VAR:?}` or quote the full pair).
3. If migrating from Compose v1, replace the old space-separated `scale svc count` form with the '=' form.
4. Run `docker compose scale --help` to confirm the expected [SERVICE=REPLICAS...] syntax.

Example fix

# before
docker compose scale web 3

# after
docker compose scale web=3

# before calling compose, verify each arg is SERVICE=INT
for arg in "$@"; do
  [[ "$arg" =~ ^[A-Za-z0-9._-]+=.+$ ]] || { echo "bad scale arg: $arg" >&2; exit 2; }
done
docker compose scale "$@"

When it happens

Trigger: Running `docker compose scale` with an argument like `web` (no '='), `=3` (empty service name), `web=` (empty replica value), or passing flags after args without '--'. cobra.MinimumNArgs(1) passes all args straight into the parser, and the first malformed one aborts the whole command.

Common situations: Muscle memory from `docker-compose scale web 3` (space-separated, old v1 syntax) instead of `web=3`; shell-quoting mistakes that swallow the '='; copy-pasting a service list where one entry is just a name; trailing empty arg from an unquoted variable like `scale ${SVC}=$REPLICAS` when REPLICAS is unset.

Related errors

• invalid scale specifier: can't parse replica value as int: %
• invalid --scale option %q. Should be SERVICE=NUM
• cannot combine %s and --remove-orphans
• no service selected
• --wait-timeout must be a non-negative integer

AI-assisted analysis of docker/compose@ddc4b044b6 (2026-08-15). Data as JSON: /api/errors/0784bfd3eeb8e520. Report an issue: GitHub.