What causes "-rate=0 requires setting -max-workers" in tsenart/vegeta?

Pass an explicit worker count, e.g. add `-max-workers=10` (or the concurrency you want) alongside `-rate=0`.

-rate=0 requires setting -max-workers

Error message

-rate=0 requires setting -max-workers

What it means

Vegeta's attack command rejects a configuration where the request rate is left at its default (Freq==0, i.e. `-rate=0` meaning 'as fast as possible') while the worker pool is also left at the default max-workers. In that mode throughput is unbounded and driven entirely by workers, so vegeta requires an explicit worker count to avoid an ambiguous/accidentally unlimited attack. This is a deliberate argument-validation guard in attack().

Source

Thrown at attack.go:119

	redirects      int
	maxBody        int64
	headers        headers
	proxyHeaders   headers
	laddr          localAddr
	keepalive      bool
	resolvers      csl
	unixSocket     string
	promAddr       string
	dnsTTL         time.Duration
	sessionTickets bool
	connectTo      map[string][]string
}

// attack validates the attack arguments, sets up the
// required resources, launches the attack and writes the results
func attack(opts *attackOpts) (err error) {
	if opts.maxWorkers == vegeta.DefaultMaxWorkers && opts.rate.Freq == 0 {
		return fmt.Errorf("-rate=0 requires setting -max-workers")
	}

	if len(opts.resolvers) > 0 {
		res, err := resolver.NewResolver(opts.resolvers)
		if err != nil {
			return err
		}
		net.DefaultResolver = res
	}

	net.DefaultResolver.PreferGo = true

	files := map[string]io.Reader{}
	for _, filename := range []string{opts.targetsf, opts.bodyf} {
		if filename == "" {
			continue
		}
		f, err := file(filename, false)

View on GitHub (pinned to cf58112690)

Solutions

Pass an explicit worker count, e.g. add `-max-workers=10` (or the concurrency you want) alongside `-rate=0`.
If you did not intend unlimited rate, set a real rate such as `-rate=100/s` and leave -max-workers at its default.
If -rate is read from a config/variable, ensure it is never the zero value unless max-workers is also set.

Example fix

// before
vegeta attack -rate=0 -duration=10s -targets=targets.txt
// after
vegeta attack -rate=0 -max-workers=50 -duration=10s -targets=targets.txt

Defensive patterns

Strategy: validation

Validate before calling

if rate == 0 && maxWorkers == vegeta.DefaultMaxWorkers {
    return fmt.Errorf("-rate=0 requires setting -max-workers")
}

Prevention

Always pass -max-workers explicitly when using -rate=0.
Wrap vegeta invocations in a script that asserts rate/max-workers combination before exec.
Keep rate and worker settings together in one config block to avoid partial overrides.

When it happens

Trigger: Running `vegeta attack -rate=0 ...` (or omitting -rate so it defaults to 0) without also passing a `-max-workers` value other than the default (DefaultMaxWorkers).

Common situations: Users copying old vegeta examples where `-rate=0` meant unlimited rate; CI scripts that pass -rate=0 for maximum throughput but forget -max-workers; users upgrading vegeta and hitting the newly enforced rule.

Related errors

AI-assisted analysis of tsenart/vegeta@cf58112690 (2026-08-31). Data as JSON: /api/errors/59200db6c35f919c. Report an issue: GitHub.