What causes "marshaling hosts option: %w" in grafana/k6?

Verify options.hosts in the script is a plain object mapping hostnames to 'host:port' strings

marshaling hosts option: %w

Error message

marshaling hosts option: %w

What it means

Returned by setFlagsFromK6Options during launch flag preparation when json.Marshal of the k6 options.Hosts value (types.NullHosts) fails. Hosts is a map of host strings, so standard marshaling practically cannot fail; if this appears it indicates a corrupted/programmatic Options value (unsupported type inside Hosts) rather than a normal script configuration mistake.

Source

Thrown at internal/js/modules/k6/browser/chromium/browser_type.go:534

	if currHostResolver, ok := flags["host-resolver-rules"]; ok {
		hostResolver = append(hostResolver, fmt.Sprintf("%s", currHostResolver))
	}

	// Add the host resolver rules.
	//
	// This is done by marshaling the k6 hosts option to JSON and then
	// unmarshaling it to a map[string]string. This is done because the
	// k6 v0.42 changed Hosts from a map to types.NullHosts and doesn't
	// expose the map anymore.
	//
	// TODO: A better way to do this would be to handle the resolver
	// rules by communicating with Chromium (and then using Hosts's
	// Match method) instead of passing the rules via the command line
	// to Chromium.
	var rules map[string]string
	b, err := json.Marshal(k6opts.Hosts)
	if err != nil {
		return fmt.Errorf("marshaling hosts option: %w", err)
	}
	if err := json.Unmarshal(b, &rules); err != nil {
		return fmt.Errorf("unmarshaling hosts option: %w", err)
	}
	for k, v := range rules {
		hostResolver = append(hostResolver, fmt.Sprintf("MAP %s %s", k, v))
	}
	if len(hostResolver) > 0 {
		sort.Strings(hostResolver)
		flags["host-resolver-rules"] = strings.Join(hostResolver, ",")
	}

	return nil
}

// makeLogger makes and returns an extension wide logger.
func makeLogger(ctx context.Context, envLookup env.LookupFunc) (*log.Logger, error) {
	var (

View on GitHub (pinned to 93accf6570)

Solutions

Verify options.hosts in the script is a plain object mapping hostnames to 'host:port' strings
If you use the Go API, ensure Options.Hosts is a types.NullHosts built from string values
Temporarily remove the hosts option to confirm the rest of the launch works, then re-add entries incrementally
Report upstream with the exact hosts value if it reproduces on stock k6

Example fix

// before
export const options = { hosts: { 'test.k6.io': null } };

// after
export const options = { hosts: { 'test.k6.io': 'localhost:3000' } };

Defensive patterns

Strategy: validation

Validate before calling

const hosts = { 'test.k6.io': 'localhost:3000' };
if (Object.values(hosts).some(v => typeof v !== 'string')) {
  throw new Error('all hosts values must be strings of the form host:port');
}
export const options = { hosts };

Type guard

function isValidHostsMap(h) {
  return typeof h === 'object' && h !== null &&
    Object.entries(h).every(([, v]) => typeof v === 'string' && /^[^:]+:\d+$/.test(v));
}

Prevention

Keep hosts mappings as 'hostname' -> 'host:port' strings
Generate hosts config from a single validated source in CI
Report stock-k6 reproductions upstream — this path is nearly unreachable with normal scripts

When it happens

Trigger: Launching with options.hosts set to a value that marshals abnormally — in practice only via the Go API constructing lib.Options with a hand-built NullHosts containing unsupported data; not reachable from a normal JS script where hosts parse into string maps.

Common situations: Custom k6 builds or extensions injecting lib.Options programmatically; extremely unusual in stock k6 usage — if seen with a plain script, suspect a k6 bug in the options type.

Related errors

AI-assisted analysis of grafana/k6@93accf6570 (2026-08-15). Data as JSON: /api/errors/602c3912325a952c. Report an issue: GitHub.