What causes "failed to parse config" in Netflix/chaosmonkey?

Inspect the wrapped error for the TOML parse position

failed to parse config

Error message

failed to parse config

What it means

Fires when viper's ReadConfig cannot parse the TOML config supplied via the io.Reader to NewFromReader. It means the provided config bytes are not valid TOML (syntax error, wrong format, empty input); the wrapped viper error identifies the exact parse problem and location.

Source

Thrown at config/monkey.go:139

// Defaults returns a Monkey config that just has the default values set
// it will not load local files or remote ones
func Defaults() *Monkey {
	v := &Monkey{v: viper.New()}
	v.setDefaults()
	return v
}

// NewFromReader returns a Monkey config which parses the initial config
// from a reader. It may load remote if configured to
// Config file must be in toml format
func NewFromReader(in io.Reader) (*Monkey, error) {
	m := &Monkey{v: viper.New()}
	m.setDefaults()
	m.v.SetConfigType("toml")
	err := m.v.ReadConfig(in)
	if err != nil {
		return nil, errors.Wrap(err, "failed to parse config")
	}

	err = m.configureRemote()
	if err != nil {
		return nil, err
	}

	return m, nil

}

// configureRemote configures viper for a remote provider if the user has
// specified one
func (m *Monkey) configureRemote() error {
	provider := m.v.GetString(param.DynamicProvider)
	endpoint := m.v.GetString(param.DynamicEndpoint)
	path := m.v.GetString(param.DynamicPath)

View on GitHub (pinned to eaa28fb761)

Solutions

Inspect the wrapped error for the TOML parse position
Validate the TOML with a linter before feeding the reader
Fix duplicate keys or invalid value types
Ensure the reader is fully written/closed before parsing

Example fix

// before
reader := strings.NewReader("key = value") // missing quotes
m, err := config.NewFromReader(reader)
// after
reader := strings.NewReader("key = \"value\"")
m, err := config.NewFromReader(reader)

Defensive patterns

Strategy: validation

Validate before calling

var probe interface{}
if err := toml.Unmarshal([]byte(tomlBytes), &probe); err != nil {
    return fmt.Errorf("invalid TOML before parsing: %w", err)
}
m, err := config.NewFromReader(bytes.NewReader(tomlBytes))

Try / catch

m, err := config.NewFromReader(r)
if err != nil {
    log.Printf("config parse failed: %v", errors.Unwrap(err))
    return err
}

Prevention

Validate generated TOML before feeding the reader
Avoid duplicate keys
Use a TOML library/linter in tests for fixtures
Ensure the reader content is complete before parsing

When it happens

Trigger: Constructing a Monkey from a reader whose content is not valid TOML (bad syntax, duplicate keys, type mismatches).

Common situations: Programmatically generated config with formatting bugs, embedded config template with a typo, hand-written test fixtures.

Understand the failure class

Parsing and encoding errors: unexpected token, malformed input — why parsers reject input and how to find the real culprit.

Related errors

AI-assisted analysis of Netflix/chaosmonkey@eaa28fb761 (2026-09-03). Data as JSON: /api/errors/62b9ef0f7c07958e. Report an issue: GitHub.