googleapis/mcp-toolbox · error

%s is not an allowed value

Error message

%s is not an allowed value

What it means

StringParameter.Parse rejects the runtime value supplied for a string parameter because it is not in the parameter's 'allowedValues' whitelist. The guard fires at tool invocation time when the caller (or LLM) passes a value outside the configured enumeration; the offending value is named in the message.

Source

Thrown at internal/util/parameters/parameters.go:700

}

var _ Parameter = &StringParameter{}

// StringParameter is a parameter representing the "string" type.
type StringParameter struct {
	CommonParameter `yaml:",inline"`
	Default         *string `yaml:"default"`
	Escape          *string `yaml:"escape"`
}

// Parse casts the value "v" as a "string".
func (p *StringParameter) Parse(v any) (any, error) {
	newV, ok := v.(string)
	if !ok {
		return nil, &ParseTypeError{p.Name, p.Type, v}
	}
	if !p.IsAllowedValues(newV) {
		return nil, fmt.Errorf("%s is not an allowed value", newV)
	}
	if p.IsExcludedValues(newV) {
		return nil, fmt.Errorf("%s is an excluded value", newV)
	}
	if p.Escape != nil {
		return applyEscape(*p.Escape, newV)
	}
	return newV, nil
}

func applyEscape(escape, v string) (any, error) {
	switch escape {
	case escapeBackticks:
		escaped := strings.ReplaceAll(v, "`", "``")
		return fmt.Sprintf("`%s`", escaped), nil
	case escapeDoubleQuotes:
		escaped := strings.ReplaceAll(v, `"`, `""`)
		return fmt.Sprintf(`"%s"`, escaped), nil

View on GitHub (pinned to 8cc6e09de2)

Solutions

1. Pass one of the values listed in the parameter's allowedValues in tools.yaml
2. Add the desired value to allowedValues if it should be permitted
3. Check for case or whitespace mismatches between the passed value and the whitelist

When it happens

Trigger: Thrown at internal/util/parameters/parameters.go:700 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of googleapis/mcp-toolbox@8cc6e09de2 (2026-09-05). Data as JSON: /api/errors/86aae2fc382030cb. Report an issue: GitHub.