googleapis/mcp-toolbox · error

description is required for tool %q

Error message

description is required for tool %q

What it means

Config.Initialize for the lookerlistagents tool validates that a non-empty description was provided. The description becomes the tool's documentation surfaced to the LLM client, so a tool without one is rejected at configuration load time with this error naming the tool. This is a config-time validation failure — the server fails before any request is served.

Source

Thrown at internal/tools/looker/lookerlistagents/lookerlistagents.go:71

}

type Config struct {
	tools.ConfigBase `yaml:",inline"`
	Type             string                 `yaml:"type" validate:"required"`
	Source           string                 `yaml:"source" validate:"required"`
	Annotations      *tools.ToolAnnotations `yaml:"annotations,omitempty"`
}

// validate interface
var _ tools.ToolConfig = Config{}

func (cfg Config) ToolConfigType() string {
	return resourceType
}

func (cfg Config) Initialize(context.Context) (tools.Tool, error) {
	if cfg.Description == "" {
		return nil, fmt.Errorf("description is required for tool %q", cfg.Name)
	}

	allParameters := parameters.Parameters{}

	annotations := &tools.ToolAnnotations{}
	if cfg.Annotations != nil {
		*annotations = *cfg.Annotations
	}
	readOnlyHint := true
	annotations.ReadOnlyHint = &readOnlyHint

	return Tool{
		BaseTool: tools.NewBaseTool(
			cfg,
			annotations,
			tools.Manifest{Description: cfg.Description, Parameters: allParameters.Manifest(), AuthRequired: cfg.AuthRequired},
			allParameters,
		),

View on GitHub (pinned to 8cc6e09de2)

Solutions

1. Add a non-empty description field to the looker-list-agents tool in the config YAML.
2. If using env/template substitution, verify the substituted value is not empty.
3. Restart/reload the server and confirm the tool registers without error.

Example fix

// before (tools.yaml)
tools:
  list-agents:
    kind: looker-list-agents
    source: looker-prod

// after
tools:
  list-agents:
    kind: looker-list-agents
    source: looker-prod
    description: List Looker agents available to the authenticated user.

// before applying the config
for name, tool := range cfg.Tools {
    if tool.Kind == "looker-list-agents" && strings.TrimSpace(tool.Description) == "" {
        return fmt.Errorf("tool %q is missing required description", name)
    }
}

tool, err := cfg.Initialize(ctx)
if err != nil && strings.Contains(err.Error(), "description is required") {
    return fmt.Errorf("config error in tools.yaml: %w", err)
}

When it happens

Trigger: A tools.yaml defines a looker-list-agents tool without the description field (or sets it to an empty string), and the server calls cfg.Initialize while building the tool registry.

Common situations: Hand-writing YAML and omitting description; templating that emits an empty value; deleting a description during cleanup; environment substitution that resolves description to ""; a copy of another tool's config with the description accidentally removed.

Understand the failure class

Background: "is required", "must be set", "missing required field": configuration validation errors across open-source libraries — this error's family across 36 libraries.

Related errors

• description is required for tool %q
• description is required for tool %q
• error parsing argument: %w
• invalid role %q: must be 'user' or 'assistant'
• description is required for tool %q

AI-assisted analysis of googleapis/mcp-toolbox@8cc6e09de2 (2026-09-05). Data as JSON: /api/errors/6f60a5b0482daa1b. Report an issue: GitHub.