googleapis/mcp-toolbox · error

description is required for tool %q

Error message

description is required for tool %q

What it means

MCP toolbox tools require a human-readable `description` field; it is what the LLM sees in the tool manifest. `Config.Initialize` validates this eagerly and refuses to construct the tool when the description is empty.

Source

Thrown at internal/tools/looker/lookergetconnections/lookergetconnections.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)
	}

	params := parameters.Parameters{}

	// finish tool setup
	return Tool{
		BaseTool: tools.NewBaseTool(
			cfg,
			tools.GetAnnotationsOrDefault(cfg.Annotations, tools.NewReadOnlyAnnotations),
			tools.Manifest{Description: cfg.Description, Parameters: params.Manifest(), AuthRequired: cfg.AuthRequired},
			params,
		),
	}, nil
}

// validate interface
var _ tools.Tool = Tool{}

View on GitHub (pinned to 8cc6e09de2)

Solutions

1. Add a non-empty `description:` to the tool entry in tools.yaml
2. If constructing Config in Go, set cfg.Description before calling Initialize
3. Validate the whole config with the toolbox CLI (e.g. `toolbox --tools-file tools.yaml --prebuilt` dry run) to surface all such validation errors at once

Example fix

// before
tools:
  list-conns:
    kind: looker-get-connections
    source: my-looker
// after
tools:
  list-conns:
    kind: looker-get-connections
    source: my-looker
    description: Fetches Looker database connections.

if cfg.Description == "" {
    return fmt.Errorf("tool %q: description must be set before Initialize", cfg.Name)
}
_ = cfg.Initialize(context.Background())

tool, err := cfg.Initialize(ctx)
if err != nil && strings.Contains(err.Error(), "description is required") {
    // fix the tool config entry and retry
    return fmt.Errorf("config error for %s: %w", cfg.Name, err)
}

When it happens

Trigger: Calling Config.Initialize on a looker-get-connections tool whose `description` field in the tool config is absent or set to "".

Common situations: Hand-edited tools.yaml where the description line was deleted; programmatically-built Config structs that leave Description zero-valued; copy-pasting a tool entry and forgetting to fill in the description.

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
• description is required for tool %q
• description is required for tool %q
• description is required for tool %q

AI-assisted analysis of googleapis/mcp-toolbox@8cc6e09de2 (2026-09-05). Data as JSON: /api/errors/efdaa30ccfd6e087. Report an issue: GitHub.