googleapis/mcp-toolbox · error

source_bucket cannot be empty for tool %q

Error message

source_bucket cannot be empty for tool %q

What it means

In cloud-storage-copyobject's Config.Initialize, source_bucket is an optional *string parameter. If it is provided (non-nil) but is the empty string, the config is invalid: the tool would render a parameter with a meaningless default. Initialize rejects it early with this error naming the tool.

Source

Thrown at internal/tools/cloudstorage/cloudstoragecopyobject/cloudstoragecopyobject.go:77

	Type              string                 `yaml:"type" validate:"required"`
	Source            string                 `yaml:"source" validate:"required"`
	Annotations       *tools.ToolAnnotations `yaml:"annotations,omitempty"`
	SourceBucket      *string                `yaml:"source_bucket,omitempty"`
	DestinationBucket *string                `yaml:"destination_bucket,omitempty"`
}

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)
	}
	if cfg.SourceBucket != nil && *cfg.SourceBucket == "" {
		return nil, fmt.Errorf("source_bucket cannot be empty for tool %q", cfg.Name)
	}
	if cfg.DestinationBucket != nil && *cfg.DestinationBucket == "" {
		return nil, fmt.Errorf("destination_bucket cannot be empty for tool %q", cfg.Name)
	}

	sourceObjectParam := parameters.NewStringParameter(sourceObjectKey, "Full source object name (path) within the source bucket, e.g. 'path/to/file.txt'.")
	destinationObjectParam := parameters.NewStringParameter(destinationObjectKey, "Full destination object name (path) within the destination bucket, e.g. 'path/to/file.txt'.")
	allParameters := parameters.Parameters{}
	if cfg.SourceBucket == nil {
		allParameters = append(allParameters, parameters.NewStringParameter(sourceBucketKey, "Name of the Cloud Storage bucket containing the source object."))
	}
	allParameters = append(allParameters, sourceObjectParam)
	if cfg.DestinationBucket == nil {
		allParameters = append(allParameters, parameters.NewStringParameter(destinationBucketKey, "Name of the Cloud Storage bucket to copy into."))
	}
	allParameters = append(allParameters, destinationObjectParam)

	return Tool{

View on GitHub (pinned to 8cc6e09de2)

Solutions

1. Set a real bucket name for source_bucket in the config, or remove the key entirely to make it a runtime parameter
2. Fix the unset environment variable that templated to an empty string
3. Add CI validation that rendered configs contain non-empty bucket names

Example fix

// before (tools.yaml)
  sourceBucket: ${SOURCE_BUCKET}   # env unset -> ""
// after
  sourceBucket: my-source-bucket   # or omit key to prompt at invocation

func checkSourceBucket(name string, ptr *string) error {
    if ptr != nil && *ptr == "" {
        return fmt.Errorf("tool %s: source_bucket set but empty (check env var)", name)
    }
    return nil
}

func hasNonEmptyBucket(p *string) bool { return p == nil || *p != "" }

tool, err := cfg.Initialize(ctx)
if err != nil && strings.Contains(err.Error(), "source_bucket cannot be empty") {
    return fmt.Errorf("set SOURCE_BUCKET env or remove the key: %w", err)
}

When it happens

Trigger: Setting `sourceBucket: ""` or `source_bucket: ""` in the tool config, or binding the pointer to an empty env-substituted value (e.g. ${SOURCE_BUCKET} unset and template resolved to empty).

Common situations: Environment variable for the bucket name not set at deploy time so the config template substitutes an empty string; YAML key present with no value (`source_bucket:`) which unmarshals to an empty string pointer.

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
• destination_bucket cannot be empty for tool %q
• description is required for tool %q
• description is required for tool %q
• doc %d: unexpected non-string key in input: %v

AI-assisted analysis of googleapis/mcp-toolbox@8cc6e09de2 (2026-09-05). Data as JSON: /api/errors/140c76eb41ad8162. Report an issue: GitHub.