googleapis/mcp-toolbox · error

invalid source for %q tool: source %q is not a compatible ty

Error message

invalid source for %q tool: source %q is not a compatible type

What it means

This error is thrown by Tool.ValidateSource when the source instance passed to the cloudstorage-delete-bucket tool does not implement the tool's compatibleSource interface. The toolbox only allows tools to run against sources that provide the required client (e.g. a Cloud Storage-backed source with storage client). It fails fast at validation time instead of panicking later during Invoke.

Source

Thrown at internal/tools/cloudstorage/cloudstoragedeletebucket/cloudstoragedeletebucket.go:100

var _ tools.Tool = Tool{}

type Tool struct {
	tools.BaseTool[Config]
}

func (t Tool) GetSourceName() string {
	return t.Cfg.Source
}

func (t Tool) ToConfig() tools.ToolConfig {
	return t.Cfg
}

func (t Tool) ValidateSource(source sources.Source) error {
	_, ok := source.(compatibleSource)
	if !ok {
		return fmt.Errorf("invalid source for %q tool: source %q is not a compatible type", t.Cfg.Type, t.Cfg.Source)
	}
	return nil
}

func (t Tool) Invoke(ctx context.Context, s sources.Source, params parameters.ParamValues, accessToken tools.AccessToken) (any, util.ToolboxError) {
	source, ok := s.(compatibleSource)
	if !ok {
		return nil, util.NewClientServerError("source used is not compatible with the tool", http.StatusInternalServerError, nil)
	}
	mapParams := params.AsMap()
	bucket, ok := mapParams[bucketKey].(string)
	if !ok || bucket == "" {
		return nil, util.NewAgentError(fmt.Sprintf("invalid or missing '%s' parameter; expected a non-empty string", bucketKey), nil)
	}

	resp, err := source.DeleteBucket(ctx, bucket)
	if err != nil {
		return nil, cloudstoragecommon.ProcessGCSError(err)

View on GitHub (pinned to 8cc6e09de2)

Solutions

  1. Set the tool's source field to the name of a cloudstorage source in tools.yaml
  2. Verify the referenced source kind is the Cloud Storage source that implements the tool's compatibleSource interface
  3. Run the toolbox and check startup logs: source type mismatch is reported at config load time

Example fix

// before
sources:
  my-db:
    kind: postgres
    uri: ...
tools:
  delete-bucket:
    kind: cloudstorage-delete-bucket
    source: my-db
// after
sources:
  my-gcs:
    kind: cloudstorage
    ...
tools:
  delete-bucket:
    kind: cloudstorage-delete-bucket
    source: my-gcs
Defensive patterns

Strategy: type-guard

Validate before calling

// before starting the server, check the source kind in your tools.yaml
sources := cfg.Sources
if _, ok := sources["my-gcs"].(cloudstorage.Source); !ok {
    log.Fatalf("tool delete-bucket requires a cloudstorage source")
}

Type guard

func isCompatibleGCSSource(s sources.Source) bool {
    _, ok := s.(cloudstorage.CompatibleSource)
    return ok
}

Prevention

When it happens

Trigger: Registering the cloudstorage-delete-bucket tool in tools.yaml but binding it to a source of the wrong kind (e.g. a postgres, bigquery, or other non-cloudstorage source, or a cloudstorage source whose client is not the storage client the interface requires).

Common situations: Copy-pasting a tools.yaml where the tool's source field points at the wrong named source; renaming sources and leaving a stale reference; mixing sources from incompatible SDK versions after an upgrade.

Related errors

AI-assisted analysis of googleapis/mcp-toolbox@8cc6e09de2 (2026-09-05). Data as JSON: /api/errors/2799927b3ec83cb9. Report an issue: GitHub.