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

Thrown by the cloudstorage-list-objects tool's ValidateSource when the source instance passed to the tool does not implement the tool's internal compatibleSource interface. The tool only works with Cloud Storage sources; wiring it to any other source kind fails this type assertion.

Source

Thrown at internal/tools/cloudstorage/cloudstoragelistobjects/cloudstoragelistobjects.go:129

// validate interface
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 := cloudstoragecommon.ResolveString(t.Cfg.Bucket, mapParams, bucketKey)
	if bucket == "" {
		return nil, util.NewAgentError(fmt.Sprintf("invalid or missing '%s' parameter; expected a non-empty string", bucketKey), nil)
	}
	prefix := cloudstoragecommon.ResolveString(t.Cfg.Prefix, mapParams, prefixKey)
	delimiter := cloudstoragecommon.ResolveString(t.Cfg.Delimiter, mapParams, delimiterKey)
	pageToken, _ := mapParams[pageTokenKey].(string)
	maxResults, _ := mapParams[maxResultsKey].(int)

View on GitHub (pinned to 8cc6e09de2)

Solutions

  1. Point the tool's `source` field at a source with kind cloud-storage
  2. Check that the referenced source name exists in the sources section of tools.yaml
  3. Rebuild/reload after fixing so ValidateSource runs against the correct source

Example fix

// before
tools:
  list-objects:
    kind: cloudstorage-list-objects
    source: my-postgres
// after
tools:
  list-objects:
    kind: cloudstorage-list-objects
    source: my-gcs
Defensive patterns

Strategy: type-guard

Validate before calling

if _, ok := src.(sources.Source); ok {
    // additionally ensure the configured source kind is cloud-storage in tools.yaml
}

Type guard

func asCloudStorageSource(s sources.Source) (CompatibleSource, bool) {
    cs, ok := s.(CompatibleSource)
    return cs, ok
}

Prevention

When it happens

Trigger: A tools.yaml binds the tool to a non-cloud-storage source (e.g. a postgres source) via `source:`; or code calls ValidateSource with the wrong sources.Source implementation.

Common situations: Copy-pasting a tool definition and forgetting to change the source reference; renaming sources and pointing the tool at the wrong one; programmatically assembling tool/source pairs.

Related errors

AI-assisted analysis of googleapis/mcp-toolbox@8cc6e09de2 (2026-09-05). Data as JSON: /api/errors/9e6bbb8df9334ddb. Report an issue: GitHub.