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

cloudstorage-get-object-metadata's ValidateSource fails when the bound source does not implement the tool's compatibleSource interface. Only Cloud Storage sources can back this tool; any other source type triggers this error during server startup.

Source

Thrown at internal/tools/cloudstorage/cloudstoragegetobjectmetadata/cloudstoragegetobjectmetadata.go:114

// 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)
	}
	object, ok := mapParams[objectKey].(string)
	if !ok || object == "" {
		return nil, util.NewAgentError(fmt.Sprintf("invalid or missing '%s' parameter; expected a non-empty string", objectKey), nil)
	}

View on GitHub (pinned to 8cc6e09de2)

Solutions

  1. Point `source:` at a cloudstorage source entry.
  2. Remove the tool from sources that cannot support it.
  3. Cross-check tool-to-source bindings in the config before startup.

Example fix

# before
source: my-mysql
# after
source: my-gcs
Defensive patterns

Strategy: validation

Validate before calling

if _, ok := src.(compatibleSource); !ok {
    return fmt.Errorf("get-object-metadata requires a cloudstorage source, got %T", src)
}

Type guard

func isCompatible(s sources.Source) bool { _, ok := s.(compatibleSource); return ok }

Prevention

When it happens

Trigger: Type assertion `source.(compatibleSource)` fails in ValidateSource because the tool's `source` config points at a non-cloudstorage source.

Common situations: Miswired YAML binding a GCS object tool to a database source; shared config files with mismatched source names; refactors that changed source kinds.

Related errors

AI-assisted analysis of googleapis/mcp-toolbox@8cc6e09de2 (2026-09-05). Data as JSON: /api/errors/7f8e714c90656c04. Report an issue: GitHub.