What causes "invalid source for %q tool: source %q is not a compatible type" in googleapis/mcp-toolbox?

Wire the tool to a firestore-kind source via the `source:` field.

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

firestore-list-collections' ValidateSource asserts the provided sources.Source implements compatibleSource. A source of any other type (non-Firestore) fails the check and returns this error instead of proceeding to Invoke.

Source

Thrown at internal/tools/firestore/firestorelistcollections/firestorelistcollections.go:104

// 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()

	// Check if parentPath is provided
	parentPath, _ := mapParams[parentPathKey].(string)
	if parentPath != "" {
		// Validate parent document path
		if err := fsUtil.ValidateDocumentPath(parentPath); err != nil {
			return nil, util.NewAgentError(fmt.Sprintf("invalid parent document path: %v", err), err)
		}

View on GitHub (pinned to 8cc6e09de2)

Solutions

Wire the tool to a firestore-kind source via the `source:` field.
Verify the source's `kind: firestore` in the config and that it initialized without error.

Example fix

// before
  list_collections:
    kind: firestore-list-collections
    source: my-mysql
// after
  list_collections:
    kind: firestore-list-collections
    source: my-firestore

Defensive patterns

Strategy: type-guard

Validate before calling

if err := listCollTool.ValidateSource(src); err != nil {
	return fmt.Errorf("firestore-list-collections requires a firestore source: %w", err)
}

Type guard

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

Try / catch

if err := tool.ValidateSource(src); err != nil {
	log.Printf("incompatible source: %v", err)
	return err
}

Prevention

Keep firestore tools under a source whose kind is firestore.
Add a startup assertion per tool that ValidateSource passes.
Re-run config validation after any source kind change.

When it happens

Trigger: Calling ValidateSource or Invoke with a sources.Source whose concrete Go type is not the Firestore source.

Common situations: Config pointing the tool at a SQL/other source; programmatic misuse passing the wrong source object.

Related errors

AI-assisted analysis of googleapis/mcp-toolbox@8cc6e09de2 (2026-09-05). Data as JSON: /api/errors/9096d5ee3a9caea0. Report an issue: GitHub.