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

Point the tool's "source" field at a source of kind "firestore-mongodb" in tools.yaml.

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-mongodb-get-schema tools require a source implementing compatibleSource (FirestoreClient() and ExecuteMQL). ValidateSource is called at startup with the configured source; if the type assertion fails because the source is a different kind, this error is returned.

Source

Thrown at internal/tools/firestoremongodb/firestoremongodbgetschema/firestoremongodbgetschema.go:103

// 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()

	collection, _ := mapParams[collectionKey].(string)

	logger, err := util.LoggerFromContext(ctx)
	if err != nil {
		return nil, util.NewClientServerError("error getting logger", http.StatusInternalServerError, err)
	}
	logger.DebugContext(ctx, fmt.Sprintf("executing `%s` tool for collection: %q", resourceType, collection))

View on GitHub (pinned to 8cc6e09de2)

Solutions

Point the tool's "source" field at a source of kind "firestore-mongodb" in tools.yaml.
Cross-check the source's "kind" in the sources: section against the tool type.
Rename the tool's source or create a new firestore-mongodb source if none exists.

Example fix

# before
schema-tool:
  type: firestore-mongodb-get-schema
  source: alloydb-src
# after
schema-tool:
  type: firestore-mongodb-get-schema
  source: firestore-mongodb-src

Defensive patterns

Strategy: validation

Validate before calling

// Check tool/source kind pairing before startup
if tool.Type == "firestore-mongodb-get-schema" && cfg.Sources[tool.Source].Kind != "firestore-mongodb" {
    return fmt.Errorf("tool %q needs a firestore-mongodb source", tool.Name)
}

Type guard

func isFirestoreMongoDBSource(s sources.Source) bool {
    _, ok := s.(interface {
        FirestoreClient() *firestore.Client
        ExecuteMQL(context.Context, string) (any, error)
    })
    return ok
}

Prevention

Verify the source kind listed under sources: matches the tool's documented requirement
Avoid sharing one source name across tools of different types
Add a config smoke test that instantiates all tools in CI

When it happens

Trigger: A "firestore-mongodb-get-schema" tool's "source" field names a source whose Go type does not implement compatibleSource, e.g. a postgres or http source, causing source.(compatibleSource) to fail.

Common situations: Wrong source name in tools.yaml after renaming sources; reusing a tool template across configs without updating the source; sharing one source name between tools of different types.

Related errors

AI-assisted analysis of googleapis/mcp-toolbox@8cc6e09de2 (2026-09-05). Data as JSON: /api/errors/387fc4ffc3077090. Report an issue: GitHub.