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

ValidateSource asserts that the tool's source implements the looker-compatibleSource interface; any source of a kind other than "looker" fails the assertion and produces this error. It is a startup guard ensuring the tool only runs against a real Looker backend.

Source

Thrown at internal/tools/looker/lookerdevmode/lookerdevmode.go:103

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)
	}
	logger, err := util.LoggerFromContext(ctx)
	if err != nil {
		return nil, util.NewClientServerError("unable to get logger from ctx", http.StatusInternalServerError, err)
	}
	mapParams := params.AsMap()
	devMode, ok := mapParams["devMode"].(bool)
	if !ok {
		return nil, util.NewAgentError(fmt.Sprintf("'devMode' must be a boolean, got %T", mapParams["devMode"]), nil)
	}

View on GitHub (pinned to 8cc6e09de2)

Solutions

  1. Bind the tool to a source with kind "looker"
  2. Verify the referenced source exists and is looker-typed
  3. Use a compatibleSource mock when unit testing

Example fix

// before
source: my-postgres
// after
source: my-looker
Defensive patterns

Strategy: type-guard

Validate before calling

if err := tool.ValidateSource(src); err != nil { return err }

Type guard

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

Try / catch

if err := tool.ValidateSource(src); err != nil { log.Fatalf("tool %s bound to wrong source: %v", name, err) }

Prevention

When it happens

Trigger: Config load where the lookerdevmode tool references a non-looker source; direct calls to ValidateSource with a foreign sources.Source.

Common situations: Wrong source name in YAML; environments where a looker source was replaced by another kind; test code passing dummy sources.

Related errors

AI-assisted analysis of googleapis/mcp-toolbox@8cc6e09de2 (2026-09-05). Data as JSON: /api/errors/27189d8041302166. Report an issue: GitHub.