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

Bind the tool to a Looker source 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

ValidateSource for looker-health-pulse asserts the given sources.Source to compatibleSource and returns this error if the assertion fails. It ensures at validation time that the tool only runs against Looker sources implementing the required interface. Non-compatible sources cannot provide the Looker API client behavior the tool needs.

Source

Thrown at internal/tools/looker/lookerhealthpulse/lookerhealthpulse.go:113

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)
	}

	sdk, err := source.GetLookerSDK(ctx, string(accessToken))
	if err != nil {
		return nil, util.NewClientServerError("error getting sdk", http.StatusInternalServerError, err)
	}

View on GitHub (pinned to 8cc6e09de2)

Solutions

Bind the tool to a Looker source in tools.yaml.
Ensure the source implements every method required by compatibleSource.
Reload the toolbox after correcting the configuration.
Add a regression test invoking ValidateSource with the production source type.

Example fix

// before
tool.ValidateSource(otherSource{}) // error: not compatible
// after
var _ compatibleSource = looker.Source{}
tool.ValidateSource(lookerSource)

Defensive patterns

Strategy: validation

Validate before calling

if err := tool.ValidateSource(src); err != nil {
    return fmt.Errorf("looker-health-pulse requires looker source, got %T", src)
}

Type guard

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

Try / catch

if err := tool.ValidateSource(src); err != nil {
    if strings.Contains(err.Error(), "not a compatible type") {
        return fmt.Errorf("check tools.yaml source binding: %w", err)
    }
    return err
}

Prevention

Point looker-health-pulse tools at kind: looker sources only
Run source validation as part of startup
Test with the actual source type used in production
Review YAML for typos in source names

When it happens

Trigger: Server-side validation passes a non-Looker source, or tools.yaml binds this tool to a source of the wrong kind, so the type assertion in ValidateSource fails.

Common situations: Wrong source reference in tools.yaml, renamed/changed source kinds, or unit tests supplying simplified stubs that don't implement compatibleSource.

Related errors

AI-assisted analysis of googleapis/mcp-toolbox@8cc6e09de2 (2026-09-05). Data as JSON: /api/errors/c00797b290522741. Report an issue: GitHub.