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 looker-kind source

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 on the looker-createviewfromtable tool asserts that the given sources.Source implements compatibleSource. A failed assertion means the source is not a Looker source, so validation fails with this error and the tool will not invoke against it.

Source

Thrown at internal/tools/looker/lookercreateviewfromtable/lookercreateviewfromtable.go:120

// 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)
	}
	logger, err := util.LoggerFromContext(ctx)
	if err != nil {
		return nil, util.NewClientServerError(fmt.Sprintf("error getting logger from context: %s", err), http.StatusInternalServerError, err)
	}

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

View on GitHub (pinned to 8cc6e09de2)

Solutions

Point the tool's source field at a looker-kind source
Ensure the Looker source is registered (init) and successfully initialized
In tests/code, pass a source implementing the compatibleSource interface

Example fix

# before
source: my-mysql
# after
source: my-looker

Defensive patterns

Strategy: type-guard

Validate before calling

if _, ok := src.(lookercreateviewfromtable.CompatibleSource); !ok {
    return fmt.Errorf("looker-create-view-from-table requires a looker source")
}

Type guard

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

Try / catch

if err := tool.ValidateSource(src); err != nil {
    if strings.Contains(err.Error(), "not a compatible type") {
        // point the tool at the looker source
    }
    return err
}

Prevention

Cross-check tool source references against declared source kinds
Validate config at boot before tools are invoked
Use the looker source in tests for looker tools
Avoid renaming source kinds without updating tool bindings

When it happens

Trigger: Calling ValidateSource with a non-Looker source — typically because the tool's YAML source reference points at a different kind of source, or a wrong Source is passed in code.

Common situations: Config mistake binding the tool to a database source; test harness passing a stub source without compatibleSource methods; source kind drift after version upgrades.

Related errors

AI-assisted analysis of googleapis/mcp-toolbox@8cc6e09de2 (2026-09-05). Data as JSON: /api/errors/5bc5d584fa70c9cd. Report an issue: GitHub.