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 type-asserts the resolved source against the tool's compatibleSource interface (Looker API access). The error fires when the tool is bound to a source that is not a Looker source, i.e. the tool kind and source kind in the YAML configuration are incompatible.

Source

Thrown at internal/tools/looker/lookergetconnectionschemas/lookergetconnectionschemas.go:107

// 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()
	conn, ok := mapParams["conn"].(string)
	if !ok {
		return nil, util.NewAgentError(fmt.Sprintf("'conn' must be a string, got %T", mapParams["conn"]), nil)
	}
	db, _ := mapParams["db"].(string)

	sdk, err := source.GetLookerSDK(ctx, string(accessToken))
	if err != nil {

View on GitHub (pinned to 8cc6e09de2)

Solutions

  1. Set the tool's `source:` to a Looker source (kind `looker`)
  2. Implement compatibleSource if a custom source must work with Looker tools
  3. Cross-check tool-to-source mappings in the config after any rename/refactor

Example fix

// before
tools:
  schemas:
    kind: looker-get-connection-schemas
    source: bq-src
// after
tools:
  schemas:
    kind: looker-get-connection-schemas
    source: looker-src
Defensive patterns

Strategy: type-guard

Validate before calling

if err := tool.ValidateSource(src); err != nil {
    return fmt.Errorf("source %T incompatible with looker-get-connection-schemas", src)
}

Type guard

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

Try / catch

if err := tool.ValidateSource(src); err != nil {
    if strings.Contains(err.Error(), "not a compatible type") {
        return errors.New("bind this tool to a looker source")
    }
    return err
}

Prevention

When it happens

Trigger: ValidateSource (or the subsequent Invoke) receives a non-Looker source, e.g. because the tools.yaml `source:` field names a source of another kind.

Common situations: Wrong source name in the tool config; renamed sources leaving stale references; custom sources missing compatibleSource methods; passing mocks in tests.

Related errors

AI-assisted analysis of googleapis/mcp-toolbox@8cc6e09de2 (2026-09-05). Data as JSON: /api/errors/79fbbdf9964f8341. Report an issue: GitHub.