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

This error is returned by the looker-get-connection-table-columns tool's ValidateSource when the sources.Source instance passed to the tool does not implement the tool's compatibleSource interface (UseClientAuthorization() bool, GetAuthTokenHeaderName() string, LookerApiSettings() *rtl.ApiSettings). The toolbox only wires Looker tools to a Looker source, so this means the tool was bound to a source of the wrong kind (e.g. a Postgres or other database source instead of a Looker source). It is a configuration/wiring error, not a runtime data error.

Source

Thrown at internal/tools/looker/lookergetconnectiontablecolumns/lookergetconnectiontablecolumns.go:109

// 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("unable to get logger from ctx", http.StatusInternalServerError, err)
	}
	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)
	}

View on GitHub (pinned to 8cc6e09de2)

Solutions

  1. Point the tool's `source` field in your config at a Looker source definition (kind: looker), not a database source.
  2. If using a custom/stub source, implement the full compatibleSource interface: UseClientAuthorization() bool, GetAuthTokenHeaderName() string, LookerApiSettings() *rtl.ApiSettings.
  3. Use prebuilt Looker configs or copy the source block from the Looker docs so source kind and tool match.
  4. If the interface changed after an upgrade, update your source implementation to satisfy the new methods (compiler will list them).

Example fix

# before (tools.yaml)
tools:
  get-columns:
    kind: looker-get-connection-table-columns
    source: my-postgres-source
# after
sources:
  my-looker:
    kind: looker
    ...
tools:
  get-columns:
    kind: looker-get-connection-table-columns
    source: my-looker
Defensive patterns

Strategy: type-guard

Validate before calling

if _, ok := source.(lookergetconnectiontablecolumns.CompatibleSource); !ok {
    return fmt.Errorf("tool %q requires a Looker source; got %T", toolName, source)
}

Type guard

func isCompatibleLookerSource(s sources.Source) bool {
    _, ok := s.(interface {
        UseClientAuthorization() bool
        GetAuthTokenHeaderName() string
        LookerApiSettings() *rtl.ApiSettings
    })
    return ok
}

Try / catch

if err := tool.ValidateSource(src); err != nil {
    log.Fatalf("tool/source mismatch: %v", err) // fail fast at startup
}

Prevention

When it happens

Trigger: Calling ValidateSource (directly or via server/tool startup validation) with a source whose concrete type does not satisfy compatibleSource — for example binding looker-get-connection-table-columns to a non-Looker source, or using a source that implements sources.Source but not the Looker-specific interface methods.

Common situations: Hand-writing a tools.yaml where the tool's source field points at the wrong source definition; refactoring/renaming sources and mis-wiring; programmatically constructing a tool + source pair in tests with a stub source that lacks LookerApiSettings/UseClientAuthorization/GetAuthTokenHeaderName; upgrading after the compatibleSource interface gained new methods so an older custom source no longer satisfies it.

Related errors

AI-assisted analysis of googleapis/mcp-toolbox@8cc6e09de2 (2026-09-05). Data as JSON: /api/errors/5156a5259693018c. Report an issue: GitHub.