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 in the config

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-create-projectfile tool asserts that the provided sources.Source implements the tool's compatibleSource interface. If the assertion fails, the source is of an incompatible type (not a Looker source) and the tool returns this error instead of proceeding to Invoke.

Source

Thrown at internal/tools/looker/lookercreateprojectfile/lookercreateprojectfile.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)
	}
	sdk, err := source.GetLookerSDK(ctx, string(accessToken))
	if err != nil {
		return nil, util.NewClientServerError("error getting sdk", http.StatusInternalServerError, err)
	}

	mapParams := params.AsMap()
	projectId, ok := mapParams["project_id"].(string)
	if !ok {
		return nil, util.NewAgentError(fmt.Sprintf("'project_id' must be a string, got %T", mapParams["project_id"]), nil)

View on GitHub (pinned to 8cc6e09de2)

Solutions

Point the tool's source field at a looker-kind source in the config
Confirm the source was registered via its init() and initialized as the Looker source type
In code/tests, pass a source implementing the compatibleSource interface

Example fix

# before
source: my-bigquery
tools:
  create-file:
    kind: looker-create-projectfile
# after
source: my-looker
tools:
  create-file:
    kind: looker-create-projectfile

Defensive patterns

Strategy: type-guard

Validate before calling

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

Type guard

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

Try / catch

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

Prevention

Match tool kind and source kind in the YAML config
Validate the toolbox config before server startup
Keep source kind names stable across upgrades
Call ValidateSource early in startup to catch misbindings

When it happens

Trigger: ValidateSource is called with a source whose concrete type is not a Looker source — typically because the tool's YAML source field points at a non-Looker source, or the wrong Source is passed programmatically.

Common situations: Config typo mapping a Looker tool to a database source; tests passing a mock source lacking the compatibleSource methods; sources refactored so kind strings no longer match.

Related errors

AI-assisted analysis of googleapis/mcp-toolbox@8cc6e09de2 (2026-09-05). Data as JSON: /api/errors/50166becf9077573. Report an issue: GitHub.