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

The looker-list-gitbranches tool's ValidateSource checks that the supplied sources.Source implements its compatibleSource interface (Looker SDK helpers). A failed type assertion means the tool was bound to a source of the wrong kind, so validation returns this error rather than allowing invocation against an incompatible source.

Source

Thrown at internal/tools/looker/lookerlistgitbranches/lookerlistgitbranches.go:106

// 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(fmt.Sprintf("error getting sdk: %v", err), http.StatusInternalServerError, err)
	}

	mapParams := params.AsMap()
	projectId := mapParams["project_id"].(string)

	resp, err := sdk.AllGitBranches(projectId, source.LookerApiSettings())

View on GitHub (pinned to 8cc6e09de2)

Solutions

  1. Point the tool's `source` field at a source of kind looker in tools.yaml.
  2. Confirm the source name matches an existing entry under `sources` with kind looker.
  3. If using a custom source, implement the full compatibleSource interface.

Example fix

# before
sources:
  looker-src:
    kind: http
tools:
  git-branches:
    kind: looker-list-gitbranches
    source: looker-src

# after
sources:
  looker-src:
    kind: looker
    # ...looker auth config...
tools:
  git-branches:
    kind: looker-list-gitbranches
    source: looker-src
Defensive patterns

Strategy: type-guard

Validate before calling

// Go: validate the source before use
if err := tool.ValidateSource(src); err != nil {
    return fmt.Errorf("tool %s bound to incompatible source: %w", tool.Name(), err)
}

Type guard

func isLookerSource(s sources.Source) bool {
    _, ok := s.(interface {
        UseClientAuthorization() bool
        GetAuthTokenHeaderName() string
        LookerApiSettings() *rtl.ApiSettings
        GetLookerSDK(context.Context, string) (*v4.LookerSDK, error)
    })
    return ok
}

Try / catch

if err := tool.ValidateSource(src); err != nil {
    if strings.Contains(err.Error(), "not a compatible type") {
        return fmt.Errorf("rebind looker-list-gitbranches to a looker-kind source: %w", err)
    }
    return err
}

Prevention

When it happens

Trigger: Calling Tool.ValidateSource(source) where source is not a Looker source — typically because the tool's `source:` references a non-Looker source in tools.yaml, or the wrong Source is passed when composing tools programmatically.

Common situations: Tool config pointing at a database source instead of the looker source; duplicated tool definitions with copy-pasted source names; a custom or upgraded Looker source that dropped a required interface method (GetLookerSDK, LookerApiSettings, etc.).

Related errors

AI-assisted analysis of googleapis/mcp-toolbox@8cc6e09de2 (2026-09-05). Data as JSON: /api/errors/f525017046e040a6. Report an issue: GitHub.