What causes "invalid source for %q tool: source %q is not a compatible type" in googleapis/mcp-toolbox?

Bind the tool to a source whose kind is `elasticsearch`

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

Same family as error 1390: ValidateSource in elasticsearchexecuteesql.go performs a type assertion of the bound source to the private `compatibleSource` interface; a source of any other type fails the assertion and the tool reports that the source is not a compatible type. The elasticsearch-execute-esql tool can only run against an Elasticsearch source.

Source

Thrown at internal/tools/elasticsearch/elasticsearchexecuteesql/elasticsearchexecuteesql.go:100

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)
	}
	paramsMap := params.AsMap()
	query, ok := paramsMap["query"].(string)
	if !ok {
		return nil, util.NewAgentError(fmt.Sprintf("unable to get cast %s", paramsMap["query"]), nil)
	}

	// Default to json format
	format := t.Cfg.Format
	if format == "" {

View on GitHub (pinned to 8cc6e09de2)

Solutions

Bind the tool to a source whose kind is `elasticsearch`
Grep tools.yaml for the source name used by this tool and confirm its `kind`
If wrapping CloudSQL/other managed Elasticsearch, ensure the correct source type is used
Restart the toolbox after fixing the config so validation re-runs

Example fix

// before (tools.yaml)
tools:
  q:
    kind: elasticsearch-execute-esql
    source: alloydb-src
// after
tools:
  q:
    kind: elasticsearch-execute-esql
    source: elasticsearch-src

Defensive patterns

Strategy: validation

Validate before calling

// Assert source kind matches the tool family before load:
// tool kind `elasticsearch-execute-esql` requires sources.<name>.kind == "elasticsearch"
if src.Kind != "elasticsearch" {
    return fmt.Errorf("tool %s requires an elasticsearch source, got %s", toolName, src.Kind)
}

Type guard

if _, ok := src.(elasticsearchexecuteesql.CompatibleSource); !ok {
    return fmt.Errorf("%T cannot back elasticsearch-execute-esql", src)
}

Try / catch

if err := tool.ValidateSource(src); err != nil {
    return nil, fmt.Errorf("fix tools.yaml source binding for %s: %w", toolName, err)
}

Prevention

Name sources after their engine to make mismatches obvious in diffs
Run `toolbox` startup validation in CI pipelines
Review source references whenever renaming or removing sources
For managed Elasticsearch-compatible services, confirm the exact supported source kind in docs

When it happens

Trigger: A tool of kind `elasticsearch-execute-esql` whose `source` references a source whose Go implementation does not implement compatibleSource (any non-Elasticsearch source kind).

Common situations: Mistyping the source name in tools.yaml so it resolves to a different source; refactoring a multi-source config and rebinding tools to the wrong source; using a managed-source wrapper that isn't recognized as Elasticsearch-compatible.

Related errors

AI-assisted analysis of googleapis/mcp-toolbox@8cc6e09de2 (2026-09-05). Data as JSON: /api/errors/cd3bbc9340bb61c6. Report an issue: GitHub.