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 source of kind `elasticsearch` in tools.yaml

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

Tool.ValidateSource in elasticsearchesql.go checks that the source attached to the tool implements the tool's private `compatibleSource` interface (i.e. it is an Elasticsearch source, not some other database source). If the Go type assertion `source.(compatibleSource)` fails, the toolbox cannot run ES|QL against that source and refuses to register/serve the tool. This is a configuration-time type mismatch between the tool and its bound source.

Source

Thrown at internal/tools/elasticsearch/elasticsearchesql/elasticsearchesql.go:102

			tools.GetAnnotationsOrDefault(c.Annotations, tools.NewReadOnlyAnnotations),
			tools.Manifest{Description: c.Description, Parameters: c.Parameters.Manifest(), AuthRequired: c.AuthRequired},
			c.Parameters,
		),
	}, nil
}

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)
	}
	var cancel context.CancelFunc
	if t.Cfg.Timeout > 0 {
		ctx, cancel = context.WithTimeout(ctx, time.Duration(t.Cfg.Timeout)*time.Second)
		defer cancel()
	} else {
		ctx, cancel = context.WithTimeout(ctx, time.Minute)
		defer cancel()
	}

View on GitHub (pinned to 8cc6e09de2)

Solutions

Point the tool's `source` field at a source of kind `elasticsearch` in tools.yaml
Check the source's `kind` value under `sources:` matches what this tool requires (elasticsearch)
If you meant a different database, use that engine's SQL tool type instead of elasticsearch-esql
Re-run toolbox and verify with `toolbox --tools-file` / the tools endpoint that the tool loads

Example fix

// before (tools.yaml)
tools:
  run-esql:
    kind: elasticsearch-esql
    source: my-postgres-source
// after
tools:
  run-esql:
    kind: elasticsearch-esql
    source: my-elasticsearch-source

Defensive patterns

Strategy: validation

Validate before calling

// Before serving/using the tool, verify the bound source kind in tools.yaml:
// yamlPath: tools.<name>.source -> sources.<srcName>.kind == "elasticsearch"
func sourceKindIs(cfg map[string]any, toolName, want string) bool {
    tools := cfg["tools"].(map[string]any)
    t := tools[toolName].(map[string]any)
    srcName := t["source"].(string)
    src := cfg["sources"].(map[string]any)[srcName].(map[string]any)
    return src["kind"] == want
}
// sourceKindIs(cfg, "run-esql", "elasticsearch")

Type guard

// Mirrors the library check: assert the source implements the compatible interface.
if esSrc, ok := src.(elasticsearchesql.CompatibleSource); ok {
    _ = esSrc // safe to use with the esql tool
} else {
    return fmt.Errorf("source %T is not an Elasticsearch source", src)
}

Try / catch

if err := tool.ValidateSource(src); err != nil {
    log.Fatalf("tool %s misconfigured: %v", toolName, err)
}

Prevention

Keep one source per engine with an unambiguous name (e.g. `elasticsearch-prod`)
After editing tools.yaml, run the toolbox once locally to catch validation errors before deploy
Lint tools.yaml in CI: assert each tool's source kind matches the tool family
Never reuse a source name across engines in multi-source configs

When it happens

Trigger: Declaring a tool of kind `elasticsearch-esql` whose `source` field references a source defined with a different SourceConfigType (e.g. a Postgres, AlloyDB or CloudSQL source) so that ValidateSource receives a sources.Source that does not satisfy compatibleSource.

Common situations: Copy-pasting a tools.yaml between configs and pointing esql tools at a non-Elasticsearch source; renaming a source but leaving the tool's `source:` key bound to an old unrelated source; composing prebuilt configs where the source kind was changed to a different database.

Related errors

AI-assisted analysis of googleapis/mcp-toolbox@8cc6e09de2 (2026-09-05). Data as JSON: /api/errors/754c5d68e0f38926. Report an issue: GitHub.