googleapis/mcp-toolbox · error

failed to read a prebuilt tool %w

Error message

failed to read a prebuilt tool %w

What it means

After listing the embedded tools directory, loadPrebuiltToolYAMLs reads each .yaml file with prebuiltConfigsFS.ReadFile. This error wraps a failure reading one specific embedded file — for embedded FSs this is rare (contents are in the binary) and usually indicates an inconsistent build or corrupted embed state.

Source

Thrown at internal/prebuiltconfigs/prebuiltconfigs.go:77

}

// Load all available pre built tools
func loadPrebuiltToolYAMLs() (map[string][]byte, []string, error) {
	toolYAMLs := make(map[string][]byte)
	var sourceTypes []string
	entries, err := prebuiltConfigsFS.ReadDir("tools")
	if err != nil {
		errMsg := fmt.Errorf("failed to read prebuilt tools %w", err)
		return nil, nil, errMsg
	}

	for _, entry := range entries {
		lowerName := strings.ToLower(entry.Name())
		if !entry.IsDir() && (strings.HasSuffix(lowerName, ".yaml")) {
			filePathInFS := path.Join("tools", entry.Name())
			content, err := prebuiltConfigsFS.ReadFile(filePathInFS)
			if err != nil {
				errMsg := fmt.Errorf("failed to read a prebuilt tool %w", err)
				return nil, nil, errMsg
			}
			sourceTypeKey := entry.Name()[:len(entry.Name())-len(".yaml")]

			sourceTypes = append(sourceTypes, sourceTypeKey)
			toolYAMLs[sourceTypeKey] = content
		}
	}
	if len(toolYAMLs) == 0 {
		errMsg := fmt.Errorf("no prebuilt tool configurations were loaded.%w", err)
		return nil, nil, errMsg
	}

	return toolYAMLs, sourceTypes, nil
}

View on GitHub (pinned to 8cc6e09de2)

Solutions

1. Identify which file fails from the wrapped error and confirm it is a regular file under internal/prebuiltconfigs/tools/.
2. Remove/rename any directory or symlink whose name ends in .yaml.
3. Rebuild the binary to refresh embedded contents.
4. If a custom prebuiltConfigsFS is injected in tests, ensure it returns readable bytes for every listed entry.

content, err := prebuiltConfigsFS.ReadFile(filePathInFS)
if err != nil {
	return fmt.Errorf("failed to read a prebuilt tool %w (path=%s)", err, filePathInFS)
}

When it happens

Trigger: ReadFile fails on a file discovered by ReadDir — e.g. the entry vanished between listing and reading in a non-embedded (OS-dir) build variant, a directory named "*.yaml" passes the suffix check but can't be ReadFile'd, or path.Join produced an unexpected path.

Common situations: Someone committed a directory named like "mydb.yaml/" which IsDir()==false handling misses in odd builds; a modified FS wrapper in tests injecting a fake that lists but cannot serve files.

Related errors

• failed to read prebuilt tools %w
• error finding YAML files in %q: %w
• error finding YML files in %q: %w
• failed to stat file: %w
• failed to rename file: %w

AI-assisted analysis of googleapis/mcp-toolbox@8cc6e09de2 (2026-09-05). Data as JSON: /api/errors/84ded948be98c436. Report an issue: GitHub.