googleapis/mcp-toolbox · error

failed to read a prebuilt tool

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.

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
• error finding YAML files in
• error finding YML files in
• error reading config folder
• error writing script

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

Appendix: 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
}

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