more than one default (nameless) group declared; only one is

Error message

more than one default (nameless) group declared; only one is allowed

What it means

A config may contain at most one nameless (default) group; the default group applies to tools without an explicit group. If two group entries with an empty name decode successfully, UnmarshalPrimitiveConfig fails at internal/server/config.go:338 with this explicit message.

Source

Thrown at internal/server/config.go:338

			}
			if _, exists := promptConfigs[name]; exists {
				return nil, nil, nil, nil, nil, nil, fmt.Errorf("prompt %q declared more than once", name)
			}
			promptConfigs[name] = c
		case "group":
			c, err := UnmarshalYAMLGroupConfig(ctx, name, resource)
			if err != nil {
				if len(file.Docs) > 1 {
					return nil, nil, nil, nil, nil, nil, fmt.Errorf("document %d: error unmarshaling %s %q: %w", docIndex, kind, name, err)
				}
				return nil, nil, nil, nil, nil, nil, fmt.Errorf("error unmarshaling %s: %w", kind, err)
			}
			if groupConfigs == nil {
				groupConfigs = make(GroupConfigs)
			}
			if _, exists := groupConfigs[name]; exists {
				if name == "" {
					return nil, nil, nil, nil, nil, nil, fmt.Errorf("more than one default (nameless) group declared; only one is allowed")
				}
				return nil, nil, nil, nil, nil, nil, fmt.Errorf("group %q declared more than once", name)
			}
			groupConfigs[name] = c
		default:
			if len(file.Docs) > 1 {
				return nil, nil, nil, nil, nil, nil, fmt.Errorf("%s invalid kind %q", formatDocLocation(docIndex, keyToken(doc.Body, "kind"), doc.Body), kind)
			}
			return nil, nil, nil, nil, nil, nil, fmt.Errorf("invalid kind %s", kind)
		}
	}
	// Fold legacy toolsets into groups. An explicit `kind: group` of the same name
	// takes precedence over a toolset (matching the prior server-side behavior); warn
	// when a toolset is shadowed this way.
	if len(toolsetGroups) > 0 {
		if groupConfigs == nil {
			groupConfigs = make(GroupConfigs)
		}

View on GitHub (pinned to 8cc6e09de2)

Solutions

Remove one of the nameless group blocks so only a single default group remains
Give the other group an explicit unique name if both sets of tools must be grouped
After merging configs, deduplicate group definitions before deploying

Example fix

# before
kind: group
:
  toolIds: [tool-a]
---
kind: group
:
  toolIds: [tool-b]
# after
kind: group
:
  toolIds: [tool-a, tool-b]

Defensive patterns

Strategy: validation

Validate before calling

// Count nameless groups in a config before parsing
text, _ := os.ReadFile(path)
docs := strings.Split(string(text), "\n---")
defaults := 0
for _, d := range docs {
	var doc struct {
		Kind string `yaml:"kind"`
		Name string `yaml:"serverRef"`
	}
	if yaml.Unmarshal([]byte(d), &doc) == nil && doc.Kind == "group" && strings.TrimSpace(d) != "" {
		// a group doc with no name key is a default group; count via key scan
		if !regexp.MustCompile(`(?m)^\s+[\w-]+:`).MatchString(strings.TrimPrefix(d, "kind: group")) == false && !strings.Contains(d, "my-group") {
			defaults++
		}
	}
}
if defaults > 1 {
	return fmt.Errorf("%d default groups found; keep at most one", defaults)
}

Try / catch

if _, _, _, _, _, _, err := server.ParseConfig(ctx, cfg); err != nil {
	if strings.Contains(err.Error(), "default (nameless) group") {
		log.Fatalf("config has multiple default groups: %v", err)
	}
	return err
}

Prevention

Declare exactly one anonymous group block per config, and only when needed
Prefer explicitly named groups over the nameless default
After merging configs, grep for repeated 'kind: group' blocks without names

When it happens

Trigger: Two 'group' resources in the YAML both have an empty name (two anonymous/default group blocks), so the second insertion into the groupConfigs map hits the exists check and the name=='' branch.

Common situations: Repeating a default group block when merging configs; YAML anchors accidentally producing two nameless groups; copy-pasting a group template and deleting its name in more than one place.

Related errors

AI-assisted analysis of googleapis/mcp-toolbox@8cc6e09de2 (2026-09-05). Data as JSON: /api/errors/530d2beeafa8d1c8. Report an issue: GitHub.