googleapis/mcp-toolbox · critical

tool type %q already registered

Error message

tool type %q already registered

What it means

This is a fail-fast panic raised during package initialization. The tool package registers its config factory under the unique `resourceType` string "mysql-list-tables-missing-unique-indexes" in the global toolRegistry via tools.Register() (internal/tools/tools.go:43). When Register() returns false because another init() already claimed that exact type string, this init() panics instead of silently overwriting, preventing ambiguous YAML `kind` resolution at startup.

Source

Thrown at internal/tools/mysql/mysqllisttablesmissinguniqueindexes/mysqllisttablesmissinguniqueindexes.go:57

        information_schema.table_constraints tco
        ON
            tab.table_schema = tco.table_schema
            AND tab.table_name = tco.table_name
            AND tco.constraint_type IN ('PRIMARY KEY', 'UNIQUE')
    WHERE
        tco.constraint_type IS NULL
        AND tab.table_schema NOT IN('mysql', 'information_schema', 'performance_schema', 'sys')
        AND tab.table_type = 'BASE TABLE'
        AND (COALESCE(?, '') = '' OR tab.table_schema = ?)
    ORDER BY
        tab.table_schema,
        tab.table_name
    LIMIT ?;
`

func init() {
	if !tools.Register(resourceType, newConfig) {
		panic(fmt.Sprintf("tool type %q already registered", resourceType))
	}
}

func newConfig(ctx context.Context, name string, decoder *yaml.Decoder) (tools.ToolConfig, error) {
	actual := Config{ConfigBase: tools.ConfigBase{Name: name}}
	if err := decoder.DecodeContext(ctx, &actual); err != nil {
		return nil, err
	}
	return actual, nil
}

type compatibleSource interface {
	MySQLPool() *sql.DB
	RunSQL(context.Context, string, []any) (any, error)
}

type Config struct {
	tools.ConfigBase `yaml:",inline"`

View on GitHub (pinned to 8cc6e09de2)

Solutions

  1. grep -rn 'mysql-list-tables-missing-unique-indexes' to locate both registrations; make the new/renamed tool use a distinct resourceType constant.
  2. Remove the duplicate import (usually in cmd/root.go) or delete the obsolete tool package.
  3. Check go.mod/replace directives for duplicate module copies that would double-register the package.
  4. Run `go build ./...` and `go test ./internal/tools/mysql/...` to verify.

Example fix

// before
copy of mysqllisttablesmissinguniqueindexes kept: const resourceType string = "mysql-list-tables-missing-unique-indexes"

// after
const resourceType string = "mysql-list-tables-missing-unique-indexes" // only in the original package; new tool gets its own unique type
Defensive patterns

Strategy: validation

Validate before calling

// Before building, assert uniqueness of the type string:
// grep -rn '"mysql-list-tables-missing-unique-indexes"' --include='*.go' . | grep 'resourceType\|tools.Register'
// Expect exactly one registration. Guard custom tool init():
func init() {
	if !tools.Register(resourceType, newConfig) {
		fmt.Printf("WARNING: %s already registered; skipping duplicate init\n", resourceType)
		return
	}
}

Type guard

func isRegistered(resourceType string) bool {
	_, exists := registrySnapshot[resourceType]
	return exists
}

Try / catch

// Panics in init() cannot be recovered in init; recover in main if needed:
func main() {
	defer func() {
		if r := recover(); r != nil {
			if s, ok := r.(string); ok && strings.Contains(s, "already registered") {
				log.Fatalf("duplicate tool registration: %s", s)
			}
			panic(r)
		}
	}()
	rootCmd.Execute()
}

Prevention

When it happens

Trigger: Two init() functions call tools.Register("mysql-list-tables-missing-unique-indexes", ...) in the same binary — e.g. a copied/renamed MySQL tool package still using this constant, or the same package being linked twice under different import paths.

Common situations: Copy-pasting a nearby mysql tool directory (mysqllisttablestats, mysqllisttables, etc.) as a template and forgetting to update resourceType at line 30; a merge that re-imports an old package into cmd/root.go; vendoring two versions of the toolbox module so the package initializes twice.

Related errors

AI-assisted analysis of googleapis/mcp-toolbox@8cc6e09de2 (2026-09-05). Data as JSON: /api/errors/519132f24a5df7d0. Report an issue: GitHub.