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 mysqlsql package registers its config factory under resourceType "mysql-sql" in the global toolRegistry via tools.Register() (internal/tools/tools.go:43). Register() returns false when the type key already exists, and init() panics because two packages claiming the same YAML `kind` would make tool config decoding ambiguous.

Source

Thrown at internal/tools/mysql/mysqlsql/mysqlsql.go:34

import (
	"context"
	"database/sql"
	"fmt"
	"net/http"

	yaml "github.com/goccy/go-yaml"
	"github.com/googleapis/mcp-toolbox/internal/sources"
	"github.com/googleapis/mcp-toolbox/internal/tools"
	"github.com/googleapis/mcp-toolbox/internal/util"
	"github.com/googleapis/mcp-toolbox/internal/util/parameters"
)

const resourceType string = "mysql-sql"

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-sql"' to find every tools.Register call with this type; keep exactly one.
2. If a fork was intended, rename the resourceType (e.g. "mysql-sql-custom") and the package directory.
3. Remove duplicate imports or extra replace directives causing the package to be linked twice.
4. Run `go build ./...` and `go test ./internal/tools/mysql/mysqlsql/...` to confirm.

Example fix

// before (forked copy)
const resourceType string = "mysql-sql"

// after
const resourceType string = "mysql-sql-v2"

// Verify uniqueness before building:
// grep -rn '"mysql-sql"' --include='*.go' . | grep 'resourceType\|tools.Register'
// Exactly one match expected. Soft guard for forked tools:
func init() {
	if !tools.Register(resourceType, newConfig) {
		fmt.Printf("WARNING: %s already registered; skipping duplicate init\n", resourceType)
		return
	}
}

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

// Recover at the process boundary:
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()
}

When it happens

Trigger: Another init() also registers "mysql-sql" — e.g. a copied fork of mysqlsql.go left in the tree, this package imported twice via different module paths, or a custom tool reusing the built-in mysql-sql type string.

Common situations: Forking mysqlsql to tweak behavior without renaming the package/type; a vendored copy of the toolbox alongside the direct dependency; rebasing a feature branch that duplicates the package directory.

Related errors

• tool type %q already registered
• tool type %q already registered
• tool type %q already registered
• tool type %q already registered
• tool type %q already registered

AI-assisted analysis of googleapis/mcp-toolbox@8cc6e09de2 (2026-09-05). Data as JSON: /api/errors/539c80bd0b199fd3. Report an issue: GitHub.