googleapis/mcp-toolbox · critical

unable to create pool: %w

Error message

unable to create pool: %w

What it means

This error is returned by the cloud-sql-mysql source's Initialize when initCloudSQLMySQLConnectionPool fails to build the MySQL connection pool. It wraps earlier failures: resolving IAM principal email from ADC, invalid ipType options, driver registration failure, or sql.Open errors. Note sql.Open rarely fails eagerly — most real problems surface at the subsequent PingContext step (a separate 'unable to connect successfully' error).

Source

Thrown at internal/sources/cloudsqlmysql/cloud_sql_mysql.go:74

	Project      string         `yaml:"project" validate:"required"`
	Region       string         `yaml:"region" validate:"required"`
	Instance     string         `yaml:"instance" validate:"required"`
	IPType       sources.IPType `yaml:"ipType"`
	User         string         `yaml:"user"`
	Password     string         `yaml:"password"`
	Database     string         `yaml:"database"`
	ReadOnly     bool           `yaml:"readOnly"`
	SQLCommenter *bool          `yaml:"sqlCommenter"`
}

func (r Config) SourceConfigType() string {
	return SourceType
}

func (r Config) Initialize(ctx context.Context, tracer trace.Tracer) (sources.Source, error) {
	pool, err := initCloudSQLMySQLConnectionPool(ctx, tracer, r.Name, r.Project, r.Region, r.Instance, r.IPType.String(), r.User, r.Password, r.Database, r.ReadOnly)
	if err != nil {
		return nil, fmt.Errorf("unable to create pool: %w", err)
	}

	err = pool.PingContext(ctx)
	if err != nil {
		pool.Close()
		return nil, fmt.Errorf("unable to connect successfully: %w", err)
	}

	s := &Source{
		Config: r,
		Pool:   pool,
	}
	return s, nil
}

var _ sources.Source = &Source{}

type Source struct {

View on GitHub (pinned to 8cc6e09de2)

Solutions

1. If using IAM auth (no user/password), ensure ADC is set up (GOOGLE_APPLICATION_CREDENTIALS or gcloud auth application-default login)
2. Provide BOTH user and password, or neither — a password without a username is rejected
3. Set ipType to a valid value (public or private)
4. Check the wrapped error to see which pool-init step failed (config, driver registration, or sql.Open)
5. Ensure the Cloud SQL Admin API is enabled and the caller has cloudsql.instances.get permission

Example fix

// before
user: ""
password: "secret"  # password without username is rejected
// after
user: "myuser"
password: "secret"  # or remove both to use IAM auth via ADC

// Before starting the toolbox, verify ADC and instance coordinates
cmd := exec.Command("gcloud", "sql", "instances", "describe", instance,
    "--project", project, "--format", "value(state)")
out, err := cmd.Output()
// out == "RUNNABLE" means the instance is up; also ensure IAM auth users have
// cloudsql.instances.login permission when omitting user/password

src, err := cfg.Initialize(ctx, tracer)
if err != nil {
    if strings.Contains(err.Error(), "unable to create pool") {
        return fmt.Errorf("check ADC/credentials and pool config: %w", err)
    }
    return err
}

When it happens

Trigger: Config.Initialize for a source of type 'cloud-sql-mysql' — password provided without a username, GetIAMPrincipalEmailFromADC fails (no Application Default Credentials or missing Cloud SQL scopes), sources.GetCloudSQLOpts receives an invalid ipType, mysql.RegisterDriver fails, or sql.Open gets a malformed DSN.

Common situations: Setting readOnly:true with an invalid iam principal, ADC not configured (GOOGLE_APPLICATION_CREDENTIALS unset) when relying on IAM auth, password set but user left empty (explicitly rejected), invalid ipType value other than public/private, duplicate driver registration edge cases.

Related errors

• unable to connect successfully: %w
• unable to connect successfully: %w
• unable to create connection pool: %w
• unable to create pool: %w
• unable to execute query: %w

AI-assisted analysis of googleapis/mcp-toolbox@8cc6e09de2 (2026-09-05). Data as JSON: /api/errors/65ef617927b0d893. Report an issue: GitHub.