golang/go · error

value for default= must be goVERSION

Error message

value for default= must be goVERSION

What it means

Thrown by CheckGodebug for the special key 'default'. The default GODEBUG setting specifies which Go language version's defaults to use and must be of the form 'goVERSION' (e.g. 'go1.22'). The check verifies the value has the 'go' prefix via strings.HasPrefix and that the remainder passes gover.IsValid. If either fails, this error is returned.

Source

Thrown at src/cmd/go/internal/modload/init.go:2297

	return url + ".v" + m
}

func CheckGodebug(verb, k, v string) error {
	if strings.ContainsAny(k, " \t") {
		return fmt.Errorf("key contains space")
	}
	if strings.ContainsAny(v, " \t") {
		return fmt.Errorf("value contains space")
	}
	if strings.ContainsAny(k, ",") {
		return fmt.Errorf("key contains comma")
	}
	if strings.ContainsAny(v, ",") {
		return fmt.Errorf("value contains comma")
	}
	if k == "default" {
		if !strings.HasPrefix(v, "go") || !gover.IsValid(v[len("go"):]) {
			return fmt.Errorf("value for default= must be goVERSION")
		}
		if gover.Compare(v[len("go"):], gover.Local()) > 0 {
			return fmt.Errorf("default=%s too new (toolchain is go%s)", v, gover.Local())
		}
		return nil
	}
	if godebugs.Lookup(k) != nil {
		return nil
	}
	for _, info := range godebugs.Removed {
		if info.Name == k {
			if info.Old(v) {
				return fmt.Errorf("removed GODEBUG %q set to old value %q (https://go.dev/doc/godebug#go-1%v)", k, v, info.Removed)
			}
			// Using a removed GODEBUG setting with a non-old value is ok (see go.dev/issue/76163).
			return nil
		}
	}

View on GitHub (pinned to b6b368adc5)

Solutions

1. Find the 'godebug default=...' line in go.mod or go.work.
2. Ensure the value starts with 'go' followed by a valid Go version, e.g. 'go1.22', 'go1.23.1'.
3. Run 'go mod tidy' or re-run the go command to confirm the file is accepted.

Example fix

// before
godebug default=1.22

// after
godebug default=go1.22

// Validate a 'default' godebug value.
func validateGodebugDefault(v string) error {
    if !strings.HasPrefix(v, "go") {
        return fmt.Errorf("value for default= must be goVERSION")
    }
    // gover.IsValid checks the version portion
    if !gover.IsValid(v[len("go"):]) {
        return fmt.Errorf("value for default= must be goVERSION")
    }
    return nil
}

When it happens

Trigger: A godebug directive 'default=X' where X does not start with 'go' or the part after 'go' is not a valid Go version string. Examples: 'default=1.22' (missing 'go' prefix), 'default=goXYZ' (invalid version), 'default=go' (empty version).

Common situations: Typing the version without the 'go' prefix because semver conventions suggest '1.22'. Using a malformed version string. Copy-pasting from documentation that used shorthand.

Related errors

• key contains space
• value contains space
• key contains comma
• value contains comma
• default=%s too new (toolchain is go%s)

AI-assisted analysis of golang/go@b6b368adc5 (2026-08-12). Data as JSON: /api/errors/be2921a5057c6fb7. Report an issue: GitHub.