What causes "key contains space" in golang/go?

Remove whitespace from the key: use a single-token key like 'godebug mykey=value'.

key contains space

Error message

key contains space

What it means

CheckGodebug rejected a godebug directive because the KEY contains a space or tab (strings.ContainsAny(k, " \t")). Applies to both go.mod and go.work godebug lines (the same validator runs for both, via 1066/1070). Directives must be one key=value per line with no internal whitespace in the key.

Source

Thrown at src/cmd/go/internal/modload/init.go:2284

	f := func(c rune) bool {
		return c > '9' || c < '0'
	}
	s := strings.FieldsFunc(path, f)
	if len(s) > 0 {
		m = s[0]
	}

	m = strings.TrimLeft(m, "0")

	if m == "" {
		return url + ".v1"
	}
	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

View on GitHub (pinned to b6b368adc5)

Solutions

Remove whitespace from the key: use a single-token key like 'godebug mykey=value'.
If you meant multiple settings, put one godebug directive per line.
Consult the known GODEBUG keys list and use an exact, single-token name.
Re-run to surface any subsequent value/comma errors.

Example fix

// before (go.mod)
godebug panic nil=0      // key 'panic nil' has a space

// after
godebug panicnil=0

Defensive patterns

Strategy: validation

Validate before calling

// Validate godebug keys for whitespace before commit.
func validGodebugKey(k string) error {
    if strings.ContainsAny(k, " \t") {
        return errors.New("godebug key contains whitespace")
    }
    return nil
}

Type guard

func isGodebugKeyWellFormed(k string) bool {
    return !strings.ContainsAny(k, " \t,") && k != ""
}

Try / catch

out, err := exec.Command("go", "build", "./...").CombinedOutput()
if err != nil && bytes.Contains(out, []byte("key contains space")) {
    // surface and stop; user must edit the godebug line
    return fmt.Errorf("godebug key has whitespace; fix directive: %s", out)
}
return err

Prevention

Use single-token keys only; no spaces or tabs.
Add godebug lines via 'go mod edit -godebug=key=value'.
Lint godebug directives in pre-commit.
Copy keys exactly from the official GODEBUG list.

When it happens

Trigger: A godebug line like 'godebug my key=value' or 'godebug "key with space"=value' triggers the first check in CheckGodebug.

Common situations: Pasting a multi-token GODEBUG env value ('GODEBUG=a b=c') into a godebug directive; hand-typing a descriptive key; misreading the directive grammar (which expects key=value, not a description).

Related errors

AI-assisted analysis of golang/go@b6b368adc5 (2026-08-12). Data as JSON: /api/errors/0537a3aa90361ded. Report an issue: GitHub.