invalid flag in : without argument (see…

Error message

invalid flag in %s: %s without argument (see https://go.dev/s/invalidflag)

What it means

Thrown by checkFlags when a flag that expects an argument (e.g. `-I`) appears as the last token in the list with no value following it. The error reports the flag and notes it is missing its argument.

Solutions

Append the missing argument after the flag (e.g. `-I ./include`).
Remove the dangling flag if the path is not needed.
Re-run `go build` after the fix to confirm the error clears.
Lint #cgo directives in vendored C packages before upgrading.

Example fix

// before
// #cgo CFLAGS: -O2 -I

// after
// #cgo CFLAGS: -O2 -I ./include

Defensive patterns

Strategy: validation

Validate before calling

// Ensure argument-taking flags are not dangling
flags := strings.Fields(os.Getenv("CGO_CFLAGS"))
argTaking := map[string]bool{"-I": true, "-L": true, "-D": true, "-U": true, "-include": true}
if argTaking[flags[len(flags)-1]] {
    log.Fatalf("%s has no argument", flags[len(flags)-1])
}

Prevention

Never end a #cgo CFLAGS line with a bare -I/-L/-D.
Lint generated cgo flag lists.
Re-read edited #cgo directives before commit.

When it happens

Trigger: A #cgo CFLAGS line or CGO_CFLAGS list ends with a bare `-I` (or similar argument-taking flag): i+1 is out of range, so checkFlags returns the `without argument` form.

Common situations: Truncation while editing a #cgo line; a generated/templated flag list that dropped the trailing path; mis-ordered flags where the intended value landed on the next line.

Related errors

AI-assisted analysis of golang/go@b6b368adc5 (2026-08-12). Data as JSON: /api/errors/5900a7cb3a7a4100. Report an issue: GitHub.

Appendix: source

Thrown at src/cmd/go/internal/work/security.go:462

					load.SafeArg(list[i+1][4:]) &&
					!strings.Contains(list[i+1][4:], ",") {
					i++
					continue Args
				}

				// Permit -I= /path, -I $SYSROOT.
				if i+1 < len(list) && arg == "-I" {
					if (strings.HasPrefix(list[i+1], "=") || strings.HasPrefix(list[i+1], "$SYSROOT")) &&
						load.SafeArg(list[i+1][1:]) {
						i++
						continue Args
					}
				}

				if i+1 < len(list) {
					return fmt.Errorf("invalid flag in %s: %s %s (see https://go.dev/s/invalidflag)", source, arg, list[i+1])
				}
				return fmt.Errorf("invalid flag in %s: %s without argument (see https://go.dev/s/invalidflag)", source, arg)
			}
		}
	Bad:
		return fmt.Errorf("invalid flag in %s: %s (see https://go.dev/s/invalidflag)", source, arg)
	}
	return nil
}

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