golang/go · error

unknown -mode %v

Error message

unknown -mode %v

What it means

Thrown by 'go tool cover' parseFlags when -mode is set to a value outside the allowed set. Valid modes are set, count, atomic, regonly, and testmain. Each maps to a counter statement strategy and a coverage.CounterMode; anything else falls through the switch's default branch.

Source

Thrown at src/cmd/cover/cover.go:168

	if *mode != "" {
		switch *mode {
		case "set":
			counterStmt = setCounterStmt
			cmode = coverage.CtrModeSet
		case "count":
			counterStmt = incCounterStmt
			cmode = coverage.CtrModeCount
		case "atomic":
			counterStmt = atomicCounterStmt
			cmode = coverage.CtrModeAtomic
		case "regonly":
			counterStmt = nil
			cmode = coverage.CtrModeRegOnly
		case "testmain":
			counterStmt = nil
			cmode = coverage.CtrModeTestMain
		default:
			return fmt.Errorf("unknown -mode %v", *mode)
		}

		if flag.NArg() == 0 {
			return fmt.Errorf("missing source file(s)")
		} else {
			if *pkgcfg != "" {
				if *output != "" {
					return fmt.Errorf("please use '-outfilelist' flag instead of '-o'")
				}
				var err error
				if outputfiles, err = readOutFileList(*outfilelist); err != nil {
					return err
				}
				covervarsoutfile = outputfiles[0]
				outputfiles = outputfiles[1:]
				numInputs := len(flag.Args())
				numOutputs := len(outputfiles)
				if numOutputs != numInputs {

View on GitHub (pinned to b6b368adc5)

Solutions

1. Use one of: set, count, atomic, regonly, testmain (lowercase)
2. Check `go tool cover -help` for the modes supported by your Go version
3. Upgrade or downgrade Go if a mode you expect is missing

Example fix

# before
go tool cover -mode=total file.go
# after
go tool cover -mode=count file.go

# Validate mode against the allowed enum
MODES="set count atomic regonly testmain"
if ! echo " $MODES " | grep -q " $MODE "; then
  echo "unknown -mode $MODE; pick from: $MODES" >&2; exit 2
fi

When it happens

Trigger: `go tool cover -mode=total` or `-mode=SET` (case-sensitive) or any unrecognized mode string.

Common situations: Misspelling a mode name. Using an uppercase variant. Assuming an older/newer mode name exists on this toolchain version (regonly/testmain were added later).

Related errors

• -var: %q is not a valid identifier
• %s: pkgconfig requires perblock/perfunc value
• too many options
• missing source file(s)
• please use '-outfilelist' flag instead of '-o'

AI-assisted analysis of golang/go@b6b368adc5 (2026-08-12). Data as JSON: /api/errors/f01831aaf9dd6bfd. Report an issue: GitHub.