What causes "expected $$ marker, but found %q (recompile package)" in golang/go?

Recompile the named package: go build or go build -a .

expected $$ marker, but found %q (recompile package)

Error message

expected $$ marker, but found %q (recompile package)

What it means

Returned when the mapped export-data buffer does not end with the "\n$$\n" section marker that delimits Go export data. strings.CutSuffix fails, the last up-to-10 bytes are captured, and the message tells the user to recompile the package — the object file is truncated or from an incompatible format.

Source

Thrown at src/cmd/compile/internal/noder/import.go:278

	// as a single large string. This reduces heap fragmentation and
	// allows returning individual substrings very efficiently.
	var mapped string
	mapped, err = base.MapFile(r.File(), pos, end-pos)
	if err != nil {
		return
	}

	// check for end-of-section marker "\n$$\n" and remove it
	const marker = "\n$$\n"

	var ok bool
	data, ok = strings.CutSuffix(mapped, marker)
	if !ok {
		cutoff := data // include last 10 bytes in error message
		if len(cutoff) >= 10 {
			cutoff = cutoff[len(cutoff)-10:]
		}
		err = fmt.Errorf("expected $$ marker, but found %q (recompile package)", cutoff)
		return
	}

	return
}

// addFingerprint reads the linker fingerprint included at the end of
// the exportdata.
func addFingerprint(path string, data string) error {
	var fingerprint goobj.FingerprintType

	pos := len(data) - len(fingerprint)
	if pos < 0 {
		return fmt.Errorf("missing linker fingerprint in exportdata, but found %q", data)
	}
	buf := []byte(data[pos:])

	copy(fingerprint[:], buf)

View on GitHub (pinned to b6b368adc5)

Solutions

Recompile the named package: go build <pkg> or go build -a <pkg>.
go clean -cache to discard all suspect export data.
Confirm the toolchain version is consistent across the build (go env GOROOT GOTOOLCHAIN).
If it persists, capture the object file and report a Go bug with the toolchain versions involved.

Defensive patterns

Strategy: retry

Validate before calling

// Treat missing $$ marker as stale and force a rebuild.
if !strings.HasSuffix(mapped, "\n$$\n") {
    os.Remove(objectFile) // force recompile
    return rebuild(pkg)
}

Prevention

Avoid interrupting builds mid-write (use go build, not hand-stitched object archives).
go clean -cache after toolchain upgrades.

When it happens

Trigger: After reading and mapping the export data section, strings.CutSuffix(mapped, "\n$$\n") returns ok=false because the marker is absent; the trailing bytes are shown as evidence.

Common situations: Truncated or partial object file (build killed mid-write), object file from a much older/newer Go that uses a different export-data framing, corrupted build cache entry, or a hand-edited object/archive.

Related errors

AI-assisted analysis of golang/go@b6b368adc5 (2026-08-12). Data as JSON: /api/errors/27f319e0873c8105. Report an issue: GitHub.