diff --git a/hbrtl/json.go b/hbrtl/json.go
index db44c44..ece4e87 100644
--- a/hbrtl/json.go
+++ b/hbrtl/json.go
@@ -58,7 +58,17 @@ func HbJsonEncode(t *hbrt.Thread) {
 	t.RetString(string(data))
 }
 
-// HB_JSONDECODE(cJSON) → xValue
+// HB_JSONDECODE(cJSON [, @xOut]) → xValue | nBytesParsed
+//
+// Two Harbour-spec calling forms:
+//
+//   - 1 arg : returns the decoded value (or NIL on parse error).
+//   - 2 args: writes the decoded value into the byref @xOut and
+//     returns the number of bytes parsed (0 on error). This is the
+//     form mod_harbour / fivenode PRG code uses for ctx_get and the
+//     like. Five previously implemented only the 1-arg form, so any
+//     PRG that relied on the byref output saw NIL and silently fell
+//     through to a default.
 func HbJsonDecode(t *hbrt.Thread) {
 	nParams := t.ParamCount()
 	t.Frame(nParams, 0)
@@ -66,10 +76,21 @@ func HbJsonDecode(t *hbrt.Thread) {
 	s := t.Local(1).AsString()
 	var raw interface{}
 	if err := json.Unmarshal([]byte(s), &raw); err != nil {
-		t.RetNil()
+		if nParams >= 2 {
+			t.SetLocal(2, hbrt.MakeNil())
+			t.RetInt(0)
+		} else {
+			t.RetNil()
+		}
 		return
 	}
-	t.RetVal(goToValue(raw))
+	v := goToValue(raw)
+	if nParams >= 2 {
+		t.SetLocal(2, v)
+		t.RetInt(int64(len(s)))
+		return
+	}
+	t.RetVal(v)
 }
 
 // === Five Extensions ===