diff --git a/reversal/tokeniser.go b/reversal/tokeniser.go
index f49887b..a36169d 100644
--- a/reversal/tokeniser.go
+++ b/reversal/tokeniser.go
@@ -115,7 +115,19 @@ func WithSignals() TokeniserOption {
 // WithWeights overrides the default signal weights for disambiguation.
 // All signal keys must be present; omitted keys silently disable those signals.
 func WithWeights(w map[string]float64) TokeniserOption {
-	return func(t *Tokeniser) { t.weights = w }
+	return func(t *Tokeniser) {
+		if len(w) == 0 {
+			t.weights = nil
+			return
+		}
+		// Copy the map so callers can safely reuse or mutate their input after
+		// constructing the tokeniser.
+		copied := make(map[string]float64, len(w))
+		for key, value := range w {
+			copied[key] = value
+		}
+		t.weights = copied
+	}
 }
 
 // NewTokeniser creates a Tokeniser for English ("en").
diff --git a/reversal/tokeniser_test.go b/reversal/tokeniser_test.go
index be505fc..05a9364 100644
--- a/reversal/tokeniser_test.go
+++ b/reversal/tokeniser_test.go
@@ -896,7 +896,7 @@ func TestDisambiguationStats_NoAmbiguous(t *testing.T) {
 func TestWithWeights_Override(t *testing.T) {
 	setup(t)
 	// Override noun_determiner to 0 — "The commit" should no longer resolve as noun
-	tok := NewTokeniser(WithWeights(map[string]float64{
+	weights := map[string]float64{
 		"noun_determiner":   0.0,
 		"verb_auxiliary":    0.25,
 		"following_class":   0.15,
@@ -904,7 +904,8 @@ func TestWithWeights_Override(t *testing.T) {
 		"verb_saturation":   0.10,
 		"inflection_echo":   0.03,
 		"default_prior":     0.02,
-	}))
+	}
+	tok := NewTokeniser(WithWeights(weights))
 	tokens := tok.Tokenise("The commit")
 	// With noun_determiner zeroed, default_prior (verb) should win
 	if tokens[1].Type != TokenVerb {
@@ -912,6 +913,29 @@ func TestWithWeights_Override(t *testing.T) {
 	}
 }
 
+func TestWithWeights_CopiesInputMap(t *testing.T) {
+	setup(t)
+	weights := map[string]float64{
+		"noun_determiner":   0.35,
+		"verb_auxiliary":    0.25,
+		"following_class":   0.15,
+		"sentence_position": 0.10,
+		"verb_saturation":   0.10,
+		"inflection_echo":   0.03,
+		"default_prior":     0.02,
+	}
+	tok := NewTokeniser(WithWeights(weights))
+
+	// Mutate the caller's map after construction; the tokeniser should keep
+	// using the original copied values.
+	weights["noun_determiner"] = 0
+
+	tokens := tok.Tokenise("The commit")
+	if tokens[1].Type != TokenNoun {
+		t.Fatalf("with copied weights, 'commit' Type = %v, want TokenNoun", tokens[1].Type)
+	}
+}
+
 // --- Benchmarks ---
 
 func benchSetup(b *testing.B) {