fix: correct SDPA mask mode and slice logits to last position

pkg/ml/backend_mlx.go

@@ -87,6 +87,8 @@ func (b *MLXBackend) Generate(ctx context.Context, prompt string, opts GenOpts)
 		}
 
 		logits := b.model.Forward(input, b.caches)
+		// Take last position: [B, L, V] → [B, V]
+		logits = lastPosition(logits)
 		next := sampler.Sample(logits)
 		mlx.Materialize(next)
 
@@ -101,6 +103,19 @@ func (b *MLXBackend) Generate(ctx context.Context, prompt string, opts GenOpts)
 	return b.tok.Decode(output), nil
 }
 
+// lastPosition extracts the last sequence position from [B, L, V] logits → [B, V].
+func lastPosition(logits *mlx.Array) *mlx.Array {
+	shape := logits.Shape()
+	if len(shape) == 3 && shape[1] > 1 {
+		L := shape[1]
+		logits = mlx.Slice(logits, []int32{0, L - 1, 0}, []int32{shape[0], L, shape[2]})
+		logits = mlx.Reshape(logits, shape[0], shape[2])
+	} else if len(shape) == 3 && shape[1] == 1 {
+		logits = mlx.Reshape(logits, shape[0], shape[2])
+	}
+	return logits
+}
+
 // Chat formats messages and generates a response.
 func (b *MLXBackend) Chat(ctx context.Context, messages []Message, opts GenOpts) (string, error) {
 	// Format as Gemma chat
@@ -148,6 +163,7 @@ func (b *MLXBackend) Chat(ctx context.Context, messages []Message, opts GenOpts)
 		}
 
 		logits := b.model.Forward(input, b.caches)
+		logits = lastPosition(logits)
 		next := sampler.Sample(logits)
 		mlx.Materialize(next)
 

pkg/mlx/fast.go

@@ -48,7 +48,7 @@ func RoPE(x *Array, dims int, traditional bool, base float32, scale float32, off
 
 // ScaledDotProductAttention computes attention using a fused Metal kernel.
 func ScaledDotProductAttention(query, key, value *Array, scale float32, causal bool) *Array {
-	mode := "none"
+	mode := ""
 	if causal {
 		mode = "causal"
 	}
@@ -67,7 +67,7 @@ func ScaledDotProductAttention(query, key, value *Array, scale float32, causal b
 
 // ScaledDotProductAttentionWithMask computes attention with an explicit mask.
 func ScaledDotProductAttentionWithMask(query, key, value, mask *Array, scale float32) *Array {
-	cMode := C.CString("none")
+	cMode := C.CString("array")
 	defer C.free(unsafe.Pointer(cMode))
 
 	sinksArr := C.mlx_array_new()

pkg/mlx/model/gemma3.go

@@ -95,9 +95,6 @@ type MLP struct {
 	DownProj *mlx.Linear
 }
 
-// debugOnce is a temporary flag for shape debugging (remove after fix).
-var debugOnce = true
-
 // compiledGELU is a singleton for the compiled GELU function.
 var compiledGELU *mlx.CompiledFunc
 
@@ -372,13 +369,6 @@ func (a *Attention) forward(x *mlx.Array, c cache.Cache, B, L int32, isSliding b
 	k := a.KProj.Forward(x)
 	v := a.VProj.Forward(x)
 
-	if debugOnce {
-		slog.Info("mlx: debug", "x_shape", x.Shape(), "q_shape", q.Shape(), "k_shape", k.Shape(), "v_shape", v.Shape(),
-			"B", B, "L", L, "num_heads", cfg.NumAttentionHeads, "num_kv_heads", cfg.NumKeyValueHeads, "head_dim", cfg.HeadDim,
-			"q_scales", a.QProj.Scales != nil)
-		debugOnce = false
-	}
-
 	// Reshape to [B, num_heads, L, head_dim]
 	q = mlx.AsStrided(q, []int32{B, cfg.NumAttentionHeads, L, cfg.HeadDim},
 		[]int64{int64(L * cfg.NumAttentionHeads * cfg.HeadDim), int64(cfg.HeadDim), int64(cfg.NumAttentionHeads * cfg.HeadDim), 1}, 0)