diff --git a/dflash/model.py b/dflash/model.py
index 5a1d33a..f717625 100644
--- a/dflash/model.py
+++ b/dflash/model.py
@@ -104,6 +104,11 @@ def dflash_generate(
     start = num_input_tokens
     draft_prefill = True
 
+    stop_token_tensor = (
+        torch.as_tensor(stop_token_ids, device=output_ids.device, dtype=output_ids.dtype)
+        if stop_token_ids is not None else None
+    )
+
     while start < max_length:
         block_output_ids = output_ids[:, start : start + block_size].clone()
         block_position_ids = position_ids[:, start : start + block_size]
@@ -142,15 +147,16 @@ def dflash_generate(
         if block_size > 1:
             target_hidden = extract_context_feature(output.hidden_states, model.target_layer_ids)[:, :acceptance_length + 1, :]
 
-        if stop_token_ids is not None and any(
-            stop_token_id in output_ids[:, num_input_tokens:] for stop_token_id in stop_token_ids
-        ):
+        if stop_token_tensor is not None and torch.isin(
+            output_ids[0, num_input_tokens : start + 1], stop_token_tensor
+        ).any().item():
             break
 
     output_ids = output_ids[:, :min(start + 1, max_length)]
-    if stop_token_ids is not None:
-        stop_token_ids = torch.tensor(stop_token_ids, device=output_ids.device)
-        stop_token_indices = torch.isin(output_ids[0][num_input_tokens:], stop_token_ids).nonzero(as_tuple=True)[0]
+    if stop_token_tensor is not None:
+        stop_token_indices = torch.isin(
+            output_ids[0][num_input_tokens:], stop_token_tensor
+        ).nonzero(as_tuple=True)[0]
         if stop_token_indices.numel() > 0:
             output_ids = output_ids[:, : num_input_tokens + stop_token_indices[0] + 1]
 
diff --git a/pyproject.toml b/pyproject.toml
index 7215065..b8a1fc7 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -36,3 +36,8 @@ mlx = [
     "mlx==0.31.2",
     "mlx-lm==0.31.3",
 ]
+test = [
+    "pytest>=8",
+    "torch",
+    "transformers==4.57.1",
+]
diff --git a/tests/__init__.py b/tests/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/tests/test_model.py b/tests/test_model.py
new file mode 100644
index 0000000..68413f6
--- /dev/null
+++ b/tests/test_model.py
@@ -0,0 +1,135 @@
+"""Unit tests for dflash.model helpers and the dflash_generate stop-token check.
+
+These tests intentionally cover only the pure-Python / pure-tensor logic so they
+run on CPU without needing weights or transformers Qwen3 wiring.
+"""
+
+import pytest
+
+torch = pytest.importorskip("torch")
+
+from dflash.model import build_target_layer_ids, extract_context_feature, sample
+
+
+def test_build_target_layer_ids_single_draft_layer():
+    assert build_target_layer_ids(num_target_layers=24, num_draft_layers=1) == [12]
+
+
+def test_build_target_layer_ids_endpoints_for_two_draft_layers():
+    layers = build_target_layer_ids(num_target_layers=64, num_draft_layers=2)
+    assert layers == [1, 64 - 3]
+
+
+def test_build_target_layer_ids_evenly_interpolates():
+    layers = build_target_layer_ids(num_target_layers=64, num_draft_layers=4)
+    assert layers[0] == 1
+    assert layers[-1] == 64 - 3
+    assert len(layers) == 4
+    assert layers == sorted(layers)
+
+
+def test_extract_context_feature_concatenates_offset_layers():
+    # extract_context_feature reads hidden_states[layer_id + 1] for each layer_id
+    # (the +1 offset skips the embedding output).
+    bsz, seq, hidden = 2, 5, 8
+    hidden_states = [torch.full((bsz, seq, hidden), float(i)) for i in range(6)]
+    out = extract_context_feature(hidden_states, layer_ids=[0, 2, 4])
+    assert out.shape == (bsz, seq, hidden * 3)
+    # hidden_states[1] is all 1.0, hidden_states[3] is all 3.0, hidden_states[5] is all 5.0
+    assert torch.equal(out[..., :hidden], torch.full((bsz, seq, hidden), 1.0))
+    assert torch.equal(out[..., hidden : 2 * hidden], torch.full((bsz, seq, hidden), 3.0))
+    assert torch.equal(out[..., 2 * hidden :], torch.full((bsz, seq, hidden), 5.0))
+
+
+def test_sample_temperature_zero_is_argmax():
+    logits = torch.tensor([[[2.0, 1.5, 1.0, 0.5], [0.1, 0.9, 0.2, 0.0]]])
+    out = sample(logits, temperature=0.0)
+    assert out.shape == (1, 2)
+    assert out[0, 0].item() == 0
+    assert out[0, 1].item() == 1
+
+
+def test_sample_with_temperature_returns_in_range():
+    torch.manual_seed(0)
+    logits = torch.randn(1, 4, 32)
+    out = sample(logits, temperature=1.0)
+    assert out.shape == (1, 4)
+    assert (out >= 0).all() and (out < 32).all()
+
+
+def _legacy_in_loop_check(output_ids, num_input_tokens, stop_token_ids):
+    """Reproduces the pre-fix in-loop early-exit check from dflash_generate."""
+    return any(
+        stop_token_id in output_ids[:, num_input_tokens:]
+        for stop_token_id in stop_token_ids
+    )
+
+
+def _new_in_loop_check(output_ids, num_input_tokens, cursor, stop_token_tensor):
+    """The new in-loop early-exit check, scoped to the actually written slice."""
+    return torch.isin(
+        output_ids[0, num_input_tokens : cursor + 1], stop_token_tensor
+    ).any().item()
+
+
+def test_stop_token_check_does_not_scan_uninitialized_buffer():
+    """Regression test for the pre-fix in-loop check.
+
+    dflash_generate pre-allocates output_ids with mask_token_id past the
+    cursor. If mask_token_id is also a stop token (a model-config-dependent
+    edge case the maintainer already cares about — see PR #76 "Preserve
+    output tokens that equal mask_token_id"), the legacy
+    `stop_id in output_ids[:, num_input_tokens:]` check fires on every
+    iteration because the still-pristine tail of the buffer is full of
+    mask tokens equal to a stop token. Generation aborts after the first
+    block even though no real stop was emitted.
+
+    The new check restricts the scan to positions that have actually been
+    written (`[num_input_tokens, cursor + 1]`) and does not regress.
+    """
+    mask_token_id = 99
+    stop_token_ids = [99, 1]  # collides with mask_token_id
+    num_input_tokens = 10
+    max_length = 50
+    block_size = 8
+
+    output_ids = torch.full(
+        (1, max_length + block_size), mask_token_id, dtype=torch.long
+    )
+    output_ids[:, :num_input_tokens] = 5
+
+    # Simulate one block of generation: 4 accepted draft tokens + 1 bonus,
+    # none of which are stop tokens. After dflash_generate's
+    # `start += acceptance_length + 1`, the cursor lands on the bonus token.
+    written = torch.tensor([5, 6, 7, 8, 10])  # no stop tokens, no mask tokens
+    output_ids[0, num_input_tokens : num_input_tokens + written.numel()] = written
+    cursor = num_input_tokens + written.numel() - 1
+
+    # Pre-fix check spuriously fires because it scans the still-mask-filled
+    # tail past the cursor.
+    assert _legacy_in_loop_check(output_ids, num_input_tokens, stop_token_ids) is True
+
+    stop_tensor = torch.as_tensor(stop_token_ids, dtype=output_ids.dtype)
+    assert _new_in_loop_check(output_ids, num_input_tokens, cursor, stop_tensor) is False
+
+
+def test_stop_token_check_detects_real_stop_after_cursor_advance():
+    """When a real stop token is written and the cursor has advanced, the
+    new check fires (matching the legacy semantic for the common case)."""
+    mask_token_id = 0
+    stop_token_ids = [7, 11]
+    num_input_tokens = 4
+    max_length = 32
+    block_size = 4
+
+    output_ids = torch.full(
+        (1, max_length + block_size), mask_token_id, dtype=torch.long
+    )
+    output_ids[:, :num_input_tokens] = 5
+
+    # Simulate one block worth of generation; cursor advances by 4.
+    output_ids[0, num_input_tokens : num_input_tokens + 4] = torch.tensor([3, 7, 6, 8])
+    cursor = num_input_tokens + 3
+
+    stop_tensor = torch.as_tensor(stop_token_ids, dtype=output_ids.dtype)
+    assert _new_in_loop_check(output_ids, num_input_tokens, cursor, stop_tensor) is True