Fix AssertionError during eval when val set size is not divisible by train_batch_size

skyrl/train/dataset/preprocess.py

@@ -196,6 +196,7 @@ def compute_prompt_mini_batch_boundaries(
     train_batch_size: int,
     is_stepwise: bool,
     n_samples_per_prompt: int,
+    is_training: bool = True,
 ) -> List[Tuple[int, int]]:
     """Compute mini-batch ``(start, end)`` slices from a flat ``uids`` list.
 
@@ -206,10 +207,12 @@ def compute_prompt_mini_batch_boundaries(
         train_batch_size: Number of prompts in a training batch. For sanity check.
         is_stepwise: Whether the training is step-wise. For sanity check.
         n_samples_per_prompt: how many samples per prompt. For sanity check.
+        is_training: Whether this is a training batch (strict validation) or eval batch (allows partial batches).
+            Defaults to True for backward compatibility.
     Returns:
         List of (start, end) indices of the mini-batches. The length of the list is the number of
-        mini-batches, guaranteed to be `train_batch_size // mini_batch_size` regardless of whether
-        the training is step-wise or not.
+        mini-batches, guaranteed to be `train_batch_size // mini_batch_size` during training, but may differ
+        during evaluation if the final batch is partial.
 
     Consecutive equal entries in ``uids`` belong to the same prompt. Each mini batch spans exactly
     ``mini_batch_size`` prompts (the last may be smaller if the total prompt count is not divisible
@@ -244,23 +247,35 @@ def compute_prompt_mini_batch_boundaries(
             prompt_end_indices.append(i)
     prompt_end_indices.append(len(uids))
 
-    # seen_uids should equal to the number of prompts and equal to `train_batch_size`
+    # Check that num_prompts matches expected batch size
     num_prompts = len(prompt_end_indices)
-    assert num_prompts == train_batch_size and len(seen_uids) == train_batch_size
-    assert train_batch_size % mini_batch_size == 0
-
-    # Compute boundaries.
+    if is_training:
+        assert (
+            num_prompts == train_batch_size and len(seen_uids) == train_batch_size
+        ), f"Expected {train_batch_size} prompts in training batch, got {num_prompts}."
+        assert train_batch_size % mini_batch_size == 0
+    else:
+        if num_prompts != train_batch_size:
+            logger.info(
+                f"Partial batch detected during eval: got {num_prompts} prompts but "
+                f"train_batch_size={train_batch_size}. Using actual batch size for mini-batch boundaries."
+            )
+
+    # Compute boundaries. Handle partial batches during eval.
     boundaries: List[Tuple[int, int]] = []
     start_seq = 0
+
     for i in range(0, num_prompts, mini_batch_size):
-        end_prompt_idx = i + mini_batch_size - 1  # i + mini_batch_size is next mini-batch's first prompt's end index
+        end_prompt_idx = min(i + mini_batch_size - 1, num_prompts - 1)
         end_seq = prompt_end_indices[end_prompt_idx]
         boundaries.append((start_seq, end_seq))
         start_seq = end_seq
-    assert len(boundaries) == train_batch_size // mini_batch_size
+
+    if is_training:
+        assert len(boundaries) == train_batch_size // mini_batch_size
 
     # Assert that the mini-batch boundaries are uniform for non-step-wise training.
-    if not is_stepwise:
+    if not is_stepwise and is_training:
         expected_num_seq_in_mini_batch = n_samples_per_prompt * mini_batch_size
         for i, (start, end) in enumerate(boundaries):
             assert start == i * expected_num_seq_in_mini_batch

skyrl/train/trainer.py

@@ -592,13 +592,17 @@ def init_weight_sync_state(self):
         self.dispatch.init_weight_sync_state(self.inference_engine_client)
         logger.info("Initialized weight sync state for policy model and inference engines.")
 
-    def convert_to_training_input(self, generator_output: GeneratorOutput, uids: List[str]) -> TrainingInputBatch:
+    def convert_to_training_input(
+        self, generator_output: GeneratorOutput, uids: List[str], is_training: bool = True
+    ) -> TrainingInputBatch:
         """Converts lists to a padded batch of tensors for training
 
         Args:
             generator_output (GeneratorOutput): Generated rollouts and associated data.
             uids (List[str]): List of prompt-unique identifiers for each generator ouput in the same
                 order as `generator_output`. Used to identify which prompt each generated rollout belongs to.
+            is_training (bool): Whether this batch is for training (strict batch size) or evaluation
+                (allows partial batches). Defaults to True for backward compatibility.
         Returns:
             training_input (TrainingInputBatch): Padded batch of tensors for training. It preserves the
                 order of `generator_output` and hence `uids`.
@@ -680,11 +684,21 @@ def convert_to_training_input(self, generator_output: GeneratorOutput, uids: Lis
         n_samples_per_prompt = self.cfg.generator.n_samples_per_prompt
         is_stepwise = self.cfg.generator.step_wise_trajectories
         training_input.metadata["policy_mini_batch_boundaries"] = compute_prompt_mini_batch_boundaries(
-            uids, self.cfg.trainer.policy_mini_batch_size, train_batch_size, is_stepwise, n_samples_per_prompt
+            uids,
+            self.cfg.trainer.policy_mini_batch_size,
+            train_batch_size,
+            is_stepwise,
+            n_samples_per_prompt,
+            is_training=is_training,
         )
         if self.cfg.trainer.critic.model.path is not None:
             training_input.metadata["critic_mini_batch_boundaries"] = compute_prompt_mini_batch_boundaries(
-                uids, self.cfg.trainer.critic_mini_batch_size, train_batch_size, is_stepwise, n_samples_per_prompt
+                uids,
+                self.cfg.trainer.critic_mini_batch_size,
+                train_batch_size,
+                is_stepwise,
+                n_samples_per_prompt,
+                is_training=is_training,
             )
 
         # 5. Record metadata and metrics.

tests/train/test_prompt_mini_batch.py

@@ -198,10 +198,88 @@ def test_same_step_count_as_non_stepwise(self):
         )
 
         assert len(stepwise_bounds) == len(non_stepwise_bounds) == 2
-
         # Non-step-wise boundaries should be uniform
         assert non_stepwise_bounds == [(0, 640), (640, 1280)]
 
+    def test_eval_partial_batch_nonstepwise(self):
+        """Test eval mode with partial batches during non-stepwise training.
+
+        This addresses the issue where evaluation crashes when val set size is
+        not divisible by train_batch_size. With is_training=False, partial
+        batches should be allowed.
+        """
+        train_batch_size = 4
+        spp = 2
+        is_stepwise = False
+        mini_batch_size = 2
+
+        # Only 3 prompts instead of 4 (partial batch)
+        uids = ["p0", "p0", "p1", "p1", "p2", "p2"]
+
+        # Should work fine with is_training=False
+        boundaries = compute_prompt_mini_batch_boundaries(
+            uids, mini_batch_size, train_batch_size, is_stepwise, spp, is_training=False
+        )
+        # With 3 prompts and mini_batch_size=2, we get 2 mini-batches:
+        # First mini-batch: prompts 0-1 (sequences 0-4)
+        # Second mini-batch: prompt 2 (sequences 4-6)
+        assert boundaries == [(0, 4), (4, 6)]
+
+    def test_eval_partial_batch_single_minibatch(self):
+        """Test eval mode with partial batch that fits in single mini-batch."""
+        train_batch_size = 4
+        spp = 2
+        is_stepwise = False
+        mini_batch_size = 2
+
+        # Only 1 prompt instead of 4 (very partial batch)
+        uids = ["p0", "p0"]
+
+        boundaries = compute_prompt_mini_batch_boundaries(
+            uids, mini_batch_size, train_batch_size, is_stepwise, spp, is_training=False
+        )
+        # With 1 prompt and mini_batch_size=2, we get 1 mini-batch
+        assert boundaries == [(0, 2)]
+
+    def test_eval_rejects_noncontiguous_uids(self):
+        """Test that eval mode still enforces contiguous uids."""
+        train_batch_size = 4
+        spp = 2
+        is_stepwise = False
+        mini_batch_size = 2
+        # Non-contiguous uids: p0 appears at index 0-1 and 4-5
+        uids = ["p0", "p0", "p1", "p1", "p0", "p0"]
+
+        with pytest.raises(AssertionError, match="uid 'p0' appears in non-contiguous positions"):
+            compute_prompt_mini_batch_boundaries(
+                uids, mini_batch_size, train_batch_size, is_stepwise, spp, is_training=False
+            )
+
+    def test_eval_stepwise_partial_batch(self):
+        """Test eval mode with stepwise training and partial batch."""
+        mini_batch_size = 2
+        train_batch_size = 4
+        spp = 2
+        is_stepwise = True
+
+        # Only 3 prompts instead of 4
+        uids = _make_uids_stepwise(
+            [
+                ("p0", 2, [3, 2]),  # 5 seqs
+                ("p1", 2, [1, 4]),  # 5 seqs
+                ("p2", 2, [2, 1]),  # 3 seqs
+            ]
+        )
+
+        # Should work fine with is_training=False
+        boundaries = compute_prompt_mini_batch_boundaries(
+            uids, mini_batch_size, train_batch_size, is_stepwise, spp, is_training=False
+        )
+        # With 3 prompts and mini_batch_size=2, we get 2 mini-batches:
+        # First: prompts 0-1 (sequences 0-10)
+        # Second: prompt 2 (sequences 10-13)
+        assert boundaries == [(0, 10), (10, 13)]
+
 
 # ---------------------------------------------------------------------------
 # Tests for MeshDispatch.stage_chunks