Hash-based filtering of stale LILO data in adapter (#4993)

Summary:
Pull Request resolved: #4993

When building the RankingDataset for PairwiseGP model fitting, exclude LILO
trial data whose input hash doesn't match the current experiment state. This
ensures PairwiseGP is only fitted on labels that are consistent with the
current metric data and LLM messages.

Changes:
- Add `_get_fresh_pairwise_trial_indices` helper to `adapter_utils.py`:
  uses `get_current_lilo_hash` from `hash_utils` to compute the current hash
  and returns trial indices whose stamped hash matches, or `None` if not a
  LILO experiment (preserving BOPE compatibility)
- Filter pairwise data in `TorchAdapter._convert_experiment_data` before
  calling `prep_pairwise_data`, ensuring stale rows are excluded
- Add tests for hash-based filtering logic

Reviewed By: saitcakmak

Differential Revision: D95284286

Author: ItsMrLin

ax/adapter/adapter_utils.py

@@ -44,6 +44,8 @@
     get_weighted_mc_objective_and_objective_thresholds,
     pareto_frontier_evaluator,
 )
+from ax.utils.common.constants import Keys
+from ax.utils.common.hash_utils import get_current_lilo_hash
 from ax.utils.common.logger import get_logger
 from ax.utils.common.typeutils import (
     assert_is_instance_of_tuple,
@@ -1270,6 +1272,57 @@ def process_contextual_datasets(
     return contextual_datasets
 
 
+def _get_fresh_pairwise_trial_indices(
+    experiment: Experiment,
+) -> set[int] | None:
+    """Return trial indices whose pairwise labels match current experiment state.
+
+    LILO (Language-in-the-Loop) trials are stamped with a hash of the
+    experiment state (metric data + LLM messages) at labeling time.  When
+    the experiment state changes (new data, updated LLM messages), old labels
+    become stale and should be excluded from PairwiseGP model fitting.
+
+    Design note: we intentionally compare each trial's stamped hash against
+    the *current* experiment state rather than the most-recently-stamped LILO
+    hash.  This is because the LLM prompt includes a full experiment summary
+    (via ``get_llm_messages_with_experiment_summary``), so any change to
+    input metric data -- even from non-LILO trials -- alters the context
+    under which labels would be produced and warrants relabeling.
+
+    Returns:
+        A set of trial indices whose LILO input hash matches the current
+        experiment state, or ``None`` if hash-based filtering is not
+        applicable (e.g., no trials have a LILO input hash -- the experiment
+        uses BOPE or another non-LILO pairwise workflow).
+    """
+    # Collect trials that have been stamped with a LILO input hash.
+    stamped_trials = {
+        idx: trial
+        for idx, trial in experiment.trials.items()
+        if Keys.LILO_INPUT_HASH in trial._properties
+    }
+    if not stamped_trials:
+        # Not a LILO experiment -- no filtering needed.
+        return None
+
+    current_hash = get_current_lilo_hash(experiment)
+    if current_hash is None:
+        return None
+
+    fresh_indices: set[int] = set()
+    for idx, trial in experiment.trials.items():
+        trial_hash = trial._properties.get(Keys.LILO_INPUT_HASH)
+        if trial_hash is None:
+            # Trial without hash (non-LILO trial) -- always include.
+            fresh_indices.add(idx)
+        elif trial_hash == current_hash:
+            # Hash matches -- labels are fresh.
+            fresh_indices.add(idx)
+        # else: stale hash -- excluded.
+
+    return fresh_indices
+
+
 def prep_pairwise_data(
     X: Tensor,
     Y: Tensor,

ax/adapter/tests/test_adapter_utils.py

@@ -8,9 +8,11 @@
 
 
 import numpy as np
+import pandas as pd
 import torch
 from ax.adapter.adapter_utils import (
     _get_adapter_training_data,
+    _get_fresh_pairwise_trial_indices,
     arm_to_np_array,
     can_map_to_binary,
     extract_objective_weight_matrix,
@@ -25,6 +27,9 @@
 from ax.adapter.torch import TorchAdapter
 from ax.adapter.transforms.choice_encode import ChoiceToNumericChoice
 from ax.core.arm import Arm
+from ax.core.data import Data
+from ax.core.derived_metric import DerivedMetric
+from ax.core.experiment import Experiment
 from ax.core.metric import Metric
 from ax.core.objective import MultiObjective, Objective, ScalarizedObjective
 from ax.core.optimization_config import MultiObjectiveOptimizationConfig
@@ -34,6 +39,8 @@
 from ax.core.types import ComparisonOp
 from ax.exceptions.core import UserInputError
 from ax.generators.torch.botorch_modular.generator import BoTorchGenerator
+from ax.utils.common.constants import Keys
+from ax.utils.common.hash_utils import compute_lilo_input_hash
 from ax.utils.common.testutils import TestCase
 from ax.utils.testing.core_stubs import (
     get_experiment_with_observations,
@@ -555,3 +562,81 @@ def test_extract_objective_weight_matrix(self) -> None:
         )
         result = extract_objective_weight_matrix(multi, outcomes)
         np.testing.assert_array_equal(result, [[1.0, 0.0, 0.0], [0.0, 0.0, -1.0]])
+
+    def test_get_fresh_pairwise_trial_indices(self) -> None:
+        """Verify _get_fresh_pairwise_trial_indices hash-based filtering."""
+        search_space = get_search_space_for_range_values()
+        exp = Experiment(name="test", search_space=search_space)
+
+        # Register a DerivedMetric with pairwise name so the function can
+        # look up input_metric_names.
+        pairwise_metric = DerivedMetric(
+            name=Keys.PAIRWISE_PREFERENCE_QUERY.value,
+            input_metric_names=["latency"],
+        )
+        exp.add_tracking_metric(pairwise_metric)
+
+        # Helper to create trial data.
+        def _attach(
+            trial_index: int, arms: dict[str, float], exp: Experiment = exp
+        ) -> None:
+            rows = [
+                {
+                    "trial_index": trial_index,
+                    "arm_name": name,
+                    "metric_name": "latency",
+                    "metric_signature": "latency",
+                    "mean": val,
+                    "sem": 0.1,
+                }
+                for name, val in arms.items()
+            ]
+            exp.attach_data(Data(df=pd.DataFrame(rows)))
+
+        # Create two trials with data.
+        for i in range(2):
+            trial = exp.new_batch_trial()
+            trial.add_arm(Arm(name=f"{i}_0", parameters={"x": float(i)}))
+            trial.mark_running(no_runner_required=True)
+            trial.mark_completed()
+            _attach(i, {f"{i}_0": float(i + 1)})
+
+        with self.subTest("no_hashes_returns_none"):
+            # No trials have LILO_INPUT_HASH -- not a LILO experiment.
+            result = _get_fresh_pairwise_trial_indices(exp)
+            self.assertIsNone(result)
+
+        # Stamp trial 0 with the current hash.
+        current_hash = compute_lilo_input_hash(exp, ["latency"])
+        exp.trials[0]._properties[Keys.LILO_INPUT_HASH] = current_hash
+
+        with self.subTest("fresh_hash_included"):
+            result = _get_fresh_pairwise_trial_indices(exp)
+            assert result is not None
+            self.assertIn(0, result)
+            # Trial 1 has no hash -- always included.
+            self.assertIn(1, result)
+
+        # Stamp trial 1 with a stale hash.
+        exp.trials[1]._properties[Keys.LILO_INPUT_HASH] = "stale_hash_value"
+
+        with self.subTest("stale_hash_excluded"):
+            result = _get_fresh_pairwise_trial_indices(exp)
+            assert result is not None
+            self.assertIn(0, result)
+            self.assertNotIn(1, result)
+
+        with self.subTest("all_stale"):
+            # Make both hashes stale by adding new data.
+            trial2 = exp.new_batch_trial()
+            trial2.add_arm(Arm(name="2_0", parameters={"x": 10.0}))
+            trial2.mark_running(no_runner_required=True)
+            trial2.mark_completed()
+            _attach(2, {"2_0": 999.0})
+            # Now both trial 0 and trial 1 have stale hashes.
+            result = _get_fresh_pairwise_trial_indices(exp)
+            assert result is not None
+            # Trial 0 and 1 are stale, trial 2 has no hash -- included.
+            self.assertNotIn(0, result)
+            self.assertNotIn(1, result)
+            self.assertIn(2, result)

ax/adapter/torch.py

@@ -17,6 +17,7 @@
 import numpy.typing as npt
 import torch
 from ax.adapter.adapter_utils import (
+    _get_fresh_pairwise_trial_indices,
     arm_to_np_array,
     array_to_observation_data,
     extract_objective_thresholds,
@@ -468,6 +469,26 @@ def _convert_experiment_data(
                 Yvar = torch.from_numpy(sem).double().square().view(-1, 1)
             group_indices = torch.from_numpy(trial_indices_np[to_keep])
             if outcome == Keys.PAIRWISE_PREFERENCE_QUERY.value:
+                # Filter out stale LILO trials whose input hash no longer
+                # matches the current experiment state.
+                fresh_indices = _get_fresh_pairwise_trial_indices(
+                    experiment=self._experiment,
+                )
+                if fresh_indices is not None:
+                    fresh_mask = torch.tensor(
+                        [int(gi.item()) in fresh_indices for gi in group_indices],
+                        dtype=torch.bool,
+                    )
+                    X = X[fresh_mask]
+                    Y = Y[fresh_mask]
+                    group_indices = group_indices[fresh_mask]
+                    # Narrow the NaN-filtered to_keep mask further so
+                    # candidate_metadata stays aligned.
+                    to_keep_indices = np.where(to_keep)[0]
+                    fresh_mask_np = fresh_mask.numpy()
+                    to_keep = np.zeros_like(to_keep)
+                    to_keep[to_keep_indices[fresh_mask_np]] = True
+
                 dataset = prep_pairwise_data(
                     X=X.to(device=self.device),
                     Y=Y.to(dtype=torch.long, device=self.device),