feat: avoid excessive low divergence iteration (#73)

* feat: adjust scoring to avoid useless iteration

Adjusts the scoring function to avoid targeting meaninglessly low KL divergences.
Below a threshold value, the KL divergence score switches to the refusal count.
Adds config option kl_divergence_target (defaulting to 0.01).

* fix: Clean up parameter selection in objective

Create variables for num_layers and last_layer_index
* Improves readability and makes choices explicit

* feat: Print the parameters of the selected model

config.default.toml

@@ -49,6 +49,10 @@ residual_plot_style = "dark_background"
 # This is used to ensure balanced co-optimization of KL divergence and refusal count.
 kl_divergence_scale = 1.0
 
+# The KL divergence to target. Below this value, an objective based on the refusal count is used.
+# This helps prevent the sampler from extensively exploring parameter combinations that "do nothing".
+kl_divergence_target = 0.01
+
 # Number of abliteration trials to run during optimization.
 n_trials = 200
 

src/heretic/config.py

@@ -119,6 +119,14 @@ class Settings(BaseSettings):
         ),
     )
 
+    kl_divergence_target: float = Field(
+        default=0.01,
+        description=(
+            "The KL divergence to target. Below this value, an objective based on the refusal count is used."
+            'This helps prevent the sampler from extensively exploring parameter combinations that "do nothing".'
+        ),
+    )
+
     n_trials: int = Field(
         default=200,
         description="Number of abliteration trials to run during optimization.",

src/heretic/evaluator.py

@@ -76,9 +76,19 @@ class Evaluator:
         refusals = self.count_refusals()
         print(f"  * Refusals: [bold]{refusals}[/]/{len(self.bad_prompts)}")
 
+        kl_divergence_scale = self.settings.kl_divergence_scale
+        kl_divergence_target = self.settings.kl_divergence_target
+
+        refusals_score = refusals / self.base_refusals
+
+        if kl_divergence >= kl_divergence_target:
+            kld_score = kl_divergence / kl_divergence_scale
+        else:
+            kld_score = refusals_score * kl_divergence_target / kl_divergence_scale
+
         score = (
-            (kl_divergence / self.settings.kl_divergence_scale),
-            (refusals / self.base_refusals),
+            kld_score,
+            refusals_score,
         )
 
         return score, kl_divergence, refusals

src/heretic/main.py

@@ -27,6 +27,7 @@ from optuna import Trial, TrialPruned
 from optuna.exceptions import ExperimentalWarning
 from optuna.samplers import TPESampler
 from optuna.study import StudyDirection
+from optuna.trial import TrialState
 from pydantic import ValidationError
 from questionary import Choice
 from rich.traceback import install
@@ -264,6 +265,8 @@ def run():
             ],
         )
 
+        last_layer_index = len(model.get_layers()) - 1
+
         # Discrimination between "harmful" and "harmless" inputs is usually strongest
         # in layers slightly past the midpoint of the layer stack. See the original
         # abliteration paper (https://arxiv.org/abs/2406.11717) for a deeper analysis.
@@ -273,8 +276,8 @@ def run():
         # work with conditional or variable-range parameters.
         direction_index = trial.suggest_float(
             "direction_index",
-            0.4 * (len(model.get_layers()) - 1),
-            0.9 * (len(model.get_layers()) - 1),
+            0.4 * last_layer_index,
+            0.9 * last_layer_index,
         )
 
         if direction_scope == "per layer":
@@ -293,8 +296,8 @@ def run():
             )
             max_weight_position = trial.suggest_float(
                 f"{component}.max_weight_position",
-                0.6 * (len(model.get_layers()) - 1),
-                len(model.get_layers()) - 1,
+                0.6 * last_layer_index,
+                1.0 * last_layer_index,
             )
             # For sampling purposes, min_weight is expressed as a fraction of max_weight,
             # again because multivariate TPE doesn't support variable-range parameters.
@@ -307,7 +310,7 @@ def run():
             min_weight_distance = trial.suggest_float(
                 f"{component}.min_weight_distance",
                 1.0,
-                0.6 * (len(model.get_layers()) - 1),
+                0.6 * last_layer_index,
             )
 
             parameters[component] = AbliterationParameters(
@@ -378,13 +381,27 @@ def run():
     # If no trials at all have been evaluated, the study must have been stopped
     # by pressing Ctrl+C while the first trial was running. In this case, we just
     # re-raise the interrupt to invoke the standard handler defined below.
-    if not study.best_trials:
+    completed_trials = [t for t in study.trials if t.state == TrialState.COMPLETE]
+    if not completed_trials:
         raise KeyboardInterrupt
 
-    best_trials = sorted(
-        study.best_trials,
-        key=lambda trial: trial.user_attrs["refusals"],
+    # Get the Pareto front of trials. We can't use study.best_trials directly
+    # as get_score() doesn't return the pure KL divergence and refusal count.
+    # Note: Unlike study.best_trials, this does not handle objective constraints.
+    sorted_trials = sorted(
+        completed_trials,
+        key=lambda trial: (
+            trial.user_attrs["refusals"],
+            trial.user_attrs["kl_divergence"],
+        ),
     )
+    min_divergence = math.inf
+    best_trials = []
+    for trial in sorted_trials:
+        kl_divergence = trial.user_attrs["kl_divergence"]
+        if kl_divergence < min_divergence:
+            min_divergence = kl_divergence
+            best_trials.append(trial)
 
     choices = [
         Choice(
@@ -426,6 +443,9 @@ def run():
 
         print()
         print(f"Restoring model from trial [bold]{trial.user_attrs['index']}[/]...")
+        print("* Parameters:")
+        for name, value in get_trial_parameters(trial).items():
+            print(f"  * {name} = [bold]{value}[/]")
         print("* Reloading model...")
         model.reload_model()
         print("* Abliterating...")