feat: make response prefix logic configurable

src/heretic/config.py

@@ -142,6 +142,45 @@ class Settings(BaseSettings):
         description="Maximum number of tokens to generate for each response.",
     )
 
+    response_prefix: str | None = Field(
+        default=None,
+        description=(
+            "Common prefix to assume for all responses, so that evaluation happens "
+            "at the point where responses start to differ for different prompts. "
+            "If not set, the prefix is determined automatically by comparing multiple responses."
+        ),
+    )
+
+    chain_of_thought_skips: list[tuple[str, str]] = Field(
+        default=[
+            # Most thinking models.
+            (
+                "<think>",
+                "<think></think>",
+            ),
+            # gpt-oss.
+            (
+                "<|channel|>analysis<|message|>",
+                "<|channel|>analysis<|message|><|end|><|start|>assistant<|channel|>final<|message|>",
+            ),
+            # Unknown, suggested by user.
+            (
+                "<thought>",
+                "<thought></thought>",
+            ),
+            # Unknown, suggested by user.
+            (
+                "[THINK]",
+                "[THINK][/THINK]",
+            ),
+        ],
+        description=(
+            "List of pairs of the form (cot_initializer, closed_cot_block) used to skip "
+            "the Chain-of-Thought block in responses, so that evaluation happens "
+            "at the start of the actual response."
+        ),
+    )
+
     print_responses: bool = Field(
         default=False,
         description="Whether to print prompt/response pairs when counting refusals.",

src/heretic/main.py

@@ -393,52 +393,44 @@ def run():
         settings.batch_size = best_batch_size
         print(f"* Chosen batch size: [bold]{settings.batch_size}[/]")
 
-    print()
-    print("Checking for common response prefix...")
-    prefix_check_prompts = good_prompts[:100] + bad_prompts[:100]
-    responses = model.get_responses_batched(prefix_check_prompts)
-
-    # Despite being located in os.path, commonprefix actually performs
-    # a naive string operation without any path-specific logic,
-    # which is exactly what we need here. Trailing spaces are removed
-    # to avoid issues where multiple different tokens that all start
-    # with a space character lead to the common prefix ending with
-    # a space, which would result in an uncommon tokenization.
-    model.response_prefix = commonprefix(responses).rstrip(" ")
-
-    # Suppress CoT output.
-    recheck_prefix = False
-    if model.response_prefix:
-        # When using any of the predefined prefixes below, we need to check that
-        # the prefix is actually complete (e.g. not missing a trailing newline).
-        recheck_prefix = True
-        if model.response_prefix.startswith("<think>"):
-            # Most thinking models.
-            model.response_prefix = "<think></think>"
-        elif model.response_prefix.startswith("<|channel|>analysis<|message|>"):
-            # gpt-oss.
-            model.response_prefix = "<|channel|>analysis<|message|><|end|><|start|>assistant<|channel|>final<|message|>"
-        elif model.response_prefix.startswith("<thought>"):
-            # Unknown, suggested by user.
-            model.response_prefix = "<thought></thought>"
-        elif model.response_prefix.startswith("[THINK]"):
-            # Unknown, suggested by user.
-            model.response_prefix = "[THINK][/THINK]"
-        else:
-            recheck_prefix = False
-
-    if model.response_prefix:
-        print(f"* Prefix found: [bold]{model.response_prefix!r}[/]")
-    else:
-        print("* None found")
-
-    if recheck_prefix:
-        print("* Rechecking with prefix...")
+    if settings.response_prefix is None:
+        print()
+        print("Checking for common response prefix...")
+        prefix_check_prompts = good_prompts[:100] + bad_prompts[:100]
         responses = model.get_responses_batched(prefix_check_prompts)
-        additional_prefix = commonprefix(responses).rstrip(" ")
-        if additional_prefix:
-            model.response_prefix += additional_prefix
-            print(f"* Extended prefix found: [bold]{model.response_prefix!r}[/]")
+
+        # Despite being located in os.path, commonprefix actually performs
+        # a naive string operation without any path-specific logic,
+        # which is exactly what we need here. Trailing spaces are removed
+        # to avoid issues where multiple different tokens that all start
+        # with a space character lead to the common prefix ending with
+        # a space, which would result in an uncommon tokenization.
+        settings.response_prefix = commonprefix(responses).rstrip(" ")
+
+        if settings.response_prefix:
+            print(f"* Prefix found: [bold]{settings.response_prefix!r}[/]")
+
+            for cot_initializer, closed_cot_block in settings.chain_of_thought_skips:
+                if settings.response_prefix.startswith(cot_initializer):
+                    settings.response_prefix = closed_cot_block
+                    print(
+                        f"* Closed Chain-of-Thought block: [bold]{settings.response_prefix!r}[/]"
+                    )
+
+                    # When using a Chain-of-Thought skip, we need to check that the prefix
+                    # is actually complete (e.g. not missing a trailing newline).
+                    print("* Rechecking with prefix...")
+                    responses = model.get_responses_batched(prefix_check_prompts)
+                    additional_prefix = commonprefix(responses).rstrip(" ")
+                    if additional_prefix:
+                        settings.response_prefix += additional_prefix
+                        print(
+                            f"* Extended prefix found: [bold]{settings.response_prefix!r}[/]"
+                        )
+
+                    break
+        else:
+            print("* None found")
 
     evaluator = Evaluator(settings, model)
 

src/heretic/model.py

@@ -59,7 +59,6 @@ class Model:
 
     def __init__(self, settings: Settings):
         self.settings = settings
-        self.response_prefix = ""
         self.needs_reload = False
 
         print()
@@ -565,10 +564,12 @@ class Model:
             ),
         )
 
-        if self.response_prefix:
+        if self.settings.response_prefix:
             # Append the common response prefix to the prompts so that evaluation happens
             # at the point where responses start to differ for different prompts.
-            chat_prompts = [prompt + self.response_prefix for prompt in chat_prompts]
+            chat_prompts = [
+                prompt + self.settings.response_prefix for prompt in chat_prompts
+            ]
 
         inputs = self.tokenizer(
             chat_prompts,