diff --git a/src/heretic/main.py b/src/heretic/main.py
index 3e6f4a6..c480888 100644
--- a/src/heretic/main.py
+++ b/src/heretic/main.py
@@ -377,7 +377,8 @@ def run():
 
     print()
     print("Checking for common response prefix...")
-    responses = model.get_responses_batched(good_prompts[:100] + bad_prompts[:100])
+    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,
@@ -388,24 +389,39 @@ def run():
     model.response_prefix = commonprefix(responses).rstrip(" ")
 
     # Suppress CoT output.
-    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]"
+    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...")
+        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}[/]")
+
     evaluator = Evaluator(settings, model)
 
     if settings.evaluate_model is not None: