Fix edge case for continue_final_message (#36404)

* Fix edge case for continue_final_message

* lstrip() correctly

* Add regression test

* Add a clearer error message when the final message is not present

* Add a clearer error message when the final message is not present

* Fix massive bug!

src/transformers/tokenization_utils_base.py

@@ -1702,13 +1702,30 @@ class PreTrainedTokenizerBase(SpecialTokensMixin, PushToHubMixin):
             if continue_final_message:
                 final_message = chat[-1]["content"]
                 if isinstance(final_message, (list, tuple)):
-                    final_message = final_message[-1]["text"]
-                try:
-                    rendered_chat = rendered_chat[: rendered_chat.rindex(final_message) + len(final_message)]
-                except:  # noqa: E722
-                    # Some chat templates like Llama-3.1 trim messages before rendering, so we must do the same here.
-                    final_message = final_message.strip()
-                    rendered_chat = rendered_chat[: rendered_chat.rindex(final_message) + len(final_message)]
+                    for content_block in reversed(final_message):
+                        if "text" in content_block:
+                            # Pick the last text block in the message (the first one we hit while iterating in reverse)
+                            final_message = content_block["text"]
+                            break
+                    else:
+                        raise ValueError(
+                            "continue_final_message is set but we could not find any text to continue"
+                            "in the final message!"
+                        )
+                if final_message.strip() not in rendered_chat:
+                    raise ValueError(
+                        "continue_final_message is set but the final message does not appear in the chat after "
+                        "applying the chat template! This can happen if the chat template deletes portions of "
+                        "the final message. Please verify the chat template and final message in your chat to "
+                        "ensure they are compatible."
+                    )
+                final_msg_loc = rendered_chat.rindex(final_message.strip())
+                if rendered_chat[final_msg_loc : final_msg_loc + len(final_message.lstrip())] == final_message:
+                    # The template preserves spacing or the message doesn't have trailing spacing, so things are simple
+                    rendered_chat = rendered_chat[: final_msg_loc + len(final_message.lstrip())]
+                else:
+                    # The message has trailing spacing that was trimmed, so we must be more cautious
+                    rendered_chat = rendered_chat[: final_msg_loc + len(final_message.strip())]
             rendered.append(rendered_chat)
 
         if not is_batched:

tests/test_tokenization_common.py

@@ -1565,6 +1565,33 @@ class TokenizerTesterMixin:
                     "<|im_start|>system\nsystem message<|im_end|>\n<|im_start|>user\nuser message<|im_end|>\n<|im_start|>assistant\nassistant message",
                 )
 
+    @require_jinja
+    def test_continue_final_message_with_decoy_earlier_message(self):
+        """Regression test for chat templates where an earlier message has similar content to the final message
+        https://github.com/huggingface/transformers/issues/35433"""
+
+        dummy_template = """
+        {%- for message in messages %}
+            {{- "<|im_start|>" + message['role'] + "\n" + message['content'] | trim + "<|im_end|>" + "\n"}}
+        {%- endfor %}"""
+        dummy_conversation = [
+            {"role": "user", "content": "hi 0"},
+            {"role": "assistant", "content": "bye: 0"},
+            {"role": "user", "content": "hi 1"},
+            {"role": "assistant", "content": "bye: "},
+        ]
+        tokenizers = self.get_tokenizers()
+        for tokenizer in tokenizers:
+            with self.subTest(f"{tokenizer.__class__.__name__}"):
+                prefill_output = tokenizer.apply_chat_template(
+                    dummy_conversation, chat_template=dummy_template, tokenize=False, continue_final_message=True
+                )
+                # Assert that the final message is unterminated
+                self.assertEqual(
+                    prefill_output,
+                    "<|im_start|>user\nhi 0<|im_end|>\n<|im_start|>assistant\nbye: 0<|im_end|>\n<|im_start|>user\nhi 1<|im_end|>\n<|im_start|>assistant\nbye:",
+                )
+
     @require_jinja
     def test_chat_template_dict(self):
         dummy_template_1 = "{{'a'}}"