apply_chat_template(tokenize=True) crashes on assistant messages with tool calls and no content #45290

System Info

Transformers 5.5..0

Who can help?

@zucchini-nlp @Rocketknight1

Information

• The official example scripts
• My own modified scripts

Tasks

• An officially supported task in the examples folder (such as GLUE/SQuAD, ...)
• My own task or dataset (give details below)

Reproduction

ProcessorMixin.apply_chat_template raises KeyError: 'content' when tokenize=True and the conversation contains an assistant message with tool_calls but no content key.

from transformers import AutoProcessor

processor = AutoProcessor.from_pretrained("Qwen/Qwen2.5-VL-3B-Instruct")

messages = [
    [
        {"role": "user", "content": [{"type": "text", "text": "dummy"}]},
        {"role": "assistant", "tool_calls": [{"type": "function", "function": {"name": "foo", "arguments": {}}}]},
    ]
]

processor.apply_chat_template(messages, tokenize=True)

KeyError: 'content'
  File "processing_utils.py", line 1807, in apply_chat_template
    visuals = [content for content in message["content"] if content["type"] in ["image", "video"]]
                                      ~~~~~~~^^^^^^^^^^^

It comes from these lines

where it's assumed that all turns contain a content key. However in the codebase, AFAICT, content is always assumed to be optional for assistant turns with tool calls.

Possible fix

Guard the access with .get("content") or skip messages without content:

for message in conversation:
    content = message.get("content") or []
    visuals = [c for c in content if isinstance(c, dict) and c.get("type") in ["image", "video"]]

Expected behavior

to pass with no content