[Mengxi-Xu]

What does this PR do?

Fixes an uncaught IndexError in render_jinja_template() when continue_final_message is set and a conversation is an empty list.

Problem

In render_jinja_template(), when continue_final_message is set (truthy), the code accesses chat[-1] to read the final message's continue field. If a conversation is an empty list ([]), chat[-1] raises IndexError: list index out of range instead of the documented ValueError.

The function already raises ValueError for the case "final message has no continue field" — but it never validates the case "conversation itself is empty", so the empty-list case falls through to chat[-1] and crashes with the wrong exception type.

Fix

Add an explicit empty-conversation check at the start of the if continue_final_message: branch, raising ValueError("continue_final_message is set but the conversation is empty!") — consistent with the existing ValueError raised for the missing-continue-field case in the same function.

Reproducer

from transformers.utils.chat_template_utils import render_jinja_template

template = "{% for m in messages %}{{ m['role'] }}:{{ m['content'] }}\n{% endfor %}"

# On main: IndexError: list index out of range (chat_template_utils.py:755, chat[-1])
# With this fix: ValueError: continue_final_message is set but the conversation is empty!
render_jinja_template(
    conversations=[[]],
    chat_template=template,
    continue_final_message="content",
    add_generation_prompt=False,
)

continue_final_message=True (bool) reproduces identically — it is normalized to "content" before reaching chat[-1].

Dependencies

None.

Who can review?

@ArthurZucker @itazap (tokenizers / chat template)

[github-actions]

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[Rocketknight1]

Yeah, makes sense to me! It's an edge case but the fix is small so I'm happy with it

[HuggingFaceDocBuilderDev]

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