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[Bugfix][Multi Modal] Fix incorrect Molmo token processing #26873

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Merged
DarkLight1337 merged 1 commit into from
Oct 15, 2025
Merged

[Bugfix][Multi Modal] Fix incorrect Molmo token processing #26873

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7 changes: 5 additions & 2 deletions vllm/model_executor/models/molmo.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1264,13 +1264,16 @@ def _apply_hf_processor_tokens_only(
) -> list[int]:
processor = self.info.get_hf_processor()

# Apply the chat template to the tokens
# The chat template is already applied to the prompt tokens
# Use message_format="none" to avoid applying it again
# Prepend an empty space if `always_start_with_space` is True
tokens = processor.processor.get_tokens_input( # type: ignore
self.info.get_tokenizer().decode(prompt_tokens),
message_format=processor.message_format,
message_format="none",
always_start_with_space=processor.always_start_with_space,
)
Comment on lines +1267 to 1274
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@gemini-code-assist gemini-code-assist bot Oct 15, 2025

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high

While this correctly fixes the double-templating issue, calling get_tokens_input here incurs a decode-then-encode cycle for the entire prompt on every request. This can be inefficient for long prompts.

Since the main purpose of this call (with message_format="none") is to enforce the always_start_with_space logic, we can optimize this by inlining a more efficient version of that logic. The suggestion below avoids re-encoding the prompt if it already starts with a space, which can provide a significant performance improvement.

Suggested change
# The chat template is already applied to the prompt tokens
# Use message_format="none" to avoid applying it again
# Prepend an empty space if `always_start_with_space` is True
tokens = processor.processor.get_tokens_input( # type: ignore
self.info.get_tokenizer().decode(prompt_tokens),
message_format=processor.message_format,
message_format="none",
always_start_with_space=processor.always_start_with_space,
)
tokenizer = self.info.get_tokenizer()
# The chat template is already applied. The logic below is an
# optimized reimplementation of `processor.get_tokens_input`
# with `message_format="none"`. It avoids a decode-encode cycle
# if the prompt already starts with a space, improving performance.
if processor.always_start_with_space:
decoded_prompt = tokenizer.decode(prompt_tokens)
if not decoded_prompt.startswith(" "):
tokens = tokenizer.encode(" " + decoded_prompt,
add_special_tokens=False)
else:
tokens = prompt_tokens
else:
tokens = prompt_tokens


# Prepend a BOS token id to the tokens
processed_data = self.info.ctx.call_hf_processor(
processor, # type: ignore
dict(tokens=tokens),
Expand Down