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fixing mm placeholder replacement issue with gemma3 #27538

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Merged
merged 1 commit into from
Oct 27, 2025
Merged

fixing mm placeholder replacement issue with gemma3 #27538

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2 changes: 1 addition & 1 deletion vllm/model_executor/models/gemma3_mm.py
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Original file line number Diff line number Diff line change
Expand Up @@ -403,7 +403,7 @@ def _find_mm_placeholders(

def get_repl_toks(tok: int) -> list[int]:
if tok == newline_3:
return [newline_1, newline_2]
return [newline_2, newline_1]
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@gemini-code-assist gemini-code-assist bot Oct 26, 2025

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critical

This change correctly handles the case where [newline_2, newline_1] is merged into newline_3. However, it breaks the case where [newline_1, newline_2] is merged, as the decomposition is no longer its inverse.

The root cause is the ambiguous merging in _apply_token_matches (lines 373-382) where both [newline_1, newline_2] and [newline_2, newline_1] are merged into newline_3. The decomposition here in _find_mm_placeholders can only be the inverse of one of these, making the transformation lossy and causing one of the cases to fail.

A more robust fix would be to make the merging unambiguous. For example, by only merging [newline_2, newline_1] into newline_3 in _apply_token_matches and removing the merge for [newline_1, newline_2].

Specifically, you could remove lines 373-377 in _apply_token_matches:

        token_ids = replace_token_matches(
            token_ids,
            [newline_1, newline_2],
            [newline_3],
        )

With that change, your change here to decompose newline_3 into [newline_2, newline_1] would be correct and not break other cases. Without changing _apply_token_matches, this PR trades one bug for another.

if tok == newline_4:
return [newline_2, newline_2]

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