[Bugfix] Deep-merge nested mm_processor_kwargs so request overrides do not drop configured keys

[apex-mochen]

Purpose

Follow-up to #58406. This PR fixes the concrete data-loss half of the reported mm_processor_kwargs issue: the shallow merge replaced whole nested mappings and silently dropped configured keys.

Problem

MultiModalConfig.merge_mm_processor_kwargs combined configured and request kwargs with a shallow dict union (kwargs | dict(inference_kwargs)). When both sides supply a nested mapping — e.g. configured videos_kwargs={"fps": 2, "size": {...}} and request videos_kwargs={"size": {...}} — the request's dict wholesale replaces the configured one, dropping fps (and any other key not repeated in the request).

Reproduced from the issue:

config = MultiModalConfig(mm_processor_kwargs={
    "size": {"shortest_edge": 100, "longest_edge": 1000},
    "videos_kwargs": {"fps": 2, "size": {"longest_edge": 1200}},
})
merged = config.merge_mm_processor_kwargs({
    "size": {"longest_edge": 1800},
    "videos_kwargs": {"size": {"shortest_edge": 200}},
})
# Before: merged["videos_kwargs"] == {"size": {"shortest_edge": 200}}  (fps lost)
# After:  merged["videos_kwargs"] == {"fps": 2, "size": {"longest_edge": 1200, "shortest_edge": 200}}

Solution

Add _deep_merge_mm_kwargs, which recursively merges nested Mapping values key-by-key while leaving non-mapping override behavior unchanged (request wins on leaf values, new keys are added, a non-mapping request value still replaces a configured mapping). This is backward compatible: flat kwargs continue to override exactly as before, and no behavior changes for requests that do not send nested mappings.

The flat-vs-scoped precedence contract (engine flat < engine scoped < request flat < request scoped) described in the issue is a larger design change to the overlay path and is intentionally left for a follow-up; this PR only stops the silent data loss.

Test Plan

Added test_mm_processor_kwargs_deep_merges_nested_scoped_kwargs in tests/multimodal/test_processing.py, covering the issue's repro: configured fps survives, and nested size dicts merge key-by-key.

Test Result

• ruff check and ruff format --check clean on both files.
• A standalone equivalent of the merge function reproduces the regression (fps dropped before) and confirms the merged result after, plus flat-override, new-key, and mapping-replaced-by-scalar cases.

Compatibility

No API change. Requests that previously sent nested kwargs will now retain configured sibling keys instead of dropping them, which is the intended behavior; requests without nested kwargs are unaffected.

Related Issue

Relates to #58406.

[claude]

Claude Code Review

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[apex-mochen]

Gentle nudge: this PR fixes a real regression where a request-level nested mm_processor_kwargs (e.g. �ideos_kwargs) wholesale replaced the configured one and dropped sibling keys like ps. The fix is a small recursive deep-merge with no behavior change for flat kwargs, plus a regression test. Happy to address any feedback - would appreciate a pre-run-check approval if there's bandwidth. Thanks!

[DarkLight1337]

Are you ok if we merge #56372 which fixes the problem more generally, and add you as co-author?

[apex-mochen]

Yes, absolutely happy with that - please go ahead and merge #56372. Thanks for looping me in as co-author; glad the underlying regression is getting fixed properly. I'll close this PR once #56372 lands. Thanks!

[apex-mochen]

Please go ahead — that works for me, and thank you for the co-author offer.

I checked that #56372 subsumes this PR rather than assuming it from the description: _recursively_merge_mm_processor_kwargs merges when both sides are Mapping and replaces otherwise, and merge_mm_processor_kwargs runs it across the config/inference × flat/scoped matrix with explicit precedence. The nested override that dropped configured keys here is covered by that, so there is nothing in this PR worth carrying on its own.

Closing it now instead of leaving a duplicate open. If #56372 stalls for any reason, ping me and I will reopen this — it does not need to sit open in the meantime. I am also happy to run the nested-override case against #56372 locally if a second pair of eyes would help.

[apex-mochen]

Thanks for getting #56372 merged — I’m glad the broader fix is in.

I noticed that the final squashed commit (295a0b8) does not include the Co-authored-by attribution for me that we discussed here. I closed #58408 based on the plan that #56372 would be merged with me added as a co-author.

Could you please confirm whether the attribution was inadvertently omitted during the final merge/squash? If adding it to the commit is no longer practical, an acknowledgment in this thread or another appropriate contributor record would also be appreciated. Thanks again for picking up the broader fix.

[DarkLight1337]

Sorry, I originally did edit the commit message but perhaps it got overwritten by subsequent merges, Could you open a PR that does a token change so it could be counted as your contribution?

[apex-mochen]

Thanks for the suggestion. I opened #59399 as a small test-only follow-up for the concrete #58408 regression case.

Before replying, I verified that the remote patch is limited to one file (+37/-0), has no production-code changes, and passes the local Python compilation, diff/whitespace, commit-signoff, line-ending, and scope checks. I also attempted the targeted pytest, but the current environment does not have pytest/PyTorch or the vLLM project toolchain installed, so that test could not start; this limitation is recorded in the PR description.

The added coverage checks preservation of configured video siblings such as fps, together with the flat/scoped precedence between configured and request-time values.