[Achyuthan-S]

Fixes #46459

Problem
dtype passed to AutoModelForCausalLM.from_pretrained is silently ignored for composite (multimodal) checkpoints like Qwen3.5. AutoConfig.from_pretrained absorbs it into the top-level composite config, which is then swapped to the text sub-config via get_text_config() — dropping the user's value. The model falls back to the checkpoint default (bfloat16) with no warning.

Fix
Propagate parent_config.dtype to the extracted text sub-config at both get_text_config() call sites in auto_factory.py, mirroring the existing quantization_config propagation from #45494.

Coordination
Coordinated on issue #46459 before opening this PR. Issue author (@qflen) acknowledged with approval.

Duplicate check
gh pr list --repo huggingface/transformers --state open --search "46459 in:body" returned no results. No overlapping open PRs exist.

Tests run
pytest tests/models/qwen3_5/test_modeling_qwen3_5.py::Qwen3_5ModelTest::test_automodelforcausallm_propagates_dtype
PASSED

AutoModelForCausalLM.from_pretrained("Qwen/Qwen3.5-0.8B", dtype=torch.float32)
Output: torch.float32

AI assistance disclosure
AI assistance (Claude) was used for codebase fix verification. All changed lines were reviewed and understood by personally , and the fix was validated locally end-to-end.

The tests_generate failure is a pre-existing flaky test — Kosmos2_5ModelTest::test_generate_with_cache_matches_no_cache — marked FLAKY by CircleCI. It tests cache consistency in Kosmos2.5 generation and has no relation to this change.

@Cyrilvallez @zucchini-nlp

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

This PR ensures that a composite model config’s dtype is preserved when Auto* classes extract a text-only config (via get_text_config()), and adds a regression test around Qwen3.5 autoclass loading.

Changes:

• Propagate dtype from a parent (composite) config onto the extracted text config in auto_factory.
• Add a Qwen3.5 unit test to validate dtype behavior during AutoModelForCausalLM.from_pretrained(...).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
tests/models/qwen3_5/test_modeling_qwen3_5.py	Adds a regression test intended to verify dtype survives composite-config extraction during AutoModel loading.
src/transformers/models/auto/auto_factory.py	Copies dtype from the composite/parent config to the extracted text config in both from_config and from_pretrained.

[github-actions]

[For maintainers] Suggested jobs to run (before merge)

run-slow: auto, qwen3_5