Fix: qwen3_5 model type not recognized by Transformers

[www917]

Problem

When loading a Qwen3.5 model checkpoint (e.g. Qwen/Qwen3.5-4B) with vLLM or transformers, the following error occurs:

KeyError: 'qwen3_5'

ValueError: The checkpoint you are trying to load has model type `qwen3_5`
but Transformers does not recognize this architecture.

Root Cause

The qwen3_5 model type was added to the Hugging Face transformers library in version 5.2.0 (released February 16, 2026, via PR #43830). Older versions lack the CONFIG_MAPPING entry for qwen3_5 in configuration_auto.py, which causes the KeyError.

Solution

Upgrade transformers to version >= 5.2.0:

pip install "transformers>=5.2.0"

Or install from source if no release supports it yet:

pip install git+https://github.com/huggingface/transformers.git

Changes

• requirements.txt: Added transformers>=5.2.0 as a minimum version requirement
• fix_qwen3_5_model_recognition.sh: Automated fix script that upgrades transformers and verifies the fix
• QWEN3_5_FIX.md: Documentation of the issue, root cause, and solutions

References

• transformers PR #43830 - Adding Support for Qwen3.5
• vLLM Issue #36217 - qwen3_5 not recognized
• vLLM Qwen3.5 docs