Cause

Newer Transformers requires a tokenizer when generate() uses stop strings. That can come from:

Explicit stop_strings= in the call, or
The model’s generation_config.json
naver-hyperclovax/HyperCLOVAX-SEED-Think-14B (used in test_common.py) has this in its generation config:

"stop_strings": ["<|endofturn|>", "<|stop|>"]
HfRunner in tests/conftest.py was calling:

self.model.generate(**inputs, ...)
without tokenizer=, so Transformers raised that ValueError.

Fix
HfRunner now passes the tokenizer by default in all three model.generate() paths:

generate_kwargs = dict(kwargs)
generate_kwargs.setdefault("tokenizer", self.tokenizer)
self.model.generate(..., **generate_kwargs)

Callers can still override by passing tokenizer= explicitly (as vlm_utils/core.py already does).