dit/config.json asks for enable_flashattn2, but requirements-longcat.txt ships
xformers — flash-attn 2.7.4.post1 is a source build needing ninja and a CUDA
toolchain. Upstream's Attention has no fallback: anything unselected reaches
`raise RuntimeError("Unsupported attention operations.")`, and the FA2 branch
imports flash_attn, which is not in the venv.

--attention was only ever written into the environment. Nothing read
LONGCAT_ATTENTION, so the checkpoint's config won every time.

Now both load paths — bf16 through from_pretrained and INT8 through the config
dict — are built with the backend that is actually importable, preferring what
was asked for and saying so when it falls back. None installed is an error
here, with the reason, rather than 48 blocks into the first step of a load that
already cost minutes.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0185zEBiy3KWB37xNGSrxL7w