An unsupported FASTVIDEO_ATTENTION_BACKEND name, such as a typo, was ignored
and the run fell back to automatic backend selection without telling the
user. An explicit attention_backend argument with the same name raised.
get_env_variable_attn_backend now raises ValueError for an unsupported name,
using the same parse as an explicit request (coerce_attn_backend). Every
reader goes through it: the fill_attention_backend_from_env resolution
step, FastVideoArgs.__post_init__, and the attention selector's
environment fallbacks.
The golden case environment/attention_backend_unknown_name now records the
error, and the registry description and env_vars.md table say that an
unsupported name raises.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012M91wnVFmPEvJ9h39r5BH7
