Live on zeiss, /v1/audio/generate answered in 0.25s with "ACE-Step
requirements missing", which was not the problem: the venv was there and
imported fine by hand in the same container.

The container's per-engine launchers export PYTHONHOME=/opt/coderai/python
(the standalone 3.13); PID 1 and an interactive docker exec do not. A 3.12
interpreter inheriting it aborts before running a byte -- "Current thread ...
<no Python frame>" -- so the worker's import probe failed instantly, and the
caller went down the rebuild path to report a missing file instead. Reproduced
both ways before changing anything.

longcat_worker had already solved this for its 3.10 venv, behind a docstring
saying coderai does not set those variables. It does. The helper is shared now
(codai/api/venv_env), longcat delegates to it, and every ACE-Step spawn --
probe, both venv-creation routes, the install, the service -- goes through it.
The probe also prints what the child said rather than returning a bare False.

Then response_format: declared as url | b64_wav | b64_mp3, and ignored. The
response was chosen by whether a file directory was configured, so a client
asking for base64 silently got a URL -- the same bug /v1/images had with
b64_json. Honoured now, and b64_mp3 is really encoded, because base64 of a wav
under an mp3 key is a decode failure at the client.

Also corrected in passing: versions.env pins UBUNTU_VERSION=24.04, whose
system python IS 3.12, so the baked venv needs no bundled interpreter. The
Dockerfile rewrote pyvenv.cfg at /opt/coderai/py312 unconditionally, which
would have broken a working venv when nothing was bundled there, and the build
refused outright without a relocatable 3.12 it did not need.

Verified live after the fix: b64_wav returns 5,742,296 bytes (~15s of 48 kHz
stereo), eviction by a 21 GB model stopped the worker and took the GPU from
7958 MiB to 274 MiB in under four seconds, and ACE-Step then reloaded and
generated again. Suite at its 15-failure baseline, 5299 passed.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014T3rZnLhydX9QqUcfBbqLu