The desktop smoke configured a provider as `model.provider: custom` plus
OPENAI_BASE_URL/OPENAI_API_KEY in .env, with deliberately no provider entry in
config.yaml. Every `installer-script+desktop` leg then died at its OLD
checkpoint: the v2026.8.31 app boots behind the onboarding overlay ("No usable
credentials found for custom. setup.status reports configured credentials, but
runtime resolution still failed"), so the smoke's trial click on the composer
never landed (225 retries against the covering div).
That vintage resolves a bare `custom` only from config.yaml
`custom_providers:` (name + base_url + key_env, with a bare-"custom" fallback to
the first valid entry); the env pair stays for the trees that resolve the
endpoint from the environment. Write both, replacing the entry this writer owns
rather than stacking duplicates.
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