The `provider` field in CRONJOB_SCHEMA only showed examples like
'openrouter' and 'anthropic', with no mention of the canonical
'custom:<name>' form required for custom_providers entries. When the
user has custom providers configured, LLMs tend to write the bare type
name ('custom') because the schema does not advertise the ':<name>'
suffix. The bare value then serializes into jobs.json and causes the
cron job to fail silently at run time — `_resolve_model_override`
treats it as a user-specified provider and skips the pin-to-current
fallback, but no provider ever resolves from the bare 'custom' string.
Clarifying the schema so the canonical form is discoverable addresses
the root cause at the tool-definition boundary.
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