Context

The cron fallback inheritance feature (canonical: #91362) is shipped in v2026.6.11 and works correctly for new cron jobs created without payload.fallbacks or payload.model.

However, existing cron jobs cannot clear these fields to opt into inheritance.

Current Behavior

cron update rejects:

• payload.fallbacks: null → must be array
• payload.model: null → model stuck at previous value
• payload.fallbacks: [] → treated as "explicitly set, no fallbacks" (not inheritance)

This means:

• New cron jobs: inheritance works ✅
• Existing cron jobs: can never opt into inheritance, must be deleted and recreated ❌

Steps to Reproduce

1. Create a cron job with payload.fallbacks: ["some-model"]
2. Try to update it to remove fallbacks via cron update with fallbacks: null
3. Observe: schema validation rejects it
4. Try fallbacks: [] as workaround
5. Observe: empty array disables fallbacks entirely instead of triggering inheritance

Desired Behavior

cron update should accept model: null and fallbacks: null to clear these fields, falling back to agent-level defaults (primary model / global fallback chain).

Verification

Verified by deleting and recreating a cron job without these fields — inheritance works correctly on the recreated job.

Related

• Duplicate of: #100577 (closed as already-implemented)
• Canonical fix: #91362
• Fix PR: #91379