Best possible solution:

Keep the narrow current-main cron tool-schema fix from #100801 and treat any nullable config fallback contract as a separate maintainer-approved proposal.

Do we have a high-confidence way to reproduce the issue?

Yes for the original shipped bug by source and issue evidence, but no current-main failure remains: createCronToolSchema() now accepts payload.model: null and payload.fallbacks: null in update patches.

Is this the best way to solve the issue?

No. This branch changes broader Zod config schemas, while the best and already-merged fix is the narrow cron tool TypeBox patch payload schema change.

Security review:

Security review cleared: The diff only changes local Zod schemas and tests; it adds no dependencies, code execution paths, permissions, secrets handling, or supply-chain surface.

AGENTS.md: found and applied where relevant.

What I checked:

• Current main accepts nullable fallback clears in the cron tool patch schema: createCronPatchObjectSchema() now passes nullableStringArraySchema("Fallback models, or null to clear") into cronPayloadObjectSchema(), so the model-facing cron update patch schema accepts payload.fallbacks: null. (src/agents/tools/cron-tool.ts:317, a98a64649ec0)
• Current main has regression coverage for the reported null clear: The runtime schema test now asserts that Value.Check(createCronToolSchema(), ...) accepts payload.model: null and payload.fallbacks: null in an update patch. (src/agents/tools/cron-tool.schema.test.ts:234, a98a64649ec0)
• Service layer already deletes stored fallback overrides on null: mergeCronPayload deletes next.fallbacks when a patch carries fallbacks === null, matching the now-fixed tool schema contract. (src/cron/service/jobs.ts:1166, a98a64649ec0)
• Current config contract remains non-null arrays: The current typed config contract still defines agent and tool model fallbacks as string[], so the PR's nullable config-schema widening is not needed for the cron patch fix and would be a separate config-contract change. (src/config/types.agents-shared.ts:10, a98a64649ec0)
• This PR targets config schemas rather than the cron tool schema: The live PR diff changes src/config/zod-schema.agent-model.ts, src/config/zod-schema.ts, and config-schema tests; it does not add unique behavior beyond the merged cron tool-schema fix on current main. (src/config/zod-schema.agent-model.ts:10, 21ae1964da23)
• Fix provenance is the merged canonical PR: Commit a98a64649e is the merge commit for fix(cron): accept null fallbacks in update patch payload (#100707) #100801 and changes only the cron tool schema/tests plus generated prompt snapshots for this null fallback fix. (src/agents/tools/cron-tool.ts:112, a98a64649ec0)

Likely related people:

• SunnyShu0925: Authored the merged canonical PR that parameterized cron tool fallbacks and added the runtime schema regression test now on main. (role: current-main fix author; confidence: high; commits: c2d0962a470b, ed311c9df535, a98a64649ec0; files: src/agents/tools/cron-tool.ts, src/agents/tools/cron-tool.schema.test.ts)
• brunolorente: PR fix(cron-tool): add typed properties to job/patch schemas #55043 introduced the typed cron job/patch schema properties and adjacent schema tests that define this validation layer. (role: introduced typed cron tool schema surface; confidence: high; commits: ca76e2fedc8e; files: src/agents/tools/cron-tool.ts, src/agents/tools/cron-tool.schema.test.ts, src/agents/tools/cron-tool.test.ts)
• 849261680: PR fix(cron): expose per-job fallbacks in CLI #93369 added the shipped per-job fallback CLI clear behavior and service/protocol support that the cron tool schema must expose. (role: recent cron fallback contributor; confidence: high; commits: 1662b07810eb; files: src/cli/cron-cli/register.cron-edit.ts, src/cron/service/jobs.ts, src/cron/types.ts)

Codex review notes: model internal, reasoning high; reviewed against a98a64649ec0; fix evidence: commit a98a64649ec0, main fix timestamp 2026-07-06T06:48:00-07:00.