PR #25317: feat(core): implement silent fallback for Plan Mode model routing

• Repository: google-gemini/gemini-cli
• Author: jerop
• State: closed | merged: True
• Link: https://github.com/google-gemini/gemini-cli/pull/25317

Description (problem / solution / changelog)

Summary

Implement silent fallback for Plan Mode to ensure an uninterrupted user experience when Pro models are unavailable or inaccessible.

Details

This PR introduces a "Unified Injection" approach to enforce silent fallback when the system is in Plan Mode. By intercepting the ModelPolicyChain resolution in resolvePolicyChain, all fallback actions are overridden to 'silent' if ApprovalMode.PLAN is active. This ensures that failures in the primary planning model (Pro) automatically trigger a transition to the next available model (Flash) without interrupting the user with prompts or getting "stuck".

• Exported SILENT_ACTIONS from policyCatalog.ts.
• Refactored resolvePolicyChain in policyHelpers.ts to apply silent overrides.
• Added unit tests in policyHelpers.test.ts verifying silent fallback for both legacy and dynamic routing paths.
• Updated docs/cli/plan-mode.md to document the new behavior.

Related Issues

Closes https://github.com/google-gemini/gemini-cli/issues/25110

Pre-Merge Checklist

• Updated relevant documentation and README (if needed)
• Added/updated tests (if needed)
• Noted breaking changes (if any)
• Validated on required platforms/methods:
  • MacOS
    • npm run
    • npx
    • Docker
    • Podman
    • Seatbelt
  • Windows
    • npm run
    • npx
    • Docker
  • Linux
    • npm run
    • npx
    • Docker

Changed files

• docs/cli/plan-mode.md (modified, +4/-0)
• packages/core/src/availability/policyCatalog.ts (modified, +1/-1)
• packages/core/src/availability/policyHelpers.test.ts (modified, +14/-1)
• packages/core/src/availability/policyHelpers.ts (modified, +48/-36)