feat(core): enhance availability routing with wrapped fallback and single-model policies

[adamfweidman]

Summary

Enhances the policy-driven fallback mechanism to support "upgrade" fallbacks (e.g., Flash → Pro) via wrapped candidate lists and implements strict single-model policies for user-specified custom models to prevent unwanted fallbacks.

Details

Enhances the policy-driven fallback mechanism to support "upgrade" fallbacks (e.g., Flash → Pro) via wrapped candidate lists and implements strict single-model policies for user-specified custom models to prevent unwanted fallbacks.

Related Issues

Fixes https://github.com/google-gemini/maintainers-gemini-cli/issues/1068

How to Validate

npm run preflight

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

[gemini-code-assist]

Summary of Changes

Hello @adamfweidman, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request refines the model availability routing by introducing more sophisticated fallback logic. It allows for 'upgrade' fallbacks where a failed model can fall back to a more capable model if available, and ensures that custom models, when explicitly chosen, do not inadvertently trigger fallbacks to other models. This improves the predictability and flexibility of model selection within the core system.

Highlights

• Enhanced Fallback Mechanism: The policy-driven fallback mechanism has been enhanced to support 'upgrade' fallbacks, allowing models to fall back to more capable options (e.g., Flash → Pro) using wrapped candidate lists.
• Strict Single-Model Policies: Implemented strict single-model policies for user-specified custom models, preventing unintended fallbacks to other models when a specific custom model is explicitly chosen.
• Improved Policy Context: The buildFallbackPolicyContext now returns candidates in a wrapped order, prioritizing downgrades (continuing the chain) before wrapping around to potential upgrades.

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[gemini-code-assist]

Code Review

This pull request enhances the model availability routing by introducing two key features: wrapped fallback policies to allow for model upgrades (e.g., Flash to Pro) and strict single-model policies for user-specified custom models to prevent unintended fallbacks. The implementation appears solid and well-tested. The logic for creating fallback candidate lists now correctly wraps around the policy chain. The handling of custom models is now stricter, creating a single-model chain which is the intended behavior. The fallback handler logic has also been refactored for better robustness, especially in scenarios where no suitable fallback model can be found. The accompanying test updates are thorough and cover the new functionality and edge cases well. Overall, this is a high-quality contribution that improves the flexibility and predictability of the model fallback system.

[github-actions]

Size Change: -103 B (0%)

Total Size: 21.4 MB

ℹ️ View Unchanged

Filename	Size	Change
./bundle/gemini.js	21.4 MB	-103 B (0%)
./bundle/sandbox-macos-permissive-closed.sb	1.03 kB	0 B
./bundle/sandbox-macos-permissive-open.sb	890 B	0 B
./bundle/sandbox-macos-permissive-proxied.sb	1.31 kB	0 B
./bundle/sandbox-macos-restrictive-closed.sb	3.29 kB	0 B
./bundle/sandbox-macos-restrictive-open.sb	3.36 kB	0 B
./bundle/sandbox-macos-restrictive-proxied.sb	3.56 kB	0 B

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