Issue for this PR

Closes #25150
Closes #7602
Closes #20100
Closes #16868

Type of change

• New feature

What does this PR do?

• Adds a way to add backup models (array format)
• It goes through the array until it can complete successfully. If not, it will try the last model on the retry loop (current behavior using selected model)
• Shows 'Used backup model: provider/model' at the end of prompt completion so user knows it switched to the backup and which one it used
• No backup model defined, standard retry logic is used

This improves the user experience, especially for those who like to use the free models. It sucks when the session breaks when the rate limit is hit, and it continues to retry or shows "Retrying in 4h 31m...". This way it will continue on error, switching to the backup when the provider is down, or usage is hit.

Format for backup model is an array with "model/provider", same as model. Example:

backupModel: ["opencode/big-pickle", "opencode-go/deepseek-v4-flash"]

How did you verify your code works?

• Ran bun test inside the packages/opencode directory
• Ran bun typecheck
• User testing (sending prompts and trying out the feature) works on compaction and standard prompts sent by user

Screenshots / recordings

How it looks in my commit-storyteller.md agent file in .config/opencode/agents directory:

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

Any feedback would be greatly appreciated!