feat: add customizable fallback chains and improved model matching (Fixes #1307)

[wydrox]

Replaces #1307.

Summary

This PR adds support for customizable fallback chains and improves model ID matching to handle prefixed models (like antigravity-*).

Changes in this PR (Fixes for #1307)

1. Resolved merge conflicts with dev branch.
2. CLA Signed.
3. Refactored Schema: Extracted shared FallbackChainSchema in src/config/schema.ts to deduplicate definitions.
4. Fixed Logic Bug: findVariantInChain now strictly requires model match when entry.model is defined (prevents incorrect provider-only fallback).
5. Added Tests: Comprehensive edge case tests in src/shared/agent-variant.test.ts covering model mismatches, chain ordering, and multi-provider entries.

Original Description

• Fallback chains are hardcoded in model-requirements.ts
• Variant resolution only works with exact provider/model matches
• No support for custom model names like antigravity-gemini-3-pro

Solution

1. Customizable fallback chains - Users can now define their own fallback chains in oh-my-opencode.json.
2. Model ID normalization - Strips common routing prefixes when matching models.
3. Improved variant fallback - Falls back to category/agent variant when model-based resolution fails.

Testing

All tests pass, including new edge case tests for the logic fix.

---

Summary by cubic

Adds customizable fallback chains for agents and categories and improves model matching (handles prefixed IDs like antigravity-*). Fixes #1307 by allowing user-defined chains to override built-ins and ensuring precise variant selection.

• New Features

  • Add fallback_chain config to agents and categories for custom model/provider fallbacks.
  • Normalize model IDs by stripping common prefixes for matching.
  • Clear priority: agent fallback_chain → agent variant → category fallback_chain → category variant → built-in chains.

• Bug Fixes

  • Require a model match when entry.model is set.
  • Correct handling of multi-provider entries with strict model checks.
  • Add tests for prefixed models, chain ordering, and mismatch edge cases.

^{Written for commit c1125a3. Summary will update on new commits.}

[github-actions]

All contributors have signed the CLA. Thank you! ✅
_{Posted by the CLA Assistant Lite bot.}

[cubic-dev-ai]

1 issue found across 5 files

Confidence score: 3/5

• Potential user-facing regression: src/config/schema.ts now requires model in fallback chain, which may reject previously valid provider-only configs at runtime/tests
• Score reflects a medium-severity validation change that could block configuration usage, with uncertain confidence
• Pay close attention to src/config/schema.ts - schema constraint may not match runtime expectations.

Prompt for AI agents (all issues)

Check if these issues are valid — if so, understand the root cause of each and fix them.


<file name="src/config/schema.ts">

<violation number="1" location="src/config/schema.ts:106">
P2: Fallback chain schema requires `model`, but runtime logic/tests allow provider-only entries, so valid configs without a model will now be rejected.</violation>
</file>

_{Reply with feedback, questions, or to request a fix. Tag @cubic-dev-ai to re-run a review.}

[cubic-dev-ai]

P2: Fallback chain schema requires model, but runtime logic/tests allow provider-only entries, so valid configs without a model will now be rejected.

Prompt for AI agents

Check if this issue is valid — if so, understand the root cause and fix it. At src/config/schema.ts, line 106:

<comment>Fallback chain schema requires `model`, but runtime logic/tests allow provider-only entries, so valid configs without a model will now be rejected.</comment>

<file context>
@@ -101,6 +101,13 @@ export const BuiltinCommandNameSchema = z.enum([
 
+export const FallbackChainEntrySchema = z.object({
+  providers: z.array(z.string()),
+  model: z.string(),
+  variant: z.string().optional(),
+})
</file context>

Suggested change

	model: z.string(),
	model: z.string().optional(),

[wydrox]

recheck

[wydrox]

recheck

[wydrox]

I have read the CLA Document and I hereby sign the CLA