What changed

• Add a built-in fallback entry for Google provider model google/gemini-3.1-pro-preview in src/agents/model-catalog.ts when it is missing from discovered models.
• Keep fallback behavior aligned for runtime usage: reasoning enabled, context window set to 1,000,000, and text/image input support.
• Add tests in src/agents/model-catalog.test.ts covering:
  • model injection when missing,
  • no-duplicate behavior when already present.

Why this fixes the issue

Issue 22323 is caused by discovery missing the new Gemini 3.1 Pro Preview model, which can hide it from built-in provider selection. This fallback guarantees the catalog always contains that model and does not alter discovery semantics when a matching entry already exists.

Tests run

• pnpm vitest run --config vitest.unit.config.ts src/agents/model-catalog.test.ts
• pnpm test -- src/agents/model-catalog.test.ts

Edge cases

• Existing entries for google/gemini-3.1-pro-preview are preserved and not duplicated.
• Existing cache-reset/retry behavior and partial discovery failure handling remain unchanged.

Infra flake / blockers

• None known.

Greptile Summary

Adds fallback catalog entry for google/gemini-3.1-pro-preview model when it's missing from SDK discovery, following the same pattern as the existing OpenAI Codex Spark fallback.

Key changes:

• Implements applyGoogle31ProPreviewFallback() function with deduplication check
• Configures model with reasoning: true, 1M context window, and text+image input support
• Adds comprehensive test coverage for both insertion and deduplication scenarios
• Updates existing test to use toContainEqual instead of strict equality for better flexibility

Confidence Score: 5/5

• This PR is safe to merge with minimal risk
• The implementation follows an established pattern (OpenAI Codex fallback), includes proper deduplication logic, and has comprehensive test coverage validating both insertion and non-duplication scenarios. The change is isolated, well-tested, and addresses a specific discoverability issue.
• No files require special attention

_{Last reviewed commit: c903811}