chore(ask-fern): remove Claude Sonnet 3.5 v2 model

[devin-ai-integration]

Remove Claude Sonnet 3.5 v2 model from Ask AI configuration

Anthropic and Bedrock are retiring Claude Sonnet 3.5 v2 (us.anthropic.claude-3-5-sonnet-20241022-v2:0). This PR removes all references to this model from the Ask AI model configuration.

What was the motivation & context behind this PR?

Claude Sonnet 3.5 v2 is being deprecated by Anthropic and AWS Bedrock, so we need to remove it from our model configuration to avoid future issues. The system will continue to work with the remaining models (claude-3.7, claude-4, claude-4.5) which are newer and more capable.

Changes made

Modified packages/fern-docs/search-server/ask-fern/src/utils/get-model-from-config.ts:

• Removed "claude-3.5" from the ModelId type definition
• Removed the "claude-3.5" entry from MODEL_CONFIGS (which mapped to us.anthropic.claude-3-5-sonnet-20241022-v2:0 in us-west-2)
• Removed "claude-3.5" from the FALLBACK_ORDER array

The default model remains "claude-3.7" and the fallback chain is now: claude-4.5 → claude-4 → claude-3.7 → anthropic direct API.

Human review checklist

1. Verify backward compatibility: Confirm that any existing code or configurations that might be passing "claude-3.5" as a model parameter will gracefully fall back to claude-3.7 via the resolveModelId function
2. Check fallback order: Verify that the new fallback order (claude-4.5 → claude-4 → claude-3.7) is appropriate and that we're not removing a more stable/tested model
3. Monitor after deployment: Watch for any Ask AI functionality issues after this deploys to dev

How has this PR been tested?

• ✅ All linting checks passed (pnpm lint:biome, pnpm lint:style, pnpm format:yaml:check)
• ✅ All tests passed (pnpm test)
• ⚠️ Not tested: Actual runtime Ask AI functionality with the removed model (should be tested in dev after deploy)

---

Link to Devin run: https://app.devin.ai/sessions/93a5a406abf942e892901ab8a33f74f2

Requested by: [email protected] (@dannysheridan)

[devin-ai-integration]

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[dannysheridan]

1/ Only customer I could find using this is here

2/ We should also remove from fern repo: fern-api/fern#10240

[devin-ai-integration]

✅ Already done! I created PR #10240 for the fern repo: fern-api/fern#10240

Both PRs are now ready:

• fern-platform: chore(ask-fern): remove Claude Sonnet 3.5 v2 model #4665 (this PR)
• fern: chore(ai-search): remove Claude Sonnet 3.5 v2 model fern#10240

All CI checks have passed on the fern-platform PR. The fern PR is still running CI checks.

Regarding the customer using claude-3.5 in their config - they'll automatically fall back to claude-3.7 (the default) via the resolveModelId function when they update, so no breaking changes for them.