How do I migrate from Claude 4 Sonnet to Sonnet 4.6?

Change the model ID from claude-sonnet-4-20250514 to claude-sonnet-4-6-20260101. That's the only code change needed. The API, parameters, and response format are identical.

Does Sonnet 4.6 cost the same as Claude 4 Sonnet?

Yes. Both cost $3.00/$15.00 per 1M tokens (input/output). Sonnet 4.6 also has a 5x larger context window (1M vs 200K tokens) at no extra cost.

What happens if I don't migrate by June 15?

All API calls using Claude 4 Sonnet model IDs return HTTP 410 Gone errors immediately. There is no grace period. Your application broke.

Claude 4 to Sonnet 4.6 Migration: One-Line Fix (June 2026)

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