Explain the lifecycle of a migration from Claude Sonnet 4.6 to Claude Sonnet 5?

The lifecycle begins with an audit of existing request-building code for the three specific breaking patterns: non-default sampling parameters, manual extended thinking configuration with a fixed budget_tokens value, and assistant message prefilling, since code containing none of these three patterns may genuinely need only a model-ID swap.

Next comes parameter remediation: removing sampling parameters entirely, replacing manual thinking budgets with the effort parameter, and replacing prefilling with an alternative like structured outputs or explicit system prompt instructions, addressing each of the concrete 400-error-causing patterns identified in the audit.

Following that, token and budget recalibration takes place: re-running token counting against Sonnet 5 specifically using representative prompts, and revisiting max_tokens limits to account for both the tokenizer's higher token-per-text rate and thinking now drawing from that same shared budget by default.

With the mechanical changes addressed, the process moves into behavioral validation: testing prompts as-is first, since Sonnet 5 generally performs well on existing Sonnet 4.6 prompts, then specifically checking verbosity calibration, effort-level behavior, and, if the workload touches cybersecurity-adjacent territory even tangentially, whether new cyber safeguards affect any previously-successful prompts.

The lifecycle concludes with staged rollout: shifting a small slice of production traffic first, comparing cost, latency, and output quality against the Sonnet 4.6 baseline on real production traces rather than isolated test prompts, and only expanding to full traffic once that comparison confirms the migration behaves as expected at production scale.

flowchart TD
  A[Audit code for 3 breaking patterns] --> B[Remediate: remove sampling params, migrate thinking config, replace prefill]
  B --> C[Recalibrate tokens: recount, revisit max_tokens]
  C --> D[Validate behavior: test as-is, check verbosity/effort/cyber safeguards]
  D --> E[Staged rollout: small slice first]
  E --> F{Metrics acceptable?}
  F -- Yes --> G[Expand traffic in stages]
  F -- No --> H[Hold, keep Sonnet 4.6 fallback active]

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