Claude Sonnet 5 Migration Notes

Analysis

The migration risk is behavioral

Sonnet 5 is a broad default model, so it will be tempting to update model IDs first and ask questions later. The platform migration guide argues for the opposite order. It documents adaptive thinking defaults, unsupported sampling parameters, tokenizer differences, and manual extended-thinking changes. Those are exactly the details that can break regression tests even when the output looks better to a human.

Start with representative prompts, not synthetic demos. Include prompts that depend on exact JSON structure, refusal handling, system-message placement, retrieval chunks, long code context, and cost ceilings. If a task depends on token counts, redo the accounting before raising context size or max output.

Analysis

Pricing window and entitlement check

The platform guide lists introductory API pricing of $2 input and $10 output per million tokens through August 31, 2026. After that period, the listed price returns to $3 input and $15 output per million tokens. A cost review that only uses the intro window will understate steady-state run rate.

Claude app availability is broad, but API billing, model IDs, and plan entitlements still need environment-specific verification. Team and Enterprise admins should pair Sonnet 5 rollout with model-entitlement review so a default-model change does not surprise controlled workspaces.

Analysis

Compatibility checklist

The highest-value checks are small and concrete. Confirm that client libraries do not send now-unsupported sampling parameters. Confirm that manual extended thinking is not assumed. Confirm that your eval framework distinguishes refusal stop details from ordinary completions. Confirm that cache behavior and token budgets still match production assumptions.

• Inventory every hard-coded model ID and route.
• Re-run tests that assert token counts, output length, or cache economics.
• Remove or gate unsupported sampling parameters before switching traffic.
• Inspect refusal and stop-detail telemetry separately from generic errors.
• Compare intro-window cost with post-August 31 steady-state pricing.

Analysis

Recommended rollout shape

A careful rollout is narrow: route one task family to Sonnet 5, run old and new outputs side by side, inspect the failure set, then expand. The first family should have clear success criteria and low support blast radius. Avoid using a model migration to rewrite prompts, change retrieval, and change tool schemas at the same time.

When Sonnet 5 is better, record why. The useful artifact is not "model upgraded"; it is a migration note that names the tasks, model IDs, eval delta, token-cost delta, and any prompt or client changes required. That note becomes the baseline for the next Anthropic model change.