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Anthropic: Claude Sonnet migration: test tool settings and conversation replay

Published by Nova3 AI Research

What shipped

On 2026-09-28, Anthropic documented five ways Claude Sonnet 5 code can break on Claude Sonnet 5.5: disabling up-front thinking requires thinking: {"type": "between_tools"} instead of "disabled" at high effort or below; forced tool_choice types any and tool return a 400 error; thinking blocks are tied to the model and conversation; computer_20251124 is not accepted on the Claude API and Google Cloud; and the advisor tool rejects Claude Opus 4.8, Claude Opus 4.7, and Claude Sonnet 5 as advisors.

Vendor source headline: Claude Sonnet 5.5 compatibility changes

Anthropic release notes

What this means for your business

Search request builders for the affected settings and tool identifiers before changing the model name. Add regression tests for forced tool use and conversation replay.

Nova3 implementation guidance

Analysis added .

Nova3's migration recommendation is to compare complete request paths, not just replace the model identifier. The dated vendor notice identifies request settings and replay behavior that can change. A useful acceptance set therefore includes a normal request, a tool-using request, a stored conversation and the application's error handling. Establish expected outcomes from the application's existing requirements. Passing one prompt does not demonstrate compatibility for every integration or establish that the replacement model performs better.

Build a migration acceptance set

Implementation checklist derived from the compatibility issues described in the dated source. It is not a claim that Nova3 has tested a customer's integration.

  1. Inventory request builders using the old model and identify settings mentioned in the vendor's compatibility notice.
  2. Include tool-choice requests, stored conversation replay and relevant account boundaries in the sample set.
  3. Check both returned errors and the contents delivered to downstream systems; a successful HTTP response is insufficient.
  4. Assign an owner and a rollback procedure before changing production routing ahead of the stated retirement deadline.

Still open

The supplied excerpt does not establish regional restrictions or preview status for the affected tools.

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