#anthropic api

Claude Sonnet 5 to Sonnet 5.5 migration showing which API changes return errors

Claude Sonnet 5.5 migration: which changes return a 400

Claude Sonnet 5.5 launched on September 28, 2026 at the same price as Claude Sonnet 5, $2 per million input tokens and $10 per million output tokens. Moving to it is not a model-ID swap. Several

Tue Sep 29 2026 · 6 min read · 2 views

Developer monitor showing an Anthropic API 400 error for thinking and tool_choice settings beside a code diff

Claude Opus 5.5 returns 400 errors. Here is how to fix it

If your code moves from claude-opus-5 to claude-opus-5-5 and starts returning 400 errors, one of two request fields is the cause. Either thinking is set to disabled, or toolchoice is set to any or

Fri Sep 25 2026 · 8 min read · 61 views