Opus 5.5 migration makes unchanged conversation history part of thinking validity
Anthropic binds retained thinking to the instructions, tools and messages that preceded it. Integrations that rewrite earlier context can encounter a 400 error even when thinking blocks themselves are intact.
A Claude Opus 5.5 migration can fail even when an application leaves the model's thinking blocks untouched. Anthropic says those blocks also depend on the system prompt, tool definitions and messages that came before them. Editing that earlier context can invalidate the retained reasoning.
For accounts created from August 31, 2026, at 00:00 UTC, the prefix check is enforced by default. In ordinary Messages API requests, replaying a block after an incompatible edit returns a 400 error. Older accounts enforce the check only when the mismatch-behaviour field is set.
Anthropic advises keeping history append-only. A client that rewrites old tool results or rebuilds a system prompt between turns should review that behaviour before switching models, even if its older account currently reports no error.
The optional API drop_block setting lets a request proceed without invalid reasoning, but requires the documented beta header. It changes error handling; it does not repair the history edit.
프리즘코리아 편집국 > Daniel Reed
