What
The Agent SDK is a toolkit for building your own programs on top of Claude Code. It sends a stream of messages. One field on a message names earlier messages that it replaces. That field's description now covers a new case: a response whose tool call could not be parsed. A tool call is a single action Claude asks to take. Before, it covered only the fallback after a refusal. The messages it replaces can now include user messages as well.
Separately, Claude Code now also removes worker-only fields from a control response when that response has an error_code field.
Why
A program that displays the stream should remove every message that has been replaced, including user messages. Otherwise it can go on showing messages that no longer apply.