What
At the start of a session, Claude Code attaches several 'announcement' blocks to the first message, things like sandbox instructions, environment details, the active model, proactivity notes, budget info, available tools, skill listings, output style instructions, language, and auto-mode status. A new mechanism can now move these announcements so they sit right before the closing line of the message instead of their normal position.
This behavior is controlled by a new environment variable, CLAUDE_CODE_SESSION_START_ANNOUNCEMENTS_BEFORE_PROMPT. When it's set, Claude Code checks whether the repositioning is possible (it isn't for models without system messages, when there's no user message, or when there's nothing to announce) and moves the announcements if so.
Why
Repositioning these blocks can change how the model weighs session-start context relative to the user's actual request. The feature records telemetry on whether and why the reposition happened or was skipped, which helps diagnose behavior differences tied to this setting.
New in this build: CLAUDE_CODE_SESSION_START_ANNOUNCEMENTS_BEFORE_PROMPT