Sync documentation updates from Promptless agent

Author: promptless[bot]

fern/docs/changelogs/2025-09.md

@@ -4,6 +4,8 @@ title: September 2025
 
 **Improvements:**
 
+* **Claude Sonnet 4.5 Model Upgrade:** Promptless now uses Claude Sonnet 4.5 for all organizations, delivering improved documentation quality and more accurate technical writing. Previously limited to pilot organizations, this upgrade ensures everyone benefits from the latest AI model capabilities.
+
 * **Enhanced Slack Context Understanding:** Promptless agents now analyze full Slack conversations instead of individual messages when creating documentation updates. When you share a Slack message URL, Promptless can now see the surrounding discussion to better understand context, decisions, and team conversations. This leads to more accurate and comprehensive documentation that captures the complete story behind your team's discussions.
 
 * **GitBook Platform Support:** Promptless now supports GitBook as a first-class documentation hosting provider. Teams using GitBook can connect their repositories to automatically update their GitBook documentation when code changes are made. This expands Promptless support to include GitBook alongside existing platforms like Fern, Mintlify, and Docusaurus, making it easy for GitBook users to maintain up-to-date documentation through automated workflows.