The system prompt for the SendUserMessage tool has been significantly condensed. The old version was a detailed multi-paragraph guide with examples. The new version is a concise description: "Send a message the user will read. Text outside this tool is visible in the detail view, but most won't open it — the answer lives here." The companion "Talking to the user" guide has similarly been streamlined to focus on the key pattern: replies go through SendUserMessage; text outside it should be assumed unread.
Evidence
Rewritten prompt (search for "Send a message the user will read", "## Talking to the user")
Strings lifted out of the shipped bundle, so the claim above can be checked against them.