How to make a scheduled Claude agent read from bookmarks instead of 'the last 24 hours'

Telling a scheduled agent to read "the last 24 hours" leaves gaps and duplicates. A late run misses items, and an early run reads some twice. Anthropic's agent automation guide on the claude.dev blog (October 8) calls this out and recommends a bookmark per source.

At the end of each run, the agent writes the timestamp of the newest item it read from each source to one file, bookmarks.json.

{
  "slack": "2026-09-14T13:02:11Z"
}

The next run starts from those bookmarks, so its window stretches or shrinks to cover everything since the last run. The guide keeps the file in a memory store that persists between runs.

If your prompts say "since yesterday" or "the last 7 days," change them the same way. It applies to Claude Code scheduled tasks and to agents you run from cron or launchd too.