A file one agent commits for the next to pick up: a spec, a diff, a report. Unlike a chat message it outlives the session, has a version, and can be checked. If you pass a commit, the next stage knows exactly what it received.
A summary passed along a chain changes at each step, like a game of telephone. A committed file doesn't. It also means you can open what each stage really handed over, not what it said it did.
Say it to your agent
“Write your findings to reports/auth-audit.md and commit it, with the commit hash in your final message. The next agent gets that file, and it won't see this conversation.”