Context windows end. The question is what survives - and the answer should not be "whatever was in the chat".
Write the handoff document
Every product can own a document with the slug agent-handoff. That name is a convention the product knows: it is pulled into the product's context package automatically, so the next session reads it without being told to.
save_document product=tidepool slug=agent-handoff body="..."
Worth putting in it: what you are in the middle of, what you tried that did not work, decisions taken during the session and why, and the next concrete step. Not a summary of the codebase - the tools can read that.
Log what happened, do not rewrite it
Use a named log for the running record:
append_log_entry log=changelog body="Reworked the entitlement check to run inside the transaction."
Entries are append-only and read from the tail, so the next session asks for the last few instead of reading a file that only grows. A correction is a new entry that supersedes the old one - being wrong stays visible, which is usually the useful part.
Start the next session with one call
get_product_context product=tidepool
Nine sections, budgeted to fit. It includes the handoff document, the open decisions, the board, and what changed recently. Keep the contextRevision it returns and pass it back next time to get only the delta.
What already survives without you
You do not need to write down what the product records itself: every consequential change is in the Mission Log with who did it and when, decisions carry their own rationale, and what an agent actually ran is in telemetry. Write down the things only you know.