"Summarise this" produces something nobody wanted: shorter, blander, and missing whatever the reader cared about. Useful summarisation needs to know who's reading and why.

1. Decide the reader and the decision

A summary exists to help someone decide something. A support ticket summarised for an engineer needs different content than the same ticket summarised for a manager. Name the reader and the decision before writing a single prompt.

2. Force the shape

Free-form summaries drift in length, tone and structure. Ask for a fixed structure and you get something skimmable and comparable across documents.

summarise.txt
Summarise the following for <reader>, who needs to decide <what>.

Output exactly this structure:
- One sentence: what this is.
- Three bullets: the points that affect the decision.
- One line: what action is needed, or "none".
- One line: anything important that is missing or unclear.

Rules: only what's in the source. No preamble, no "this document
discusses". If something is uncertain in the source, keep the
uncertainty — don't resolve it.

<document>

That last instruction matters. Summarisers routinely turn "the client may want X" into "the client wants X", which is the one failure that actually causes harm.

3. Handle long inputs deliberately

Beyond the context limit you have choices: summarise sections then summarise the summaries, or retrieve only the relevant parts first. The first loses detail predictably; the second loses anything you didn't search for. Pick knowingly and tell the user which happened.

4. Keep the source in reach

Every summary should link or expand to the original. A summary you can't verify is a rumour. This one design decision is most of what makes people trust the feature.

5. Test on the awkward ones

Not clean articles. Test on the messy meeting transcript, the document that's mostly a table, the one that contradicts itself, the two-line input. Those reveal how your prompt actually behaves.

Run the same document three times. If you get three materially different summaries, the prompt isn't specific enough about what matters.