Summarise something too long to send

I need to <summarise / extract from> a document of roughly <length>,
which is beyond what I can send in one call.

Recommend one approach and say what it costs me in accuracy:
- Chunk, process each, then combine the results.
- Retrieve only the relevant parts first, then process those.
- Progressive refinement — carry a running summary through the chunks.

Then implement it, and be explicit about:
- What gets lost with this approach.
- How chunk boundaries are handled so nothing is cut mid-thought.
- What the user is told about the fact this happened.

Don't silently truncate. If something is dropped, I want to know.