Long-context effects

Two distinct failure modes with two distinct signatures.

4 min read · Lesson 5 of 9 in this domain

Long contexts fail in two distinct ways with two distinct signatures, and telling them apart is the diagnostic skill being tested. "Lost in the middle" degrades recall of material in the middle of a long context while the beginning and end stay strong. Boundary degradation is different: as the total approaches the window limit, the tail gets dropped. So a symptom of "the last third of every long document is consistently missed" points at the boundary, not at a general attention span — and the fix is to find what else is consuming the window, remembering that a detailed tool schema can easily cost a couple of thousand tokens before the document is even loaded.

• "Lost in the middle" degrades recall of material in the middle of a long context.
• Boundary degradation is different: when the total approaches the window limit, the tail is systematically dropped. A consistently missing final third points here, not to general span limits.
• Remember that tool schemas consume the window too. A 12-field schema with detailed descriptions can cost ~2,500 tokens; add a system prompt and your usable space shrinks measurably.
• Diagnostic pattern to recognise: accuracy fine below a size threshold, sharp cliff near the limit, tail of the document missing → the fixed overhead plus the document is crowding the boundary.
• When passing work downstream, do not send everything. Pass a dense narrative plus a compact structured citation index (citation id, claim, source, key quote) rather than 120k of raw content or a bare summary with no attribution.

Source: Claude Docs — Context windows · Independent study aid, not affiliated with or endorsed by Anthropic.