What just happened
Most people think long Claude chats get expensive because the model is 'thinking harder'. Not exactly. A lot of the cost is just carrying conversational luggage turn-to-turn — stale tool output, dead-end attempts, repeated instructions. Two recent benchmarks (MindStudio May 1, StartupHub May 2) put numbers on a habit that pass-takers already use intuitively: hit /compact *before* you need to.
What /compact actually does, and when
- Compact at ~60% context, not 95%. MindStudio's benchmark: manual
/compactat 60% cut input tokens 35-50% on coding tasks vs auto-compaction at 95%. Waiting until the 'oh no' stage means you've already paid for the bloat. - Latency drops too. StartupHub measured 15-20% lower latency on Opus 4.7 after a manual compact. Cost goes down; the chat also gets snappier. Both wins from one keystroke.
- Restart in a fresh chat with the summary. The non-obvious move: take the compacted summary and paste it into a brand-new conversation as the system prompt. You keep task continuity with far fewer tokens.
- Compact at phase boundaries, not during a fire. End of debug session, end of feature scaffolding, end of code review. Boundary points beat reactive ones. Treat it like git commits — small, frequent, semantic.
- Specify what to keep. Use a hint:
/compact keep the current objective, key decisions, modified files, and unresolved blockers. Don't trust the default summary to preserve case-facts you'll need later.
Why this matters for the exam
The CCA-F D5 domain (15%, smallest by weight, biggest by careless-wrong-answers) hammers the progressive-summarization trap — the failure mode where context compresses away exactly the case-facts you need. /compact is the operator-side answer. Pair it with a pinned case-facts block and the trap doesn't trigger.
Three /compact mistakes
Compacting reactively, at 90%+ usage
By the time you notice latency, the bloat has already been priced in for many turns. Compact early, compact often.
Trusting the default summary to preserve case-facts
The default summary is generic. If your task depends on specific account IDs, prior decisions, or a multi-step plan, name them in the /compact hint.
Not restarting in a fresh chat after compacting
Compaction inside the existing session doesn't undo the prior tokens — they were already paid for. The full win comes from compacting AND opening a fresh chat with the summary.
Sources
Where this lands in the exam-prep map
Each blog post bridges into the evergreen pillars. These are the most relevant follow-ups for this story.
Concept
Context window
Why context decays in quality before it fills in capacity.
Open ↗Concept
Case-facts block
Pin the load-bearing facts so /compact doesn't summarize them away.
Open ↗Knowledge
Context-window deep dive
The Skilljar mirror with full mechanics on attention decay and the lost-in-the-middle effect.
Open ↗Exam Guide
Day-of distractor patterns
The progressive-summarization trap is a recurring D5 distractor.
Open ↗4 questions answered
What does /compact actually do?
Why compact at 60%, not 95%?
How is /compact different from auto-compaction?
/compact is early, scheduled, hinted: you call it at phase boundaries with a guidance string for what to keep. Both exist; the manual habit is what saves cost.Does this map to a CCA-F domain?
/compact as the operator-side fix is exam-relevant.Synthesized from research output on 2026-05-02. LinkedIn cross-post pending.
Last reviewed 2026-05-06.