Cache TTL is killing me

Before you state the obvious - no I didn't need to use Opus 4.8 for this whole thing. It was an accident, I had been experimenting and didn't change the model before switching focus.

I've been working on a feature and realised the cost was going up and up so once it was done I got a fresh session to analyse why it cost so much and this is what it said:

```

Here's the breakdown:

---
Session 239ad469 — $1,555 total

Session span: 28 hours (2026-06-02 08:51 → 2026-06-03 12:55), 1,148 turns

Cost breakdown

┌──────────────────┬────────┬──────┐
│     Category     │ Tokens │ Cost │
├──────────────────┼────────┼──────┤
│ Cache creation   │ 31.2M  │ $584 │
├──────────────────┼────────┼──────┤
│ Cache reads      │ 554M   │ $832 │
├──────────────────┼────────┼──────┤
│ Output           │ 1.73M  │ $130 │
├──────────────────┼────────┼──────┤
│ Input (uncached) │ 307K   │ $5   │
└──────────────────┴────────┴──────┘

Why so expensive

Root cause: 28-hour session on Opus 4-8. Cache TTL is 5 minutes. Every time you walked away, cache expired. On return: 31M+ tokens recreated at $18.75/MTok (cache write rate). This happened
repeatedly over the day.

- $584 in cache creates = ~1,660 full cache rebuilds of ~18K tokens each, or fewer rebuilds of a larger context
- Opus 4-8 for 1,148 turns — Sonnet would be $3–5× cheaper for most of those
- 46 subagents but only $19 — that part was fine

Fixes

1. /compact before stepping away — reduces context size, so cache rebuilds cost less
2. End session + start fresh after major pauses — don't leave Opus sessions open overnight
3. Use Sonnet by default — switch to Opus only for complex reasoningHere's the breakdown:

---
Session 239ad469 — $1,555 total

Session span: 28 hours (2026-06-02 08:51 → 2026-06-03 12:55), 1,148 turns

Cost breakdown

┌──────────────────┬────────┬──────┐
│     Category     │ Tokens │ Cost │
├──────────────────┼────────┼──────┤
│ Cache creation   │ 31.2M  │ $584 │
├──────────────────┼────────┼──────┤
│ Cache reads      │ 554M   │ $832 │
├──────────────────┼────────┼──────┤
│ Output           │ 1.73M  │ $130 │
├──────────────────┼────────┼──────┤
│ Input (uncached) │ 307K   │ $5   │
└──────────────────┴────────┴──────┘

Why so expensive

Root cause: 28-hour session on Opus 4-8. Cache TTL is 5 minutes. Every time you walked away, cache expired. On return: 31M+ tokens recreated at $18.75/MTok (cache write rate). This happened
repeatedly over the day.

- $584 in cache creates = ~1,660 full cache rebuilds of ~18K tokens each, or fewer rebuilds of a larger context
- Opus 4-8 for 1,148 turns — Sonnet would be $3–5× cheaper for most of those
- 46 subagents but only $19 — that part was fine

Fixes

1. /compact before stepping away — reduces context size, so cache rebuilds cost less
2. End session + start fresh after major pauses — don't leave Opus sessions open overnight
3. Use Sonnet by default — switch to Opus only for complex reasoning
```

I've done similar analyses of sessions before and the problem is ALWAYS the cache TTL. This is an extreme example it's true, but it's a consistent problem I have.

Any suggestions of how to handle this better? Obviously "don't leave a session overnight" (I didn't realise I'd done that) but anything else? Other than just having one session running at once and watching it like a hawk...