Heads up, maybe this was already discussed and I missed it, but if I did, I’m guessing a lot of you did too.
I was running a long task the other day and noticed my weekly budget was disappearing way faster than it should. After some digging: subagents in Claude Code use a 5-minute cache by default. The main session gets 1 hour on a subscription, so I just assumed subagents did too. They don’t.
Why that hurts: if a subagent sits idle for more than 5 minutes and Claude comes back to it, the cache is gone and the whole context gets processed again from scratch. On Opus 5.5 that’s about 25x the cost of a cached read. In a long run it keeps happening, and it quietly eats your limit.
The fix is one line in your settings:
"subagentPromptCacheTtl": "1h"
(You’ll need Claude Code 2.1.242 or newer.)
That said, don’t flip it blindly. A 1-hour cache write costs more than a 5-minute one, so it only pays off if your subagents sit around between calls and get reused. If yours are short and done quickly, the default is fine.
For reference, cache prices on the API (per million tokens):
Opus 5.5
: $4 input · $5 for a 5-min write · $8 for a 1-hr write · $0.20 cached read
Fable 5.1
: $10 input · $12.50 for a 5-min write · $20 for a 1-hr write · $0.25 cached read
Hope this saves someone’s weekly budget 🙂
submitted by
/u/Master-Biscotti-1186
Originally posted by u/Master-Biscotti-1186 on r/ClaudeCode
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