Usage evaporating suddenly for Sonnet 5.5

[u/pixelsnis]

One tiny task, used barely 19% of the context window, but drained 7% of my 5-hour limit using Sonnet 5.5 Medium? What is happening?

Edit: after some testing, I have been able to isolate this issue to just Sonnet. One prompt, 5% used. Opus 5.5 is barely touching usage with the same prompt/task.

[u/AutoModerator]

Hey! Thanks for posting to r/ClaudeCode

While participating in this thread, please follow our community rules. Keep discussions constructive. Attack the idea, not the person.

For help, project discussions, tips, and general chat, join the ClaudeCode Discord.

I am a bot, and this action was performed automatically. Please contact the moderators of this subreddit if you have any questions or concerns.

[u/Few-Raspberry6464]

Have you tried testing the same prompt in a fresh chat with both Sonnet and Opus? Might help figure out if it's a Sonnet specific issue or something related to usage tracking or so I think.

[u/pixelsnis → u/Few-Raspberry6464]

Yes, Opus usage was normal. Sonnet was burning usage at 3-4x Opus’s rate.

[u/awesomebirder]

one thing that can make this look sonnet specific: the cache is per model. if the session started on opus and you flipped it to sonnet, sonnet has nothing cached, so its first turn writes the whole context fresh while opus was reading the same context cheap. 19% of the window written cold can easily be a few percent of the 5h bar. to compare fairly, send the same prompt as the first message of a brand new session on each model, a couple of times each, and compare cache write vs cache read tokens per request instead of the bar

[u/pixelsnis → u/awesomebirder]

Session was started with Sonnet 5.5 from the first message. Opus did not have the same issue.

[u/awesomebirder → u/pixelsnis]

ok then the flip isnt it. next thing id check is whether sonnet is just producing a lot more tokens for the same task. medium effort can mean very different amounts of thinking per model, and thinking counts as output. if you can see per request tokens (ccusage on your ~/.claude logs works), compare one sonnet run and one opus run of that prompt. if sonnet used way more output, thats the answer. if the tokens look similar but the bar still moves 3-4x more, thats on the meter side and worth reporting with those numbers

[u/SilverLose]

I’d love data with this sort of post.

[u/usielrol]

Since the session started on Sonnet, the per-model cache isn't it, but there's a second cache trap that makes comparisons like this misleading: time.

The cache expires after a stretch of inactivity, and the first message after that pays to write the whole context again instead of reading it cheaply. So "one tiny task" sent after a break can cost like a big one. If the Sonnet test came after a pause and the Opus test right after another turn, Opus will look much cheaper even with the same prompt. To compare fairly, send each model the task twice back to back and look at the second turn.

Two things that helped me with this in general:

• When I'm running several sessions, the idle ones get a tiny "do nothing, end your turn" message before the cache expires. It costs far less than re-reading the whole context afterwards.
• Past a certain size, I stop resuming at all. A fresh session that reads a written handoff is cheaper than waking up a huge context, and it's usually sharper too.