Non-uniform session usage

Hi there, wanted to ask whether anyone else experienced something similar. I have $16 zai glm light subscription, and using it mainly for software architecture design and refinement (not a vibe coding).

At first I was very happy at how slow it was consuming session usage with GLM 5.2 - prompted, it started exploring design draft and related repos (some pretty big), we started discussing options, context was already like 70-80k, there were some really long thinking blocks (so - it did output a lot) - but it still has eaten like 2-3% of session. Then asked it to generate some prompts for focused research in particular areas and pasted to separate sessions (I‘m using Pi without subagents, for extra control) - similar context size, total usage at like 5–7%.

Then after some corrections copy pasted research result back to main session - still good, after some more back and forths, at context size around 120k and session usage of 10% I asked it to update draft design document. Some edits, mainly appends (file itself is a markdown with like 1.5k lines) - and bam, I‘m suddenly at 30% usage. Generate another self-review prompt and paste to separate session - usage growing slow again, like to 35%, even though that review session was doing a lot of code exploration and generated massive thinking blocks. Paste review result back to main session - it starts doing series of edit of document, and usage goes like - bam 55%, 75%, 90%.

As I suspected that something might be wrong with cache usage, I was closely monitoring it, and Pi was constantly reporting numbers like 95%+ cache hit rate. Tokscale analysing my model usage reported similar numbers.

So - anyone else having similar experience? Is that normal? Are tokens for write/edit tools somehow much more expensive compared to thinking output (but they are both just output)? Or maybe there is some subtle bug either in Pi or on Zai servers that prevents proper usage of cache in some circumstances, even if Pi thinks it is using it?