I think you might be confused a bit about compaction? The LLM API endpoint does not do compaction, it's an external agent harness that does it. And the Codex/Claude agents aren't constantly summarizing it down, they generally wait until you get within 3/4 of the max of the context size.

Compaction doesn't save them money, it just makes it possible for you to continue a session. If you compact a session too many times, besides the fact that the model basically stops being useful, you eventually just cannot do anything else in the session because all the context is taken up by compaction notes. But if you don't compact it, pretty soon the session is completely unusable because it can't output any more tokens. You can disable compaction in those agents if you want to see the difference.

Also, using a lot of context can make the model perform poorly, so compaction can improve results. If you have a much larger context size, it means you have more headroom before the model starts to perform poorly (as it grows closer to max context size). A larger context also lets you do things like handle larger documents or reason over a larger amount of data without having to break it up into subtasks. Eventually we want models' context to get much bigger so we can do more things in a session. (Some research is being done to see if we can get rid of the limit entirely)

LLM API endpoint does do compaction. OpenAI definitely does support serverside compaction, both explicit and automatic, and this is different than what could be implemented purely clientside: https://developers.openai.com/api/docs/guides/compaction (and there was rumors a few months ago on HN about how activation-preserving/latent it is, vs just summarization). Anthropic as well, in beta (new to me): https://platform.claude.com/docs/en/build-with-claude/compac...

The names for the pieces are confusing, so it's easy to talk past each other. For instance, you're saying "Codex the agent", which isn't a thing now. It's currently GPT-5.5, and at one point it was GPT-5.3-Codex, so when I say "Codex", I meant the MacOS "harness". Similar for Claude Code vs Claude Opus/Sonnet.

Anyways, I don't know specifics well enough to argue with you on anything, but there is a cost for input tokens, and you see/pay it when you use the API directly or through OpenRouter. Maybe you looked at the leaked source for the Claude Code and can tell me definitively otherwise, but Anthropics and OpenAI's incentives for when to compact are not always aligned with the users depending on pricing plans.