This was my experience too, and it seems to waste context habitually. Like I'd ask it to implement a feature by modifying a couple files, it'll plan the feature change in a document. Then it'll begin implementing the feature in the first file, it notices its context is filling up and begins "sundowning" and documents its progress in another markdown document. I ask if you finish off at least the current file, so it adds one more line, re reads both documents it made. Updated them, then decided to write another third document detailing it's progress. Realizing I should start a new chat I do so, and point it at one of the documents for tracking it's progress, you bet instead of trusting the document and simply continuing where the previous agent left off, it rereads and verifies the changes, notices there incomplete, and writes a fourth document now to track whats missing. If I'm lucky it now finishes off the changes in the first file, but usually it'll 'give up' noticing complex changes are requested but it's context limit is already full so it creates a tracking document for the agent in the next chat session to ignore and/or poison it's context with.

Switched to codex 5.1 and it's so much better, stays on task, doesn't blow up its context on pointless stuff, isn't annoyingly verbose or overly confident and prioritized exploring the codebase and understanding it before making changes. Like sonnet 4.5 will constantly "Perfect I found the bug it's X... Wait actually" like a dozen times, making a small change each time, none of which actually fixed the issue I described or allowed the tests to pass. I really don't understand what happened from sonnet 4, to 4.5, like it got smarter but also much less actually useful, it's context window awareness seems to just make it compelled to spend the last half of its context window doing nothing but writing the most verbose disorganized documentation possible, and manually fixing it instead of using the linting auto fix tools. I tried Opus once and hit the limits almost immediately, I think it started a project plan and failed due to the daily limit about 1/3 of the way through.

It really gives the impression of an incompetent, used car salesman of a developer. Like a completely shameless yes man who has no concept of objective reality. The amount of guidance necessary to get it to write code first, then after tests pass, quality checks pass, and I give approval, document it's work was insane and never once worked 100% reliably. The documentation it did make was excessively verbose and wasteful of tokens, I'd have to edit it or the next chat session would get blown up immediately just by reading the document to figure out where to start.

I swear I once saw Sonnet 4.5 make five different multi hundred line markdown docs to track the implementation of a simple feature, of which it's only added about 10 lines of code, and run none of the quality checks for. Then it gets confused because the tests say it doesn't work but the docs (that it crapped out) say it should work.

It's super weird because sonnet 4 did not have this problem and it used to be my go to coding llm, and neither have any of the chatgpt codex models. Something about sonnet 4.5 makes it simultaneously once of the smartest (excluding chatgpt codex 5/5.1) and one of the absolute dumbest coding agents. It doesn't surprise me that Opus 4.5 would be similar, just dumber at a much larger scale.