Using Opus 4.5, I have noticed that in long sessions about a complex topic, there often comes a point when Opus starts spouting utter gibberish. One or two questions earlier it was making total sense, and suddenly it seems to have forgotten everything and responds in a way that barely relates to the question I asked, and certainly not to the "conversation" we were having.

Is that a sign of having having surpassed that context window size? I guess to keep them sharp, I should start a new session often and early.

From what I understand, a token is either a word or a character, so I can use 100k words or characters before I start running into limits. But I've got the feeling that the complexity of the problem itself also matters.

It could have exceeded either its real context window size (or the artificially truncated one) and the dynamic summarization step failed to capture the important bits of information you wanted. Alternatively, the information might be stored in certain places in the context window where it failed to perform well in needle in haystack retrieval.

This is part of the reason why people use external data stores (e.g. vector databases, graph tools like Bead etc. in the hope of supplementing the agent's native context window and task management tools).

https://github.com/steveyegge/beads

The whole field is still in its infancy. Who knows, maybe in another update or two the problem might just be solved. It's not like needle in the haystack problems aren't differentiable (mathematically speaking).