So Long and Thanks for All the Context

I got a really interesting question last week from Mike Loukides, my editor at Radar, after he read the third part of this trilogy on context management. “Another issue I’ve read about,” Mike asked, “is the tendency for a model to ignore the middle of the context. I’ve seen that particularly for the models with very large context windows. Is there anything to be said about that?”

Excellent question, Mike, and yes, there is. In that same email he pointed out that clearing the context and reloading it with just what’s important does a pretty good job dealing with this “ignore the middle” problem when it happens, but that’s clearly a stopgap.

It’s worth a deeper dive into what’s actually happening when an AI starts forgetting what’s in the middle of its context, because the problem is deeper (and more interesting!) than it might seem at first. It turns out that there’s a basic problem that’s fundamental to how LLMs manage context, and we’re still learning about it as an industry. That problem is called a U-shape. There’s been a lot of really interesting research into the U-shape problem recently, and several useful techniques have emerged that can help you manage it. And it’s probably not a coincidence that I’ve had to use all of them in my ongoing experiments with AI-driven development and agentic engineering (even if I didn’t always realize that’s what I was doing at the time).

A few weeks ago, in fact, I ran into the exact failure mode that Mike described. I was running the Quality Playbook, my open source code quality engineering skill, and ran into trouble with one of its phases—the one that writes up the bugs the earlier phases find. There’s a part of the bug writeup process where it had just created a file called BUGS.md that had an overview of each of the bugs, and had to create individual writeups for each bug it found. But instead of filling in the details correctly, it produced skeletal-looking stub files, with a generic template that had blank values instead of populated ones.

The thing is, the instructions for how to write a populated writeup were in the prompt. The actual bug data was in BUGS.md. I was absolutely certain that everything the agent needed was sitting in its context window, because I could see that it hadn’t compacted yet, and the skill’s intermediate artifacts let me see that earlier phases had read and reasoned about both files (which I talked about in my last article in this series). But the agent was producing stubs anyway. It really looked like the agent had everything it needed sitting in plain sight, and just wasn’t using the information it had. Frustrating!

I thought at the time that the model was just an idiot (which, arguably, was true but beside the point). It turns out that I had run directly into the U-shaped context problem.

In the previous three articles I covered what context is and why it disappears, how to keep important information in files instead of leaving it in the agent’s context window, and how to detect and recover when context has been compacted out from under you. All three were about losing context, through fragmentation, through compaction, through long sessions that overrun the window. This article is about this entirely different U-shaped failure mode, where the context is still sitting in the window and the model just isn’t using it.

The U-shape failure, and why bigger windows don’t fix it

The U-shape is an active area of academic investigation, so I’m going to start by going into a little bit of that research, because I think it will actually help us pin down what’s going on. I’ll start with an experiment run by Nelson Liu, an AI researcher at Stanford, who tested how language models actually use the contents of long inputs by giving them documents with the relevant answer placed at different positions and measuring whether the model could still find it. An interesting thing his findings show is that the U-shape didn’t appear to be a quirk of a single model. The U-shape showed up across model families, and even models with larger context windows still exhibited it.

If you have time, it’s actually worth taking a look at the paper that Liu and his team wrote, called “Lost in the Middle: How Language Models Use Long Contexts.” (It’s surprisingly readable for an academic paper.) The result they reported was a robust U-shape: The model performed best when the relevant information was at the beginning of its context window or at the recent end and worst when it was in the middle. Performance on questions where the answer was buried mid-context fell off sharply, even when the answer was sitting right there in plain sight. The field now uses the terms primacy bias and recency bias for those two preferences, and the U-shape is what you get when you plot them together against position.

I’m going to lean a little into academia here, because a lot of researchers are still learning about how LLM context actually works and what behavior has emerged in it.

One reason the U-shape matters more than “just another LLM quirk” is that recent research has started showing it’s a structural property of how transformers work, not a learned artifact. A 2025 ICML paper called “On the Emergence of Position Bias in Transformers” explained it as the equilibrium between two opposing forces inside the model: The causal mask amplifies the influence of the first few tokens (the primacy bias), while position encodings like RoPE heavily weight the tokens closest to where the model is generating (the recency bias). The middle is where those two forces cancel out. A 2026 paper by Borun Chowdhury, a researcher at Meta, called “Lost in the Middle at Birth: An Exact Theory of Transformer Position Bias,” took the argument even further by proving mathematically that the U-shape exists at the moment of initialization, before any training has happened, with random weights.

That matters because the natural assumption about large context windows is that more room means fewer problems. Most of today’s frontier models give you a million tokens or more, with some pushing well past two million, and some have made real progress on the simplest version of the lost-in-the-middle test, the needle-in-a-haystack benchmark, where the model has to retrieve a single sentence buried in a long document. Google’s Gemini 1.5 Pro reported near-perfect single-needle recall at 1M tokens, and current Gemini 3 models are similar.

So the accurate version of “bigger windows don’t fix it” is this: Bigger windows have made simple single-fact retrieval much better. They have not made long-context agent work reliable by default. A two-million-token window means a bigger middle to fall into.

The important idea that’s emerging here is that it’s increasingly looking like the U-shape isn’t just a bug in today’s models that will eventually be worked out or trained away by more data or better fine-tuning. Instead, it seems like the U-shape may actually be a geometric property of the LLM architecture itself.

In other words, we’re all going to have to deal with the U-shape. And that means we need techniques for managing it, and any effective technique we use isn’t likely to become obsolete any time soon. And that’s my goal in this article: to show you the techniques that have emerged for managing U-shaped context memory loss that you can use today in your own work.

Five techniques to help with U-shaped context problems

The previous article in this series laid out a pattern for detecting and recovering from context loss, which I called externalize-recognize-rehydrate. The techniques below extend the same discipline to the lost-in-the-middle problem. The principle I keep coming back to is that working memory is untrustworthy, and the discipline that follows from it is to externalize what matters, curate what stays in context, and verify what the agent claims to know against what’s on disk. The five techniques are how I do that in practice, and each one is drawn from a real moment in the Quality Playbook’s development.

Curate, don’t accumulate

This is the technique which, in its most brute-force form, is exactly what Mike talked about in his email to me: just clear the context and reload it with just what matters, periodically and deliberately. In other words, don’t trust an accumulated session to stay coherent; build the artifact, then start fresh against it. And if you have the AI write down the important parts of the context (like we’ve talked about throughout this series), then you can start a new session with refreshed AI that has a more targeted, curated context as a starting point.

I ran into this during the v1.5.2 release prep for the Quality Playbook. I was using a long Claude Code session that had been working through a series of fixes. But I noticed that it was just starting to show its age: It had forgotten a couple of things it should know, and its thinking times were starting to grow.

When it came time to land the final four fixes for the release, I worked with the AI to write a context brief, or a separate document with everything the implementing session needed. The question was whether to keep using the existing session, which already “knew” the codebase from the earlier work, or open a fresh CLI session and point it at the brief. I asked another session what to do:

Should we run that in a new cli session rather than continue my current claude code session that has the existing context?

The AI gave me a good answer—start a fresh session, using a starting prompt to read the brief—and it gave three reasons that have stuck with me. First, the brief was self-contained, including file paths, line numbers, exact diffs, regression test bodies, and preflight greps. Anything the new session needed to know was already there, and continuing context bought nothing. Second, fresh context is stricter about adherence. A session that already “knows” the codebase tends to skim the new instructions and improvise from prior assumptions. Surgical fixes are exactly the case where you want the agent to read the brief carefully rather than rely on memory of what felt right last round. And third, the audit trail: The brief is the artifact, and the implementing session is reproducible from just the brief. If the same work has to be redone in six months by a different model, you point at the brief and say, “This is the input.”

The approach worked really well. I was able to pick up development seamlessly, and the model’s memory problems disappeared.

Position critical information at the edges

The U-shape says the model attends best to the beginning and end of its context. The natural move is to put your most load-bearing information in those positions and keep the middle for things you don’t need the model to focus on. Anything important that lives only in the middle of an accumulated context tends to slide out of attention.

The other side of this technique is what not to put in the middle. If something matters, don’t bury it in a long preamble of context you’ve been accumulating; move it to the edges, restate it where the model will act on it, and let the middle absorb the less important material. Luckily, there’s a useful technique that can help with this problem.

In Claude Code, for example, one really clean way to put information at the beginning of context is to use the system prompt. The CLI gives you --append-system-prompt for exactly this. (Most of the other providers’ CLI tools have similar options.) If you put your brief (or selected parts of it) there, the agent will attend to it strongly throughout the session, and that in turn will help keep the per-turn user prompt focused on the action you want the agent to take right now.

Short sessions over long ones

Don’t run one long session. Run many short ones, each reading fresh from disk. This will help you iterate on your brief and your external development context, so instead of relying

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