My habits have changed quite a bit with Opus 4.5 in the past month. I need to write about it..
That feels more like chasing than a clear line of improvement. It's interrupted very different from something like "my habits have changed quite a bit since reading The Art of Computer Programming". They're categorically different.
Why do you use the word "chasing" to describe this? I don't understand. Maybe you should try it and compare it to earlier models to see what people mean.
> Why do you use the word "chasing" to describe this?
Btw, I have tried it. It's annoying that people think the problem is not trying. It was getting old when GPT 3.5 came out. Let's update the argument...
Some of these improvements have been minor, some of them have been big enough to feel like step changes. Sonnet 3.7 + Claude Code (they came out at the same time) was a big step change; Opus 4.5 similarly feels like a big step change.
(If you don't trust vibes, METR's task completion benchmark shows huge improvements, too.)
If you're sincerely trying these models out with the intention of seeing if you can make them work for you, and doing all the things you should do in those cases, then even if you're getting negative results somehow, you need to keep trying, because there will come a point where the negative turns positive for you.
If you're someone who's been using them productively for a while now, you need to keep changing how you use them, because what used to work is no longer optimal.
So does the comment I critiqued in the sibling comment to yours. I don't know why it's so hard to believe we just haven't tried. I have a Claude subscription. I'm an ML researcher myself. Trust me, I do try.
But that last part also makes me keenly aware of their limitations and failures. Frankly I don't trust experts who aren't critiquing their field. Leave the selling points to the marketing team. The engineer and researcher's job is to be critical. To find problems. I mean how the hell do you solve problems if you're unable to identify them lol. Let the marketing team lead development direction instead? Sounds like a bad way to solve problems
> benchmark shows huge improvements
For METR I think they say a lot right here (emphasis my own) that reinforces my point
> Current frontier AIs are vastly better than humans at text prediction and knowledge tasks. They outperform experts on most *exam-style problems* for a fraction of the cost. ... And yet the best AI agents are not currently able to carry out substantive projects by themselves or directly substitute for human labor. *They are unable to reliably handle even relatively low-skill*, computer-based work like remote executive assistance. It is clear that capabilities are increasing very rapidly in some sense, but it is unclear how this corresponds to real-world impact.
It's great that they include longer tasks but also notice the biases and distribution in the human workers. This is important in properly evaluating.
Also remember what exactly I quoted. For a long time we've all known that being good at leetcode doesn't make one a good engineer. But it's an easy thing to test and the test correlates with other skills that are likely to be learned to be good at those tests (despite being able to metric hack). We're talking about massive compression machines. That pattern match. Pattern matching tends to get much more difficult as task time increases but this is not a necessary condition.
Treat every benchmark adversarialy. If you can't figure out how to metric hack it then you don't know what a benchmark is measuring (and just because you know what can hack it doesn't mean you understand it nor that that's what is being measured)
It seems to be "people keep saying the models are good"?
That's true. They are.
And the reason people keep saying it is because the frontier of what they do keeps getting pushed back.
Actual, working, useful code completion in the GPT 4 days? Amazing! It could automatically write entire functions for me!
The ability to write whole classes and utility programs in the Claude 3.5 days? Amazing! This is like having a junior programmer!
And now, with Opus 4.5 or Codex Max or Gemini 3 Pro we can write substantial programs one-shot from a single prompt and they work. Amazing!
But now we are beginning to see that programming in 6 months time might look very different to now because these AI system code very differently to us. That's exactly the point.
So what is it you are arguing against?
I think you said you didn't like that people are saying the same thing, but in this post it seems more complicated?
People have been doing this parlor trick with various "substantial" programs [1] since GPT 3. And no, the models aren't better today, unless you're talking about being better at the same kinds of programs.
[1] If I have to see one more half-baked demo of a running game or a flight sim...
Can you expand on that? It doesn't match my experience at all.
I think "substantial" is doing a lot of heavy lifting in the sentence I quoted. For example, I’m not going to argue that aspects of the process haven’t improved, or that Claude 4.5 isn't better than GPT 4 at coding, but I still can’t trust any of the things to work on any modestly complex codebase without close supervision, and that is what I understood the broad argument to be about. It's completely irrelevant to me if they slay the benchmarks or make killer one-shot N-body demos, and it's marginally relevant that they have better context windows or now hallucinate 10% less often (in that they're more useful as tools, which I don't dispute at all), but if you want to claim that they're suddenly super-capable robot engineers that I can throw at any "substantial" problem, you have to bring evidence, because that's a claim that defies my day-to-day experience. They're just constantly so full of shit, and that hasn't changed, at all.
FWIW, this line of argument usually turns into a mott and bailey fallacy, where someone makes an outrageous claim (e.g. "models have recently gained the ability to operate independently as a senior engineer!"), and when challenged on the hyperbole, retreats to a more reasonable position ("Claude 4.5 is clearly better than GPT 3!"), but with the speculative caveat that "we don't know where things will be in N years". I'm not interested in that kind of speculation.
I think they represent a meaningful step change in what models can build. For me they are the moment we went from building relatively trivial things unassisted to building quite large and complex system that take multiple hours, often still triggered by a single prompt.
Some personal examples from the past few weeks.
- A spec-compliant HTML5 parsing library by Codex 5.2: https://simonwillison.net/2025/Dec/15/porting-justhtml/
- A CLI-based transcript export and publishing tool by Opus 4.5: https://simonwillison.net/2025/Dec/25/claude-code-transcript...
- A full JavaScript interpreter in dependency/free Python (!) https://github.com/simonw/micro-javascript - and here's that transcript published using the above-mentioned tool: https://static.simonwillison.net/static/2025/claude-code-mic...
- A WebAssembly runtime in Python which I haven't yet published
The above projects all took multiple prompts, but were still mostly built by prompting Claude Code for web on my iPhone in between Christmas family things.
I have a single-prompt one:
- A Datasette plugin that integrates Cloudflare's CAPTCHA system: https://github.com/simonw/datasette-turnstile - transcript: https://gistpreview.github.io/?2d9190335938762f170b0c0eb6060...
I'm not confident any of these projects would have worked with the coding agents and models we had had four months ago. There is no chance they would've worked with the January 2025 available models.
Setting aside the fact that your examples are mostly “replicate this existing thing in language X” [2], again, I’m not saying that the models haven’t gotten better at crapping out code, or that they’re not useful tools. I use them every day. They're great tools, when someone actually intelligent is using them. I also freely concede that they're better tools than a year ago.
The devil is (as always) in the details: how many prompts did it take? what exactly did you have to prompt for? how closely did you look at the code? how closely did you test the end result? Remember that I can, with some amount of prompting, generate perfectly acceptable code for a complex, real-world app, using only GPT 4. But even the newest models generate absolute bullshit on a fairly regular basis. So telling me that you did something complex with an unspecified amount of additional prompting is fine, but not particularly responsive to the original claim.
[1] Copilot, with a liberal sprinkling of ChatGPT in the web UI. Please don’t engage in “you’re holding it wrong” or "you didn't use the right model" with me - I use enough frontier models on a regular basis to have a good sense of their common failings and happy paths. Also, I am trying to do something other than experiment with models, so if I have to switch environments every day, I’m not doing it. If I have to pay for multiple $200 memberships, I’m not doing it. If they require an exact setup to make them “work”, I am unlikely to do it. Finally, if your entire argument here hinges on a point release of a specific model in the last six weeks…yeah. Not gonna take that seriously, because it's the same exact argument, every six weeks. </caveats>
[2] Nothing really wrong with this -- most programming is an iterative exercise of replicating pre-existing things with minor tweaks -- but we're pretty far into the bailey now, I think. The original argument was that you can one-shot a complex application. Now we're in "I can replicate a large pre-existing thing with repeated hand-holding". Fine, and completely within my own envelope for model performance, but not really the original claim.
For me it is something I can describe in a single casual prompt.
For example I wrote a fully working version of https://tools.nicklothian.com/llm_comparator.html in a single prompt. I refined it and added features with more prompts, but it worked from the start.
Think: SaaS application that solves some domain specific problem in corporate accounting, versus "in-browser speadsheet", or "first-person shooter video game with AI, multi-player support, editable levels, networking and high-resolution 3D graphics" vs "flappy bird clone".
When you're working on a product of this size, you're probably solving problems like the ones cited by simonw multiple times a week, if not daily.
I think you'd get close on something like Lovable but that's not really one shot either.
In that case my Vibe-Prolog project would count: https://github.com/nlothian/Vibe-Prolog/
- It's 45K of python code
- It isn't a duplicate of another program (indeed, the reason it isn't finished is because it is stuck between ISO Prolog and SWI Prolog and I need to think about how to resolve this, but I don't know enough Prolog!)
- Not a *single* line of code is hand written.
But - importantly - this project is what convinced me that the frontier models are much better than the previous generation. There were numerous times I tried the same thing in a non-Frontier model which couldn't do it, and then I'd try it in Claude, Codex or Gemini and it would succeed.
Copilot style autocomplete or chatting with a model directly is an entirely different experience from letting the model spend half an hour writing code, running that code and iterating on the result uninterrupted.
Here's an example where I sent a prompt at 2:38pm and it churned away for 7 minutes (executing 17 bash commands), then I gave it another prompt and it churned for half an hour and shipped 7 commits with 160 passing tests: https://static.simonwillison.net/static/2025/claude-code-mic...
I completed most of that project on my phone.
edit: I wrote a different response here, then I realized we might be talking about different things.
Are you asking if I let the agents use tools without my prior approval? I do that for a certain subset of tools (e.g. run tests, do requests, run queries, certain shell commands, even use the browser if possible), but I do not let the agents do branch merges, deploys, etc. I find that the best models are just barely good enough to produce a bad first draft of a multi-file feature (e.g. adding an entirely new controller+view to a web app), and I would never ever consider YOLOing their output to production unless I didn't care at all. I try to get to tests passing clean before even looking at the code.
Also, I am happy to let Copilot burn tokens in this manner and will regularly do it for refactors or initial drafts of new features, I'm honestly not sure if the juice is worth the squeeze -- I still typically have to spend substantial time reworking whatever they create, and the revision time required scales with the amount of time they spend spinning. If I had to pay per token, I'd be much more circumspect about this approach.
Letting it burn tokens on running tests and refactors (but not letting it merge branches or deploy) is the thing that feels like a huge leap forward to me. We are talking about the same set of capabilities.
I mainly eat it clear tasks like "keep going until all these tests pass", but I do keep an eye on it and occasionally tell it to keep going.
"AI, I don't like paying for my SAP license, make me a clone with just the features I need".
- Models keep getting better[0]
- Models since GPT 3 are able to replace junior developers
The problem is we've been promised that these employees would be replaced[1] any day now, yet that's not happening.
People forget, it is harder to advance when you're already skilled. It's not hard to go from non-programmer to a junior level. Hard to go from junior to senior. And even harder to advance to staff. The difficulty level only increases. This is true for most skills and this is where there's a lot of naivity. We can be advancing faster while the actual capabilities begin to crawl forward rather than leap.
[0] Implication is not just at coding test style questions but also in more general coding development.
[1] Which has another problem in the pipeline. If you don't have junior devs and are unable to replace both mid and seniors by the time that a junior would advance to a senior then you have built a bubble. There's a lot of big bets being made that this will happen yet the evidence is not pointing that way.
The answer is: Exactly what we are saying. This is also why people keep suggesting that you need to try them out with a more open mind, or with different techniques: Because we know with absolute first-person iron-clad certainty what is possible, and if you don't think it's possible, you're missing something.
> Opus 4.5 is categorically a much better model from benchmarks and personal experience than Opus 4.1 & Sonnet models. The reason you're seeing a lot of people wax about O4.5 is that it was a real step change in reliable performance. It crossed for me a critical threshold in being able to solve problems by approaching things in systematic ways.