“Our systematic study exposes a phenomenon of constraint decay in LLM-based coding agents. While current models excel at unconstrained generation, their performance drops when forced to navigate explicit architectural rules. For end-users, this dichotomy implies that agents are reliable for rapid prototyping but remain unreliable for production-grade backend development.”

One major weakness of this study is that they didn’t fully test frontier models for cost reasons, so the specific performance results should be taken with a grain of salt. But the overall conclusion that models degrade when both behavior and architecture must be correct is interesting, and something to keep an eye on.

• today at 8:06 PM

• zdragnar

  today at 7:36 PM

  I've noticed something similar with AI assist authored books as well. Early on it does alright, but after some chapters the beginning of each chapter repeats the end of the previous, and obvious LLM tells become more frequent.

  The more it has to go on, the more it relies on repetition of what came before. It's also possible that authors start paying much less attention and put less effort into editing later chapters.

  Despite the sheer volume on Amazon, LLMs are not at the point of writing well.

  • piker

    today at 7:46 PM

    Holy crap are you reading books that advertised somehow they were written with LLM assistance? Hard no here in 2026.

• qsort

  today at 4:16 PM

  I think it's downstream of "you can't optimize for two different objectives".

  If you only have functional requirements, then in effect you're doing some form of program synthesis, and RL can optimize that very hard.

  If you have a mixture of functional and non-functional requirements, you are basically giving the model an incomplete specification, and it must in some way guess at the user's intent to fill in the blanks. This is also why adding to the prompt examples of the style of code you want (hats off to antirez for this particular tip ;)) is phenomenally powerful.

  • apsurd

    today at 4:20 PM

    Would you mind sharing antirez' suggestion?

    • qsort

      today at 4:29 PM

      I am obviously paraphrasing, but the general idea is that trying to synthesize style from a codebase into e.g. a markdown guide generally doesn't work very well. What achieves style transfer is providing the model with a lot of examples of the style, conventions, patterns you want.

      To put it in practice: if you point claude/codex to a repository and you ask it to implement feature X using style guide Y, the code will probably work, but you can usually get better results by saying "do it in the style of this file, it was done well there".

      • brandensilva

        today at 5:17 PM

        Right more simply put it's great at being a copy cat, exploring similar data points that match your token needs.

        It is not great at decision making or judgment calls that don't have a well defined spec or plan in place yet; like unofficial or unapproved tokens if you will. A lot of this stuff simply never has had specs as it has been internal to how companies work and their secret sauce.

        The closest thing we have are governance and compliance policies due to legal/business needs requiring it so it's far more well documented than operational ones in how we work. It is more about the how versus the what here I guess is what I'm saying.

        But yeah this is why it does great when there are tests, design systems, evals, and other artifacts to mirror. Far more reckless and unpredictable without these things, but still great for exploration and finding the data output you seek.

        • withinboredom

          today at 7:07 PM

          Doesn't that make sense? Its text prediction. If you give it examples, it can predict. Synthesizing "put semi-colons on new lines" requires it to generate its own examples 'in its head' (so to speak) and remember that. It won't.

          It's like when I see people feeding it a whole bunch of "best practices" and expect it to follow them. It won't. But you could ask it questions about the best practices all day long.

          • brandensilva

            today at 7:45 PM

            Yes, exactly. Any engineer deep on this stuff right now understands that grounded predictive engine sprinkled with RL training and are discovering what that means in terms of its strengths and weaknesses for company use.

      • mikeyouse

        today at 5:19 PM

        I ran into similar issues as we started to roll out LLM generated financials in our org.. I’m so used to the old SQL workflow of “grab this data from this table, that data from that table, combine it into a final result that looks like xxxx” where the tables were outputs from reports in our ERP but I was having terrible results.

        Ended up pointing Claude at a few sample files from our existing reporting, gave it read-only oauth access to the ERP and said “build a new report showing the cash by project as calculated by xxxx - yyyy + zzzz in the style of the existing reports” and it basically one-shot from there.

        Kind of crazy and I built a bunch of redundant check-sums because I honestly didn’t think it would be able to replace like 6 workdays of effort for the 2 FTEs who generate that kind of thing manually every month but so far so good..

        • BlueTierOps

          today at 6:41 PM

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      • KaiShips

        today at 7:02 PM

        [flagged]

• nijave

  today at 4:36 PM

  Hmm, I have some anecdotal evidence this is true. Interactively working out a plan with Opus on multiple occasions it'd come up with an incompatible solution, I'll add additional context/requirements, and it has a tendency to "anchor" on it's original architecture and struggles to adapt. Sometimes it tries to sneak in changes for the original plan anyway.

  • whstl

    today at 6:01 PM

    Opus does this waaaay too much for my taste. It works fine for vibe-coders but for technical work it is infuriating.

  • UncleEntity

    today at 6:56 PM

    I think the problem is they take the shortest path to the goal ...which may or may not coincide with what you have planned. Oh, and generally think instructions are merely suggestions and what you really want this this totally different thing and not the one in the plan you handed them plus, as a stoke of good luck, this other system is a lot easier to implement as well.

    I mean, I spend more tokens having them clean up all the places they didn't follow the the plan (if I catch it) or implementing what came out of a 'complete and tested' previous plan where they just stop as soon as all the pathetic new test pass and you discover half of it isn't even there when trying to implement the next thing on top of it.

    Though... I have been conducting an experiment, of sorts, where we've been cooking on these fairly complicated projects and I don't ever touch a single line of code, just yell at them a lot, and with suitable amounts of marijuana (they are very frustrating most of the time) it's been going pretty well. I also helps that they need to explain what they're doing to somebody fairly-baked -- maybe not such an HR friendly plan?

• Animats

  today at 7:03 PM

  That may be the same problem seen when prompts try to force "alignment" or "guardrails". There's a performance drop. Seemingly, a big chunk of the potential solution space has been made unreachable.

  For example, if you apply "guardrails" to an image generator of about a year ago, all the people start looking alike. Story generators start using only a few standard names.

  That was last year. Is it happening with the frontier models?

• jeremyjh

  today at 4:20 PM

  Even the strongest frontier model they used - GPT 5.2 - I would consider barely usable for agentic programming.

  I’m not really interested in analysis of the weaknesses of such models because in my experience many weaknesses disappear entirely as models get stronger and reasoning effort is turned up. Especially if you tell them what you want them to do.

  Also, it’s not surprising to learn that when more acceptance criteria are added the failure rate increases.

  • sigbottle

    today at 5:04 PM

    Wait isn't gpt 5.2 good? Or is it not thinking / not codex? 5.2 was what sparked the late 2025 openai agentic programming revolution.

• xienze

  today at 4:55 PM

  > their performance drops when forced to navigate explicit architectural rules

  Even the best models have trouble adhering to stuff as mundane as rules for how to style generated code (indent this much, name things with these patterns, etc.). Even the most die-hard AI-first coder will admit to that kind of stuff being not unheard-of. Yet they still delude themselves into thinking that these models will follow a sufficiently detailed spec to the letter, every time.

I recommend spending some time getting a few parts of the codebase idiomatic and then @-ing those files as exemplars. This works a lot better than trying to steer it with markdown. This works reasonably well for like FastAPI but JavaScript seems to be the worst, even with guidance and exemplars it'll prefer in-lining a bunch of garbage rather than use the APIs as directed.

> For end-users, this dichotomy implies that agents are reliable for rapid prototyping but remain unreliable for production-grade backend development.

Time to start writing linting tools that check the architecture and spoon feed the LLM what exactly it's doing wrong.

I reckon something like this would be good for every project out there: https://www.archunit.org/getting-started

They expand a bit more on the reasoning behind it: https://www.archunit.org/motivation

(I also wrote a simple linter for architecture/code checks that aren't well encapsulated by ones that just focus on individual files, that uses Go + goja to write rules in ECMAScript and parallelize the read only ones and also allow ones that change files as necessary, in addition to something like Ruff / Oxlint / Oxfmt / whatever is present in each stack; though it's is still in development and not as good of a focused example as ArchUnit is)

If we write software specification docs, bother describing how it evolves with ADRs, enforce code style automatically and require certain test coverage automatically (or at least should), why couldn't we go a step further, formalize those specs and ensure that any new code is also up to snuff? I don't think that's any more of a job for an LLM, than telling it how it should format code is. Also, I'm in the camp that believes that at least many of your ORM mappings and similar stuff should be the output of codegen, since you've already gone through the trouble of describing the schema/migrations to get there.

I don't think this would be only good for LLMs, though - I've seen projects that have like 3 different audit systems built in, not because of some fancy business requirement, but rather cause the devs either didn't know about the previous one(s) or just didn't feel like following what should have been the pre-established conventions, even when there were docs in place (nobody read those).

Reminds me of the recent paper about delegating document editing tasks to LLMs across different disciplines [1]. That paper found that programming was the only discipline most LLMs can perform long horizon tasks on without accumulating errors & corrupting the document.

I've only read the abstract of this one so far but it seems like this paper has zoomed in on programming with greater fidelity and shown a similar phenomenon. But not about long horizon tasks, more like "long style horizons" of larger sets of structural constraints.

[1] https://arxiv.org/abs/2604.15597

Discussion: https://news.ycombinator.com/item?id=48073246

• emp17344

  today at 3:41 PM

  If it’s not easily verifiable, LLMs aren’t good at it.

  • jeremyjh

    today at 4:03 PM

    I think that’s mostly because they get so much more of that reinforcement learning - since it is so economical. I dont know if there is any evidence of a fundamental reason they can’t be just as good at other tasks, but it might be economically infeasible for awhile yet.

    • mjburgess

      today at 4:36 PM

      No one is curating vast amounts of data for them in other domains. Programmers send programs with fixes

      • knollimar

        today at 5:43 PM

        There's no diff of my excel lambdas being fixed? :(

    • emp17344

      today at 6:19 PM

      RLVR doesn’t work for unverifiable tasks, so they won’t be able to effectively use tools to boost reliability for those tasks.

I've been experimenting quite a bit with long-horizion agentic coding[1] and I have also noticed that agents seem to perform worse when forced into certain architectural patterns. I have found that is a bit better when including the constraints along the way instead of adding them after the fact. There seems to be a side-effect I have been calling "calcification", where a pattern starts appearing in the codebase and the agent follows the pattern to the point where it dominates the context and becomes self-reinforcing. This could potentially be a strength or a weakness for existing code bases depending the codebase quality. I will have more insights on this soon as more from-scratch runs conclude that include architectural guidance from the beginning.

[1]: https://medium.com/@vishvananda/i-spent-2-billion-tokens-wri...

This sounds like another version of "As a chat becomes longer, the guardrails seem to become fuzzy". You can't use all of the context window bc at the end, the output would not respect the constraints (or guardrails) but to reliably produce production grade code you want the model to have expansive awareness which fills up the context window pretty quickly. It's like saying "Keep everything in mind from these 6 directories - and make this <insert ticket> change" - but keeping everything in mind already fills it's context window which makes it lose it's ability to follow the constraints (or guardrails).

• usrusr

  today at 7:24 PM

  So give harder guiderails? Sonarcube and the like. But I guess then the failure mode would be appeasing the linter while slowly forgetting the requirements... (or not so slowly, because the try/fail loop won't be nice to context at all..)

• whatever1

  today at 4:05 PM

  This is not a new problem though. This is why we started writing modular code, strict interfaces etc

  • lanstin

    today at 4:12 PM

    And doing incremental dev, so once a feature is done you can mostly ignore it.

    • Silhouette

      today at 4:43 PM

      If there is one good thing that the generative AI tools have shown beyond any doubt it's that the classic "good programming" practices are still useful and effective. Self-documenting code. Modular design. Clearly defined architecture. Incremental development. Coding standards. Automated tests. Automated everything.

      If there's a second thing the generative AI tools have shown beyond any doubt it's that many of the more modern (relatively speaking) "best practices" that have always been over-hyped and questionably-evidenced really do tend to produce worse results. LLMs take these methods to their logical conclusions and show us the end result much sooner. You can't just iterate your way to a solution when you don't even know what problem you're trying to solve. If you don't have a clear spec then you don't know what a correct product looks like. You need to invest time in reviewing code properly. If you don't keep the big picture in mind then the big picture becomes a mess.

      Maybe one day the LLMs will leave me out of a job but at least I'll feel validated first!

   tasks spanning eight web frameworks

• bob1029

  today at 4:56 PM

  I think web frameworks have been "in trouble" as of gpt-5.4. I can't imagine using something like React anymore.

  The most incredible combo I've seen lately is progressive enhancement of Razor Pages with javascript. With this arrangement the newest models tend to make a really good call on if something should happen server-side (cshtml) or on the client (js).

As a codebase grows, divergent structural emergence from incidental(lang and lib) details results in prolonged complexity costs. I'm working on a language that enforces structure for agents: https://github.com/hale-lang/hale

I think someone is going to figure out a framework for using LLMs for coding.

A framework would use static code checking tools to force an architecture on to LLMs instead of trying to do so in markdown.

I don't know exactly what it will look like but for example I could imagine a Java Framework where the LLM could only create subclasses of certain classes.

So my finding is: planning is worth it.

For a little complex changes, I always run codex (5.5-high) in planning mode first. I have linked various docs/{ARCHITECTURE,BACKEND-GUIDELINES,NESTJS-DI,..}.md etc. from AGENTS.md so they can quickly discover relevant docs at planning time, only if they are needed. No need to know react specific stuff when it's dealing with a backend problem for example. I typically blindly approve plans made by the agent with a fresh context, because that's as if I had prompted it. Works the best for me.

Using /goal however, it's really just constantly compacting and doing it's thing, of course it gets sloppy. If only there was a state machine that would transform tickets into a Planning Mode Prompt, then use, idk. guardian approvals (somehow a "Product Management Perspective Lens" approving or making changes to the plan) and then letting a less capable or less reasoning agent execute the plan, I think that would work the best.

Odd they used GPT-5.2 and not GPT-5.2-codex. i.e. the one optimized for coding agent tasks.

• maleldil

  today at 6:10 PM

  Considering this is from academia, there's a chance there were limitations on the available models. My research group accesses OpenAI models via Azure, and until recently (last week) the latest model was GPT 5. We just got 5.4.

  • beering

    today at 8:11 PM

    That’s wild. Are you at a university that bans using the OpenAI APIs directly?

Also they used languages with dynamic typing like Python & JS. In my experience a statically typed codebase is easier to maintain for humans so maybe it is also for agents.

When using Codex/Claude Code with Go code I cannot count the times the agent does some change, runs a build to check for errors, find some and fix them.

• acbart

  today at 3:51 PM

  It's crazy to me that people think of Python as dynamically typed by default. Strong static typing has been an option in Python for years now, and it should just be the default.

  • mrob

    today at 5:50 PM

    >Strong static typing has been an option in Python for years now, and it should just be the default.

    https://docs.python.org/3/library/typing.html

    "The Python runtime does not enforce function and variable type annotations. They can be used by third party tools such as type checkers, IDEs, linters, etc."

    Which third-party enforcement mechanism do you propose become the default?

  • epgui

    today at 3:54 PM

    The python type hints are useful for static analysis (and yes, should be the default) but it’s a joke compared to the utility of types in a language like Haskell.

  • antonvs

    today at 6:34 PM

    Typing with tools like Pyright doesn't come close to providing what a good statically typechecked language provides.

    There are many reasons for this. A big one is that many libraries are only partially typed at best, and dynamic types tend to propagate, weakening the guarantees you get from type checking.

    Dynamic idioms in general, including something as common as string-indexed dictionaries, negate type checking. Runtime metaprogramming is the same. All of these things have equivalents in a good statically checked language, but Python doesn't follow those models.

    Fundamentally, in Python static typing is an optional analysis layer over a dynamic language, and the consequences of that can't be fully mitigated. The result is a big difference in what types can guarantee.

> Our findings reveal a phenomenon of constraint decay: as structural requirements accumulate, agent performance exhibits a substantial decline.

I have exactly the inverse findings on my end. The bigger and more legacy the codebase, the more accurate the patches become.

The harness itself seems to be the most important part. I use a recursive loop that primes the root context based on the user prompt each time. My agent will often make over 100 tool calls to sql and git before it finally decides to apply a patch. If I was greenfield, there would be nothing to query or constrain against.

• richardlblair

  today at 4:03 PM

  I find the same. We have abstractions with multiple concrete implementations, examples of patterns and examples of anti patterns.

  I usually find I can achieve 90% of the outcome I'm trying to achieve. I use sonnet for planning, qwen for coding, sonnet for review.

• xcjsam

  today at 4:24 PM

  The harness mattering more than the model lines up with my experience too. What this paper measures is within-turn constraint decay. The version that bites in multi-agent setups is across-session — the architectural rules an agent wrote down on Monday don't reach the agent making the next change on Tuesday.

These things don’t think. We’re going to have to reiterate this for a long time, I fear.

• emp17344

  today at 3:30 PM

  There is now a trillion-dollar industry bent to the task of convincing people these things can think. It’s gonna cause some damage.

  • suprfnk

    today at 5:14 PM

    I don't think they think. I still use them a lot despite that, because they are very powerful parameterised code generators.

• sheeshkebab

  today at 3:40 PM

  …but they reason well enough given enough context (using their matmuls).

  • noosphr

    today at 3:51 PM

    To this day frontier models think that A and not B means A and B when the sentence gets pushed far enough back in their context window. The context length that model can reason over without obvious errors is much smaller than the advertised context. Between a 1/4th to a 1/20th what is advertised on the tin.

    • antonvs

      today at 6:37 PM

      Critiques like this tend to focus very hard on what models can't do. It's true, they have limitations.

      But they're also superhuman in so many other ways. It's valid to point out limitations, but that doesn't support the conclusion that models are not incredibly powerful and capable of the functional equivalent of reasoning at human or superhuman levels in many scenarios.

    • Npovview

      today at 4:56 PM

      Do you also happen to remember what you ate last thrusday?

      • leecommamichael

        today at 5:00 PM

        Is that the same gap as what you’re responding to? To me, it seems his critique is about advertised capability and logical statements, and your rhetorical(?) question is about memory.

      • UncleEntity

        today at 7:15 PM

        "If you have a question look in the specification for the answer and don't just guess" seems a fairly important thing to remember for more than a couple of minutes...

        • Npovview

          today at 8:00 PM

          I had a coding session where I was doing stuff across two repositories. And CC forgot in exactly which repository a particular file was so it was grepping the parent directory. I just asked it to write all important key-value pairs which it thinks are important to a file and it never did parent directory grepping.

• akomtu

  today at 6:29 PM

  There is a movie, Gold (2016), about a fake gold mine. One of its founders is a true believer: he found a few chunks of gold and started digging for more. The other founder is a nihilist: he realised that there is no gold there, but who cares if he makes the investors believe? So he does, and almost sells the company for $300M.

  In our story, investors are mining intelligence from GPUs, and they truly believe they are one inch from discovering the biggest goldmine in history. But GPUs, unlike a goldmine, cannot be inspected for traces of gold by independent contractors. To keep the hype up, the nihilists in our story dig up cheap gold-looking metals from time to time and tell investors that with a bit of alchemy - agentic workflows, etc. - those metals can be magically turned into gold.

  Investors will keep digging until the end of the age, or until they run out of money.

  • today at 6:42 PM

This is interesting, anecdotally I have felt like I was having better luck with raw sqlite than using an ORM in a recent typescript project, using raw sqlite queries vs drizzle

This research is useless and nearly all other LLM research is too.

gpt 5.2 is the strongest model they tested, a nearly 6 month old model.

Traditional research can not keep up.

• acgourley

  today at 5:31 PM

  I disagree, their findings should generalize to the frontier. Even if the latest can deal with the extra complexity, it stands to reason it will take more tokens to do less. This could be a useful insight into the next generation of evals.

Exactly why you can't remove humans in the loop to assess that the solution is not only correct (which LLMs are quite bad at, once concurrency, logic, etc are involved), but also elegant, maintainable, etc

"constraint decay" isn't this just another name for the (already well understood) idea of "context rot"?

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