Why Does Devin Sometimes Overcomplicate a Fix for What Should Be a Simple Bug?
Because a well-formed instruction to Devin usually asks it to find the root cause and fix it, and for a genuinely simple bug, the root cause and the visible symptom are the same one-line thing, but Devin doesn't reliably know that in advance. It investigates as if the fix might be deep, and if the investigation turns up a plausible related improvement along the way, an abstraction that would make the code "more correct," a helper it can generalize, a dependency that already handles the edge case, it tends to include that improvement in the diff rather than stop at the line that actually broke.
The result isn't a wrong fix. It's a right fix wrapped in code nobody asked for. A reviewer opens a PR that resolves the reported bug, and also touches three files that had nothing to do with it, introduces a new internal helper used exactly once, or replaces a working conditional with a small state machine. Independent testing of Devin has documented unnecessary complexity as its own recurring failure category, separate from Devin simply getting a task wrong, though the documented instances are new-feature and integration builds rather than diagnosed bug tickets. Below is what that category looks like in the source, why the same behavior turns up on a bug that's already fully diagnosed, and where to draw the scope line before a session starts.
What the overcomplication actually looks like
When Answer.AI ran Devin against 20 real engineering tasks spanning new builds, integrations, research, and code review, unnecessary complexity showed up as its own category of failure, separate from tasks Devin simply got wrong. On the task asking Devin to generate synthetic data and load it into Braintrust, a new-project build, the reviewer wrote that "Devin produced what can only be described as code soup - layers of abstraction that made simple operations needlessly complex." On a separate new-project task, building an integration between two applications for Spiral.computer, the reviewer called the result "really horrible spaghetti code that was way more confusing to read through than me trying to just write it from scratch." Neither task was a bug fix; both asked Devin to build something from nothing, and it still routed through more structure than the ask required.
A third example, updating an existing nbdev notebook, is smaller but shows the same reflex on a task that wasn't asking for new architecture at all. The reviewer wrote, "One curiosity that I noticed is that Devin created Python scripts to edit notebooks rather than trying to edit the notebook itself." Nothing about that task called for a script layer between Devin and the file it was supposed to change; Devin built one anyway.
None of those three tasks share a root cause, and none of them are bug fixes, but the shape repeats. A direct path existed each time, and Devin built infrastructure around it instead of taking it. Two smaller tasks from the same review show the same reflex at lower stakes. Asked to build a minimal HTMX bulk-upload example, Devin's diff, in the reviewer's words, "added many unnecessary things, like toasts (which also didn't work), and inline css styling" that the brief never called for. In a separate task asking only for a minimal DaisyUI theme, the reviewer wrote that "the theme was doing nothing. The other colors in the app were from the default theme," so even the one thing that task did ask for didn't ship working. Thoroughness, each time, moved sideways into scope nobody asked for.
“we need SSO before rollout”
“any update on single sign-on?”
“SSO came up twice on this call”
Linear issue, quotes attached
insight report in Notion
DevinThe tax rounding bug at Cobblewire
Cobblewire builds point-of-sale software for independent coffee shops and small retail counters, and Delphine Struthers runs the two-person platform team that keeps it running against a queue of support tickets from shop owners who mostly don't want to think about software at all. A ticket came in from a shop in Tacoma:
Support ticket: Split-check receipts are showing $0.01 more total tax than a single receipt for the same order. Customer noticed because she runs the register herself and checks totals by hand.
The bug was real and small. A rounding function in receipts/tax.ts rounded each split line up independently instead of rounding the order total once and dividing the tax proportionally, so two lines could each round up a cent and the split total would land a cent over the equivalent single receipt. Delphine wrote the ticket with the file, the repro (any split of an order with a tax total ending in certain fractions of a cent), and the expected behavior (split totals match the single-receipt total to the cent). It was, by the standard of a clear ticket, about as unambiguous as a bug report gets.
Devin's PR fixed it, and also introduced a new TaxAllocator class with a configurable rounding strategy, a mode parameter for three rounding modes only one of which Cobblewire used anywhere, and a 40-line test file covering all three modes. Delphine's read on it, from the PR comment she left before requesting changes:
Delphine: The fix itself is like four lines. I don't know why there's a class with a strategy pattern in here for a rounding rule we use in exactly one place. Can we just fix the function.
The scoped-down version Devin produced on the second pass was the four-line fix Delphine expected. Nothing about the ticket had been ambiguous the first time; the instruction just hadn't told Devin to stop at four lines; when a legitimate improvement was implicit in the code Devin encountered, it built the improvement in the process of fixing the bug.
Why a clear ticket doesn't rule this out
This is the part that separates overcomplication from the more familiar failure of Devin guessing wrong on a vague ticket. A vague ticket produces a wrong fix because Devin has to fill a gap with an assumption. An overcomplicated fix can happen on a ticket with no gaps at all, because Devin's default instruction pattern for a bug, as Devin's own good-vs-bad-instructions guidance models it, is built for bugs where the cause is genuinely unknown. Its own example of a well-formed bug instruction reads: "Users are reporting 500 errors on the checkout page. Use the Sentry MCP to pull the latest stack traces for the payments-api project. Check the database for any related data issues. Find the root cause, fix it, and add a regression test." That's the right instruction for a bug that could be anywhere in three systems. Applied unchanged to a bug that's already been diagnosed down to one function, "find the root cause and fix it" still tells Devin to go looking, and going looking is exactly the behavior that turned Cobblewire's rounding fix into a strategy pattern.
The instruction doesn't distinguish between "we don't know what's wrong, investigate" and "we know exactly what's wrong, apply this fix." Both get read the same way unless the ticket says otherwise.
Constraining scope before the session starts
Three changes catch most of this without adding much process:
- Say what not to touch, not just what to fix. A line like "fix only the rounding in
calculateSplitTax(); don't introduce new classes, config, or dependencies" closes the gap the default instruction pattern leaves open. This has to be written per ticket, because a ticket that's already this specific rarely needs "find the root cause" phrasing at all. - Put a standing scope rule in REVIEW.md. Devin Review checks a finished PR against instruction files in the repo, and its documentation confirms REVIEW.md is meant for exactly this: "common pitfalls or anti-patterns to watch for." A line like "flag PRs that touch more files or add more abstraction than the linked issue implies" gives the review pass something concrete to check, though it depends on someone reading the flag rather than merging past it.
- Read the diff for what's not in the ticket, not just what is. The fastest signal that a fix has drifted past scope is a new file, a new dependency, or a new parameter that isn't mentioned anywhere in the bug report. Delphine caught the
TaxAllocatorclass in about ten seconds of skimming; the habit, not the tooling, did the work.
Where the per-ticket habit stops holding
All three of those are things one person does on one ticket, and they hold up fine at Cobblewire's volume. They stop holding up because nobody writing ticket forty knows that ticket twelve already needed the same "don't introduce a class for this" caveat, so the caveat gets rewritten, or forgotten, every time a similar bug resurfaces. A REVIEW.md rule catches drift inside one PR; it has no memory of the fact that this exact category of small, well-diagnosed bug keeps arriving and keeps tempting the same overcomplication.
Modem is built for exactly that gap between what one engineer remembers and what the next brief inherits. Modem is also the company that wrote this guide; judge the fit against how often Cobblewire-sized bugs actually recur in your own queue, not against how the pitch reads here. Modem's Devin integration writes the task brief from the Modem agent, built from the customer's original report and whatever repro detail already exists in Modem's context graph, so the next engineer isn't starting from a blank repro the way a fresh ticket normally would. What it doesn't do on its own is remember Delphine's scope caveat. The "don't introduce a class for this" line still has to be written into the next brief by whoever's on call that day, the same as it does today, though it now lives on a topic with the original ticket and fix already attached instead of in one person's head. Modem's handoff does support a mid-task follow-up, so a brief that turns out too permissive can be narrowed without restarting the session. Below the point where these bugs recur, writing "fix only this function" by hand is the whole job; past it, the context that makes writing it fast needs somewhere to live besides one engineer's memory.
For the broader set of failure modes a vague or underspecified ticket produces, not just overcomplication, see how to stop Devin from inventing a fix when the ticket is vague. And for how teams route bugs to Devin in the first place, the options are compared in the 6 best ways to hand customer-reported bugs to Devin.
The one line that catches most of it
Before the next bug ticket goes to Devin, add one sentence naming the file or function the fix belongs in and stating that nothing outside it should change. Whether that sentence gets typed fresh each time or shows up because a topic already remembers the last time this bug shape came through, it's what turned Cobblewire's strategy-pattern PR back into a four-line fix. Writing it once costs less than the review comment asking for the smaller version after the fact; writing it from scratch every time a similar ticket resurfaces is the part actually worth fixing.
