Over-editing refers to a model modifying code beyond what is necessary

Code for this post is available here.

AI-assisted coding has become the norm and with tools like Cursor, GitHub Copilot, Claude Code, Codex, we are increasingly letting models touch our code. If you have used any of these tools in the past year, you have probably experienced something like this: you ask the model to fix a simple bug (perhaps a single off-by-one error, or maybe a wrong operator). The model fixes the bug but half the function has been rewritten. An extra helper function has appeared. A perfectly reasonable variable name has been renamed. New input validation has been added. And the diff is enormous.

I refer to this as the Over-Editing problem where models have the tendency to rewrite code that didn’t need rewriting. This matters more than it might seem. Code review is already a bottleneck and reviewers need to understand what changed, why it changed, and whether the change is safe. A model that rewrites entire functions, even correctly, makes this job dramatically harder as the code is now completely unrecognizable.

In this post, I will investigate this problem: whether existing LLMs have a tendency to over-edit and whether we can train models to be more faithful editors.

Over-Editing

Over-editing refers to a model modifying code beyond what is strictly necessary to fix the problem at hand. To be precise: a model is over-editing if its output is functionally correct but structurally diverges from the original code more than the minimal fix requires.

The example in Figure 1 illustrates this well. The bug is a single off-by-one error in a range() call (range(len(x) - 1) should be range(len(x))) and the correct fix is a single line. GPT-5.4 (with high reasoning effort) responds by rewriting the entire function: it adds explicit None checks, introduces np.asarray conversions with dtype=float, adds finite-value masking, validates array sizes, changes the curve_fit call signature, and replaces the plotting logic entirely. While the output passes the tests and is functionally correct, the diff is enormous, and none of those additions were asked for or even necessary.

It helps to think about this in terms of the kind of work being done. Software engineering broadly splits into two modes: green-field (building something new from scratch) and brown-field (working within an existing codebase). Specifically in brown-field, the existing code has been understood by the team and has been deliberately written the way it was. The model’s job is to fix the issue and nothing else.

A common piece of advice for working with AI coding tools is to simply write more tests because if the tests pass, the code is fine. However, Over-editing is a brown-field failure where unlike correctness failures, it is invisible to test suites. As models generate more code, engineers have more to review and over-editing makes that harder. There is more complex logic to parse, more lines of code to read, and a higher chance that overall codebase quality quietly degrades.

Measuring Over-Editing

To study over-editing, we first need a dataset of code edits where the “ground truth” edit is well-defined with some degree of “minimality”. Rather than using another LLM to introduce bugs (which is what most existing benchmarks do), we programmatically corrupt 400 problems from BigCodeBench which gives us more fine-grained control: things like flipping a comparison operator (< → <=), swapping + for -, or changing boolean values (True → False). Each corrupted sample remains syntactically valid and verified to break the corresponding test cases. This ensures that the ground truth edit is exactly the reversal of the corruption and nothing more, thus making this edit minimal by construction. We can then evaluate not just whether a model fixes the bug, but how much else it changed in the process.

Metrics

Most coding benchmarks evaluate models on correctness using some variant of Pass@1. However, Pass@1 is necessary but not sufficient. A model can score perfectly on Pass@1 while completely rewriting every function it touches. For this experiment, we need metrics that capture how much the model changed beyond what was required.

Token-level Levenshtein Distance. Unlike standard Levenshtein which counts the minimum number of character insertions, deletions, and substitutions to transform one string into another, we use a Python token-level variant. The code is first passed through Python’s tokenizer, which splits it into its atomic syntactic units (def, add, (, a, ,, b, ), :, return, a, +, b). Levenshtein is then computed over this token sequence rather than raw characters.

Added Cognitive Complexity. Cognitive Complexity measures how hard code is to understand. It penalizes nesting, recursion, mixed logical operators, and non-obvious control flow. Since all our corruptions change values rather than structure, the correct fix should always add zero Cognitive Complexity. Any increase in the model’s output was introduced unprompted and is unnecessary.

Do Models Over-Edit?

Yes, even frontier ones. Among the latest frontier models, GPT-5.4 over-edits the most. It has a high Levenshtein distance and adds significant cognitive complexity compared to models like Claude Opus 4.6, which tends to be a more faithful editor.