Cursor can rewrite a 200-line file, hand you a tidy green-and-red diff, and leave you unsure whether the model fixed one bug or quietly re-indented everything. Difftastic, an open-source structural diff tool, closes that gap by comparing syntax trees instead of lines. This tutorial turns it into a Cursor AI edits checker: a small Python audit that reports how much of every change is real code and how much is formatting noise.
Plenty of guides already cover the steering side of the problem. One walks through tuning Cursor rules for diff versus rewrite edits, and another builds an edit tracer that separates patches from rewrites. This one adds the missing layer. Rules ask the model to behave, and line counts approximate what happened. A syntax-aware diff tells you what actually changed in the program.
You will finish with a working repository: a rule file for Cursor, a Git difftool alias, an audit script with a churn budget, a pre-commit hook, and a pull request check. Plan on about 45 minutes. Every command was written for Linux and macOS, and the Windows notes sit in the troubleshooting section.
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Why Line Diffs Hide What Cursor Really Changed
A line diff answers one question: which lines differ as text. It cannot say whether a differing line carries meaning. Change the indentation of a function from four spaces to two and Git reports every line as removed and re-added. Move a long argument list onto separate lines and the same thing happens. The reviewer sees a wall of red and green, and the one-line logic fix hides somewhere inside it.
AI-assisted editing makes this worse because a model has no attachment to the original formatting. Ask it to fix a bug and it may regenerate the whole file in its own style. Developers already sense the cost. According to the 2025 Stack Overflow Developer Survey, 84% of respondents use or plan to use AI tools, and 46% actively distrust the accuracy of those tools while 33% trust them. The survey press release adds that 45% of developers say debugging AI-generated code takes more time, and that 18% of respondents used Cursor. Reviewing a noisy diff is a large part of that debugging time.
Difftastic attacks the noise directly. It parses both versions of a file with a grammar for the language, compares the two trees, and highlights only the tokens whose meaning changed. When a file differs in whitespace or layout only, it reports that no syntactic changes exist. That single verdict, formatting-only versus real change, is what the audit script in this tutorial builds on.
| Tool | What it compares | Sees a re-indent as | Best use in this workflow |
|---|---|---|---|
git diff | Lines of text | Every line changed | Raw churn count and the starting point for everything else |
git diff -w | Lines, ignoring whitespace | No change | Measuring how much of the churn is whitespace |
git diff --color-moved | Lines, with moved blocks flagged | Every line changed | Spotting code that moved without edits |
difft (Difftastic) | Syntax trees | No syntactic changes | Deciding whether a file changed in meaning |
| Cursor review UI | Lines, per change and per file | Every line changed | Accepting or rejecting individual hunks |
Each tool answers a different question, so the audit uses three of them together. Git supplies the raw numbers, -w isolates pure whitespace, and Difftastic delivers the verdict on meaning. Cursor stays in charge of the accept and reject decisions.
How Cursor Applies Edits: Patch, Rewrite, or Both
Cursor splits an edit into two jobs. A large model decides what should change, then Cursor applies that decision to your file and shows it in the editor. The model may express its answer as a targeted patch, as a block of replacement code, or as a full copy of the file. Which one you get depends on the model, the prompt, the size of the file, and the mode you work in. You rarely see the choice, only the result.
Cursor does not document a patch-only guarantee. The release notes reviewed for this article describe agent editing, rules, browser use, pull request workflows and cloud agents, but none of them promises that the editor never regenerates a whole file. Treat any claim of guaranteed minimal diffs, including a natural-language rule, as a request the model usually honors rather than a constraint the editor enforces. The Composer setup guide for diff versus rewrite explains the settings that influence the behavior, and none of them replace measuring the outcome.
The review surfaces matter too. According to Cursor release notes, the editor reportedly offers per-change Keep and Undo plus file-level Keep File and Undo File, while the Agents Window shows a Git-style diff with file-level discard. The setting that controls inline diffs reportedly lives under Cursor Settings, then Agents, then Applying Changes. If your build places it elsewhere, search the settings panel for Inline Diffs. In either surface the diff you review is line-based, which is why a second opinion from a syntax-aware tool earns its place.
Two recent Cursor changes make an independent audit more valuable. Per the release notes, nested AGENTS.md rules now apply when an agent edits or creates files in subdirectories, so instructions reach deeper into a repository than before. At the same time, the model picker keeps growing. Claude Sonnet 5.5 and Claude Haiku 5.5 arrived as recent additions, and Cursor’s own site lists Composer 2.5, Grok 4.7 and Claude Opus 5.5 among the choices. More models mean more edit styles, and an audit gives you a neutral way to compare them.
The mechanism you rely on will keep shifting. Cursor added remote control for local agents on October 6, 2026, and announced its Agent SDK. Your audit script does not care about any of that. It looks at the files that changed in Git, so it keeps working whichever surface or model produced the edit.
Prerequisites and Versions
Everything here runs locally and costs nothing beyond what you already pay for Cursor. Check each item before you start, because version drift causes most first-run failures.
- Cursor: the current stable build for your platform. Record the version from the About dialog so you can compare results after updates.
- Git: any current release. The audit script was run against Git 2.43.0.
- Python 3: any current 3.x release. The script uses only the standard library and was run on Python 3.12.3.
- Difftastic: release 0.71.0 or newer. That is the newest tag we could confirm at publication. The executable is called
difft. - Rust toolchain (current stable): only needed if you install Difftastic with
cargo. - A Git repository with at least one commit. The tutorial builds a throwaway sample so your real code stays untouched.
- GitHub (optional) for the pull request check in Step 12. Any CI system works if you can run a Python script and install a binary.
You do not need an API key, a paid plan or a special Cursor setting. On October 12, 2026, Cursor became part of The X Subscription and existing subscribers could keep their plans or switch, according to a pricing breakdown from MegaBrain. The workflow below behaves the same on any plan, because it audits files rather than calling a model. Browse the software category for more editor and tooling guides.
Steps 1-5: Install Difftastic and Wire It Into Git
Step 1: Confirm Git and Python. Open a terminal. Cursor’s integrated terminal works fine, and using it keeps the whole loop inside one window.
git --version
python3 --versionYou should see version strings for both. If Python reports a 2.x version or is missing, install Python 3 before going further.
Step 2: Install Difftastic. Homebrew and several other package managers carry it, and building from source with Cargo works on any platform with a Rust toolchain. Linux users can also follow Digitash’s guide to installing and using Difftastic on Linux for a second walkthrough.
# macOS or Linux with Homebrew
brew install difftastic
# any platform with a Rust toolchain
cargo install --locked difftasticCargo compiles the tool, so expect a few minutes on a laptop. Make sure ~/.cargo/bin is on your PATH afterwards.
Step 3: Verify the binary and watch it ignore formatting. Check the version first, then compare two small files that differ only in layout.
difft --version
printf 'function add(a, b) {\n return a + b;\n}\n' > a.js
printf 'function add(a,\n b) {\n return a + b;\n}\n' > b.js
diff -u a.js b.js
difft a.js b.jsThe classic diff -u prints a hunk that removes and adds lines, because the text changed. Difftastic prints No syntactic changes. because both files parse to the same tree. That message is the signal the audit script reads, so seeing it here confirms your install can do the job. Now change a + b to a - b in b.js and run difft again. This time it highlights only the operator.
Step 4: Register Difftastic as a Git difftool. A difftool runs only when you ask for it, so your normal git diff stays unchanged. That separation matters, because several editor features and scripts parse the standard diff format.
git config --global diff.tool difftastic
git config --global difftool.difftastic.cmd 'difft "$LOCAL" "$REMOTE"'
git config --global difftool.prompt false
git config --global pager.difftool true
git config --global alias.dft difftoolAfter this, git dft shows a syntax-aware diff of your working tree against the index, and git dft HEAD~1 compares against the previous commit. The pager setting keeps long output scrollable, and the prompt setting stops Git from asking permission for each file.
Step 5: Add a one-off external diff for logs and patches. You sometimes want Difftastic inside git log -p or git show. Git supports that through an environment variable, so you do not need to change any permanent configuration.
GIT_EXTERNAL_DIFF=difft git log -p --ext-diff -n 3
GIT_EXTERNAL_DIFF=difft git show --ext-diff HEADLine-ending churn is the one source of noise that neither Git nor Difftastic fixes for you. If a Windows machine and a Linux machine touch the same repository, a file can flip between CRLF and LF and show as fully rewritten. The guide to gitattributes for Cursor diffs covers the normalization that prevents it, and you should read it before trusting any churn number from a mixed-platform team.
Steps 6-7: Build a Sample Repo and Capture a Baseline
Step 6: Create a throwaway repository. Testing on a scratch project lets you break things without risk, and it gives you files with known properties. The generator below writes three TypeScript files of roughly 30 lines each. You will edit them by hand and later let Cursor edit them, so the audit has something to measure.
mkdir diff-audit-lab && cd diff-audit-lab
git init
mkdir -p src scripts .cursor/rules .githooks
python3 - <<'EOF'
cart = ["export function total(items: Item[]): number {"]
for i in range(30):
cart.append(" const v%d = items[%d] ? items[%d].price * items[%d].qty : 0;" % (i, i, i, i))
cart += [" return v0 + v1;", "}"]
open("src/cart.ts", "w").write("\n".join(cart) + "\n")
pricing = ["export function discount(p: number): number {"]
for i in range(30):
pricing.append(" if (p > %d) { p = p - %d; }" % (i * 10, i))
pricing += [" return p;", "}"]
open("src/pricing.ts", "w").write("\n".join(pricing) + "\n")
rates = ["export const rates = {"] + [" zone%d: %d," % (i, i * 3) for i in range(25)] + ["};"]
open("src/shipping.ts", "w").write("\n".join(rates) + "\n")
EOF
git add . && git commit -m "baseline"The code is deliberately plain. Each file uses four-space indentation and one statement per line, which makes a re-indent easy to produce and easy to recognize.
Step 7: Reproduce the problem by hand. Before any AI gets involved, simulate the exact failure you want to catch. Re-indent src/cart.ts from four spaces to two and compare the three views of the same change.
sed -i 's/^ / /' src/cart.ts # on macOS use: sed -i '' 's/^ / /' src/cart.ts
git diff --stat
git diff -w --stat
git dft --statOn the sample repository the first command reports the damage: every content line in the file counts as changed.
src/cart.ts | 62 ++++++++++++++++++++++++++++++-------------------------------
1 file changed, 31 insertions(+), 31 deletions(-)The second command prints nothing, because ignoring whitespace erases the entire change. The Difftastic view agrees and reports no syntactic changes. Here lies the whole case for this tutorial in one experiment. Git counts 62 touched lines, a reviewer would see 31 removals and 31 additions, and the program did not change at all. Restore the file with git checkout src/cart.ts before you continue.
Step 8: Write the Diff Audit Script
The audit script answers four questions about any change set. How many lines moved in total? How many of those lines sit in files where nothing meaningful changed? Did any single file lose most of its original lines? And did the change touch more files than the task warranted? Earlier posts in this cluster counted lines only, for example the Composer rewrite audit tool. This version adds the syntax verdict, which separates a formatting sweep from a real refactor.
Save the following as scripts/diffaudit.py. It has no dependencies beyond Git, Python 3 and the difft binary.
#!/usr/bin/env python3
"""diffaudit: compare line churn with syntax-level change for an AI-edited change set."""
import argparse
import fnmatch
import json
import os
import shutil
import subprocess
import sys
import tempfile
from pathlib import Path
def git(*args, binary=False, check=True):
result = subprocess.run(["git", *args], capture_output=True)
if check and result.returncode != 0:
sys.exit("git %s failed: %s" % (" ".join(args), result.stderr.decode().strip()))
return result.stdout if binary else result.stdout.decode("utf-8", "replace")
def numstat(mode, base, head, ignore_ws=False):
args = ["diff", "--numstat", "--no-renames", "-z"]
if ignore_ws:
args.append("-w")
if mode == "staged":
args.append("--cached")
args.append(base)
if mode == "range":
args.append(head)
stats = {}
for record in git(*args).split("\0"):
if not record:
continue
added, deleted, path = record.split("\t", 2)
if added == "-":
stats[path] = None
else:
stats[path] = (int(added), int(deleted))
return stats
def read_old(base, path):
result = subprocess.run(["git", "show", "%s:%s" % (base, path)], capture_output=True)
return result.stdout if result.returncode == 0 else None
def read_new(mode, head, path):
if mode == "worktree":
target = Path(path)
return target.read_bytes() if target.is_file() else None
ref = ":" + path if mode == "staged" else "%s:%s" % (head, path)
result = subprocess.run(["git", "show", ref], capture_output=True)
return result.stdout if result.returncode == 0 else None
def syntax_verdict(difft, old, new, workdir, path):
old_file = Path(workdir, "old", path)
new_file = Path(workdir, "new", path)
for target, data in ((old_file, old), (new_file, new)):
target.parent.mkdir(parents=True, exist_ok=True)
target.write_bytes(data)
result = subprocess.run(
[difft, "--exit-code", "--color", "never", "--display", "inline", str(old_file), str(new_file)],
capture_output=True,
text=True,
)
text = result.stdout + result.stderr
if "No syntactic changes" in text:
return "formatting"
return "syntactic"
def audit(args):
stats = numstat(args.mode, args.base, args.head)
stats_w = numstat(args.mode, args.base, args.head, ignore_ws=True)
difft = None if args.no_syntax else shutil.which(os.environ.get("DIFFT", "difft"))
if not args.no_syntax and difft is None:
sys.exit("difft not found on PATH. Install Difftastic or pass --no-syntax.")
rows = []
with tempfile.TemporaryDirectory() as workdir:
for path, counts in sorted(stats.items()):
if any(fnmatch.fnmatch(path, glob) for glob in args.exclude):
continue
if counts is None:
rows.append({"path": path, "kind": "binary", "added": 0, "deleted": 0,
"removed_share": 0.0, "ws_churn": 0, "suspect": False})
continue
added, deleted = counts
ws_added, ws_deleted = stats_w.get(path, (0, 0))
ws_churn = (added + deleted) - (ws_added + ws_deleted)
old = read_old(args.base, path)
new = read_new(args.mode, args.head, path)
if old is None:
kind = "added"
elif new is None:
kind = "deleted"
elif difft:
kind = syntax_verdict(difft, old, new, workdir, path)
else:
kind = "unchecked"
old_lines = len(old.splitlines()) if old else 0
share = deleted / old_lines if old_lines else 0.0
suspect = old_lines >= args.min_lines and share >= args.rewrite_share
rows.append({"path": path, "kind": kind, "added": added, "deleted": deleted,
"removed_share": round(share, 2), "ws_churn": ws_churn, "suspect": suspect})
churn = sum(r["added"] + r["deleted"] for r in rows)
noise = sum(r["added"] + r["deleted"] for r in rows if r["kind"] == "formatting")
noise_ratio = round(noise / churn, 2) if churn else 0.0
problems = []
if churn > args.max_churn:
problems.append("churn %d exceeds budget %d" % (churn, args.max_churn))
if len(rows) > args.max_files:
problems.append("%d files touched, budget is %d" % (len(rows), args.max_files))
if noise_ratio > args.max_noise:
problems.append("formatting-only churn is %d%% of the change, budget is %d%%"
% (round(noise_ratio * 100), round(args.max_noise * 100)))
for row in rows:
if row["suspect"]:
problems.append("%s removed %d%% of its original lines" % (row["path"], round(row["removed_share"] * 100)))
return {"rows": rows, "churn": churn, "noise": noise, "noise_ratio": noise_ratio, "problems": problems}
def render(report, label):
rows = report["rows"]
width = max([len(r["path"]) for r in rows] + [4])
lines = ["diffaudit: %s" % label,
"%-*s %-10s %9s %7s %6s" % (width, "file", "kind", "+/-", "removed", "ws")]
for r in rows:
flag = " <-- rewrite?" if r["suspect"] else ""
lines.append("%-*s %-10s %4d/%-4d %6d%% %6d%s" % (
width, r["path"], r["kind"], r["added"], r["deleted"],
round(r["removed_share"] * 100), r["ws_churn"], flag))
lines.append("total churn: %d lines, formatting-only: %d (%d%%)" % (
report["churn"], report["noise"], round(report["noise_ratio"] * 100)))
if report["problems"]:
lines.append("FAIL")
lines.extend(" - " + p for p in report["problems"])
else:
lines.append("PASS")
return "\n".join(lines)
def main():
parser = argparse.ArgumentParser(description=__doc__)
parser.add_argument("--staged", action="store_true", help="audit the index against --base")
parser.add_argument("--range", nargs=2, metavar=("BASE", "HEAD"), help="audit two refs")
parser.add_argument("--base", default="HEAD")
parser.add_argument("--max-churn", type=int, default=400)
parser.add_argument("--max-files", type=int, default=12)
parser.add_argument("--max-noise", type=float, default=0.25)
parser.add_argument("--rewrite-share", type=float, default=0.5)
parser.add_argument("--min-lines", type=int, default=20)
parser.add_argument("--exclude", action="append", default=[], metavar="GLOB", help="skip matching paths")
parser.add_argument("--no-syntax", action="store_true", help="skip Difftastic, line stats only")
parser.add_argument("--json", action="store_true")
args = parser.parse_args()
args.head = None
if args.range:
args.mode, args.base, args.head = "range", args.range[0], args.range[1]
label = "%s..%s" % (args.base, args.head)
elif args.staged:
args.mode, label = "staged", "index vs %s" % args.base
else:
args.mode, label = "worktree", "working tree vs %s" % args.base
report = audit(args)
print(json.dumps(report, indent=2) if args.json else render(report, label))
sys.exit(1 if report["problems"] else 0)
if __name__ == "__main__":
main()A few design choices deserve a closer look, because they decide whether the numbers can be trusted.
- It reads Git, not the editor. The script compares a base revision with the working tree, the index or a second revision. Nothing depends on how Cursor produced the edit.
- It disables rename detection.
--no-renamesmakes a moved file appear as a deletion plus an addition. That is noisy but honest, and it keeps the parsing of-zoutput simple. - It writes temporary copies that keep the original path. Difftastic picks a grammar from the file extension, so the temp files must end in
.ts,.pyor whatever the source used. - It merges stdout and stderr before searching for the verdict. The wording
No syntactic changesis the contract. If a future release rewords it, one string insyntax_verdictneeds an update. - It measures the share of original lines removed. A file that loses 90% of its lines while the task asked for a small fix is a rewrite candidate, whatever the verdict says.
- It exits with code 1 on any violation. That makes the same command usable by hand, in a Git hook and in CI.
| Flag | Default | What it controls |
|---|---|---|
--staged | off | Audit the index against the base instead of the working tree |
--range BASE HEAD | none | Audit two revisions, used by CI |
--base REF | HEAD | Revision to compare against |
--max-churn N | 400 | Fail when added plus deleted lines exceed N |
--max-files N | 12 | Fail when more than N files changed |
--max-noise R | 0.25 | Fail when formatting-only churn exceeds this share of the total |
--rewrite-share R | 0.5 | Flag files that lost at least this share of their original lines |
--min-lines N | 20 | Ignore the rewrite check for files shorter than N lines |
--exclude GLOB | none | Skip matching paths such as lockfiles, repeatable |
--no-syntax | off | Skip Difftastic and report line statistics only |
--json | off | Print the report as JSON for logging or comparison |
The defaults are starting points and nothing more. Step 13 explains how to tune them for your repository. Run the script now against your hand-made re-indent to see the failure mode.
python3 scripts/diffaudit.pyWith src/cart.ts re-indented and nothing else changed, the report flags the file as formatting-only and fails the run:
diffaudit: working tree vs HEAD
file kind +/- removed ws
src/cart.ts formatting 31/31 94% 62
total churn: 62 lines, formatting-only: 62 (100%)
FAIL
- formatting-only churn is 100% of the change, budget is 25%
- src/cart.ts removed 94% of its original linesThe ws column shows how many of those lines disappear when whitespace is ignored. Here it equals the whole churn, which confirms a pure layout change. Restore the file again with git checkout src/cart.ts.
Step 9: Add a Cursor Rule That Asks for Small Diffs
A rule cannot enforce anything, but a good one shifts the odds. Cursor reads project rules from .cursor/rules, and each rule file can declare a description and whether it always applies. Create .cursor/rules/small-diffs.mdc with the following content.
---
description: Keep every edit as small as the request allows
alwaysApply: true
---
- Change only the lines the request needs. Leave the rest of each file as it is.
- Never reformat, re-indent, reorder imports or rename symbols unless the request says so.
- Do not rewrite a whole file. If more than a third of a file would change, stop and ask first.
- Keep the existing quote style, trailing commas, line endings and indentation.
- When you finish, list each file you touched with a one-line reason.
- Run `python3 scripts/diffaudit.py` and fix any FAIL it reports before you stop.Each line targets a specific failure. The first two cover the habit of tidying code nobody asked about. The third gives the model an explicit threshold and a way out, which works better than a flat ban because the model can still ask when a large change is truly needed. The last line closes the loop, so an agent that can run terminal commands checks its own work. The tutorial on linting AI edits for rewrite damage pairs well with this rule if you want style checks on top of the audit.
Cursor also reads AGENTS.md files, and according to its release notes nested ones now apply when an agent edits files in a subdirectory. A short root file duplicates the rule for agents that read it first.
# AGENTS.md
## Editing policy
- Prefer targeted edits over regenerating files.
- Do not change formatting outside the lines you edit.
- Before finishing, run `python3 scripts/diffaudit.py` and report its summary line.Keep the two files consistent, because contradictory instructions produce unpredictable edits. Commit both, since a rule that lives only on your machine protects only you.
Step 10: Run a Real Cursor Edit and Read the Report
Commit the rule files and the script first so the base revision includes them. Then open the repository in Cursor, choose the model you normally use, and run two prompts. The first should pass. The second invites a rewrite, and you want to watch the audit catch it.
git add .cursor AGENTS.md scripts
git commit -m "add small-diff rule and diff audit"A prompt that should pass
Ask for a narrow, well-located change: In src/pricing.ts, change the discount for prices above 50 from 5 to 6. A well-behaved edit touches one line. Run the audit after you accept the change.
diffaudit: working tree vs HEAD
file kind +/- removed ws
src/pricing.ts syntactic 1/1 3% 0
total churn: 2 lines, formatting-only: 0 (0%)
PASSOne file, two churn lines (one removed, one added), and a syntactic verdict. That is the shape of a healthy AI edit. Everything else in the file survived untouched.
A prompt that invites a rewrite
Now ask for something vague: Clean up src/cart.ts and src/shipping.ts. Vague verbs like clean up, modernize and refactor give the model permission to regenerate. If it does, the audit looks like the report below. Your numbers will differ, since they depend on the model and on how each file is formatted.
diffaudit: working tree vs HEAD
file kind +/- removed ws
src/cart.ts formatting 31/31 94% 62 <-- rewrite?
src/pricing.ts syntactic 1/1 3% 0
src/shipping.ts syntactic 26/26 96% 0 <-- rewrite?
total churn: 116 lines, formatting-only: 62 (53%)
FAIL
- formatting-only churn is 53% of the change, budget is 25%
- src/cart.ts removed 94% of its original lines
- src/shipping.ts removed 96% of its original linesRead it row by row. src/cart.ts is a formatting-only rewrite, so you can reject it without reading a single line of code. src/shipping.ts is different. Almost every line changed, and the syntactic verdict says some of it matters, so it deserves a careful look. Run git dft src/shipping.ts to see which tokens changed, then keep the meaningful hunks and undo the rest with Cursor’s Undo controls or git restore -p.
The decision tree is short. A formatting verdict means reject the file. A syntactic verdict with a high removed share means inspect the tokens. A syntactic verdict with a low removed share means review normally. Over a few days of real use you will notice which prompts trigger the first branch, and you can rewrite them before sending.
Steps 11-12: Enforce the Budget With a Pre-Commit Hook and CI
Step 11: Add a pre-commit hook. The rule asks, the script measures, and the hook decides. A local hook catches oversized AI edits before they enter your history. Store it in the repository so teammates get it too, and point Git at the folder.
cat > .githooks/pre-commit <<'EOF'
#!/bin/sh
if ! python3 scripts/diffaudit.py --staged $DIFFAUDIT_ARGS; then
echo "diffaudit failed. Shrink the change, or raise the budget for this commit:"
echo " DIFFAUDIT_ARGS='--max-churn 2000' git commit"
exit 1
fi
EOF
chmod +x .githooks/pre-commit
git config core.hooksPath .githooksThe DIFFAUDIT_ARGS variable gives you a deliberate escape hatch for legitimate large changes such as a planned migration. That beats a blanket bypass, because the exception is explicit and visible in your shell history. Test the hook by staging the re-indented cart.ts from Step 7 and trying to commit. The commit should stop with the audit report on screen.
Hooks only protect the machine they run on, and core.hooksPath is local configuration. A teammate who clones the repository still has to run the git config line once. A short note in the README or a setup script handles that.
Step 12: Add a pull request check. CI is the backstop for everyone who skipped the hook. The workflow below installs Difftastic with Cargo, finds the merge base with the target branch, and audits the whole pull request as a single change set.
# .github/workflows/diff-audit.yml
name: diff-audit
on:
pull_request:
jobs:
audit:
runs-on: ubuntu-latest
steps:
- uses: actions/checkout@v4
with:
fetch-depth: 0
- uses: dtolnay/rust-toolchain@stable
- uses: Swatinem/rust-cache@v2
- name: Install Difftastic
run: cargo install --locked difftastic
- name: Audit the pull request
run: |
BASE=$(git merge-base "origin/${{ github.base_ref }}" HEAD)
python3 scripts/diffaudit.py --range "$BASE" HEAD --exclude "*.lock" --exclude "package-lock.json"Two details trip people up here. The default checkout is shallow, so without fetch-depth: 0 the merge-base lookup fails with a bad-revision error. And the action versions shown are examples, so pin whichever releases your organization has approved. With the rust cache in place, the Cargo install stays quick after the first run.
A failing check does not have to block merges. Many teams start by letting it fail loudly while still allowing a maintainer override, then switch to a required check once the thresholds feel fair. Either way, the failure message names the file and the reason, so the author knows what to fix without opening the logs.
Step 13: Tune Thresholds Per Repository and Model
Defaults that suit a small service will annoy a monorepo team, and the reverse is also true. Measure your own history before you set a budget. The loop below audits the last 30 non-merge commits with every limit switched off, then prints churn and file count per commit.
for c in $(git rev-list --no-merges -n 30 HEAD); do
git rev-parse --verify -q "$c~1" >/dev/null || continue
printf '%s ' "$(git rev-parse --short "$c")"
python3 scripts/diffaudit.py --range "$c~1" "$c" --json \
--max-churn 100000 --max-files 1000 --max-noise 1 --rewrite-share 2 \
| python3 -c 'import sys, json; d = json.load(sys.stdin); print(d["churn"], len(d["rows"]))'
doneSort the output and look at the upper range. A reasonable starting budget sits slightly above what your careful human commits already reach, so legitimate work passes and outliers stand out. If your typical feature commit runs a couple of hundred lines, a 400-line ceiling is fair. If your team commits large generated updates, raise the ceiling and exclude those paths instead.
Model choice is the second tuning axis. Different models express edits differently, and the picker now offers many, including Claude Sonnet 5.5 (which Anthropic’s release notes place in late September 2026), Composer 2.5 and Grok 4.7. Run one identical prompt per model on its own branch and save the JSON report. Reject any tidy theory about which model rewrites least, since the answer shifts with the prompt and the language. Your repository will show you.
for model in sonnet-5.5 composer-2.5 grok-4.7; do
git switch -c "audit/$model" main
echo "Run the same prompt in Cursor with $model, accept the edits, then press Enter"
read _
mkdir -p audit
python3 scripts/diffaudit.py --json > "audit/$model.json"
git restore --source=main --staged --worktree .
git switch main
doneCompare the three JSON files on churn, noise_ratio and the number of rows. The restore step discards the experimental edits, so run this only on a scratch branch or a copy of the repository. The reports sit in an untracked audit folder and survive the branch switches.
Price is the third axis. Third-party models bill separately on the new plans, and a model that regenerates whole files burns more output tokens than one that patches a few lines. A cheaper model that stays inside your diff budget is often the better deal than a stronger model that rewrites everything and sends you into review.
Common Pitfalls That Skew the Numbers
The audit is simple, which means a handful of setup mistakes can quietly distort it. These are the ones that cost the most time.
Trusting the Cursor UI line counts. The review panel counts changed lines, and a re-indent inflates that figure exactly as Git does. Use the panel to accept or reject hunks, and use the audit to judge the size and kind of the change. The Fast Apply safety net guide shows how to catch bad rewrites during review itself.
Forgetting untracked files. git diff ignores files Git has never seen. If Cursor creates a new module, the audit will not count it until you run git add -N path to record intent. A change set that looks small may hide a pile of new files.
Mixed line endings. A file that flips between CRLF and LF shows as fully rewritten, and the syntax verdict will correctly call it formatting-only. That is a true finding, but the cure is repository configuration, not a stricter rule for the model.
Counting lockfiles and generated code. A single dependency bump can add thousands of lines to a lockfile and drown the signal. Exclude those paths with --exclude and keep the budget focused on code people write and review.
Letting format-on-save run. If your editor formats files when Cursor writes them, a perfectly minimal AI edit turns into a repo-wide style change. The audit will blame the model for what the formatter did. Check your formatter settings before you tune any prompt.
Copying thresholds from a blog post. The defaults in this tutorial are placeholders. A repository of 30-line scripts and a repository of 3,000-line classes need different budgets. Measure first, as Step 13 shows.
Treating a pass as proof of correctness. A small, syntactic diff can still be wrong. The audit tells you the edit was focused. It says nothing about whether the logic is right, so keep your tests and your review habits.
Troubleshooting: 10 Problems and Fixes
| Symptom | Likely cause | Fix |
|---|---|---|
difft not found on PATH | Binary not installed, or ~/.cargo/bin missing from PATH | Reinstall in Step 2, open a new shell, or set DIFFT=/full/path/to/difft |
Every file reports syntactic, even pure re-indents | Language not supported, or the extension is unknown, so Difftastic falls back to a text diff | Run difft --list-languages and confirm your extension appears |
Every file reports formatting | The verdict string changed, or difft is a wrapper that prints other text | Run difft a.js b.js by hand and update the string in syntax_verdict |
git dft asks “Viewing 1/3” for each file | Difftool prompt still enabled | Run git config --global difftool.prompt false |
| Hook never runs | The hook is not executable, or core.hooksPath is unset in this clone | Run chmod +x .githooks/pre-commit and set core.hooksPath again |
| CI fails with a bad revision error | Shallow checkout has no merge base | Set fetch-depth: 0 on the checkout step |
| New files missing from the report | Untracked files are invisible to git diff | Run git add -N . before the audit |
| Report flags a tiny file as a rewrite | Small files lose a large share with one edit | Raise --min-lines so short files skip the rewrite check |
| Script errors on Windows | The python3 launcher is missing, or paths use backslashes | Use py -3 scripts/diffaudit.py and run it from Git Bash or WSL |
| Cursor ignores the rule | Rules are advisory, the file is outside .cursor/rules, or the front matter is malformed | Check the .mdc header, restate the rule in the prompt, and rely on the hook for enforcement |
If a symptom is not in the table, start with the smallest possible reproduction. Run git diff --numstat to confirm what Git sees, then run difft on the two versions of one file by hand. The script only glues those two observations together, so one of them will point at the cause.
Advanced Tips for Heavier Cursor Workflows
Format only what you edit. VS Code-based editors support a format-on-save mode that touches only modified lines. Setting "editor.formatOnSaveMode": "modifications" keeps a formatter from sweeping untouched code, and it needs a Git repository to know what changed. Because Cursor builds on the VS Code editor, the setting is worth trying, and the audit will show whether it helped.
Compare models with worktrees. The branch loop in Step 13 works but forces you to switch contexts. git worktree add ../lab-sonnet main gives each model its own directory, so you can open several Cursor windows side by side and audit them in parallel.
Script the loop with headless runs. Cursor offers a command-line agent, and its changelog reportedly describes cleaner edit display and stricter approval for risky recursive or wildcard deletes in the 2026.10.08 release. If you drive the agent from a script, run the audit as the next command and fail the job on exit code 1. A benchmark of Cursor CLI diff versus rewrite edits shows one way to structure repeated runs.
Log reports as JSON lines. Append the output of --json to a file after every accepted AI edit, along with the model name. After a week you can plot churn and noise ratio by model and by prompt style, and your thresholds stop being guesses.
Look at moved code separately. When a file shows high churn with a syntactic verdict, run git diff --color-moved=dimmed-zebra. Blocks that moved without edits are dimmed, which separates reorganization from new logic. Trying --histogram or --patience can also produce a cleaner line diff for files with repeated structure.
Review hunks on purpose. After a borderline audit, use git add -p to stage only the hunks you want and git restore -p to throw away the rest. Cursor’s own Keep and Undo controls do the same job inside the editor, and the terminal route gives you a second pair of eyes with a different view of the diff.
Build your own patch applier. If you want to understand why edits arrive as patches or rewrites at all, the walkthrough on building a fast-apply patch engine explains the trade-offs from the implementer’s side.
Watch the ecosystem. Tools around Difftastic keep growing. The GitComet Git client merged optional Difftastic-based comparisons on October 19, 2026, with changed-token highlighting and a fallback to line comparison. Your audit will likely get friendlier front ends over time, and the verdict logic stays the same.
What the Finished Project Looks Like
When every step is done, the repository contains a small, self-explanatory set of files. Each one has a single job.
diff-audit-lab/
.cursor/rules/small-diffs.mdc asks the model for minimal edits
AGENTS.md repeats the policy for agents
scripts/diffaudit.py measures churn and syntax-level change
.githooks/pre-commit blocks oversized staged changes
.github/workflows/diff-audit.yml audits whole pull requests
src/ sample code used in the experimentsThe workflow runs in a loop. You prompt Cursor with a narrow request, review the hunks in the editor, and run the audit. If it passes, you commit. If it fails, the report tells you whether to reject a formatting sweep or inspect a heavy rewrite. The hook and the CI check repeat that decision automatically for anything that slips through.
Rules, measurement and enforcement each cover a gap the others leave open. The rule improves your odds, the audit gives you facts, and the hook and the pipeline make the facts matter.
FAQ: Difftastic and Cursor AI Edits
Does Cursor edit files with diffs or full rewrites?
It depends on the model, the prompt and the file, and Cursor does not document a patch-only guarantee. The editor shows you a line-based diff either way, so the review screen alone cannot tell you whether the model rewrote the file. Measuring the Git change with a script, as in this tutorial, is the reliable way to find out.
Can a Cursor rule force small diffs?
No. A rule is an instruction the model usually follows, and no official Cursor source confirms a rule syntax that guarantees minimal diffs. Use the rule to improve the odds, then use the audit, the hook and CI to verify the result.
Is Difftastic part of Cursor or an AI tool?
No. Difftastic is a standalone open-source structural diff utility with the command-line executable difft. No official Cursor integration or partnership has been confirmed, which is why this tutorial connects the two through Git instead.
Which languages does Difftastic support?
It supports many languages through tree-sitter grammars, and the exact list changes between releases. Run difft --list-languages to see what your install supports. For an unsupported file type it falls back to a plain line diff, and the audit then reports that file as syntactic.
How do I audit an AI session I already committed?
Use the range mode. Run python3 scripts/diffaudit.py --range BASE HEAD with the commit before the session as BASE. The script compares the two revisions and applies the same budget and rewrite checks.
Will the pre-commit hook slow my commits?
The cost grows with the number of changed files, because Difftastic runs once per changed file. A normal AI edit touching a few files finishes quickly. For a huge change, raise the budget for that commit with DIFFAUDIT_ARGS or run the audit in CI only.
Does this work with other AI editors?
Yes. The script reads Git history, the index and the working tree, so it audits changes from any editor, agent or human. Only the rule file is specific to Cursor.
Which model rewrites files the least?
No universal answer exists. Results shift with the prompt, the language and the size of the file. Run the same prompt across the models in your picker, save the JSON reports, and compare churn and noise ratio on your own code.
