You typed “clean up this section” or “make the style consistent” into Composer. git status now shows 14 files changed, 500+ lines moved, naming conventions rewritten, three tests broken. The model wasn’t being greedy — Composer defaults to treating any ambiguous instruction as “do what you think is right,” and agent mode pushes that harder.
Stopping the bleed is git checkout. Stopping the next over-edit is fixing the boundaries you hand Composer.
Common causes
1. Unbounded verbs in the prompt
“Clean up,” “refactor,” “improve,” “polish,” “modernize” — no implicit limit. The model treats them as “do everything you’d consider good,” so renames, extractions, reordered params and added comments all happen at once.
How to judge: lift the first verb in your prompt. “Clean up” is unbounded; “rename foo to bar” is not.
2. Agent mode has multi-file + shell power by default
In Agent mode Composer can chain read_file → grep → edit_file across the whole repo. You asked one thing; it “also fixed” eight more it considered related.
How to judge: Composer’s mode dropdown shows “Agent”; one reply expanded into a dozen tool calls.
3. No “do not touch” list
The model doesn’t know which files are sacred. migrations/, generated/, vendor/, schema.prisma, vendored SDK wrappers — all high-risk and silently edited if you don’t say “off limits.”
How to judge: git diff --stat and look for changes in directories you’d never have asked it to touch.
4. No commit between multiple prompts
Prompt → edits → unhappy → next prompt → more edits. After three rounds the diffs are tangled and you can’t separate what came from where.
How to judge: git log --oneline — if the last commit is from this morning or days ago, you’re in tangled territory.
5. Wrong model picked for the task
opus / gpt-5 in agent mode lean toward sweeping rewrites; sonnet / haiku stay closer to surgical patches. Using a big model for a surgical task tends to over-edit.
How to judge: look at the model name bottom-right; opus on a surgical task is a mismatch.
6. .cursorrules doesn’t enforce “minimal diff”
Without rules the model defaults to “good-engineer style” learned in training — which includes refactoring code it considers ugly. You must say “don’t refactor unless asked.”
How to judge: cat .cursorrules at repo root. No “minimal edits” / “don’t refactor” clause means you need one.
Before you start
- Identify which entry point misbehaved: Composer / Cmd+K / chat. Cmd+K is local, Composer is global; very different behavior.
- The moment you see misedits, stop prompting.
git diff > /tmp/diff.patchfirst; then decide whether to revert. - Note Cursor version and active model. The same prompt on a different model often behaves very differently.
Info to collect
- The exact prompt that triggered the misedit.
git diff --statshowing which files changed by how many lines.- Current model + mode (Ask / Edit / Agent) + whether Max mode is on.
- Contents of
.cursorrules; whether.cursorignoreexists.
Shortest fix path
In “stop bleeding first, fix workflow second” order.
Step 1: Triage and preserve evidence
git diff > /tmp/composer-misedit.patch # keep the patch as evidence
git stash # set it aside, don't drop
# Or revert outright:
git checkout HEAD -- . # discards all uncommitted changesStep 2: Cherry-pick what to keep
If 5 of 14 files are worth keeping:
git stash pop # bring the patch back
# Use Cursor source control panel to discard per-file
# Or via CLI:
git checkout HEAD -- path/to/unwanted-fileOr git add -p for interactive hunk staging.
Step 3: Rewrite the prompt with explicit scope
Swap unbounded verbs for surgical ones:
Before: refactor the auth module
After: In src/auth/login.ts only:
1. Replace the magic string "DEFAULT_TIMEOUT" with the constant from config.
2. Do NOT change function signatures.
3. Do NOT touch any other file.
4. Do NOT add or remove comments.
Output a unified diff, ≤ 30 lines.Step 4: Drop to Edit mode or Cmd+K
Composer input → mode dropdown → “Edit” instead of “Agent.” Edit mode only modifies files you @ and won’t iterate across the repo.
For surgical work, use Cmd+K (select the lines first, then trigger) — power is limited to the selection.
Step 5: Bake “stay conservative” into .cursorrules
# .cursorrules
- Make the minimum change required to satisfy the prompt.
- Do NOT refactor, rename, or reformat code that is not explicitly mentioned.
- Do NOT touch files in: migrations/, generated/, vendor/, schema.prisma.
- If the task seems to require changes outside the listed files, STOP and ask first.
- Preserve existing comments and import order unless asked to change them.Step 6: Commit before dangerous tasks
Make git commit -m "wip: before composer" a habit before any big Composer task. Reverting then costs seconds.
How to verify the fix
- Rerun the same prompt; confirm changes stay inside @ed files.
- Have a teammate open the same workspace and run the same prompt — should match.
- Run tests + lint to confirm no style or behavior drift slipped through.
If it still fails
- Reduce the prompt to its smallest: single verb + single file + single target line.
- Roll back the most recent
.cursorruleschange and confirm which rule wasn’t taking effect. - Search forum.cursor.com for “composer over-edits”; include version + model + prompt + diff.
- Grab View → Output → Cursor logs and post to Bug Reports.
Prevention
- Commit before every Composer session so you start clean.
- Standardize a prompt template with a “Constraints” section: allowed files / forbidden files / max diff lines.
- Default to Edit mode or Cmd+K; only flip to Agent when you genuinely need multi-step work.
- Lock in
.cursorrules“minimum diff” and a sacred-paths list. - Drop loud comments in critical files:
// DO NOT MODIFY without ticket #1234— models usually skip them.