Cursor: the diff envelope

When you're prompting an LLM inside an IDE — Cursor, Zed, IntelliJ AI Assistant, JetBrains AI — the model isn't free to chat. It needs to produce output the IDE can parse and apply directly to your buffer. The right output shape is a unified diff wrapped in a code block.

This module's flow:

Cursor's user-message → diff round-trip:

A short system prompt + a focused user request gets you there. Here's the system prompt that makes the model behave like an IDE-native pair:

You are an expert pair programmer working inside an IDE. The user shows you code; you respond with a unified diff (--- / +++ / hunk markers) only — no prose unless explicitly asked. Always preserve unrelated lines exactly.

Now the user message is just the file and the change request:

File: src/utils/email.ts

export function isValidEmail(s: string): boolean {
  return s.includes("@") && s.includes(".");
}

Task: tighten the validation. Reject leading/trailing whitespace, require at least one char before "@", require a dot AFTER "@", and a TLD of at least 2 letters. Keep the same signature.

Claude Sonnet 4.5 returned:

--- src/utils/email.ts
+++ src/utils/email.ts
@@ -1,3 +1,11 @@
 export function isValidEmail(s: string): boolean {
-  return s.includes("@") && s.includes(".");
+  if (s !== s.trim()) return false;
+  
+  const atIndex = s.indexOf("@");
+  if (atIndex <= 0) return false;
+  
+  const domain = s.slice(atIndex + 1);
+  const lastDotIndex = domain.lastIndexOf(".");
+  if (lastDotIndex === -1) return false;
+  
+  return domain.slice(lastDotIndex + 1).length >= 2;
 }

Captured from Claude Sonnet 4.5 (claude-sonnet-4-5) on 2026-04-27. Re-runs may differ slightly.

This output is paste-ready. The IDE can apply it directly. There's no prose to strip, no explanation to ignore, no alternative implementations.

The system prompt is doing four things at once:

Phrase	Effect
"expert pair programmer"	Sets the persona toward concise senior-engineer voice
"working inside an IDE"	Frames the output for tool consumption
"unified diff only"	Locks the output format
"preserve unrelated lines exactly"	Stops the model from sneaking in formatting changes

That last constraint matters more than it looks. Without it, the model often "improves" indentation, changes single quotes to double quotes, or adds a missing trailing newline. Each of those creates noise in the diff and in the eventual git history. The constraint forces the diff to contain only your intended change.

A useful extension: ask the model to also add the test in the same diff:

Task: tighten the validation as described, AND add a unit test in src/utils/email.test.ts covering the new rules. Both changes in the same diff.

The diff envelope handles multi-file changes naturally — each file gets its own --- / +++ pair. The IDE applies all hunks together, and you get an atomic change.

When the diff comes back wrong, the fix is rarely "rerun with a better prompt." It's usually "show me what you tried, and apply the patch I describe." Cursor, Zed, and similar tools let you do this through their chat surface. The conversation pattern: model proposes diff → you apply mentally → you correct one specific line → model rewrites the diff. Three turns, one merged change.

The four IDE-style envelopes you'll meet across this module, side by side:

Next up: the SEARCH/REPLACE block format used by Aider. :::