diff --git a/packages/natives/package.json b/packages/natives/package.json
index d4db21043..97ff3e4f6 100644
--- a/packages/natives/package.json
+++ b/packages/natives/package.json
@@ -43,7 +43,8 @@
   },
   "devDependencies": {
     "@napi-rs/cli": "catalog:",
-    "@types/bun": "catalog:"
+    "@types/bun": "catalog:",
+    "diff": "catalog:"
   },
   "engines": {
     "bun": ">=1.3.14"
diff --git a/packages/natives/test/diff-parity.test.ts b/packages/natives/test/diff-parity.test.ts
new file mode 100644
index 000000000..c5c24d361
--- /dev/null
+++ b/packages/natives/test/diff-parity.test.ts
@@ -0,0 +1,121 @@
+import { describe, expect, test } from "bun:test";
+import { diffLineRuns, diffLines, diffWords, structuredPatchHunks } from "@oh-my-pi/pi-natives";
+import * as Diff from "diff";
+
+/** Normalize jsdiff change objects (added/removed may be undefined). */
+function jsChanges(changes: Diff.Change[]) {
+	return changes.map(c => ({ value: c.value, count: c.count, added: !!c.added, removed: !!c.removed }));
+}
+
+function natChanges(changes: { value: string; count: number; added: boolean; removed: boolean }[]) {
+	return changes.map(c => ({ value: c.value, count: c.count, added: c.added, removed: c.removed }));
+}
+
+function jsHunks(oldText: string, newText: string, context: number) {
+	return Diff.structuredPatch("", "", oldText, newText, "", "", { context }).hunks.map(h => ({
+		oldStart: h.oldStart,
+		oldLines: h.oldLines,
+		newStart: h.newStart,
+		newLines: h.newLines,
+		lines: h.lines,
+	}));
+}
+
+function assertParity(oldText: string, newText: string) {
+	expect(natChanges(diffLines(oldText, newText))).toEqual(jsChanges(Diff.diffLines(oldText, newText)));
+	for (const context of [0, 3, 4]) {
+		expect(structuredPatchHunks(oldText, newText, context)).toEqual(jsHunks(oldText, newText, context));
+	}
+	const jsRuns = Diff.diffArrays(oldText.split("\n"), newText.split("\n")).map(c => ({
+		count: c.value.length,
+		added: !!c.added,
+		removed: !!c.removed,
+	}));
+	expect(diffLineRuns(oldText, newText).map(c => ({ count: c.count, added: c.added, removed: c.removed }))).toEqual(
+		jsRuns,
+	);
+}
+
+/** Deterministic LCG so failures are reproducible. */
+function makeRng(seed: number) {
+	let state = seed >>> 0;
+	return () => {
+		state = (state * 1664525 + 1013904223) >>> 0;
+		return state / 0x1_0000_0000;
+	};
+}
+
+function randomText(rng: () => number, lines: number, opts: { crlf?: boolean; unicode?: boolean } = {}) {
+	const words = opts.unicode
+		? ["alpha", "béta", "γάμμα", "デルタ", "🚀rocket", "ω"]
+		: ["alpha", "beta", "gamma", "delta", "epsilon", "zeta"];
+	const eol = opts.crlf ? "\r\n" : "\n";
+	const out: string[] = [];
+	for (let i = 0; i < lines; i++) {
+		const n = 1 + Math.floor(rng() * 4);
+		const parts: string[] = [];
+		for (let j = 0; j < n; j++) parts.push(words[Math.floor(rng() * words.length)]!);
+		out.push(parts.join(" "));
+	}
+	let text = out.join(eol);
+	if (rng() > 0.5) text += eol;
+	return text;
+}
+
+function mutate(rng: () => number, text: string, density: number) {
+	const lines = text.split("\n");
+	for (let i = 0; i < lines.length; i++) {
+		const roll = rng();
+		if (roll < density / 3) lines[i] = `${lines[i]} edited`;
+		else if (roll < (density * 2) / 3) {
+			lines.splice(i, 1);
+			i--;
+		} else if (roll < density) lines.splice(i, 0, `inserted ${Math.floor(rng() * 1000)}`);
+	}
+	return lines.join("\n");
+}
+
+describe("native diff parity with jsdiff", () => {
+	test("fixed edge cases", () => {
+		assertParity("", "");
+		assertParity("", "a\nb\n");
+		assertParity("a\nb\n", "");
+		assertParity("same\ntext\n", "same\ntext\n");
+		assertParity("a\nb\nc", "a\nx\nc"); // no trailing newline
+		assertParity("a\r\nb\r\nc\r\n", "a\r\nx\r\nc\r\n"); // CRLF
+		assertParity("a\nb\n", "a\r\nb\r\n"); // mixed EOL rewrite
+		assertParity("líne ünicode 🚀\nsecond\n", "líne ünicode 🚀\nsécond\n");
+		assertParity("lone\rcarriage\n", "lone\rreturn\n"); // \r is not a line break
+	});
+
+	test("word diff parity", () => {
+		const cases: [string, string][] = [
+			["foo bar baz", "foo qux baz"],
+			["  leading space", "leading  space"],
+			["tab\tsep", "tab sep"],
+			["punct, mark!", "punct; mark?"],
+			["ünïcode wörds", "ünïcode words"],
+			["", "new words"],
+			["same same", "same same"],
+		];
+		for (const [a, b] of cases) {
+			expect(natChanges(diffWords(a, b))).toEqual(jsChanges(Diff.diffWords(a, b)));
+		}
+	});
+
+	test("seeded random documents", () => {
+		const rng = makeRng(0xc0ffee);
+		for (let round = 0; round < 30; round++) {
+			const crlf = round % 3 === 1;
+			const unicode = round % 4 === 2;
+			const base = randomText(rng, 5 + Math.floor(rng() * 120), { crlf, unicode });
+			assertParity(base, mutate(rng, base, round % 2 === 0 ? 0.01 : 0.2));
+		}
+	});
+
+	test("10k-line document", () => {
+		const rng = makeRng(0xbeef);
+		const base = randomText(rng, 10_000);
+		assertParity(base, mutate(rng, base, 0.01));
+	});
+});