diff --git a/packages/cli/src/commands/history.test.ts b/packages/cli/src/commands/history.test.ts
index c6edea726b..a4baaa61f0 100644
--- a/packages/cli/src/commands/history.test.ts
+++ b/packages/cli/src/commands/history.test.ts
@@ -1,5 +1,5 @@
 // @vitest-environment node
-import { spawn, type SpawnSyncOptionsWithStringEncoding } from "node:child_process";
+import { spawn } from "node:child_process";
 import {
   chmodSync,
   existsSync,
@@ -36,33 +36,6 @@ const clock = { at: 0 };
 const advance = (ms: number) => void (clock.at += ms);
 
 const tracked = vi.hoisted(() => [] as Array<{ action: string; via: string }>);
-const outputRace = vi.hoisted(() => ({ enabled: false, replacements: 0 }));
-vi.mock("node:child_process", async (importOriginal) => {
-  const original = await importOriginal<typeof import("node:child_process")>();
-  const { renameSync, writeFileSync } = await import("node:fs");
-  const { join } = await import("node:path");
-  return {
-    ...original,
-    spawnSync(
-      command: string,
-      args: readonly string[],
-      options: SpawnSyncOptionsWithStringEncoding,
-    ) {
-      const result = original.spawnSync(command, args, options);
-      if (
-        outputRace.enabled &&
-        command === "git" &&
-        args[0] === "diff" &&
-        typeof options.cwd === "string"
-      ) {
-        renameSync(join(options.cwd, "diff"), join(options.cwd, "written-diff"));
-        writeFileSync(join(options.cwd, "diff"), "replacement-file-content");
-        outputRace.replacements++;
-      }
-      return result;
-    },
-  };
-});
 vi.mock("../telemetry/events.js", () => ({
   trackHistoryAction: (props: { action: string; via: string }) => tracked.push(props),
 }));
@@ -304,23 +277,6 @@ describe.each(["direct", "preview"])("hyperframes history (%s)", (mode) => {
     expect(diff).toContain(`-${before}+${after.trimEnd()}`);
   });
 
-  it("reads the written diff when its pathname is replaced after Git exits", async () => {
-    const { hf, turn } = await setup();
-    const entry = await turn("claude", "Retitle", "index.html", "A2");
-    outputRace.enabled = true;
-    outputRace.replacements = 0;
-    try {
-      const result = await hf("show", entry.id, "--diff", "--json");
-
-      expect(outputRace.replacements).toBe(1);
-      expect(result.code, result.err).toBe(0);
-      expect(JSON.parse(result.out).diff).toMatch(/-A\n\\ No newline at end of file\n\+A2/);
-      expect(result.out).not.toContain("replacement-file-content");
-    } finally {
-      outputRace.enabled = false;
-    }
-  });
-
   it("peek reads a file as it was without writing, and restore puts every file back", async () => {
     const { read, hf, turn } = await setup();
     const entry = await turn("claude", "Retitle", "index.html", "A2");
diff --git a/packages/cli/src/commands/history.ts b/packages/cli/src/commands/history.ts
index 0dc5d03e4c..e157688e3d 100644
--- a/packages/cli/src/commands/history.ts
+++ b/packages/cli/src/commands/history.ts
@@ -1,16 +1,3 @@
-import { spawnSync } from "node:child_process";
-import {
-  closeSync,
-  fstatSync,
-  mkdirSync,
-  mkdtempSync,
-  openSync,
-  readSync,
-  rmSync,
-  writeFileSync,
-} from "node:fs";
-import { tmpdir } from "node:os";
-import { dirname, join } from "node:path";
 import { defineCommand, type ArgsDef } from "citty";
 import {
   HISTORY_START,
@@ -25,6 +12,7 @@ import {
 import type { Example } from "./_examples.js";
 import { trackHistoryAction } from "../telemetry/events.js";
 import { setCommandExitCode } from "../utils/commandResult.js";
+import { unifiedDiff } from "../utils/unifiedDiff.js";
 import {
   Refusal,
   endTurn,
@@ -204,48 +192,18 @@ function print(json: boolean, data: object, text: string): void {
 }
 
 async function textDiff(reader: Reader, entry: HistoryEntry): Promise<string> {
-  const work = mkdtempSync(join(tmpdir(), "hf-history-diff-"));
-  try {
-    const out: string[] = [];
-    for (const file of entry.files) {
-      const sides = await Promise.all(
-        [file.before, file.after].map((hash) => (hash ? reader.blob(hash) : Buffer.alloc(0))),
-      );
-      if (sides.some((bytes) => bytes.subarray(0, 8000).includes(0))) {
-        out.push(`Binary ${file.path} changed`);
-        continue;
-      }
-      const [a, b] = ["a", "b"].map((side, index) => {
-        const path = join(side, file.path);
-        mkdirSync(dirname(join(work, path)), { recursive: true });
-        writeFileSync(join(work, path), sides[index]!);
-        return path;
-      });
-      const outputPath = join(work, "diff");
-      const outputFd = openSync(outputPath, "w+", 0o600);
-      try {
-        const diff = spawnSync("git", ["diff", "--no-index", "--no-color", "--no-prefix", a!, b!], {
-          cwd: work,
-          encoding: "utf-8",
-          stdio: ["ignore", outputFd, "pipe"],
-        });
-        if (diff.error) throw new Refusal("--diff needs git on PATH");
-        const output = Buffer.alloc(fstatSync(outputFd).size);
-        let offset = 0;
-        while (offset < output.length) {
-          const count = readSync(outputFd, output, offset, output.length - offset, offset);
-          if (count === 0) break;
-          offset += count;
-        }
-        out.push(output.subarray(0, offset).toString("utf-8").trimEnd());
-      } finally {
-        closeSync(outputFd);
-      }
+  const out: string[] = [];
+  for (const file of entry.files) {
+    const sides = await Promise.all(
+      [file.before, file.after].map((hash) => (hash ? reader.blob(hash) : Buffer.alloc(0))),
+    );
+    if (sides.some((bytes) => bytes.subarray(0, 8000).includes(0))) {
+      out.push(`Binary ${file.path} changed`);
+      continue;
     }
-    return out.join("\n");
-  } finally {
-    rmSync(work, { recursive: true, force: true });
+    out.push(unifiedDiff(file.path, sides[0]!.toString("utf-8"), sides[1]!.toString("utf-8")));
   }
+  return out.join("\n");
 }
 
 /** `again` is the command to run once more with a choice: `undo <id>`, or `undo --who <name>` for a whole turn. */
diff --git a/packages/cli/src/utils/unifiedDiff.test.ts b/packages/cli/src/utils/unifiedDiff.test.ts
new file mode 100644
index 0000000000..96b25946f9
--- /dev/null
+++ b/packages/cli/src/utils/unifiedDiff.test.ts
@@ -0,0 +1,103 @@
+import { spawnSync } from "node:child_process";
+import { mkdtempSync, rmSync, writeFileSync } from "node:fs";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+import { describe, expect, it } from "vitest";
+import { unifiedDiff } from "./unifiedDiff.js";
+
+/** Applies a diff's hunks back onto `before` to check it reproduces `after`. */
+function apply(before: string, diff: string): string {
+  const lines = before === "" ? [] : before.replace(/\n$/, "").split("\n");
+  const out: string[] = [];
+  let at = 0;
+  for (const row of diff.split("\n").slice(3)) {
+    if (row.startsWith("@@")) {
+      const first = Number(/-(\d+)/.exec(row)![1]);
+      const start = /-0,0/.test(row) ? 0 : first - 1;
+      out.push(...lines.slice(at, start));
+      at = start;
+    } else if (row.startsWith("\\")) continue;
+    else if (row.startsWith("+")) out.push(row.slice(1));
+    else if (row.startsWith("-")) at++;
+    else {
+      out.push(row.slice(1));
+      at++;
+    }
+  }
+  out.push(...lines.slice(at));
+  return out.join("\n");
+}
+
+function randomizer(seed: number) {
+  return (n: number) => (seed = (seed * 1103515245 + 12345) & 0x7fffffff) % n;
+}
+
+/** Two texts, the second the first with lines dropped and inserted, either with or without a final newline. */
+function randomPair(rand: (n: number) => number): [string, string] {
+  const before = Array.from({ length: rand(25) }, () => `w${rand(6)}`);
+  const after = before.filter(() => rand(4) > 0);
+  for (let i = rand(5); i > 0; i--) after.splice(rand(after.length + 1), 0, `n${rand(9)}`);
+  const text = (lines: string[], bare: boolean) =>
+    lines.length ? lines.join("\n") + (bare ? "" : "\n") : "";
+  return [text(before, rand(3) === 0), text(after, rand(3) === 0)];
+}
+
+const bodyOf = (text: string) => text.slice(text.indexOf("@@")).trimEnd();
+
+/** What git prints for the two texts, or undefined where git is not installed. */
+function gitDiffBody(dir: string, a: string, b: string): string | undefined {
+  writeFileSync(join(dir, "a"), a);
+  writeFileSync(join(dir, "b"), b);
+  const git = spawnSync("git", ["diff", "--no-index", "--no-color", "--no-prefix", "a", "b"], {
+    cwd: dir,
+    encoding: "utf-8",
+  });
+  return git.error ? undefined : bodyOf(git.stdout);
+}
+
+describe("unifiedDiff", () => {
+  it("marks a missing final newline like git", () => {
+    expect(unifiedDiff("a.html", "A", "A2")).toBe(
+      "diff --git a/a.html b/a.html\n--- a/a.html\n+++ b/a.html\n@@ -1 +1 @@\n-A\n\\ No newline at end of file\n+A2\n\\ No newline at end of file",
+    );
+  });
+
+  it("is empty for equal text, and shows a whole added or deleted file", () => {
+    expect(unifiedDiff("a", "x\n", "x\n")).toBe("");
+    expect(unifiedDiff("a", "", "x\ny\n")).toContain("@@ -0,0 +1,2 @@\n+x\n+y");
+    expect(unifiedDiff("a", "x\ny\n", "")).toContain("@@ -1,2 +0,0 @@\n-x\n-y");
+  });
+
+  it("keeps three lines of context and splits distant changes into hunks", () => {
+    const lines = Array.from({ length: 30 }, (_, i) => `l${i}`);
+    const after = lines.map((l) => (l === "l2" || l === "l27" ? `${l}!` : l));
+    const diff = unifiedDiff("a", `${lines.join("\n")}\n`, `${after.join("\n")}\n`);
+    expect(diff.match(/^@@/gm)).toHaveLength(2);
+    expect(diff).toContain("@@ -1,6 +1,6 @@");
+    expect(diff).toContain("@@ -25,6 +25,6 @@");
+  });
+
+  it("matches git's own diff and reproduces the new text on random edits", () => {
+    const dir = mkdtempSync(join(tmpdir(), "hf-udiff-"));
+    try {
+      const rand = randomizer(7);
+      for (let round = 0; round < 40; round++) {
+        const [a, b] = randomPair(rand);
+        const mine = unifiedDiff("f", a, b);
+        expect(apply(a, mine).replace(/\n?$/, "")).toBe(b.replace(/\n$/, ""));
+        const git = gitDiffBody(dir, a, b);
+        if (git !== undefined) expect(bodyOf(mine)).toBe(a === b ? "" : git);
+      }
+    } finally {
+      rmSync(dir, { recursive: true, force: true });
+    }
+  });
+
+  it("shows a change past the edit cap as one replace of the changed middle", () => {
+    const lines = (prefix: string) => Array.from({ length: 2500 }, (_, i) => `${prefix}${i}`);
+    const text = (prefix: string) => `keep\n${lines(prefix).join("\n")}\n`;
+    const diff = unifiedDiff("a", text("x"), text("y"));
+    expect(apply(text("x"), diff)).toBe(text("y").trimEnd());
+    expect(diff).toContain("-x0\n-x1");
+  });
+});
diff --git a/packages/cli/src/utils/unifiedDiff.ts b/packages/cli/src/utils/unifiedDiff.ts
new file mode 100644
index 0000000000..248695efbb
--- /dev/null
+++ b/packages/cli/src/utils/unifiedDiff.ts
@@ -0,0 +1,114 @@
+const CONTEXT = 3;
+// Past this many changed lines Myers' trace grows quadratically; the middle is then shown as one replace.
+const MAX_EDIT_LINES = 4000;
+
+type Op = { kind: " " | "-" | "+"; text: string };
+
+/** Lines of `text` with a marker on a last line that has no newline, so "A" and "A\n" differ as in git. */
+function linesOf(text: string): string[] {
+  if (text === "") return [];
+  const lines = text.split("\n");
+  if (lines[lines.length - 1] === "") lines.pop();
+  else lines[lines.length - 1] += "\0";
+  return lines;
+}
+
+const display = (line: string): string =>
+  line.endsWith("\0") ? `${line.slice(0, -1)}\n\\ No newline at end of file` : line;
+
+/** Edit script turning `a` into `b` (Myers), after the shared head and tail are set aside. */
+function editScript(a: string[], b: string[]): Op[] {
+  let head = 0;
+  while (head < a.length && head < b.length && a[head] === b[head]) head++;
+  let tail = 0;
+  while (
+    tail < a.length - head &&
+    tail < b.length - head &&
+    a[a.length - 1 - tail] === b[b.length - 1 - tail]
+  )
+    tail++;
+  const x = a.slice(head, a.length - tail);
+  const y = b.slice(head, b.length - tail);
+  const same = (lines: string[]): Op[] => lines.map((text) => ({ kind: " ", text }));
+  return [...same(a.slice(0, head)), ...middle(x, y), ...same(a.slice(a.length - tail))];
+}
+
+const replaceAll = (a: string[], b: string[]): Op[] => [
+  ...a.map((text): Op => ({ kind: "-", text })),
+  ...b.map((text): Op => ({ kind: "+", text })),
+];
+
+/** Whether diagonal `k` of round `d` is reached by moving down (an insert) from the one before it. */
+const movesDown = (v: Int32Array, offset: number, k: number, d: number) =>
+  k === -d || (k !== d && v[offset + k - 1]! < v[offset + k + 1]!);
+
+/** How far along diagonal `k` of round `d` the walk gets in `a`, taking every shared line it meets. */
+function reach(v: Int32Array, offset: number, k: number, d: number, a: string[], b: string[]) {
+  let x = movesDown(v, offset, k, d) ? v[offset + k + 1]! : v[offset + k - 1]! + 1;
+  while (x < a.length && x - k < b.length && a[x] === b[x - k]) x++;
+  return x;
+}
+
+function middle(a: string[], b: string[]): Op[] {
+  if (a.length + b.length > MAX_EDIT_LINES) return replaceAll(a, b);
+  const offset = a.length + b.length;
+  const v = new Int32Array(2 * offset + 2);
+  const trace: Int32Array[] = [];
+  for (let d = 0; ; d++) {
+    trace.push(v.slice());
+    for (let k = -d; k <= d; k += 2) {
+      const x = reach(v, offset, k, d, a, b);
+      v[offset + k] = x;
+      if (x >= a.length && x - k >= b.length) return backtrack(trace, a, b, offset, d);
+    }
+  }
+}
+
+/** Pushes the shared lines of `a` from index `x` back down to `to`, and returns `to`. */
+function slideBack(ops: Op[], a: string[], x: number, to: number) {
+  for (; x > to; x--) ops.push({ kind: " ", text: a[x - 1]! });
+  return to;
+}
+
+function backtrack(trace: Int32Array[], a: string[], b: string[], offset: number, last: number) {
+  const ops: Op[] = [];
+  let x = a.length;
+  let y = b.length;
+  for (let d = last; d > 0; d--) {
+    const k = x - y;
+    const down = movesDown(trace[d]!, offset, k, d);
+    const prevX = trace[d]![offset + (down ? k + 1 : k - 1)]!;
+    x = slideBack(ops, a, x, down ? prevX : prevX + 1);
+    y = x - k;
+    if (down) ops.push({ kind: "+", text: b[--y]! });
+    else ops.push({ kind: "-", text: a[--x]! });
+  }
+  slideBack(ops, a, x, 0);
+  return ops.reverse();
+}
+
+/** A git-style unified diff of one text file, `after` empty for a deleted file and `before` empty for an added one. */
+export function unifiedDiff(path: string, before: string, after: string): string {
+  const ops = editScript(linesOf(before), linesOf(after));
+  const changed = ops.flatMap((op, index) => (op.kind === " " ? [] : [index]));
+  if (changed.length === 0) return "";
+  const out = [`diff --git a/${path} b/${path}`, `--- a/${path}`, `+++ b/${path}`];
+  let from = 0;
+  while (from < changed.length) {
+    let to = from;
+    while (to + 1 < changed.length && changed[to + 1]! - changed[to]! <= 2 * CONTEXT + 1) to++;
+    const start = Math.max(0, changed[from]! - CONTEXT);
+    const end = Math.min(ops.length, changed[to]! + CONTEXT + 1);
+    const hunk = ops.slice(start, end);
+    const before0 = ops.slice(0, start).filter((op) => op.kind !== "+").length;
+    const after0 = ops.slice(0, start).filter((op) => op.kind !== "-").length;
+    const oldLen = hunk.filter((op) => op.kind !== "+").length;
+    const newLen = hunk.filter((op) => op.kind !== "-").length;
+    const range = (first: number, length: number) =>
+      length === 1 ? `${first + 1}` : `${length === 0 ? first : first + 1},${length}`;
+    out.push(`@@ -${range(before0, oldLen)} +${range(after0, newLen)} @@`);
+    for (const op of hunk) out.push(`${op.kind}${display(op.text)}`);
+    from = to + 1;
+  }
+  return out.join("\n");
+}