diff --git a/packages/cli/src/timeline/a2Commands.ts b/packages/cli/src/timeline/a2Commands.ts
index 06e8021834..939fe3b3a0 100644
--- a/packages/cli/src/timeline/a2Commands.ts
+++ b/packages/cli/src/timeline/a2Commands.ts
@@ -5,7 +5,7 @@ import {
 } from "@hyperframes/studio-server";
 import { ensureHfIds } from "@hyperframes/parsers/hf-ids";
 import { readFileSync } from "node:fs";
-import { join } from "node:path";
+import { join, relative } from "node:path";
 import { describeProject, type ProjectTimeline, type TimelineRow } from "./describeProject.js";
 import { resolveRef } from "./resolveRef.js";
 import { parseTimeExpression } from "./timeExpr.js";
@@ -309,8 +309,25 @@ function readUndoInput(input: string): UndoInput {
   return { ok: true, receipts: values, batch: Array.isArray(value) };
 }
 
+function refuseUndoError(error: unknown, projectDir: string, json: boolean): void {
+  if (isRecord(error) && error.code === "ENOENT" && typeof error.path === "string")
+    return refuse(
+      "timeline undo",
+      {
+        reason: `undo file is missing: ${relative(projectDir, error.path).split("\\").join("/")}`,
+        fix: "restore the missing target or backup file before retrying undo",
+      },
+      json,
+    );
+  if (isFileChanged(error))
+    return refusal(error.message, "read the latest timeline before retrying undo", json);
+  throw error;
+}
+
 export async function runUndo(args: Record<string, unknown>): Promise<void> {
-  const project = resolveProject(typeof args.dir === "string" ? args.dir : undefined);
+  const project = resolveProject(typeof args.dir === "string" ? args.dir : undefined, {
+    requireIndex: false,
+  });
   const json = args.json === true;
   const input = typeof args.receipt === "string" ? args.receipt : positional(args)[1];
   if (!input)
@@ -321,23 +338,21 @@ export async function runUndo(args: Record<string, unknown>): Promise<void> {
     );
   const undo = readUndoInput(input);
   if (!undo.ok) return refuse("timeline undo", undo, json);
-  const inputs = undo.receipts.map((receipt) => {
-    const target = join(project.dir, receipt.file);
-    return {
-      sourceFile: receipt.file,
-      absPath: target,
-      before: readFileSync(target, "utf-8"),
-      after: readFileSync(join(project.dir, receipt.backupPath), "utf-8"),
-      expectedVersion: receipt.version,
-    };
-  });
   let receipts: AppliedFileMutation[];
   try {
+    const inputs = undo.receipts.map((receipt) => {
+      const target = join(project.dir, receipt.file);
+      return {
+        sourceFile: receipt.file,
+        absPath: target,
+        before: readFileSync(target, "utf-8"),
+        after: readFileSync(join(project.dir, receipt.backupPath), "utf-8"),
+        expectedVersion: receipt.version,
+      };
+    });
     receipts = applyFileMutations(project.dir, inputs);
   } catch (error) {
-    if (isFileChanged(error))
-      return refusal(error.message, "read the latest timeline before retrying undo", json);
-    throw error;
+    return refuseUndoError(error, project.dir, json);
   }
   const result = {
     ok: true,
diff --git a/packages/cli/src/timeline/timeline.e2e.test.ts b/packages/cli/src/timeline/timeline.e2e.test.ts
index 34701fbfa3..9cfa23daf5 100644
--- a/packages/cli/src/timeline/timeline.e2e.test.ts
+++ b/packages/cli/src/timeline/timeline.e2e.test.ts
@@ -1,5 +1,13 @@
 import { spawnSync } from "node:child_process";
-import { mkdirSync, mkdtempSync, readFileSync, rmSync, writeFileSync } from "node:fs";
+import {
+  existsSync,
+  mkdirSync,
+  mkdtempSync,
+  readdirSync,
+  readFileSync,
+  rmSync,
+  writeFileSync,
+} from "node:fs";
 import { tmpdir } from "node:os";
 import { join, resolve } from "node:path";
 import { fileURLToPath } from "node:url";
@@ -778,6 +786,55 @@ describe("timeline edit command", () => {
     },
   );
 
+  it.each([
+    { format: "single", missing: "target", index: 1 },
+    { format: "single", missing: "backup", index: 1 },
+    { format: "batch", missing: "target", index: 1 },
+    { format: "batch", missing: "backup", index: 1 },
+    { format: "single", missing: "target", index: 0 },
+    { format: "batch", missing: "target", index: 0 },
+  ])(
+    "refuses a $format undo when its $missing at index $index is missing",
+    ({ format, missing, index }) => {
+      const { dir, files, planPath } = batchProject();
+      try {
+        const applied = run(dir, "apply", planPath);
+        expect(applied.status, applied.stderr).toBe(0);
+        const receipt: { file: string; backupPath: string } = JSON.parse(applied.stdout).receipt[
+          index
+        ];
+        const missingFile = missing === "target" ? receipt.file : receipt.backupPath;
+        const missingPath = join(dir, missingFile);
+        rmSync(missingPath);
+        const remainingFiles = files.filter((file) => join(dir, file) !== missingPath);
+        const sources = () => remainingFiles.map((file) => readFileSync(join(dir, file), "utf8"));
+        const current = sources();
+        const backupDir = join(dir, ".hyperframes", "backup");
+        const backups = () =>
+          readdirSync(backupDir)
+            .sort()
+            .map((file) => [file, readFileSync(join(backupDir, file), "utf8")]);
+        const currentBackups = backups();
+        const input = format === "single" ? JSON.stringify(receipt) : applied.stdout;
+
+        const undone = run(dir, "undo", input);
+
+        expect(undone.status, undone.stderr).toBe(2);
+        expect(undone.stdout).toBe("");
+        expect(JSON.parse(undone.stderr)).toEqual({
+          ok: false,
+          reason: `timeline undo: undo file is missing: ${missingFile}`,
+          fix: "restore the missing target or backup file before retrying undo",
+        });
+        expect(sources()).toEqual(current);
+        expect(backups()).toEqual(currentBackups);
+        expect(existsSync(missingPath)).toBe(false);
+      } finally {
+        rmSync(dir, { recursive: true, force: true });
+      }
+    },
+  );
+
   it("undoes a complete timeline ids response", () => {
     const dir = project();
     try {