diff --git a/test/e2e/suite/undoCapable.ts b/test/e2e/suite/undoCapable.ts
new file mode 100644
index 0000000..c61dabd
--- /dev/null
+++ b/test/e2e/suite/undoCapable.ts
@@ -0,0 +1,41 @@
+import * as fs from "fs";
+import * as path from "path";
+import * as vscode from "vscode";
+
+/**
+ * #54: `vscode.commands.executeCommand("undo")` is non-functional in some headless
+ * `.vscode-test` instances (it does not restore the buffer), which false-fails
+ * every undo-dependent E2E (#38, #40) and makes `main` E2E red there. This is a
+ * RUNTIME probe: it performs a real edit-then-undo on a scratch buffer and reports
+ * whether undo actually restored it. Undo-dependent suites gate on it — running
+ * normally where undo works (real coverage), skipping with a loud warning where it
+ * doesn't (no false red, no silent loss — the skip is logged). Memoized per run.
+ */
+let cached: boolean | undefined;
+
+export async function undoWorks(): Promise<boolean> {
+  if (cached !== undefined) return cached;
+  const abs = path.join(process.env.E2E_WORKSPACE!, "docs/.undo-probe.md");
+  fs.mkdirSync(path.dirname(abs), { recursive: true });
+  fs.writeFileSync(abs, "undo probe baseline\n", "utf8");
+  const doc = await vscode.workspace.openTextDocument(vscode.Uri.file(abs));
+  await vscode.window.showTextDocument(doc);
+  await new Promise((r) => setTimeout(r, 250));
+  const edit = new vscode.WorkspaceEdit();
+  edit.insert(doc.uri, doc.positionAt(doc.getText().length), "PROBE-EDIT-MARKER");
+  await vscode.workspace.applyEdit(edit);
+  await new Promise((r) => setTimeout(r, 250));
+  if (!doc.getText().includes("PROBE-EDIT-MARKER")) {
+    cached = false; // even the edit didn't take — treat as not undo-capable
+    return cached;
+  }
+  await vscode.commands.executeCommand("undo");
+  await new Promise((r) => setTimeout(r, 250));
+  cached = !doc.getText().includes("PROBE-EDIT-MARKER"); // undo removed the marker → undo works
+  return cached;
+}
+
+/** Loud, single-line reason logged when an undo suite skips (no silent loss — #54). */
+export const UNDO_SKIP_REASON =
+  "[E2E] SKIPPING undo-dependent suite — executeCommand('undo') is non-functional in this " +
+  "VS Code test instance (see vscode-cowriting-plugin#54). These tests run where undo works.";
diff --git a/test/e2e/suite/undoMarks.test.ts b/test/e2e/suite/undoMarks.test.ts
index ddeff88..7f6d6d0 100644
--- a/test/e2e/suite/undoMarks.test.ts
+++ b/test/e2e/suite/undoMarks.test.ts
@@ -3,6 +3,7 @@ import * as fs from "fs";
 import * as path from "path";
 import * as vscode from "vscode";
 import type { CowritingApi } from "../../../src/extension";
+import { undoWorks, UNDO_SKIP_REASON } from "./undoCapable";
 
 const WS = process.env.E2E_WORKSPACE!;
 const settle = () => new Promise((r) => setTimeout(r, 400));
@@ -34,6 +35,14 @@ suite("F10 #38 — undo does not mis-attribute restored text (host E2E, no LLM)"
   const DOC_REL = "docs/undo38.md";
   const BASE = "Alpha bravo charlie.\n";
 
+  // #54: skip (loudly) where executeCommand("undo") is non-functional; run where it works.
+  suiteSetup(async function () {
+    if (!(await undoWorks())) {
+      console.warn(UNDO_SKIP_REASON);
+      this.skip();
+    }
+  });
+
   test("undo of a deletion of baseline text leaves it unattributed (not human)", async () => {
     const { doc, key } = await freshDoc(DOC_REL, BASE);
     const api = await getApi();