diff --git a/test/e2e/fixtures/workspace/docs/crossrung.md b/test/e2e/fixtures/workspace/docs/crossrung.md
new file mode 100644
index 0000000..c7abd46
--- /dev/null
+++ b/test/e2e/fixtures/workspace/docs/crossrung.md
@@ -0,0 +1,5 @@
+# Crossrung doc
+
+The portable record travels with ordinary git.
+
+Nothing else here.
diff --git a/test/e2e/fixtures/workspace/docs/v2.md b/test/e2e/fixtures/workspace/docs/v2.md
new file mode 100644
index 0000000..8aa4b8d
--- /dev/null
+++ b/test/e2e/fixtures/workspace/docs/v2.md
@@ -0,0 +1,3 @@
+# Future doc
+
+Anchored text from the future.
diff --git a/test/e2e/suite/crossrung.test.ts b/test/e2e/suite/crossrung.test.ts
new file mode 100644
index 0000000..aa188e9
--- /dev/null
+++ b/test/e2e/suite/crossrung.test.ts
@@ -0,0 +1,129 @@
+import * as assert from "assert";
+import * as fs from "fs";
+import * as path from "path";
+import { execFileSync } from "child_process";
+import * as vscode from "vscode";
+import type { CowritingApi } from "../../../src/extension";
+import type { Artifact } from "../../../src/model";
+
+const WS = process.env.E2E_WORKSPACE!;
+const PROJECT_ROOT = path.resolve(__dirname, "../../../..");
+const STANDIN = path.join(PROJECT_ROOT, "scripts", "crossrung-reply.mjs");
+
+function sidecarPath(docRel: string): string {
+  return path.join(WS, ".threads", `${docRel}.json`);
+}
+async function open(docRel: string): Promise<vscode.TextDocument> {
+  const uri = vscode.Uri.file(path.join(WS, docRel));
+  const doc = await vscode.workspace.openTextDocument(uri);
+  await vscode.window.showTextDocument(doc);
+  return doc;
+}
+async function getApi(): Promise<CowritingApi> {
+  const ext = vscode.extensions.getExtension("benstull.vscode-cowriting-plugin")!;
+  const api = (await ext.activate()) as CowritingApi;
+  assert.ok(api?.versionGuard, "extension exports versionGuard (F5)");
+  return api;
+}
+// The extension host is Electron: run the stand-in with the same binary in
+// plain-node mode — a forge's write is just a local process to git (fork e).
+function runStandin(...args: string[]): void {
+  execFileSync(process.execPath, [STANDIN, ...args], {
+    encoding: "utf8",
+    env: { ...process.env, ELECTRON_RUN_AS_NODE: "1" },
+  });
+}
+function commentsOf(api: CowritingApi, docRel: string, threadId: string): number {
+  const docs = (
+    api.threadController as unknown as {
+      docs: Map<string, { vsThreads: Map<string, vscode.CommentThread> }>;
+    }
+  ).docs;
+  const vsThread = docs.get(docRel)?.vsThreads.get(threadId);
+  return vsThread ? vsThread.comments.length : -1;
+}
+async function until(cond: () => boolean, ms = 5000): Promise<void> {
+  const t0 = Date.now();
+  while (!cond()) {
+    if (Date.now() - t0 > ms) throw new Error("timed out waiting for condition");
+    await new Promise((r) => setTimeout(r, 100));
+  }
+}
+
+suite("F5 cross-rung round-trip (host E2E)", () => {
+  test("stand-in reply lands in the editor's thread via external change (PUC-2)", async () => {
+    const DOC = "docs/crossrung.md";
+    const doc = await open(DOC);
+    const api = await getApi();
+    const target = "portable record";
+    const start = doc.getText().indexOf(target);
+    const editor = vscode.window.activeTextEditor!;
+    editor.selection = new vscode.Selection(doc.positionAt(start), doc.positionAt(start + target.length));
+    const threadId = await api.threadController.createThreadOnSelection("Editor note");
+    assert.ok(threadId, "thread created in the editor");
+
+    // the Gitea-rung stand-in appends a reply OUT OF EDITOR (spec fork e)
+    runStandin(sidecarPath(DOC), threadId!, "Reply from the forge", "--author-id", "gitea", "--author-email", "forge@example.org");
+
+    // watcher → external change → re-render with the new message (the F2 path)
+    await until(() => commentsOf(api, DOC, threadId!) === 2);
+    const rendered = api.threadController.getRendered(DOC);
+    const mine = rendered.find((r) => r.id === threadId)!;
+    assert.strictEqual(mine.orphaned, false, "anchor still resolves after the foreign write");
+    const anchorLine = doc.positionAt(doc.getText().indexOf(target)).line;
+    assert.strictEqual(mine.range.startLine, anchorLine, "rendered at the re-resolved anchor");
+
+    const art = JSON.parse(fs.readFileSync(sidecarPath(DOC), "utf8")) as Artifact;
+    const msgs = art.threads.find((t) => t.id === threadId)!.messages;
+    assert.strictEqual(msgs.length, 2);
+    assert.deepStrictEqual(msgs[1].author, { kind: "human", id: "gitea", email: "forge@example.org" });
+  });
+
+  test("newer-major sidecar: warning + read-only, file bytes never change (PUC-4)", async () => {
+    const DOC = "docs/v2.md";
+    const scPath = sidecarPath(DOC);
+    fs.mkdirSync(path.dirname(scPath), { recursive: true });
+    // a v2 sidecar: valid v1 sections plus future content this build can't know
+    const v2 = {
+      schemaVersion: 2,
+      document: { path: DOC },
+      anchors: {
+        a_1: { fingerprint: { text: "Anchored text", before: "", after: " from the future.", lineHint: 2 } },
+      },
+      threads: [
+        {
+          id: "t_1",
+          anchorId: "a_1",
+          status: "open",
+          messages: [{ id: "m_1", author: { kind: "human", id: "future" }, body: "from v2", createdAt: "2026-06-10T00:00:00.000Z" }],
+        },
+      ],
+      attributions: [],
+      proposals: [],
+      v2OnlySection: { shiny: true },
+    };
+    fs.writeFileSync(scPath, JSON.stringify(v2, null, 2) + "\n", "utf8");
+    const before = fs.readFileSync(scPath, "utf8");
+
+    const doc = await open(DOC);
+    const api = await getApi();
+    api.threadController.renderAll(doc); // render best-effort: the v1 subset shows
+    const rendered = api.threadController.getRendered(DOC);
+    assert.strictEqual(rendered.length, 1, "renders what it understands");
+
+    // write gestures are refused + warned (PUC-4)
+    const target = "Anchored text";
+    const start = doc.getText().indexOf(target);
+    const editor = vscode.window.activeTextEditor!;
+    editor.selection = new vscode.Selection(doc.positionAt(start), doc.positionAt(start + target.length));
+    const created = await api.threadController.createThreadOnSelection("should not land");
+    assert.strictEqual(created, undefined, "createThread refused on newer major");
+    assert.strictEqual(api.versionGuard.wasWarned(DOC), true, "user warned once (PUC-4)");
+
+    // an edit + save must NOT rewrite the sidecar (INV-16)
+    await editor.edit((b) => b.insert(doc.positionAt(0), "edited "));
+    await doc.save();
+    await new Promise((r) => setTimeout(r, 500));
+    assert.strictEqual(fs.readFileSync(scPath, "utf8"), before, "sidecar bytes untouched (INV-16)");
+  });
+});