diff --git a/package.json b/package.json
index ef48956..c060878 100644
--- a/package.json
+++ b/package.json
@@ -26,6 +26,8 @@
     "lint": "pnpm -r --workspace-concurrency=1 lint",
     "format": "pnpm --filter @etherpad/desktop format",
     "test": "pnpm -r --workspace-concurrency=1 test",
+    "test:vectors": "pnpm --filter @etherpad/shell exec vitest run tests/wire/vectors.spec.ts",
+    "test:smoke": "pnpm --filter @etherpad/shell exec vitest run tests/wire/smoke.spec.ts",
     "test:watch": "pnpm --filter @etherpad/desktop test:watch",
     "test:e2e": "pnpm --filter @etherpad/desktop test:e2e",
     "mobile:dev": "pnpm --filter @etherpad/mobile dev",
diff --git a/packages/shell/tests/fixtures/wire-vectors.json b/packages/shell/tests/fixtures/wire-vectors.json
new file mode 100644
index 0000000..6b8ff01
--- /dev/null
+++ b/packages/shell/tests/fixtures/wire-vectors.json
@@ -0,0 +1,7 @@
+[
+  {"name":"plain-insert","initialText":"abc\n","changeset":"Z:4>3=3+3$XYZ","pool":{"numToAttrib":{},"nextNum":0},"resultText":"abcXYZ\n"},
+  {"name":"plain-delete","initialText":"abcdef\n","changeset":"Z:7<3=1-3$","pool":{"numToAttrib":{},"nextNum":0},"resultText":"aef\n"},
+  {"name":"formatted-insert","initialText":"abc\n","changeset":"Z:4>4=3*0+4$bold","pool":{"numToAttrib":{"0":["bold","true"]},"nextNum":1},"resultText":"abcbold\n"},
+  {"name":"multiline-insert","initialText":"abc\n","changeset":"Z:4>8=3|2+8$one\ntwo\n","pool":{"numToAttrib":{},"nextNum":0},"resultText":"abcone\ntwo\n\n"},
+  {"name":"attrib-reuse","initialText":"abc\n","changeset":"Z:4>2*0+1=3*0+1$AB","pool":{"numToAttrib":{"0":["bold","true"]},"nextNum":1},"resultText":"AabcB\n"}
+]
diff --git a/packages/shell/tests/wire/smoke.spec.ts b/packages/shell/tests/wire/smoke.spec.ts
new file mode 100644
index 0000000..bcd6ad6
--- /dev/null
+++ b/packages/shell/tests/wire/smoke.spec.ts
@@ -0,0 +1,96 @@
+import { describe, it, expect, beforeAll } from 'vitest';
+
+/**
+ * Downstream wire-compatibility — live smoke test (headless-light).
+ *
+ * Phase 2 of ether/etherpad#7923. This proves the desktop/mobile shell could
+ * talk to a real Etherpad server *without* booting Electron. The full Electron
+ * e2e stays in this repo's own CI; this gate is deliberately headless-light —
+ * a plain HTTP roundtrip against the contract the shell depends on:
+ *   1. create a pad via the HTTP API,
+ *   2. fetch `/p/<pad>` (the exact URL the shell would load in its webview)
+ *      and assert HTTP 200,
+ *   3. read the pad text back via the API to confirm content.
+ *
+ * Env contract:
+ *   - ETHERPAD_SMOKE_URL    server base URL (default http://localhost:9003)
+ *   - ETHERPAD_SMOKE_APIKEY HTTP API key (required to actually run)
+ *
+ * Unless BOTH a reachable server and an API key are present, this SKIPS
+ * cleanly — it must never fail CI for lack of test infrastructure.
+ */
+
+const BASE = (process.env.ETHERPAD_SMOKE_URL || 'http://localhost:9003').replace(/\/$/, '');
+const APIKEY = process.env.ETHERPAD_SMOKE_APIKEY || '';
+const API_VERSION = '1.2.13';
+
+async function reachable(): Promise<boolean> {
+  try {
+    const res = await fetch(`${BASE}/api`, { signal: AbortSignal.timeout(2000) });
+    return res.ok;
+  } catch {
+    return false;
+  }
+}
+
+interface ApiResponse<T> {
+  code: number;
+  message: string;
+  data: T;
+}
+
+async function api<T>(fn: string, params: Record<string, string>): Promise<ApiResponse<T>> {
+  const qs = new URLSearchParams({ apikey: APIKEY, ...params });
+  const res = await fetch(`${BASE}/api/${API_VERSION}/${fn}?${qs.toString()}`, {
+    signal: AbortSignal.timeout(5000),
+  });
+  expect(res.status).toBe(200);
+  return (await res.json()) as ApiResponse<T>;
+}
+
+let serverUp = false;
+
+beforeAll(async () => {
+  // No key means the smoke can't run, so don't waste a network call + timeout
+  // probing reachability — it can't change the (skip) outcome.
+  if (APIKEY) serverUp = await reachable();
+});
+
+describe('live server smoke (shell HTTP contract)', () => {
+  it('completes a create -> fetch /p/<pad> -> getText roundtrip', async () => {
+    if (!serverUp || !APIKEY) {
+      const why = !APIKEY ? `ETHERPAD_SMOKE_APIKEY not set` : `no Etherpad reachable at ${BASE}`;
+      console.warn(
+        `[smoke] ${why} — skipping live smoke test. ` +
+          `Set ETHERPAD_SMOKE_URL + ETHERPAD_SMOKE_APIKEY to run it.`,
+      );
+      return; // skip cleanly: never fail the gate without a reachable server + key
+    }
+
+    const padID = `phase2-smoke-${Date.now()}`;
+    const text = 'phase2 wire-compat smoke\n';
+
+    const created = await api<null>('createPad', { padID, text });
+    expect(created.code, created.message).toBe(0);
+
+    try {
+      // The exact URL the shell loads in its webview.
+      const padRes = await fetch(`${BASE}/p/${encodeURIComponent(padID)}`, {
+        signal: AbortSignal.timeout(5000),
+      });
+      expect(padRes.status).toBe(200);
+
+      const got = await api<{ text: string }>('getText', { padID });
+      expect(got.code, got.message).toBe(0);
+      // Etherpad guarantees a pad's text ends with exactly one trailing
+      // newline, so setting "X\n" reads back as "X\n\n". Normalize the
+      // trailing newline(s) on both sides before comparing.
+      const trimTrailing = (s: string) => s.replace(/\n*$/, '\n');
+      expect(trimTrailing(got.data.text)).toBe(trimTrailing(text));
+    } finally {
+      // Guaranteed cleanup even if an assertion above throws; swallow delete
+      // errors so cleanup never masks the real failure.
+      await api<null>('deletePad', { padID }).catch(() => {});
+    }
+  });
+});
diff --git a/packages/shell/tests/wire/vectors.spec.ts b/packages/shell/tests/wire/vectors.spec.ts
new file mode 100644
index 0000000..df722ea
--- /dev/null
+++ b/packages/shell/tests/wire/vectors.spec.ts
@@ -0,0 +1,88 @@
+import { describe, it, expect } from 'vitest';
+import { readFileSync } from 'node:fs';
+import { fileURLToPath } from 'node:url';
+import { resolve, dirname } from 'node:path';
+
+/**
+ * Downstream wire-compatibility — fixture-integrity guard.
+ *
+ * Phase 2 of ether/etherpad#7923. Phase 1 added a canonical wire-format
+ * fixture (`wire-vectors.json`) that every Etherpad client must decode
+ * identically. The Rust/CLI clients re-implement changeset decoding and
+ * therefore run these vectors through their own decoder.
+ *
+ * The desktop/mobile apps are thin shells: they load an Etherpad server URL
+ * and embed CORE'S editor (Ace) inside a webview, so there is NO local
+ * changeset decoder to exercise. This test is therefore a *fixture-integrity
+ * guard* rather than a decode test — it asserts the shape/contract of the
+ * vendored fixture so that:
+ *   - a malformed or empty fixture injected into this repo fails loudly, and
+ *   - the contract the embedded editor relies on is documented in-repo.
+ *
+ * The fixture path is overridable via `ETHERPAD_WIRE_VECTORS`, defaulting to
+ * the vendored copy next to this test.
+ */
+
+const here = dirname(fileURLToPath(import.meta.url));
+const DEFAULT_VECTORS_PATH = resolve(here, '../fixtures/wire-vectors.json');
+const VECTORS_PATH = process.env.ETHERPAD_WIRE_VECTORS || DEFAULT_VECTORS_PATH;
+
+interface WireVector {
+  name: string;
+  initialText: string;
+  changeset: string;
+  pool: { numToAttrib: Record<string, unknown>; nextNum: number };
+  resultText: string;
+}
+
+function loadVectors(): WireVector[] {
+  const raw = readFileSync(VECTORS_PATH, 'utf8');
+  return JSON.parse(raw) as WireVector[];
+}
+
+describe('wire-vectors fixture integrity', () => {
+  const vectors = loadVectors();
+
+  it('is a non-empty array', () => {
+    expect(Array.isArray(vectors)).toBe(true);
+    expect(vectors.length).toBeGreaterThan(0);
+  });
+
+  it('has unique vector names', () => {
+    const names = vectors.map((v) => v.name);
+    expect(new Set(names).size).toBe(names.length);
+  });
+
+  describe.each(vectors.map((v) => [v.name, v] as const))('vector %s', (_name, v) => {
+    it('has all five fields with the right types', () => {
+      expect(typeof v.name).toBe('string');
+      expect(v.name.length).toBeGreaterThan(0);
+      expect(typeof v.initialText).toBe('string');
+      expect(typeof v.changeset).toBe('string');
+      expect(v.changeset.length).toBeGreaterThan(0);
+      expect(typeof v.resultText).toBe('string');
+      expect(typeof v.pool).toBe('object');
+      expect(v.pool).not.toBeNull();
+    });
+
+    it('changeset uses the canonical Z: header', () => {
+      expect(v.changeset.startsWith('Z:')).toBe(true);
+    });
+
+    it('pool.numToAttrib is a plain object and pool.nextNum is a number', () => {
+      expect(typeof v.pool.numToAttrib).toBe('object');
+      expect(v.pool.numToAttrib).not.toBeNull();
+      expect(Array.isArray(v.pool.numToAttrib)).toBe(false);
+      expect(typeof v.pool.nextNum).toBe('number');
+      expect(Number.isInteger(v.pool.nextNum)).toBe(true);
+      expect(v.pool.nextNum).toBeGreaterThanOrEqual(0);
+    });
+
+    it('initialText and resultText are non-empty and newline-terminated', () => {
+      expect(v.initialText.length).toBeGreaterThan(0);
+      expect(v.initialText.endsWith('\n')).toBe(true);
+      expect(v.resultText.length).toBeGreaterThan(0);
+      expect(v.resultText.endsWith('\n')).toBe(true);
+    });
+  });
+});