diff --git a/frontend/js/api.js b/frontend/js/api.js
index fa401f4..4648764 100644
--- a/frontend/js/api.js
+++ b/frontend/js/api.js
@@ -1,11 +1,22 @@
-// Single API client: one place for fetch, JSON, and the error envelope.
-const BASE = "/api/v1";
+// Single API client: one place for fetch, JSON, the error envelope, and request
+// cancellation. Every method accepts an optional { signal } from cancellable().
+export const BASE = "/api/v1";
 
-async function request(path, options = {}) {
-  const response = await fetch(BASE + path, {
-    headers: { "Content-Type": "application/json" },
-    ...options,
-  });
+async function request(path, { signal, ...options } = {}) {
+  let response;
+  try {
+    response = await fetch(BASE + path, {
+      headers: { "Content-Type": "application/json" },
+      signal,
+      ...options,
+    });
+  } catch (error) {
+    // A caller-cancelled fetch is not a failure; tag it so views can ignore it.
+    if (error.name === "AbortError") {
+      error.code = "cancelled";
+    }
+    throw error;
+  }
   let body = null;
   try {
     body = await response.json();
@@ -22,17 +33,37 @@ async function request(path, options = {}) {
   return body;
 }
 
+// Cancellation: hand callers a signal to pass into any request and a cancel() to
+// abort the in-flight fetch. Aborted requests reject with code "cancelled".
+export function cancellable() {
+  const controller = new AbortController();
+  return { signal: controller.signal, cancel: () => controller.abort() };
+}
+
 export const api = {
-  listAssets: (params = {}) =>
-    request("/inventory/assets?" + new URLSearchParams(params).toString()),
-  listClusters: (params = {}) =>
-    request("/duplicates/clusters?" + new URLSearchParams(params).toString()),
-  getCluster: (id) => request(`/duplicates/clusters/${encodeURIComponent(id)}`),
-  decide: (id, payload) =>
+  listAssets: (params = {}, opts = {}) =>
+    request("/inventory/assets?" + new URLSearchParams(params).toString(), opts),
+  listClusters: (params = {}, opts = {}) =>
+    request("/duplicates/clusters?" + new URLSearchParams(params).toString(), opts),
+  getCluster: (id, opts = {}) =>
+    request(`/duplicates/clusters/${encodeURIComponent(id)}`, opts),
+  decide: (id, payload, opts = {}) =>
     request(`/duplicates/clusters/${encodeURIComponent(id)}/decision`, {
       method: "POST",
       body: JSON.stringify(payload),
+      ...opts,
     }),
   thumbnailUrl: (assetId, size = 512) =>
     `${BASE}/assets/${encodeURIComponent(assetId)}/thumbnail?size=${size}`,
+
+  // ── Jobs ────────────────────────────────────────────────────────────────
+  startJob: (payload, opts = {}) =>
+    request("/jobs", { method: "POST", body: JSON.stringify(payload), ...opts }),
+  getJob: (id, opts = {}) => request(`/jobs/${encodeURIComponent(id)}`, opts),
+  cancelJob: (id, opts = {}) =>
+    request(`/jobs/${encodeURIComponent(id)}/cancel`, { method: "POST", ...opts }),
+  jobEvents: (id, after = 0, opts = {}) =>
+    request(`/jobs/${encodeURIComponent(id)}/events?after=${after}`, opts),
+  jobEventsStreamUrl: (id, after = 0) =>
+    `${BASE}/jobs/${encodeURIComponent(id)}/events/stream?after=${after}`,
 };
diff --git a/frontend/js/events.js b/frontend/js/events.js
new file mode 100644
index 0000000..0bf905e
--- /dev/null
+++ b/frontend/js/events.js
@@ -0,0 +1,62 @@
+// Job activity adapter: prefer SSE (resumable, pushed), fall back to polling the
+// same durable events when EventSource is missing or the stream errors. The cursor
+// is the event `seq`, shared by both transports, so switching mid-stream never
+// drops or repeats an event. subscribeJob returns a stop() handle.
+import { api } from "./api.js";
+
+// Mirrors ACTIVE_STATES in services/jobs.py; a job outside this set is terminal
+// and the poller stops.
+const ACTIVE = new Set(["queued", "running", "cancelling", "retry_queued"]);
+
+export function subscribeJob(jobId, { onEvent, onDone, onError, after = 0, pollMs = 1000 } = {}) {
+  let cursor = after;
+  let stopped = false;
+  let source = null;
+  let timer = null;
+
+  function stop() {
+    if (stopped) return;
+    stopped = true;
+    if (source) source.close();
+    if (timer) clearTimeout(timer);
+    if (onDone) onDone(cursor);
+  }
+
+  async function poll() {
+    if (stopped) return;
+    try {
+      const data = await api.jobEvents(jobId, cursor);
+      for (const event of data.events) {
+        cursor = event.seq;
+        if (onEvent) onEvent(event);
+      }
+      if (!ACTIVE.has(data.state)) return stop();
+    } catch (error) {
+      if (!stopped && onError) onError(error);
+    }
+    if (!stopped) timer = setTimeout(poll, pollMs);
+  }
+
+  if (typeof EventSource === "function") {
+    source = new EventSource(api.jobEventsStreamUrl(jobId, cursor));
+    source.onmessage = (message) => {
+      const event = JSON.parse(message.data);
+      cursor = Number(message.lastEventId) || cursor;
+      if (onEvent) onEvent({ seq: cursor, ...event });
+    };
+    // Server sends `event: done` when the job reaches a terminal state.
+    source.addEventListener("done", stop);
+    // Network/stream error: drop SSE and continue from the same cursor via polling
+    // so no consumer notices the transport switch.
+    source.onerror = () => {
+      if (stopped || !source) return;
+      source.close();
+      source = null;
+      poll();
+    };
+  } else {
+    poll();
+  }
+
+  return { stop };
+}
diff --git a/frontend/js/store.js b/frontend/js/store.js
new file mode 100644
index 0000000..611e327
--- /dev/null
+++ b/frontend/js/store.js
@@ -0,0 +1,23 @@
+// Shared observable store: one state object, subscribe for changes, set() to
+// patch. Views read state and re-render on notify. Nothing here touches the URL
+// (the router owns navigable state) or the network (the api client owns that) —
+// this is only the in-memory glue between them.
+export function createStore(initial = {}) {
+  let state = { ...initial };
+  const listeners = new Set();
+
+  function set(patch) {
+    const next = typeof patch === "function" ? patch(state) : patch;
+    state = { ...state, ...next };
+    for (const listener of listeners) listener(state);
+  }
+
+  return {
+    get: () => state,
+    set,
+    subscribe(listener) {
+      listeners.add(listener);
+      return () => listeners.delete(listener);
+    },
+  };
+}
diff --git a/frontend/js/tests/harness.html b/frontend/js/tests/harness.html
new file mode 100644
index 0000000..e797219
--- /dev/null
+++ b/frontend/js/tests/harness.html
@@ -0,0 +1,10 @@
+<!DOCTYPE html>
+<html lang="en">
+  <head>
+    <meta charset="utf-8" />
+    <title>Shell unit tests</title>
+  </head>
+  <body>
+    <script type="module" src="./unit.js"></script>
+  </body>
+</html>
diff --git a/frontend/js/tests/unit.js b/frontend/js/tests/unit.js
new file mode 100644
index 0000000..d24fdf7
--- /dev/null
+++ b/frontend/js/tests/unit.js
@@ -0,0 +1,150 @@
+// In-browser unit tests for the shell primitives. No server, no JS toolchain:
+// globals (fetch, EventSource) are stubbed and the modules are exercised directly.
+// Driven by tests/e2e/test_frontend_shell.py, which loads harness.html and reads
+// window.__RESULTS__. Covers API errors + cancellation, store transitions,
+// routing, and the SSE→polling event fallback (US02-05 acceptance).
+import { createStore } from "../store.js";
+import { parseHash, navigate } from "../router.js";
+import { api, cancellable } from "../api.js";
+import { subscribeJob } from "../events.js";
+
+const cases = [];
+function ok(name, cond) {
+  cases.push({ name, ok: !!cond });
+}
+async function throws(name, fn, predicate) {
+  try {
+    await fn();
+    cases.push({ name, ok: false });
+  } catch (error) {
+    cases.push({ name, ok: !!predicate(error) });
+  }
+}
+
+const realFetch = window.fetch;
+const realES = window.EventSource;
+function jsonResponse(status, body) {
+  return { ok: status < 400, status, statusText: "", json: async () => body };
+}
+const tick = (ms = 0) => new Promise((r) => setTimeout(r, ms));
+
+async function run() {
+  // ── store ────────────────────────────────────────────────────────────────
+  {
+    const store = createStore({ n: 0 });
+    const seen = [];
+    const off = store.subscribe((s) => seen.push(s.n));
+    store.set({ n: 1 });
+    store.set((s) => ({ n: s.n + 1 }));
+    ok("store applies object and updater patches", store.get().n === 2);
+    ok("store notifies each change", seen.length === 2 && seen[1] === 2);
+    off();
+    store.set({ n: 99 });
+    ok("store unsubscribe stops notifications", seen.length === 2);
+  }
+
+  // ── router ───────────────────────────────────────────────────────────────
+  {
+    navigate("/inventory", { q: "beach", offset: 0, empty: "" });
+    const { path, params } = parseHash();
+    ok("router restores path", path === "/inventory");
+    ok("router restores non-empty filters", params.q === "beach" && params.offset === "0");
+    ok("router drops empty filters", !("empty" in params));
+    location.hash = "";
+    ok("router defaults to /inventory", parseHash().path === "/inventory");
+  }
+
+  // ── api errors + cancellation ────────────────────────────────────────────
+  {
+    window.fetch = async () =>
+      jsonResponse(404, { error: { code: "not_found", message: "unknown job x" } });
+    await throws(
+      "api surfaces the error envelope",
+      () => api.getJob("x"),
+      (e) => e.status === 404 && e.code === "not_found" && e.message === "unknown job x"
+    );
+
+    window.fetch = async () => {
+      const err = new Error("aborted");
+      err.name = "AbortError";
+      throw err;
+    };
+    const { signal, cancel } = cancellable();
+    cancel();
+    await throws(
+      "api tags aborted requests as cancelled",
+      () => api.getJob("x", { signal }),
+      (e) => e.code === "cancelled"
+    );
+    window.fetch = realFetch;
+  }
+
+  // ── event fallback: polling when EventSource is unavailable ───────────────
+  {
+    window.EventSource = undefined;
+    const pages = [
+      jsonResponse(200, { state: "running", events: [{ seq: 1, type: "queued", message: null }] }),
+      jsonResponse(200, {
+        state: "succeeded",
+        events: [{ seq: 2, type: "state:succeeded", message: null }],
+      }),
+    ];
+    let call = 0;
+    window.fetch = async () => pages[Math.min(call++, pages.length - 1)];
+    const got = [];
+    await new Promise((resolve) => {
+      subscribeJob("job1", {
+        pollMs: 1,
+        onEvent: (e) => got.push(e.seq),
+        onDone: resolve,
+      });
+    });
+    ok("polling delivers events in order", got.join(",") === "1,2");
+    ok("polling stops on terminal state", call >= 2);
+    window.fetch = realFetch;
+  }
+
+  // ── event stream: SSE path, then done ─────────────────────────────────────
+  {
+    class FakeES {
+      constructor() {
+        this.listeners = {};
+        setTimeout(() => {
+          this.onmessage?.({ lastEventId: "5", data: JSON.stringify({ type: "state:running", message: "go" }) });
+          this.listeners.done?.();
+        }, 0);
+      }
+      addEventListener(type, fn) {
+        this.listeners[type] = fn;
+      }
+      close() {
+        this.closed = true;
+      }
+    }
+    window.EventSource = FakeES;
+    let polled = false;
+    window.fetch = async () => {
+      polled = true;
+      return jsonResponse(200, { state: "succeeded", events: [] });
+    };
+    const got = [];
+    await new Promise((resolve) => {
+      subscribeJob("job2", { onEvent: (e) => got.push(e), onDone: resolve });
+    });
+    ok("SSE delivers event with seq from lastEventId", got.length === 1 && got[0].seq === 5);
+    ok("SSE carries type from payload", got[0].type === "state:running");
+    ok("SSE path does not poll", polled === false);
+    window.fetch = realFetch;
+    window.EventSource = realES;
+  }
+
+  await tick();
+  const failed = cases.filter((c) => !c.ok);
+  window.__RESULTS__ = { passed: cases.length - failed.length, failed: failed.length, cases };
+  const pre = document.createElement("pre");
+  pre.id = "results";
+  pre.textContent = cases.map((c) => `${c.ok ? "PASS" : "FAIL"} ${c.name}`).join("\n");
+  document.body.appendChild(pre);
+}
+
+run();
diff --git a/photo_pipeline/api/routes/jobs.py b/photo_pipeline/api/routes/jobs.py
index 1ba42b4..be9eb6f 100644
--- a/photo_pipeline/api/routes/jobs.py
+++ b/photo_pipeline/api/routes/jobs.py
@@ -102,10 +102,13 @@ async def stream_events(job_id: str, request: Request, after: int = 0):
         for _ in range(600):
             for event in service.events_after(job_id, cursor):
                 cursor = event["seq"]
+                # Default (unnamed) event so a browser EventSource receives every
+                # event via onmessage without enumerating the open-ended type set
+                # (state:*, claimed, …); the type travels in the JSON payload,
+                # matching the polling fallback's {seq, type, message} shape.
                 yield (
                     f"id: {event['seq']}\n"
-                    f"event: {event['type']}\n"
-                    f"data: {json.dumps({'message': event['message']})}\n\n"
+                    f"data: {json.dumps({'type': event['type'], 'message': event['message']})}\n\n"
                 )
             snapshot = service.get(job_id)
             if snapshot is None or snapshot["state"] not in ACTIVE_STATES:
diff --git a/tests/e2e/test_frontend_shell.py b/tests/e2e/test_frontend_shell.py
new file mode 100644
index 0000000..69d1a2f
--- /dev/null
+++ b/tests/e2e/test_frontend_shell.py
@@ -0,0 +1,143 @@
+"""Static application shell: the JS unit suite plus browser journeys asserting
+asset loading, deep links, reload-restores-filters, JSON-only APIs, and a clean
+console/network (US02-05).
+
+The unit suite (frontend/js/tests/) runs in the browser over the server's own
+static mount — ES modules can't import over file://, so the running server serves
+them; the tests themselves stub fetch/EventSource and never touch the API.
+"""
+
+import socket
+import subprocess
+import sys
+import time
+from pathlib import Path
+from types import SimpleNamespace
+
+import httpx
+import numpy as np
+import pytest
+from PIL import Image
+
+REPO = Path(__file__).resolve().parents[2]
+
+
+def _image(path, seed):
+    rng = np.random.default_rng(seed)
+    Image.fromarray(rng.integers(0, 256, (96, 128, 3), dtype=np.uint8)).save(path, quality=90)
+
+
+def _free_port():
+    with socket.socket() as sock:
+        sock.bind(("127.0.0.1", 0))
+        return sock.getsockname()[1]
+
+
+@pytest.fixture
+def server(tmp_path):
+    data = tmp_path / "data"
+    data.mkdir()
+    lib = tmp_path / "lib"
+    lib.mkdir()
+    _image(lib / "beach.jpg", 1)
+    _image(lib / "city.jpg", 2)
+
+    from photo_pipeline.config import Config
+    from photo_pipeline.db import create_db_engine, create_session_factory, run_migrations
+    from photo_pipeline.services.inventory import InventoryService
+
+    config = Config.from_env(
+        {"PHOTO_PIPELINE_DATA_DIR": str(data), "PHOTO_PIPELINE_LIBRARY_ROOTS": str(lib)}
+    )
+    run_migrations(config.database_url)
+    engine = create_db_engine(config.database_url)
+    InventoryService(create_session_factory(engine)).scan(lib)
+    engine.dispose()
+
+    port = _free_port()
+    env = {
+        "PATH": __import__("os").environ.get("PATH", ""),
+        "PHOTO_PIPELINE_DATA_DIR": str(data),
+        "PHOTO_PIPELINE_LIBRARY_ROOTS": str(lib),
+        "PHOTO_PIPELINE_HOST": "127.0.0.1",
+        "PHOTO_PIPELINE_PORT": str(port),
+    }
+    proc = subprocess.Popen(
+        [sys.executable, "-m", "photo_pipeline", "serve"],
+        cwd=str(REPO),
+        env=env,
+        stdout=subprocess.PIPE,
+        stderr=subprocess.PIPE,
+    )
+    base = f"http://127.0.0.1:{port}"
+    deadline = time.monotonic() + 30
+    ready = False
+    while time.monotonic() < deadline:
+        if proc.poll() is not None:
+            out, err = proc.communicate()
+            pytest.fail(f"server exited: {err.decode(errors='replace')}")
+        try:
+            if httpx.get(f"{base}/api/v1/health/ready", timeout=1).status_code == 200:
+                ready = True
+                break
+        except httpx.HTTPError:
+            time.sleep(0.2)
+    if not ready:
+        proc.terminate()
+        pytest.fail("server never became ready")
+    try:
+        yield SimpleNamespace(base=base)
+    finally:
+        proc.terminate()
+        try:
+            proc.wait(timeout=10)
+        except subprocess.TimeoutExpired:
+            proc.kill()
+
+
+def test_js_unit_suite_passes(page, server):
+    page.goto(f"{server.base}/app/js/tests/harness.html")
+    page.locator("#results").wait_for()
+    results = page.evaluate("window.__RESULTS__")
+    failing = [c["name"] for c in results["cases"] if not c["ok"]]
+    assert results["failed"] == 0, f"JS unit failures: {failing}"
+    assert results["passed"] >= 12
+
+
+def test_shell_loads_assets_without_console_or_network_errors(page, server):
+    errors, failed = [], []
+    page.on("console", lambda m: errors.append(m.text) if m.type == "error" else None)
+    page.on("requestfailed", lambda r: failed.append(r.url))
+
+    page.goto(f"{server.base}/app/#/inventory")
+    page.get_by_test_id("asset-row").first.wait_for()
+    assert page.locator("nav a[data-nav]").count() >= 2
+    assert errors == [], f"console errors: {errors}"
+    assert failed == [], f"failed requests: {failed}"
+
+
+def test_deep_link_and_reload_restore_view_and_filters(page, server):
+    # Deep-link straight to a filtered inventory view, then reload: the hash router
+    # must restore both the view and the filter with no server-rendered state.
+    page.goto(f"{server.base}/app/#/inventory?q=beach")
+    search = page.get_by_label("Filter by path")
+    search.wait_for()
+    assert search.input_value() == "beach"
+
+    page.reload()
+    page.get_by_label("Filter by path").wait_for()
+    assert page.get_by_label("Filter by path").input_value() == "beach"
+    assert "q=beach" in page.evaluate("location.hash")
+
+
+def test_api_is_json_only_and_html_never_intercepts(server):
+    # /api/v1 always answers JSON — the static SPA mount must never shadow it.
+    ok = httpx.get(f"{server.base}/api/v1/inventory/assets", timeout=5)
+    assert ok.headers["content-type"].startswith("application/json")
+
+    missing = httpx.get(f"{server.base}/api/v1/does-not-exist", timeout=5)
+    assert missing.status_code == 404
+    assert missing.headers["content-type"].startswith("application/json")
+
+    shell = httpx.get(f"{server.base}/app/", timeout=5)
+    assert shell.headers["content-type"].startswith("text/html")