diff --git a/backend/apps/outputs/outputs.py b/backend/apps/outputs/outputs.py
index f18654de..fa38e207 100644
--- a/backend/apps/outputs/outputs.py
+++ b/backend/apps/outputs/outputs.py
@@ -445,18 +445,25 @@ def runtime_status_payload(workspace_id: str, instance: int = 1) -> dict:
         is_new = is_new_mode(folder) if os.path.isdir(folder) else False
         return {
             "running": False,
+            "ready": False,
             "port": None,
+            "serving_url": None,
             "has_backend_file": False,
             "backend_url": None,
             "frontend_port": None,
             "frontend_url": None,
             "is_new_mode": is_new,
         }
+    # The one address a client can actually open (ENG-190: `port` is the OPTIONAL API backend, which legitimately 404s at /, and readers kept treating it as the app).
+    serving_url = rt.frontend_url if rt.is_new_mode else (f"http://127.0.0.1:{rt.port}" if rt.ready and rt.port else None)
     return {
         "running": rt.running,
+        # 'spawned' vs 'serving': ready flips only once the primary port answered the bind poll (and un-flips when the process dies or is frozen).
+        "ready": rt.ready,
         "port": rt.port,
+        "serving_url": serving_url,
         "has_backend_file": rt.has_backend_file,
-        # For old-mode: backend.py serves; backend_url is its port. For new-mode: backend.py is optional (gated by BACKEND_PORT!=NONE); only populated if the agent ran bash backend_init.sh.
+        # For old-mode: backend.py serves; backend_url is its port. For new-mode: backend.py is optional (gated by BACKEND_PORT!=NONE); only populated if the agent ran bash backend_init.sh. 404 at / is NORMAL here: it serves /api routes, not the app.
         "backend_url": f"http://127.0.0.1:{rt.port}" if rt.running and rt.port else None,
         # New-mode only: where the Vite dev server is reachable. Old-mode workspaces report null and the editor falls back to the legacy /api/outputs/workspace/{ws}/serve/... path.
         "frontend_port": rt.frontend_port,
diff --git a/backend/apps/outputs/runtime.py b/backend/apps/outputs/runtime.py
index c43bf432..201bb23e 100644
--- a/backend/apps/outputs/runtime.py
+++ b/backend/apps/outputs/runtime.py
@@ -169,6 +169,17 @@ class AppRuntime:
     def is_new_mode(self) -> bool:
         return is_new_mode(self.workspace_path)
 
+    @property
+    def ready(self) -> bool:
+        """True only when the runtime is actually SERVING (process alive, not frozen, and its
+        primary port answered the bind poll), so callers can tell 'spawned' from 'serving'.
+        Old-mode workspaces have no bind poll; a live process is their best readiness signal."""
+        if not self.running or self.p_suspended:
+            return False
+        if self.is_new_mode:
+            return self.p_frontend_ready
+        return self.port is not None
+
     @property
     def frontend_url(self) -> Optional[str]:
         # Gated on `_frontend_ready` (set by the background bind-poll task in p_start_new_mode) so the preview pane only switches over once Vite is actually accepting connections. Without this, the editor flashes a "Site can't be reached" error while `npm install` is running. Also gated on `running`: a vite that crashed or got orphaned still has _frontend_ready=True, and handing the webview that dead port is the ERR_FAILED you see on reopen. No live process, no URL. And gated on `not _suspended`: a SIGSTOP'd idle runtime is "running" (returncode is None) but frozen, so its port won't answer.
diff --git a/backend/tests/test_runtime_status_ready.py b/backend/tests/test_runtime_status_ready.py
new file mode 100644
index 00000000..f0582f3b
--- /dev/null
+++ b/backend/tests/test_runtime_status_ready.py
@@ -0,0 +1,60 @@
+"""ENG-190: the status payload distinguishes 'spawned' from 'serving' and names the one address a
+client can open, instead of running=True with an unactionable port."""
+
+from backend.apps.outputs.runtime import AppRuntime
+
+
+def p_rt(tmp_path, new_mode: bool) -> AppRuntime:
+    ws = tmp_path / "ws"
+    ws.mkdir(exist_ok=True)
+    if new_mode:
+        (ws / "run.sh").write_text("#!/bin/bash\n")
+    rt = AppRuntime(workspace_id="ws-test", workspace_path=str(ws))
+    return rt
+
+
+def test_ready_false_while_spawned_but_not_serving(tmp_path, monkeypatch):
+    rt = p_rt(tmp_path, new_mode=True)
+    monkeypatch.setattr(type(rt), "running", property(lambda self: True))
+    rt.frontend_port = 5173
+    rt.p_frontend_ready = False
+    assert rt.ready is False
+    assert rt.frontend_url is None
+
+
+def test_ready_true_once_port_answers_and_falls_with_process(tmp_path, monkeypatch):
+    rt = p_rt(tmp_path, new_mode=True)
+    monkeypatch.setattr(type(rt), "running", property(lambda self: True))
+    rt.frontend_port = 5173
+    rt.p_frontend_ready = True
+    assert rt.ready is True
+    assert rt.frontend_url == "http://127.0.0.1:5173/"
+    monkeypatch.setattr(type(rt), "running", property(lambda self: False))
+    assert rt.ready is False
+    assert rt.frontend_url is None
+
+
+def test_frozen_runtime_is_not_ready(tmp_path, monkeypatch):
+    rt = p_rt(tmp_path, new_mode=True)
+    monkeypatch.setattr(type(rt), "running", property(lambda self: True))
+    rt.frontend_port = 5173
+    rt.p_frontend_ready = True
+    rt.p_suspended = True
+    assert rt.ready is False
+
+
+def test_status_payload_carries_ready_and_serving_url(tmp_path, monkeypatch):
+    from backend.apps.outputs import outputs as outputs_mod
+    from backend.apps.outputs.runtime import manager
+
+    rt = p_rt(tmp_path, new_mode=True)
+    monkeypatch.setattr(type(rt), "running", property(lambda self: True))
+    rt.frontend_port = 5173
+    rt.p_frontend_ready = True
+    monkeypatch.setitem(manager.runtimes, "ws-test", rt)
+    payload = outputs_mod.runtime_status_payload("ws-test")
+    assert payload["ready"] is True
+    assert payload["serving_url"] == "http://127.0.0.1:5173/"
+    rt.p_frontend_ready = False
+    payload = outputs_mod.runtime_status_payload("ws-test")
+    assert payload["running"] is True and payload["ready"] is False and payload["serving_url"] is None