diff --git a/DEPLOYMENT_PROFILES.md b/DEPLOYMENT_PROFILES.md
index 3268f9a..722560d 100644
--- a/DEPLOYMENT_PROFILES.md
+++ b/DEPLOYMENT_PROFILES.md
@@ -263,6 +263,10 @@ next one starts.
    origin (`app/auth_middleware.py` gates writes on the same list).
    Verify: `curl -sD - -o /dev/null -H "Origin: https://<new-domain>" https://<new-domain>/api/healthz`
    must echo the origin back in `access-control-allow-origin`.
+   (On Railway the service now also trusts its own `RAILWAY_PUBLIC_DOMAIN`
+   origin *in addition to* this list, so a stale configured value can no
+   longer lock the service out of its own domain — but explicit origins for
+   any other dashboard host still need this step.)
 2. **Replace secret reference variables with concrete values.**
    `REGENGINE_BASIC_AUTH_USERNAME`, `REGENGINE_BASIC_AUTH_PASSWORD`, and
    `REGENGINE_WEBHOOK_HMAC_SECRET` may reference the old service. Deleting
diff --git a/app/cors.py b/app/cors.py
index db97ea7..3a414c0 100644
--- a/app/cors.py
+++ b/app/cors.py
@@ -10,14 +10,42 @@
 def cors_origins_from_env() -> list[str]:
     raw_origins = os.getenv("REGENGINE_CORS_ORIGINS")
     if not raw_origins or not raw_origins.strip():
-        return list(DEFAULT_CORS_ORIGINS)
-
-    origins: list[str] = []
-    for raw_origin in raw_origins.split(","):
-        origin = _normalize_cors_origin(raw_origin)
-        if origin and origin not in origins:
-            origins.append(origin)
-    return origins or list(DEFAULT_CORS_ORIGINS)
+        origins = list(DEFAULT_CORS_ORIGINS)
+    else:
+        origins = []
+        for raw_origin in raw_origins.split(","):
+            origin = _normalize_cors_origin(raw_origin)
+            if origin and origin not in origins:
+                origins.append(origin)
+        origins = origins or list(DEFAULT_CORS_ORIGINS)
+
+    # The service always trusts its own platform-issued domain, IN ADDITION to
+    # whatever is configured. A union, not a fallback, on purpose: in the
+    # August 2026 cutover REGENGINE_CORS_ORIGINS was set — to the previous
+    # service's URL, via a Railway reference variable — so a fallback would
+    # have changed nothing, and the service rejected every browser request
+    # from its own domain for three days (see DEPLOYMENT_PROFILES.md,
+    # "Moving the demo to a new service"). Trusting RAILWAY_PUBLIC_DOMAIN is
+    # safe because whoever controls that variable controls the deployment
+    # itself; it widens nothing beyond the service's own canonical origin.
+    platform_origin = _platform_origin()
+    if platform_origin and platform_origin not in origins:
+        origins.append(platform_origin)
+    return origins
+
+
+def _platform_origin() -> str | None:
+    domain = os.getenv("RAILWAY_PUBLIC_DOMAIN", "").strip().rstrip("/")
+    if not domain:
+        return None
+    if "://" not in domain:
+        domain = f"https://{domain}"
+    try:
+        return _normalize_cors_origin(domain)
+    except ValueError:
+        # A malformed platform value must degrade to "no extra origin", never
+        # crash startup — this runs while the ASGI app is being constructed.
+        return None
 
 
 def _normalize_cors_origin(raw_origin: str) -> str | None:
diff --git a/tests/test_api.py b/tests/test_api.py
index 33f498f..88e7f07 100644
--- a/tests/test_api.py
+++ b/tests/test_api.py
@@ -9,6 +9,7 @@
 from fastapi.testclient import TestClient
 
 from app.build_info import BRANCH_ENV_VARS, COMMIT_SHA_ENV_VARS, DEPLOYMENT_ID_ENV_VARS
+from app.cors import DEFAULT_CORS_ORIGINS
 from app.main import app, controller, cors_origins_from_env, scenario_saves
 from app.schemas.simulation import SimulationConfig
 from app.regengine_client import LiveIngestResult, LiveRegEngineDeliveryError
@@ -201,6 +202,42 @@ def test_cors_origins_can_be_configured_without_wildcard_credentials(monkeypatch
         cors_origins_from_env()
 
 
+def test_cors_allowlist_always_trusts_the_platform_domain(monkeypatch):
+    monkeypatch.setenv("RAILWAY_PUBLIC_DOMAIN", "demo.up.railway.app")
+
+    # With no explicit config, the platform origin joins the local defaults.
+    monkeypatch.delenv("REGENGINE_CORS_ORIGINS", raising=False)
+    assert cors_origins_from_env() == [
+        *DEFAULT_CORS_ORIGINS,
+        "https://demo.up.railway.app",
+    ]
+
+    # An explicit-but-stale list can no longer lock the service out of its own
+    # domain — the regression that kept the nightly smokes red after the
+    # August 2026 cutover (issues #80/#81).
+    monkeypatch.setenv("REGENGINE_CORS_ORIGINS", "https://old.up.railway.app")
+    assert cors_origins_from_env() == [
+        "https://old.up.railway.app",
+        "https://demo.up.railway.app",
+    ]
+
+    # No duplicate when the platform origin is already configured.
+    monkeypatch.setenv("REGENGINE_CORS_ORIGINS", "https://demo.up.railway.app")
+    assert cors_origins_from_env() == ["https://demo.up.railway.app"]
+
+
+def test_cors_platform_domain_never_crashes_startup(monkeypatch):
+    monkeypatch.delenv("REGENGINE_CORS_ORIGINS", raising=False)
+
+    # A malformed platform value degrades to "no extra origin", not a raise —
+    # this path runs while the ASGI app is being constructed.
+    monkeypatch.setenv("RAILWAY_PUBLIC_DOMAIN", "demo.up.railway.app/?bad=1")
+    assert cors_origins_from_env() == list(DEFAULT_CORS_ORIGINS)
+
+    monkeypatch.setenv("RAILWAY_PUBLIC_DOMAIN", "   ")
+    assert cors_origins_from_env() == list(DEFAULT_CORS_ORIGINS)
+
+
 def test_basic_auth_is_optional_but_enforced_when_configured(monkeypatch):
     health_response = client.get("/api/health")
     assert health_response.status_code == 200