diff --git a/frontend/index.html b/frontend/index.html
index cd324d0..ebf59d5 100644
--- a/frontend/index.html
+++ b/frontend/index.html
@@ -2635,13 +2635,25 @@ function decodeResponse() {
 }
 
 // ===== pysim integration =====
-// When served by pysim-otaman-server itself (same origin), the API is relative;
-// otherwise default to the local server on 127.0.0.1:8080.
-let pysimBase = window.PYSIM_EMBEDDED ? '' : 'http://127.0.0.1:8080';
-if (window.PYSIM_EMBEDDED) {
-	const urlEl = document.getElementById('pysim-url');
-	if (urlEl) { urlEl.value = ''; urlEl.placeholder = '(same origin)'; }
+// Default to a local server on 127.0.0.1:8080. If this PWA is served by
+// pysim-otaman-server itself (same origin), detect the co-located API once
+// and switch to a relative base; otherwise keep the default and wait for the
+// user to point at a server.
+let pysimBase = 'http://127.0.0.1:8080';
+
+async function pysimDetectSameOrigin() {
+	try {
+		const res = await fetch('/api/version', { method: 'GET', cache: 'no-store' });
+		if (res.ok) {
+			pysimBase = '';
+			const urlEl = document.getElementById('pysim-url');
+			if (urlEl) { urlEl.value = ''; urlEl.placeholder = '(same origin)'; }
+		}
+	} catch (e) {
+		// no co-located API (file://, plain static server) — keep the default
+	}
 }
+pysimDetectSameOrigin();
 
 function esc(str) {
 	return str.replace(/&/g, '&amp;').replace(/</g, '&lt;').replace(/>/g, '&gt;');
diff --git a/frontend/sw.js b/frontend/sw.js
index 653cef8..e3b50f1 100644
--- a/frontend/sw.js
+++ b/frontend/sw.js
@@ -32,6 +32,7 @@ self.addEventListener('activate', e => {
 
 self.addEventListener('fetch', e => {
   if (!e.request.url.startsWith('http')) return;
+  if (new URL(e.request.url).pathname.startsWith('/api/')) return;  // live data, never cache
   if (e.request.method !== 'GET') return;
   const isNavigate = e.request.mode === 'navigate' || e.request.url.endsWith('sw.js');
   if (isNavigate) {
diff --git a/pysim_otaman_server/server.py b/pysim_otaman_server/server.py
index 6a5894b..cd21639 100644
--- a/pysim_otaman_server/server.py
+++ b/pysim_otaman_server/server.py
@@ -36,7 +36,6 @@ _STATIC_MIME = {
     '.woff': 'font/woff',
     '.woff2': 'font/woff2',
 }
-_EMBEDDED_MARKER = "<script>window.PYSIM_EMBEDDED='1'</script>"
 
 
 class StderrApduTracer(ApduTracer):
@@ -801,10 +800,6 @@ class PysimHandler(BaseHTTPRequestHandler):
             return
         with open(fs_path, 'rb') as f:
             data = f.read()
-        if rel == 'index.html':
-            # Tell the frontend it is served by this server (same origin), so
-            # it can default to a relative API base instead of 127.0.0.1:8080.
-            data = data.replace(b'</head>', _EMBEDDED_MARKER.encode() + b'</head>', 1)
         content_type = _STATIC_MIME.get(os.path.splitext(rel)[1].lower(), 'application/octet-stream')
         self.send_response(200)
         self.send_header('Content-Type', content_type)