diff --git a/design/src/compare.ts b/design/src/compare.ts
index 547c85558..cd1be0ab6 100644
--- a/design/src/compare.ts
+++ b/design/src/compare.ts
@@ -391,6 +391,17 @@ export function generateCompareHtml(images: string[]): string {
 <div id="feedback-result"></div>
 
 <script>
+  // Feature-detect: are we being served over HTTP (by serve.ts or the
+  // daemon), or opened directly as a file:// URL? In file:// mode the
+  // board JS falls through to a DOM-only success path with no server
+  // round-trips. Using location.protocol instead of an injected global
+  // means the same generated HTML works at both / (legacy --no-daemon)
+  // and /boards/<id>/ (daemon) — relative URLs resolve against
+  // location.pathname in both cases.
+  function hasServer() {
+    return location.protocol === 'http:' || location.protocol === 'https:';
+  }
+
   // View toggle
   document.querySelectorAll('.view-toggle button').forEach(function(btn) {
     btn.addEventListener('click', function() {
@@ -465,8 +476,8 @@ export function generateCompareHtml(images: string[]): string {
   });
 
   function postFeedback(feedback) {
-    if (!window.__GSTACK_SERVER_URL) return Promise.resolve(null);
-    return fetch(window.__GSTACK_SERVER_URL + '/api/feedback', {
+    if (!hasServer()) return Promise.resolve(null);
+    return fetch('./api/feedback', {
       method: 'POST',
       headers: { 'Content-Type': 'application/json' },
       body: JSON.stringify(feedback),
@@ -509,7 +520,7 @@ export function generateCompareHtml(images: string[]): string {
   }
 
   function startProgressPolling() {
-    if (!window.__GSTACK_SERVER_URL) return;
+    if (!hasServer()) return;
     var pollCount = 0;
     var maxPolls = 150; // 5 min at 2s intervals
     var pollInterval = setInterval(function() {
@@ -523,7 +534,7 @@ export function generateCompareHtml(images: string[]): string {
           '</div>';
         return;
       }
-      fetch(window.__GSTACK_SERVER_URL + '/api/progress')
+      fetch('./api/progress')
         .then(function(r) { return r.json(); })
         .then(function(data) {
           if (data.status === 'serving') {
@@ -563,7 +574,7 @@ export function generateCompareHtml(images: string[]): string {
     postFeedback(feedback).then(function(result) {
       if (result && result.received) {
         showRegeneratingState();
-      } else if (window.__GSTACK_SERVER_URL) {
+      } else if (hasServer()) {
         showPostFailure(feedback);
       }
     });
@@ -578,7 +589,7 @@ export function generateCompareHtml(images: string[]): string {
     postFeedback(feedback).then(function(result) {
       if (result && result.received) {
         showPostSubmitState();
-      } else if (window.__GSTACK_SERVER_URL) {
+      } else if (hasServer()) {
         showPostFailure(feedback);
       } else {
         // DOM-only mode (legacy / test)
diff --git a/design/src/serve.ts b/design/src/serve.ts
index 9fd5fd665..6ee1abf17 100644
--- a/design/src/serve.ts
+++ b/design/src/serve.ts
@@ -1,12 +1,18 @@
 /**
  * HTTP server for the design comparison board feedback loop.
  *
- * Replaces the broken file:// + DOM polling approach. The server:
- * 1. Serves the comparison board HTML over HTTP
- * 2. Injects __GSTACK_SERVER_URL so the board POSTs feedback here
- * 3. Prints feedback JSON to stdout (agent reads it)
- * 4. Stays alive across regeneration rounds (stateful)
- * 5. Auto-opens in the user's default browser
+ * Legacy single-process path: spawned by `$D compare --serve --no-daemon`.
+ * The daemon (`design/src/daemon.ts`) handles default invocations and hosts
+ * multiple boards under `/boards/<id>/`; this file stays as the escape hatch
+ * for tests and debugging. Board JS uses relative URLs and a
+ * location.protocol feature-detect, so the same generated HTML works at
+ * both `/` (here) and `/boards/<id>/` (daemon).
+ *
+ * The server:
+ * 1. Serves the comparison board HTML over HTTP at `/`
+ * 2. Prints feedback JSON to stdout (agent reads it)
+ * 3. Stays alive across regeneration rounds (stateful)
+ * 4. Auto-opens in the user's default browser
  *
  * State machine:
  *
@@ -69,17 +75,14 @@ export async function serve(options: ServeOptions): Promise<void> {
     fetch(req) {
       const url = new URL(req.url);
 
-      // Serve the comparison board HTML
+      // Serve the comparison board HTML. The board JS uses relative paths
+      // (./api/feedback, ./api/progress) and a location.protocol
+      // feature-detect, so no per-request injection is needed.
       if (
         req.method === "GET" &&
         (url.pathname === "/" || url.pathname === "/index.html")
       ) {
-        // Inject the server URL so the board can POST feedback
-        const injected = htmlContent.replace(
-          "</head>",
-          `<script>window.__GSTACK_SERVER_URL = ${JSON.stringify(url.origin)};</script>\n</head>`,
-        );
-        return new Response(injected, {
+        return new Response(htmlContent, {
           headers: { "Content-Type": "text/html; charset=utf-8" },
         });
       }
diff --git a/design/test/feedback-roundtrip.test.ts b/design/test/feedback-roundtrip.test.ts
index cd757f38b..e8d63db23 100644
--- a/design/test/feedback-roundtrip.test.ts
+++ b/design/test/feedback-roundtrip.test.ts
@@ -55,7 +55,7 @@ beforeAll(async () => {
   serverState = 'serving';
 
   // This server mirrors the real serve.ts behavior:
-  // - Injects __GSTACK_SERVER_URL into the HTML
+  // - Serves board HTML at / (board JS uses relative URLs)
   // - Handles POST /api/feedback with file writes
   // - Handles GET /api/progress for regeneration polling
   // - Handles POST /api/reload for board swapping
@@ -67,11 +67,7 @@ beforeAll(async () => {
       const url = new URL(req.url);
 
       if (req.method === 'GET' && (url.pathname === '/' || url.pathname === '/index.html')) {
-        const injected = currentHtml.replace(
-          '</head>',
-          `<script>window.__GSTACK_SERVER_URL = '${url.origin}';</script>\n</head>`
-        );
-        return new Response(injected, {
+        return new Response(currentHtml, {
           headers: { 'Content-Type': 'text/html; charset=utf-8' },
         });
       }
@@ -140,14 +136,15 @@ describe('Submit: browser click → feedback.json on disk', () => {
     if (fs.existsSync(feedbackPath)) fs.unlinkSync(feedbackPath);
     serverState = 'serving';
 
-    // Navigate to the board (served with __GSTACK_SERVER_URL injected)
+    // Navigate to the board (board JS uses relative URLs + location.protocol detect)
     await handleWriteCommand('goto', [baseUrl], bm);
 
-    // Verify __GSTACK_SERVER_URL was injected
-    const hasServerUrl = await handleReadCommand('js', [
-      '!!window.__GSTACK_SERVER_URL'
+    // Verify the board detects HTTP mode (so postFeedback will actually fetch
+    // instead of falling into the file:// DOM-only path)
+    const httpDetected = await handleReadCommand('js', [
+      "location.protocol === 'http:' || location.protocol === 'https:'"
     ], bm);
-    expect(hasServerUrl).toBe('true');
+    expect(httpDetected).toBe('true');
 
     // User picks variant A, rates it 5 stars
     await handleReadCommand('js', [
diff --git a/design/test/serve.test.ts b/design/test/serve.test.ts
index f222a6364..d3a30bcdc 100644
--- a/design/test/serve.test.ts
+++ b/design/test/serve.test.ts
@@ -65,11 +65,9 @@ describe('Serve HTTP endpoints', () => {
         const url = new URL(req.url);
 
         if (req.method === 'GET' && url.pathname === '/') {
-          const injected = htmlContent.replace(
-            '</head>',
-            `<script>window.__GSTACK_SERVER_URL = '${url.origin}';</script>\n</head>`
-          );
-          return new Response(injected, {
+          // Board JS uses relative URLs (./api/feedback, ./api/progress)
+          // and a location.protocol feature-detect; no injection needed.
+          return new Response(htmlContent, {
             headers: { 'Content-Type': 'text/html; charset=utf-8' },
           });
         }
@@ -118,12 +116,17 @@ describe('Serve HTTP endpoints', () => {
     server.stop();
   });
 
-  test('GET / serves HTML with injected __GSTACK_SERVER_URL', async () => {
+  test('GET / serves HTML with relative-path board JS (no injection)', async () => {
     const res = await fetch(baseUrl);
     expect(res.status).toBe(200);
     const html = await res.text();
-    expect(html).toContain('__GSTACK_SERVER_URL');
-    expect(html).toContain(baseUrl);
+    // No more per-origin URL injection; board JS uses relative paths.
+    expect(html).not.toContain('__GSTACK_SERVER_URL');
+    expect(html).not.toContain(baseUrl);
+    // Board JS calls relative endpoints so the same HTML works at / and at
+    // /boards/<id>/ (daemon mode).
+    expect(html).toContain("fetch('./api/feedback'");
+    expect(html).toContain("fetch('./api/progress')");
     expect(html).toContain('Design Exploration');
   });