From 23089661b90d26bdb60533a64302c639c40c7122 Mon Sep 17 00:00:00 2001
From: Eduard Fischer-Szava <50618110+EduardF1@users.noreply.github.com>
Date: Sun, 7 Jun 2026 03:21:51 +0200
Subject: [PATCH] feat(start): support serving assets via a relative base url

Fixes #5966
---
 packages/react-router/src/Asset.tsx                      | 8 +++++++-
 .../src/start-manifest-plugin/manifestBuilder.ts         | 9 ++++++++-
 2 files changed, 15 insertions(+), 2 deletions(-)

diff --git a/packages/react-router/src/Asset.tsx b/packages/react-router/src/Asset.tsx
index e3fa1c06de..2bf529819c 100644
--- a/packages/react-router/src/Asset.tsx
+++ b/packages/react-router/src/Asset.tsx
@@ -136,7 +136,13 @@ function Script({
       const normSrc = (() => {
         try {
           const base = document.baseURI || window.location.href
-          return new URL(attrs.src, base).href
+          // If the src is explicitly relative and base is present, construct it carefully
+          // Without returning absolute URLs that break iframe/embedded deployments
+          const url = new URL(attrs.src, base)
+          if (attrs.src.startsWith('./') && !base.startsWith('http')) {
+            return attrs.src
+          }
+          return url.href
         } catch {
           return attrs.src
         }
diff --git a/packages/start-plugin-core/src/start-manifest-plugin/manifestBuilder.ts b/packages/start-plugin-core/src/start-manifest-plugin/manifestBuilder.ts
index 6f27f37d4d..3e31cb38b5 100644
--- a/packages/start-plugin-core/src/start-manifest-plugin/manifestBuilder.ts
+++ b/packages/start-plugin-core/src/start-manifest-plugin/manifestBuilder.ts
@@ -263,7 +263,14 @@ export function createManifestAssetResolvers(
       return cachedPath
     }
 
-    const assetPath = joinURL(basePath, fileName)
+    let assetPath = joinURL(basePath, fileName)
+    if (basePath === './') {
+      // Avoid inserting a leading slash if base path is purely relative
+      assetPath = './' + fileName
+    } else if (basePath.startsWith('./') && assetPath.startsWith('/')) {
+      assetPath = '.' + assetPath
+    }
+
     assetPathByFileName.set(fileName, assetPath)
     return assetPath
   }