fix(react-router-dev): Fix preset future flags merge (#14369)

* fix: preset future flags are ignored during config resolution

Fixes a bug where future flags defined by presets were completely ignored.
The config resolution was incorrectly reading from reactRouterUserConfig.future
instead of the merged userAndPresetConfigs.future, causing all preset-defined
future flags to be lost.

This fix ensures presets can properly enable experimental features as intended
by the preset system design.

Fixes #14368

* Add changeset

Author: juanpprieto

.changeset/fix-preset-future-flags.md

@@ -0,0 +1,9 @@
+---
+"@react-router/dev": patch
+---
+
+Fix preset future flags being ignored during config resolution
+
+Fixes a bug where future flags defined by presets were completely ignored. The config resolution was incorrectly reading from `reactRouterUserConfig.future` instead of the merged `userAndPresetConfigs.future`, causing all preset-defined future flags to be lost.
+
+This fix ensures presets can properly enable experimental features as intended by the preset system design.

integration/vite-presets-test.ts

@@ -114,6 +114,17 @@ const files = {
           }),
         },
 
+        // Ensure presets can set future flags
+        {
+          name: "test-preset",
+          reactRouterConfig: async () => ({
+            future: {
+              v8_middleware: true,
+              unstable_optimizeDeps: true,
+            },
+          }),
+        },
+
         // Ensure presets can set buildEnd option (this is critical for Vercel support)
         {
           name: "test-preset",
@@ -128,6 +139,7 @@ const files = {
                   "export const buildManifest = " + serializeJs(buildManifest, { space: 2, unsafe: true }) + ";",
                   "export const reactRouterConfig = " + serializeJs(reactRouterConfig, { space: 2, unsafe: true }) + ";",
                   "export const assetsDir = " + JSON.stringify(viteConfig.build.assetsDir) + ";",
+                  "export const futureFlags = " + JSON.stringify(reactRouterConfig.future) + ";",
                 ].join("\\n"),
                 "utf-8"
               );
@@ -229,6 +241,15 @@ test.describe("Vite / presets", async () => {
         "unstable_routeConfig",
       ]);
 
+      // Ensure future flags from presets are properly merged
+      expect(buildEndArgsMeta.futureFlags).toEqual({
+        v8_middleware: true,
+        unstable_optimizeDeps: true,
+        unstable_splitRouteModules: false,
+        unstable_subResourceIntegrity: false,
+        unstable_viteEnvironmentApi: false,
+      });
+
       // Ensure we get a valid build manifest
       expect(buildEndArgsMeta.buildManifest).toEqual({
         routeIdToServerBundleId: {

packages/react-router-dev/config/config.ts

@@ -587,15 +587,15 @@ async function resolveConfig({
   }
 
   let future: FutureConfig = {
-    v8_middleware: reactRouterUserConfig.future?.v8_middleware ?? false,
+    v8_middleware: userAndPresetConfigs.future?.v8_middleware ?? false,
     unstable_optimizeDeps:
-      reactRouterUserConfig.future?.unstable_optimizeDeps ?? false,
+      userAndPresetConfigs.future?.unstable_optimizeDeps ?? false,
     unstable_splitRouteModules:
-      reactRouterUserConfig.future?.unstable_splitRouteModules ?? false,
+      userAndPresetConfigs.future?.unstable_splitRouteModules ?? false,
     unstable_subResourceIntegrity:
-      reactRouterUserConfig.future?.unstable_subResourceIntegrity ?? false,
+      userAndPresetConfigs.future?.unstable_subResourceIntegrity ?? false,
     unstable_viteEnvironmentApi:
-      reactRouterUserConfig.future?.unstable_viteEnvironmentApi ?? false,
+      userAndPresetConfigs.future?.unstable_viteEnvironmentApi ?? false,
   };
 
   let reactRouterConfig: ResolvedReactRouterConfig = deepFreeze({