diff --git a/packages/react-router/src/Asset.tsx b/packages/react-router/src/Asset.tsx
index ef82d6637a..7a81b87f17 100644
--- a/packages/react-router/src/Asset.tsx
+++ b/packages/react-router/src/Asset.tsx
@@ -1,5 +1,5 @@
 import * as React from 'react'
-import { isServer } from '@tanstack/router-core'
+import { isServer } from '@tanstack/router-core/isServer'
 import { useRouter } from './useRouter'
 import type { RouterManagedTag } from '@tanstack/router-core'
 
diff --git a/packages/react-router/src/Match.tsx b/packages/react-router/src/Match.tsx
index e50f9b7b7d..c5faa866fc 100644
--- a/packages/react-router/src/Match.tsx
+++ b/packages/react-router/src/Match.tsx
@@ -6,9 +6,9 @@ import {
   getLocationChangeInfo,
   isNotFound,
   isRedirect,
-  isServer,
   rootRouteId,
 } from '@tanstack/router-core'
+import { isServer } from '@tanstack/router-core/isServer'
 import { CatchBoundary, ErrorComponent } from './CatchBoundary'
 import { useRouterState } from './useRouterState'
 import { useRouter } from './useRouter'
diff --git a/packages/react-router/src/Matches.tsx b/packages/react-router/src/Matches.tsx
index 8c1b661b86..de8a2ea73e 100644
--- a/packages/react-router/src/Matches.tsx
+++ b/packages/react-router/src/Matches.tsx
@@ -1,6 +1,7 @@
 import * as React from 'react'
 import warning from 'tiny-warning'
-import { isServer, rootRouteId } from '@tanstack/router-core'
+import { rootRouteId } from '@tanstack/router-core'
+import { isServer } from '@tanstack/router-core/isServer'
 import { CatchBoundary, ErrorComponent } from './CatchBoundary'
 import { useRouterState } from './useRouterState'
 import { useRouter } from './useRouter'
diff --git a/packages/react-router/src/ScriptOnce.tsx b/packages/react-router/src/ScriptOnce.tsx
index c37bb51a30..f552becb9f 100644
--- a/packages/react-router/src/ScriptOnce.tsx
+++ b/packages/react-router/src/ScriptOnce.tsx
@@ -1,4 +1,4 @@
-import { isServer } from '@tanstack/router-core'
+import { isServer } from '@tanstack/router-core/isServer'
 import { useRouter } from './useRouter'
 
 /**
diff --git a/packages/react-router/src/link.tsx b/packages/react-router/src/link.tsx
index 1a8eda4eb5..2fb69f8b52 100644
--- a/packages/react-router/src/link.tsx
+++ b/packages/react-router/src/link.tsx
@@ -5,10 +5,10 @@ import {
   exactPathTest,
   functionalUpdate,
   isDangerousProtocol,
-  isServer,
   preloadWarning,
   removeTrailingSlash,
 } from '@tanstack/router-core'
+import { isServer } from '@tanstack/router-core/isServer'
 import { useRouterState } from './useRouterState'
 import { useRouter } from './useRouter'
 
diff --git a/packages/react-router/src/scroll-restoration.tsx b/packages/react-router/src/scroll-restoration.tsx
index 8a88e9a671..f8c3c4559a 100644
--- a/packages/react-router/src/scroll-restoration.tsx
+++ b/packages/react-router/src/scroll-restoration.tsx
@@ -1,10 +1,10 @@
 import {
   defaultGetScrollRestorationKey,
   escapeHtml,
-  isServer,
   restoreScroll,
   storageKey,
 } from '@tanstack/router-core'
+import { isServer } from '@tanstack/router-core/isServer'
 import { useRouter } from './useRouter'
 import { ScriptOnce } from './ScriptOnce'
 
diff --git a/packages/router-core/src/index.ts b/packages/router-core/src/index.ts
index 5fe3a1af74..1897f07880 100644
--- a/packages/router-core/src/index.ts
+++ b/packages/router-core/src/index.ts
@@ -105,7 +105,6 @@ export {
 export { encode, decode } from './qss'
 export { rootRouteId } from './root'
 export type { RootRouteId } from './root'
-export { isServer } from './isServer'
 
 export { BaseRoute, BaseRouteApi, BaseRootRoute } from './route'
 export type {
diff --git a/packages/router-core/src/isServer.ts b/packages/router-core/src/isServer.ts
deleted file mode 100644
index 98f73e1261..0000000000
--- a/packages/router-core/src/isServer.ts
+++ /dev/null
@@ -1,24 +0,0 @@
-/**
- * Static server/client detection for tree-shaking support.
- *
- * This re-exports `isServer` from the `isServer/` directory which uses
- * conditional exports to provide different values based on the environment:
- *
- * - `browser` condition → `false` (client)
- * - `node`/`worker`/`deno`/`bun` → `true` (server)
- * - `development` condition → `undefined` (for tests, falls back to router.isServer)
- *
- * The bundler resolves the correct file at build time based on export conditions,
- * and since the value is a literal constant, dead code can be eliminated.
- *
- * @example
- * ```typescript
- * import { isServer } from '@tanstack/router-core'
- *
- * // The ?? operator provides fallback for development/test mode
- * if (isServer ?? router.isServer) {
- *   // Server-only code - eliminated in client bundles
- * }
- * ```
- */
-export { isServer } from './isServer/development'
diff --git a/packages/router-core/src/load-matches.ts b/packages/router-core/src/load-matches.ts
index e524032878..1cca25ed05 100644
--- a/packages/router-core/src/load-matches.ts
+++ b/packages/router-core/src/load-matches.ts
@@ -1,10 +1,10 @@
 import { batch } from '@tanstack/store'
 import invariant from 'tiny-invariant'
+import { isServer } from '@tanstack/router-core/isServer'
 import { createControlledPromise, isPromise } from './utils'
 import { isNotFound } from './not-found'
 import { rootRouteId } from './root'
 import { isRedirect } from './redirect'
-import { isServer } from './isServer'
 import type { NotFoundError } from './not-found'
 import type { ParsedLocation } from './location'
 import type {
diff --git a/packages/router-core/src/router.ts b/packages/router-core/src/router.ts
index 852e8d324f..6ffbd0b493 100644
--- a/packages/router-core/src/router.ts
+++ b/packages/router-core/src/router.ts
@@ -1,5 +1,6 @@
 import { Store, batch } from '@tanstack/store'
 import { createBrowserHistory, parseHref } from '@tanstack/history'
+import { isServer } from '@tanstack/router-core/isServer'
 import {
   createControlledPromise,
   decodePath,
@@ -26,7 +27,6 @@ import {
   trimPath,
   trimPathRight,
 } from './path'
-import { isServer } from './isServer'
 import { createLRUCache } from './lru-cache'
 import { isNotFound } from './not-found'
 import { setupScrollRestoration } from './scroll-restoration'
diff --git a/packages/router-core/src/scroll-restoration.ts b/packages/router-core/src/scroll-restoration.ts
index fbc5eb9c58..ec1925c137 100644
--- a/packages/router-core/src/scroll-restoration.ts
+++ b/packages/router-core/src/scroll-restoration.ts
@@ -1,5 +1,5 @@
+import { isServer } from '@tanstack/router-core/isServer'
 import { functionalUpdate } from './utils'
-import { isServer } from './isServer'
 import type { AnyRouter } from './router'
 import type { ParsedLocation } from './location'
 import type { NonNullableUpdater } from './utils'
diff --git a/packages/router-core/tsconfig.json b/packages/router-core/tsconfig.json
index e99dc8130a..2685821ee8 100644
--- a/packages/router-core/tsconfig.json
+++ b/packages/router-core/tsconfig.json
@@ -1,4 +1,9 @@
 {
   "extends": "../../tsconfig.json",
+  "compilerOptions": {
+    "paths": {
+      "@tanstack/router-core/isServer": ["./src/isServer/development.ts"]
+    }
+  },
   "include": ["src", "vite.config.ts", "tests", "vite-minify-plugin.ts"]
 }
diff --git a/packages/router-core/vite.config.ts b/packages/router-core/vite.config.ts
index c6f7fc9220..a1a2f56562 100644
--- a/packages/router-core/vite.config.ts
+++ b/packages/router-core/vite.config.ts
@@ -1,3 +1,4 @@
+import path from 'node:path'
 import { defineConfig, mergeConfig } from 'vitest/config'
 import { tanstackViteConfig } from '@tanstack/config/vite'
 import minifyScriptPlugin from './vite-minify-plugin'
@@ -12,6 +13,14 @@ const config = defineConfig({
     watch: false,
     environment: 'jsdom',
     typecheck: { enabled: true },
+    alias: {
+      // For tests only, resolve to development.ts which returns undefined
+      // so that router.isServer fallback is used
+      '@tanstack/router-core/isServer': path.resolve(
+        __dirname,
+        'src/isServer/development.ts',
+      ),
+    },
   },
 })
 
@@ -27,5 +36,6 @@ export default mergeConfig(
       './src/isServer/development.ts',
     ],
     srcDir: './src',
+    externalDeps: ['@tanstack/router-core/isServer'],
   }),
 )
diff --git a/packages/router-ssr-query-core/src/index.ts b/packages/router-ssr-query-core/src/index.ts
index 768fbaab6d..1ef00b1a7a 100644
--- a/packages/router-ssr-query-core/src/index.ts
+++ b/packages/router-ssr-query-core/src/index.ts
@@ -2,7 +2,8 @@ import {
   dehydrate as queryDehydrate,
   hydrate as queryHydrate,
 } from '@tanstack/query-core'
-import { isRedirect, isServer } from '@tanstack/router-core'
+import { isRedirect } from '@tanstack/router-core'
+import { isServer } from '@tanstack/router-core/isServer'
 import type { AnyRouter } from '@tanstack/router-core'
 import type {
   QueryClient,
diff --git a/packages/solid-router/src/Asset.tsx b/packages/solid-router/src/Asset.tsx
index 13d30918e0..3d89c2e1d3 100644
--- a/packages/solid-router/src/Asset.tsx
+++ b/packages/solid-router/src/Asset.tsx
@@ -1,6 +1,6 @@
 import { Link, Meta, Style, Title } from '@solidjs/meta'
 import { onCleanup, onMount } from 'solid-js'
-import { isServer } from '@tanstack/router-core'
+import { isServer } from '@tanstack/router-core/isServer'
 import { useRouter } from './useRouter'
 import type { RouterManagedTag } from '@tanstack/router-core'
 import type { JSX } from 'solid-js'
diff --git a/packages/solid-router/src/Match.tsx b/packages/solid-router/src/Match.tsx
index 3b2660c4c2..98d185513f 100644
--- a/packages/solid-router/src/Match.tsx
+++ b/packages/solid-router/src/Match.tsx
@@ -6,9 +6,9 @@ import {
   getLocationChangeInfo,
   isNotFound,
   isRedirect,
-  isServer,
   rootRouteId,
 } from '@tanstack/router-core'
+import { isServer } from '@tanstack/router-core/isServer'
 import { Dynamic } from 'solid-js/web'
 import { CatchBoundary, ErrorComponent } from './CatchBoundary'
 import { useRouterState } from './useRouterState'
diff --git a/packages/solid-router/src/Matches.tsx b/packages/solid-router/src/Matches.tsx
index 8848421082..a39f09a4db 100644
--- a/packages/solid-router/src/Matches.tsx
+++ b/packages/solid-router/src/Matches.tsx
@@ -1,6 +1,7 @@
 import * as Solid from 'solid-js'
 import warning from 'tiny-warning'
-import { isServer, rootRouteId } from '@tanstack/router-core'
+import { rootRouteId } from '@tanstack/router-core'
+import { isServer } from '@tanstack/router-core/isServer'
 import { CatchBoundary, ErrorComponent } from './CatchBoundary'
 import { useRouterState } from './useRouterState'
 import { useRouter } from './useRouter'
diff --git a/packages/solid-router/src/ScriptOnce.tsx b/packages/solid-router/src/ScriptOnce.tsx
index 703acf4015..a0981a26ba 100644
--- a/packages/solid-router/src/ScriptOnce.tsx
+++ b/packages/solid-router/src/ScriptOnce.tsx
@@ -1,4 +1,4 @@
-import { isServer } from '@tanstack/router-core'
+import { isServer } from '@tanstack/router-core/isServer'
 import { useRouter } from './useRouter'
 
 export function ScriptOnce({
diff --git a/packages/solid-router/src/Transitioner.tsx b/packages/solid-router/src/Transitioner.tsx
index e6ed2937de..c061d769d4 100644
--- a/packages/solid-router/src/Transitioner.tsx
+++ b/packages/solid-router/src/Transitioner.tsx
@@ -2,9 +2,9 @@ import * as Solid from 'solid-js'
 import {
   getLocationChangeInfo,
   handleHashScroll,
-  isServer,
   trimPathRight,
 } from '@tanstack/router-core'
+import { isServer } from '@tanstack/router-core/isServer'
 import { useRouter } from './useRouter'
 import { useRouterState } from './useRouterState'
 import { usePrevious } from './utils'
diff --git a/packages/solid-router/src/scroll-restoration.tsx b/packages/solid-router/src/scroll-restoration.tsx
index 8a88e9a671..f8c3c4559a 100644
--- a/packages/solid-router/src/scroll-restoration.tsx
+++ b/packages/solid-router/src/scroll-restoration.tsx
@@ -1,10 +1,10 @@
 import {
   defaultGetScrollRestorationKey,
   escapeHtml,
-  isServer,
   restoreScroll,
   storageKey,
 } from '@tanstack/router-core'
+import { isServer } from '@tanstack/router-core/isServer'
 import { useRouter } from './useRouter'
 import { ScriptOnce } from './ScriptOnce'
 
diff --git a/packages/vue-router/src/Asset.tsx b/packages/vue-router/src/Asset.tsx
index d30ec2cf6d..80f61398f9 100644
--- a/packages/vue-router/src/Asset.tsx
+++ b/packages/vue-router/src/Asset.tsx
@@ -1,5 +1,5 @@
 import * as Vue from 'vue'
-import { isServer } from '@tanstack/router-core'
+import { isServer } from '@tanstack/router-core/isServer'
 import { useRouter } from './useRouter'
 import type { RouterManagedTag } from '@tanstack/router-core'
 
diff --git a/packages/vue-router/src/Match.tsx b/packages/vue-router/src/Match.tsx
index 1933c0b5ee..4156611a59 100644
--- a/packages/vue-router/src/Match.tsx
+++ b/packages/vue-router/src/Match.tsx
@@ -6,9 +6,9 @@ import {
   getLocationChangeInfo,
   isNotFound,
   isRedirect,
-  isServer,
   rootRouteId,
 } from '@tanstack/router-core'
+import { isServer } from '@tanstack/router-core/isServer'
 import { CatchBoundary, ErrorComponent } from './CatchBoundary'
 import { ClientOnly } from './ClientOnly'
 import { useRouterState } from './useRouterState'
diff --git a/packages/vue-router/src/Matches.tsx b/packages/vue-router/src/Matches.tsx
index 9fefac59a7..1335447621 100644
--- a/packages/vue-router/src/Matches.tsx
+++ b/packages/vue-router/src/Matches.tsx
@@ -1,6 +1,6 @@
 import * as Vue from 'vue'
 import warning from 'tiny-warning'
-import { isServer } from '@tanstack/router-core'
+import { isServer } from '@tanstack/router-core/isServer'
 import { CatchBoundary } from './CatchBoundary'
 import { useRouterState } from './useRouterState'
 import { useRouter } from './useRouter'
diff --git a/packages/vue-router/src/ScriptOnce.tsx b/packages/vue-router/src/ScriptOnce.tsx
index 1d63983b65..2d8e0cfa2c 100644
--- a/packages/vue-router/src/ScriptOnce.tsx
+++ b/packages/vue-router/src/ScriptOnce.tsx
@@ -1,5 +1,5 @@
 import * as Vue from 'vue'
-import { isServer } from '@tanstack/router-core'
+import { isServer } from '@tanstack/router-core/isServer'
 import { useRouter } from './useRouter'
 
 export const ScriptOnce = Vue.defineComponent({
diff --git a/packages/vue-router/src/Transitioner.tsx b/packages/vue-router/src/Transitioner.tsx
index 88c8ac28a5..90f807ca75 100644
--- a/packages/vue-router/src/Transitioner.tsx
+++ b/packages/vue-router/src/Transitioner.tsx
@@ -2,9 +2,9 @@ import * as Vue from 'vue'
 import {
   getLocationChangeInfo,
   handleHashScroll,
-  isServer,
   trimPathRight,
 } from '@tanstack/router-core'
+import { isServer } from '@tanstack/router-core/isServer'
 import { useRouter } from './useRouter'
 import { useRouterState } from './useRouterState'
 import { usePrevious } from './utils'
diff --git a/packages/vue-router/src/scroll-restoration.tsx b/packages/vue-router/src/scroll-restoration.tsx
index 8a4570f2c6..156bc0a93e 100644
--- a/packages/vue-router/src/scroll-restoration.tsx
+++ b/packages/vue-router/src/scroll-restoration.tsx
@@ -2,10 +2,10 @@ import * as Vue from 'vue'
 import {
   defaultGetScrollRestorationKey,
   escapeHtml,
-  isServer,
   restoreScroll,
   storageKey,
 } from '@tanstack/router-core'
+import { isServer } from '@tanstack/router-core/isServer'
 import { useRouter } from './useRouter'
 import { ScriptOnce } from './ScriptOnce'