diff --git a/packages/router-core/src/searchMiddleware.ts b/packages/router-core/src/searchMiddleware.ts
index e90d453350f..1d60cc9587c 100644
--- a/packages/router-core/src/searchMiddleware.ts
+++ b/packages/router-core/src/searchMiddleware.ts
@@ -21,13 +21,14 @@ export function retainSearchParams<TSearchSchema extends object>(
     if (keys === true) {
       return { ...search, ...result }
     }
-    // add missing keys from search to result
+    const copy = { ...result }
+    // add missing keys from search to copy
     keys.forEach((key) => {
-      if (!(key in result)) {
-        result[key] = search[key]
+      if (!(key in copy)) {
+        copy[key] = search[key]
       }
     })
-    return result
+    return copy
   }
 }
 
@@ -55,7 +56,7 @@ export function stripSearchParams<
     if (input === true) {
       return {}
     }
-    const result = next(search) as Record<string, unknown>
+    const result = { ...next(search) } as Record<string, unknown>
     if (Array.isArray(input)) {
       input.forEach((key) => {
         delete result[key]
diff --git a/packages/router-core/tests/searchMiddleware.test.ts b/packages/router-core/tests/searchMiddleware.test.ts
new file mode 100644
index 00000000000..10c131bfd51
--- /dev/null
+++ b/packages/router-core/tests/searchMiddleware.test.ts
@@ -0,0 +1,138 @@
+import { describe, expect, test } from 'vitest'
+import { retainSearchParams, stripSearchParams } from '../src/searchMiddleware'
+
+describe('searchMiddleware - mutation prevention', () => {
+  describe('retainSearchParams', () => {
+    test('should not mutate original search object', () => {
+      const originalSearch = { id: '1', filter: 'active', page: '2' }
+      const originalCopy = { ...originalSearch }
+
+      const middleware = retainSearchParams(['id', 'filter'])
+
+      // next() returns object without 'id' and 'filter' keys
+      // so retainSearchParams should add them from search
+      const result = middleware({
+        search: originalSearch,
+        next: () => ({ page: 'new' }) as any,
+      })
+
+      expect(originalSearch).toEqual(originalCopy)
+      expect(originalSearch).toEqual({ id: '1', filter: 'active', page: '2' })
+      expect(result).toEqual({ id: '1', filter: 'active', page: 'new' })
+      expect(result).not.toBe(originalSearch)
+    })
+
+    test('should work correctly when same reference is reused', () => {
+      const sharedSearch = { id: '1', filter: 'active', page: '1' }
+      const middleware = retainSearchParams(['id', 'filter'])
+
+      // next() returns object without 'id' and 'filter' keys
+      const result1 = middleware({
+        search: sharedSearch,
+        next: () => ({ page: '2' }) as any,
+      })
+
+      expect(sharedSearch).toEqual({ id: '1', filter: 'active', page: '1' })
+      expect(result1).toEqual({ id: '1', filter: 'active', page: '2' })
+
+      const result2 = middleware({
+        search: sharedSearch,
+        next: () => ({ page: '3' }) as any,
+      })
+
+      expect(sharedSearch).toEqual({ id: '1', filter: 'active', page: '1' })
+      expect(result2).toEqual({ id: '1', filter: 'active', page: '3' })
+    })
+
+    test('should handle retainSearchParams(true) correctly', () => {
+      const originalSearch = { id: '1', filter: 'active' }
+      const originalCopy = { ...originalSearch }
+
+      const middleware = retainSearchParams(true)
+
+      const result = middleware({
+        search: originalSearch,
+        next: () => ({ id: '2' }) as any,
+      })
+
+      expect(originalSearch).toEqual(originalCopy)
+      expect(result).toEqual({ id: '2', filter: 'active' })
+    })
+  })
+
+  describe('stripSearchParams', () => {
+    test('should not mutate original search object (array input)', () => {
+      const originalSearch = { id: '1', filter: 'active', page: '1' }
+      const originalCopy = { ...originalSearch }
+
+      const middleware = stripSearchParams(['filter', 'page'])
+
+      const result = middleware({
+        search: originalSearch,
+        next: (search) => search,
+      })
+
+      expect(originalSearch).toEqual(originalCopy)
+      expect(originalSearch).toEqual({ id: '1', filter: 'active', page: '1' })
+      expect(result).toEqual({ id: '1' })
+      expect(result).not.toBe(originalSearch)
+    })
+
+    test('should not mutate original search object (object input)', () => {
+      const originalSearch = { id: '1', filter: 'active', status: 'done' }
+      const originalCopy = { ...originalSearch }
+
+      const middleware = stripSearchParams({ filter: 'active', status: 'done' })
+
+      const result = middleware({
+        search: originalSearch,
+        next: (search) => search,
+      })
+
+      expect(originalSearch).toEqual(originalCopy)
+      expect(originalSearch).toEqual({
+        id: '1',
+        filter: 'active',
+        status: 'done',
+      })
+      expect(result).toEqual({ id: '1' })
+      expect(result).not.toBe(originalSearch)
+    })
+
+    test('should work correctly when same reference is reused', () => {
+      const sharedSearch = { id: '1', filter: 'active', page: '1' }
+      const middleware = stripSearchParams(['filter', 'page'])
+
+      const result1 = middleware({
+        search: sharedSearch,
+        next: (search) => search,
+      })
+
+      expect(sharedSearch).toEqual({ id: '1', filter: 'active', page: '1' })
+      expect(result1).toEqual({ id: '1' })
+
+      const result2 = middleware({
+        search: sharedSearch,
+        next: (search) => search,
+      })
+
+      expect(sharedSearch).toEqual({ id: '1', filter: 'active', page: '1' })
+      expect(result2).toEqual({ id: '1' })
+    })
+
+    test('should handle stripSearchParams(true) correctly', () => {
+      const originalSearch = { id: '1', filter: 'active' }
+      const originalCopy = { ...originalSearch }
+
+      const middleware = stripSearchParams(true)
+
+      const result = middleware({
+        search: originalSearch,
+        next: (search) => search,
+      })
+
+      expect(originalSearch).toEqual(originalCopy)
+      expect(result).toEqual({})
+    })
+  })
+})