diff --git a/packages/react-router/tests/link.test.tsx b/packages/react-router/tests/link.test.tsx
index 92c215a0e9a..80578b6f4a3 100644
--- a/packages/react-router/tests/link.test.tsx
+++ b/packages/react-router/tests/link.test.tsx
@@ -4024,8 +4024,8 @@ describe('Link', () => {
       ),
     })
 
-    const parseParams = vi.fn()
-    const stringifyParams = vi.fn()
+    let parseParams: any
+    let stringifyParams: any
 
     const PostComponent = () => {
       const params = useParams({ strict: false })
@@ -4036,14 +4036,14 @@ describe('Link', () => {
       getParentRoute: () => rootRoute,
       path: '$postId',
       parseParams: (params) => {
-        parseParams(params)
+        parseParams = structuredClone(params) // clone object, because source will get mutated
         return {
           status: 'parsed',
           postId: params.postId,
         }
       },
       stringifyParams: (params) => {
-        stringifyParams(params)
+        stringifyParams = structuredClone(params) // clone object, because source will get mutated
         return {
           status: 'stringified',
           postId: params.postId,
@@ -4061,7 +4061,7 @@ describe('Link', () => {
       name: 'Go to post',
     })
 
-    expect(stringifyParams).toHaveBeenCalledWith({ postId: 2 })
+    expect(stringifyParams).toEqual({ postId: 2 })
 
     expect(postLink).toHaveAttribute('href', '/2')
 
@@ -4070,7 +4070,7 @@ describe('Link', () => {
     const posts2Text = await screen.findByText('Post: 2')
     expect(posts2Text).toBeInTheDocument()
 
-    expect(parseParams).toHaveBeenCalledWith({ status: 'parsed', postId: '2' })
+    expect(parseParams).toEqual({ postId: '2' })
   })
 
   test('when navigating to /$postId with params.parse and params.stringify', async () => {
@@ -4086,8 +4086,8 @@ describe('Link', () => {
       ),
     })
 
-    const parseParams = vi.fn()
-    const stringifyParams = vi.fn()
+    let parseParams: any
+    let stringifyParams: any
 
     const PostComponent = () => {
       const params = useParams({ strict: false })
@@ -4099,14 +4099,14 @@ describe('Link', () => {
       path: '$postId',
       params: {
         parse: (params) => {
-          parseParams(params)
+          parseParams = structuredClone(params) // clone object, because source will get mutated
           return {
             status: 'parsed',
             postId: params.postId,
           }
         },
         stringify: (params) => {
-          stringifyParams(params)
+          stringifyParams = structuredClone(params) // clone object, because source will get mutated
           return {
             status: 'stringified',
             postId: params.postId,
@@ -4125,7 +4125,7 @@ describe('Link', () => {
       name: 'Go to post',
     })
 
-    expect(stringifyParams).toHaveBeenCalledWith({ postId: 2 })
+    expect(stringifyParams).toEqual({ postId: 2 })
 
     expect(postLink).toHaveAttribute('href', '/2')
 
@@ -4134,7 +4134,7 @@ describe('Link', () => {
     const posts2Text = await screen.findByText('Post: 2')
     expect(posts2Text).toBeInTheDocument()
 
-    expect(parseParams).toHaveBeenCalledWith({ status: 'parsed', postId: '2' })
+    expect(parseParams).toEqual({ postId: '2' })
   })
 
   test('when navigating to /$postId with params.parse and params.stringify handles falsey inputs', async () => {
diff --git a/packages/router-core/src/router.ts b/packages/router-core/src/router.ts
index c4409961b83..64b47c8b674 100644
--- a/packages/router-core/src/router.ts
+++ b/packages/router-core/src/router.ts
@@ -1472,20 +1472,20 @@ export class RouterCore<
         : this.resolvePathWithBase(fromPath, '.')
 
       // Resolve the next params
-      let nextParams =
+      const nextParams =
         dest.params === false || dest.params === null
           ? {}
           : (dest.params ?? true) === true
             ? fromParams
-            : {
-                ...fromParams,
-                ...functionalUpdate(dest.params as any, fromParams),
-              }
+            : Object.assign(
+                fromParams,
+                functionalUpdate(dest.params as any, fromParams),
+              )
 
       // Interpolate the path first to get the actual resolved path, then match against that
       const interpolatedNextTo = interpolatePath({
         path: nextTo,
-        params: nextParams ?? {},
+        params: nextParams,
         parseCache: this.parsePathnameCache,
       }).interpolatedPath
 
@@ -1495,23 +1495,20 @@ export class RouterCore<
 
       // If there are any params, we need to stringify them
       if (Object.keys(nextParams).length > 0) {
-        destRoutes
-          .map((route) => {
-            return (
-              route.options.params?.stringify ?? route.options.stringifyParams
-            )
-          })
-          .filter(Boolean)
-          .forEach((fn) => {
-            nextParams = { ...nextParams!, ...fn!(nextParams) }
-          })
+        for (const route of destRoutes) {
+          const fn =
+            route.options.params?.stringify ?? route.options.stringifyParams
+          if (fn) {
+            Object.assign(nextParams, fn(nextParams))
+          }
+        }
       }
 
       const nextPathname = interpolatePath({
         // Use the original template path for interpolation
         // This preserves the original parameter syntax including optional parameters
         path: nextTo,
-        params: nextParams ?? {},
+        params: nextParams,
         leaveWildcards: false,
         leaveParams: opts.leaveParams,
         decodeCharMap: this.pathParamsDecodeCharMap,
@@ -1521,20 +1518,20 @@ export class RouterCore<
       // Resolve the next search
       let nextSearch = fromSearch
       if (opts._includeValidateSearch && this.options.search?.strict) {
-        let validatedSearch = {}
+        const validatedSearch = {}
         destRoutes.forEach((route) => {
-          try {
-            if (route.options.validateSearch) {
-              validatedSearch = {
-                ...validatedSearch,
-                ...(validateSearch(route.options.validateSearch, {
+          if (route.options.validateSearch) {
+            try {
+              Object.assign(
+                validatedSearch,
+                validateSearch(route.options.validateSearch, {
                   ...validatedSearch,
                   ...nextSearch,
-                }) ?? {}),
-              }
+                }),
+              )
+            } catch {
+              // ignore errors here because they are already handled in matchRoutes
             }
-          } catch {
-            // ignore errors here because they are already handled in matchRoutes
           }
         })
         nextSearch = validatedSearch
diff --git a/packages/solid-router/tests/link.test.tsx b/packages/solid-router/tests/link.test.tsx
index f1243d9f7fb..9eba08fd094 100644
--- a/packages/solid-router/tests/link.test.tsx
+++ b/packages/solid-router/tests/link.test.tsx
@@ -3592,8 +3592,8 @@ describe('Link', () => {
       ),
     })
 
-    const parseParams = vi.fn()
-    const stringifyParams = vi.fn()
+    let parseParams: any
+    let stringifyParams: any
 
     const PostComponent = () => {
       const params = useParams({ strict: false })
@@ -3604,14 +3604,14 @@ describe('Link', () => {
       getParentRoute: () => rootRoute,
       path: '$postId',
       parseParams: (params) => {
-        parseParams(params)
+        parseParams = structuredClone(params) // clone object, because source will get mutated
         return {
           status: 'parsed',
           postId: params.postId,
         }
       },
       stringifyParams: (params) => {
-        stringifyParams(params)
+        stringifyParams = structuredClone(params) // clone object, because source will get mutated
         return {
           status: 'stringified',
           postId: params.postId,
@@ -3629,7 +3629,7 @@ describe('Link', () => {
       name: 'Go to post',
     })
 
-    expect(stringifyParams).toHaveBeenCalledWith({ postId: 2 })
+    expect(stringifyParams).toEqual({ postId: 2 })
 
     expect(postLink).toHaveAttribute('href', '/2')
 
@@ -3638,7 +3638,7 @@ describe('Link', () => {
     const posts2Text = await screen.findByText('Post: 2')
     expect(posts2Text).toBeInTheDocument()
 
-    expect(parseParams).toHaveBeenCalledWith({ status: 'parsed', postId: '2' })
+    expect(parseParams).toEqual({ postId: '2' })
   })
 
   test('when navigating to /$postId with params.parse and params.stringify', async () => {
@@ -3654,8 +3654,8 @@ describe('Link', () => {
       ),
     })
 
-    const parseParams = vi.fn()
-    const stringifyParams = vi.fn()
+    let parseParams: any
+    let stringifyParams: any
 
     const PostComponent = () => {
       const params = useParams({ strict: false })
@@ -3667,14 +3667,14 @@ describe('Link', () => {
       path: '$postId',
       params: {
         parse: (params) => {
-          parseParams(params)
+          parseParams = structuredClone(params) // clone object, because source will get mutated
           return {
             status: 'parsed',
             postId: params.postId,
           }
         },
         stringify: (params) => {
-          stringifyParams(params)
+          stringifyParams = structuredClone(params) // clone object, because source will get mutated
           return {
             status: 'stringified',
             postId: params.postId,
@@ -3693,7 +3693,7 @@ describe('Link', () => {
       name: 'Go to post',
     })
 
-    expect(stringifyParams).toHaveBeenCalledWith({ postId: 2 })
+    expect(stringifyParams).toEqual({ postId: 2 })
 
     expect(postLink).toHaveAttribute('href', '/2')
 
@@ -3702,7 +3702,7 @@ describe('Link', () => {
     const posts2Text = await screen.findByText('Post: 2')
     expect(posts2Text).toBeInTheDocument()
 
-    expect(parseParams).toHaveBeenCalledWith({ status: 'parsed', postId: '2' })
+    expect(parseParams).toEqual({ postId: '2' })
   })
 
   test('when navigating to /$postId with params.parse and params.stringify handles falsey inputs', async () => {