diff --git a/.changeset/start-middleware-registration.md b/.changeset/start-middleware-registration.md
new file mode 100644
index 00000000000..a8a8e30b273
--- /dev/null
+++ b/.changeset/start-middleware-registration.md
@@ -0,0 +1,5 @@
+---
+'@tanstack/start-client-core': patch
+---
+
+Avoid an unused array when registering server-function middleware.
diff --git a/packages/start-client-core/src/createServerFn.ts b/packages/start-client-core/src/createServerFn.ts
index bfa48039cf2..c363fdad786 100644
--- a/packages/start-client-core/src/createServerFn.ts
+++ b/packages/start-client-core/src/createServerFn.ts
@@ -102,13 +102,14 @@ export const createServerFn: CreateServerFn<Register> = (options, __opts) => {
       // this is primarily useful for letting users create their own abstractions on top of `createServerFn`
 
       const newMiddleware = [...(resolvedOptions.middleware || [])]
-      middleware.map((m) => {
-        if (TSS_SERVER_FUNCTION_FACTORY in m) {
-          if (m.options.middleware) {
-            newMiddleware.push(...m.options.middleware)
+      // forEach skips holes and ignores items appended during iteration
+      middleware.forEach((item) => {
+        if (TSS_SERVER_FUNCTION_FACTORY in item) {
+          if (item.options.middleware) {
+            newMiddleware.push(...item.options.middleware)
           }
         } else {
-          newMiddleware.push(m)
+          newMiddleware.push(item)
         }
       })
 
diff --git a/packages/start-client-core/tests/createServerFn.test.ts b/packages/start-client-core/tests/createServerFn.test.ts
new file mode 100644
index 00000000000..5958720a7b5
--- /dev/null
+++ b/packages/start-client-core/tests/createServerFn.test.ts
@@ -0,0 +1,54 @@
+import { expect, test } from 'vitest'
+import { createMiddleware } from '../src/createMiddleware'
+import { createServerFn } from '../src/createServerFn'
+
+test('appends factory middleware in order without changing source builders', () => {
+  const first = createMiddleware({ type: 'function' })
+  const second = createMiddleware({ type: 'function' })
+  const third = createMiddleware({ type: 'function' })
+  const factory = createServerFn().middleware([second])
+  const base = createServerFn({ method: 'POST' }).middleware([first])
+  const middlewares = Object.freeze([factory, third, second])
+
+  const combined = base.middleware(middlewares)
+
+  expect(combined.options.middleware).toEqual([first, second, third, second])
+  expect(combined.options.method).toBe('POST')
+  expect(base.options.middleware).toEqual([first])
+  expect(factory.options.middleware).toEqual([second])
+  expect(middlewares).toEqual([factory, third, second])
+  expect(combined.middleware([]).options.middleware).toEqual([
+    first,
+    second,
+    third,
+    second,
+  ])
+})
+
+test('ignores empty slots in a middleware array', () => {
+  const middleware = createMiddleware({ type: 'function' })
+  const middlewares: Array<typeof middleware> = []
+  middlewares[1] = middleware
+
+  expect(createServerFn().middleware(middlewares).options.middleware).toEqual([
+    middleware,
+  ])
+  expect(0 in middlewares).toBe(false)
+})
+
+test('does not register middleware appended while reading the input', () => {
+  const first = createMiddleware({ type: 'function' })
+  const second = createMiddleware({ type: 'function' })
+  const middlewares = [first]
+  Object.defineProperty(middlewares, 0, {
+    get() {
+      middlewares.push(second)
+      return first
+    },
+  })
+
+  expect(createServerFn().middleware(middlewares).options.middleware).toEqual([
+    first,
+  ])
+  expect(middlewares).toHaveLength(2)
+})