diff --git a/packages/router-generator/package.json b/packages/router-generator/package.json
index 4d1704e54a..b58185a427 100644
--- a/packages/router-generator/package.json
+++ b/packages/router-generator/package.json
@@ -72,7 +72,7 @@
     "recast": "^0.23.11",
     "source-map": "^0.7.4",
     "tsx": "^4.19.2",
-    "zod": "^3.24.2"
+    "zod": "^3.25.0 || ^4.0.0"
   },
   "devDependencies": {
     "@tanstack/react-router": "workspace:*"
diff --git a/packages/router-generator/src/config.ts b/packages/router-generator/src/config.ts
index 0d1ae47339..31ffbc9068 100644
--- a/packages/router-generator/src/config.ts
+++ b/packages/router-generator/src/config.ts
@@ -4,6 +4,29 @@ import { z } from 'zod'
 import { virtualRootRouteSchema } from './filesystem/virtual/config'
 import type { GeneratorPlugin } from './plugin/types'
 
+// Helper to create a function schema compatible with both Zod v3 and v4
+function createFunctionSchema() {
+  // Try Zod v4 syntax first
+  if (typeof (z as any).function === 'function') {
+    try {
+      // Check if this is Zod v4 by testing for the new API
+      const testSchema = z.string()
+      if ('_zod' in testSchema) {
+        // Zod v4: use new function API
+        return (z as any).function({
+          input: [],
+          output: z.array(z.string()),
+        })
+      }
+    } catch (e) {
+      // Fall through to v3
+    }
+  }
+
+  // Zod v3: use old function API
+  return (z as any).function().returns(z.array(z.string()))
+}
+
 export const baseConfigSchema = z.object({
   target: z.enum(['react', 'solid', 'vue']).optional().default('react'),
   virtualRouteConfig: virtualRootRouteSchema.or(z.string()).optional(),
@@ -40,7 +63,7 @@ export const configSchema = baseConfigSchema.extend({
   routeTreeFileFooter: z
     .union([
       z.array(z.string()).optional().default([]),
-      z.function().returns(z.array(z.string())),
+      createFunctionSchema(),
     ])
     .optional(),
   autoCodeSplitting: z.boolean().optional(),
diff --git a/packages/router-plugin/package.json b/packages/router-plugin/package.json
index 37be7ce691..b6d05b783c 100644
--- a/packages/router-plugin/package.json
+++ b/packages/router-plugin/package.json
@@ -117,7 +117,7 @@
     "babel-dead-code-elimination": "^1.0.11",
     "chokidar": "^3.6.0",
     "unplugin": "^2.1.2",
-    "zod": "^3.24.2"
+    "zod": "^3.25.0 || ^4.0.0"
   },
   "devDependencies": {
     "@types/babel__core": "^7.20.5",
diff --git a/packages/router-plugin/src/core/config.ts b/packages/router-plugin/src/core/config.ts
index f93166d392..2dba87c01f 100644
--- a/packages/router-plugin/src/core/config.ts
+++ b/packages/router-plugin/src/core/config.ts
@@ -10,6 +10,29 @@ import type {
 } from '@tanstack/router-core'
 import type { CodeSplitGroupings } from './constants'
 
+// Helper to create a function schema compatible with both Zod v3 and v4
+function createGenericFunctionSchema(): any {
+  // Try Zod v4 syntax first
+  if (typeof (z as any).function === 'function') {
+    try {
+      // Check if this is Zod v4 by testing for the new API
+      const testSchema = z.string()
+      if ('_zod' in testSchema) {
+        // Zod v4: use new function API with any input/output
+        return (z as any).function({
+          input: [z.any()],
+          output: z.any(),
+        })
+      }
+    } catch (e) {
+      // Fall through to v3
+    }
+  }
+
+  // Zod v3: use old function API
+  return (z as any).function()
+}
+
 export const splitGroupingsSchema = z
   .array(
     z.array(
@@ -73,7 +96,7 @@ export type CodeSplittingOptions = {
 }
 
 const codeSplittingOptionsSchema = z.object({
-  splitBehavior: z.function().optional(),
+  splitBehavior: createGenericFunctionSchema().optional(),
   defaultBehavior: splitGroupingsSchema.optional(),
   deleteNodes: z.array(z.string()).optional(),
   addHmr: z.boolean().optional().default(true),
diff --git a/packages/start-plugin-core/package.json b/packages/start-plugin-core/package.json
index 5711e0419c..e6a6798a65 100644
--- a/packages/start-plugin-core/package.json
+++ b/packages/start-plugin-core/package.json
@@ -80,7 +80,7 @@
     "ufo": "^1.5.4",
     "vitefu": "^1.1.1",
     "xmlbuilder2": "^4.0.3",
-    "zod": "^3.24.2"
+    "zod": "^3.25.0 || ^4.0.0"
   },
   "devDependencies": {
     "@types/babel__code-frame": "^7.0.6",
diff --git a/packages/start-plugin-core/src/schema.ts b/packages/start-plugin-core/src/schema.ts
index 235796a129..649b62e29b 100644
--- a/packages/start-plugin-core/src/schema.ts
+++ b/packages/start-plugin-core/src/schema.ts
@@ -3,6 +3,33 @@ import { z } from 'zod'
 import { configSchema, getConfig } from '@tanstack/router-plugin'
 import type { TanStackStartVitePluginCoreOptions } from './types'
 
+// Helper to create a function schema compatible with both Zod v3 and v4
+function createFunctionSchema(
+  args: any,
+  returns: any,
+): any {
+  // Try Zod v4 syntax first
+  if (typeof (z as any).function === 'function') {
+    try {
+      // Check if this is Zod v4 by testing for the new API
+      const testSchema = z.string()
+      if ('_zod' in testSchema) {
+        // Zod v4: use new function API
+        return (z as any).function({
+          input: Array.isArray(args) ? args : [args],
+          output: returns,
+        })
+      }
+    } catch (e) {
+      // Fall through to v3
+    }
+  }
+
+  // Zod v3: use old function API
+  const argsArray = Array.isArray(args) ? args : [args]
+  return (z as any).function().args(...argsArray).returns(returns)
+}
+
 const tsrConfig = configSchema
   .omit({ autoCodeSplitting: true, target: true, verboseFileRoutes: true })
   .partial()
@@ -94,16 +121,13 @@ const pagePrerenderOptionsSchema = z.object({
   crawlLinks: z.boolean().optional(),
   retryCount: z.number().optional(),
   retryDelay: z.number().optional(),
-  onSuccess: z
-    .function()
-    .args(
-      z.object({
-        page: pageBaseSchema,
-        html: z.string(),
-      }),
-    )
-    .returns(z.any())
-    .optional(),
+  onSuccess: createFunctionSchema(
+    z.object({
+      page: pageBaseSchema,
+      html: z.string(),
+    }),
+    z.any(),
+  ).optional(),
   headers: z.record(z.string(), z.string()).optional(),
 })
 
@@ -159,16 +183,13 @@ const tanstackStartOptionsSchema = z
     serverFns: z
       .object({
         base: z.string().optional().default('/_serverFn'),
-        generateFunctionId: z
-          .function()
-          .args(
-            z.object({
-              filename: z.string(),
-              functionName: z.string(),
-            }),
-          )
-          .returns(z.string().optional())
-          .optional(),
+        generateFunctionId: createFunctionSchema(
+          z.object({
+            filename: z.string(),
+            functionName: z.string(),
+          }),
+          z.string().optional(),
+        ).optional(),
       })
       .optional()
       .default({}),
@@ -184,7 +205,7 @@ const tanstackStartOptionsSchema = z
       .object({
         enabled: z.boolean().optional(),
         concurrency: z.number().optional(),
-        filter: z.function().args(pageSchema).returns(z.any()).optional(),
+        filter: createFunctionSchema(pageSchema, z.any()).optional(),
         failOnError: z.boolean().optional(),
         autoStaticPathsDiscovery: z.boolean().optional(),
         maxRedirects: z.number().min(0).optional(),