Add support for customizable functionId generation.

docs/start/framework/react/server-functions.md

@@ -224,6 +224,52 @@ Cache server function results at build time for static generation. See [Static S
 
 Handle request cancellation with `AbortSignal` for long-running operations.
 
+### Function ID generation
+
+Server functions are addressed by a generated, stable function ID under the hood. These IDs are embedded into the client/SSR builds and used by the server to locate and import the correct module at runtime.
+
+Defaults:
+
+- In development, IDs are URL-safe strings derived from `${filename}--${functionName}` to aid debugging.
+- In production, IDs are SHA256 hashes of the same seed to keep bundles compact and avoid leaking file paths.
+- If two server functions end up with the same ID (including when using a custom generator), the system de-duplicates by appending an incrementing suffix like `_1`, `_2`, etc.
+- IDs are stable for a given file/function tuple for the lifetime of the process (hot updates keep the same mapping).
+
+Customization:
+
+You can customize function ID generation by providing a `generateFunctionId` function when configuring the TanStack Start Vite plugin:
+
+Example:
+
+```ts
+// vite.config.ts
+import { defineConfig } from 'vite'
+import react from '@vitejs/plugin-react'
+import { tanstackStart } from '@tanstack/react-start/plugin/vite'
+
+export default defineConfig({
+  plugins: [
+    tanstackStart({
+      serverFns: {
+        generateFunctionId: ({ filename, functionName }) => {
+          // Return a custom ID string. If you return undefined, the default is used.
+          // For example, always hash (even in dev):
+          // return createHash('sha256').update(`${filename}--${functionName}`).digest('hex')
+          return undefined
+        },
+      },
+    }),
+    react(),
+  ],
+})
+```
+
+Tips:
+
+- Prefer deterministic inputs (filename + functionName) so IDs remain stable between builds.
+- If you don’t want file paths in dev IDs, return a hash in all environments.
+- Ensure the returned ID is **URL-safe**.
+
 ---
 
 > **Note**: Server functions use a compilation process that extracts server code from client bundles while maintaining seamless calling patterns. On the client, calls become `fetch` requests to the server.

e2e/react-start/custom-basepath/tests/navigation.spec.ts

@@ -41,9 +41,7 @@ test('Server function URLs correctly include app basepath', async ({
   const form = page.locator('form')
   const actionUrl = await form.getAttribute('action')
 
-  expect(actionUrl).toBe(
-    '/custom/basepath/_serverFn/src_routes_logout_tsx--logoutFn_createServerFn_handler',
-  )
+  expect(actionUrl).toMatch(/^\/custom\/basepath\/_serverFn\//)
 })
 
 test('client-side redirect', async ({ page, baseURL }) => {

e2e/react-start/server-functions/tests/server-functions.spec.ts

@@ -6,6 +6,20 @@ import type { Page } from '@playwright/test'
 
 const PORT = await getTestServerPort(packageJson.name)
 
+test('Server function URLs correctly include constant ids', async ({
+  page,
+}) => {
+  for (const currentPage of ['/submit-post-formdata', '/formdata-redirect']) {
+    await page.goto(currentPage)
+    await page.waitForLoadState('networkidle')
+
+    const form = page.locator('form')
+    const actionUrl = await form.getAttribute('action')
+
+    expect(actionUrl).toMatch(/^\/_serverFn\/constant_id/)
+  }
+})
+
 test('invoking a server function with custom response status code', async ({
   page,
 }) => {

e2e/react-start/server-functions/vite.config.ts

@@ -3,12 +3,25 @@ import tsConfigPaths from 'vite-tsconfig-paths'
 import { tanstackStart } from '@tanstack/react-start/plugin/vite'
 import viteReact from '@vitejs/plugin-react'
 
+const FUNCTIONS_WITH_CONSTANT_ID = [
+  'src/routes/submit-post-formdata.tsx/greetUser_createServerFn_handler',
+  'src/routes/formdata-redirect/index.tsx/greetUser_createServerFn_handler',
+]
+
 export default defineConfig({
   plugins: [
     tsConfigPaths({
       projects: ['./tsconfig.json'],
     }),
-    tanstackStart(),
+    tanstackStart({
+      serverFns: {
+        generateFunctionId: (opts) => {
+          const id = `${opts.filename}/${opts.functionName}`
+          if (FUNCTIONS_WITH_CONSTANT_ID.includes(id)) return 'constant_id'
+          else return undefined
+        },
+      },
+    }),
     viteReact(),
   ],
 })

packages/directive-functions-plugin/package.json

@@ -71,6 +71,7 @@
     "@babel/types": "^7.27.7",
     "@tanstack/router-utils": "workspace:*",
     "babel-dead-code-elimination": "^1.0.10",
+    "pathe": "^2.0.3",
     "tiny-invariant": "^1.3.3"
   },
   "devDependencies": {

packages/directive-functions-plugin/src/compilers.ts

@@ -5,6 +5,7 @@ import {
   deadCodeElimination,
   findReferencedIdentifiers,
 } from 'babel-dead-code-elimination'
+import path from 'pathe'
 import { generateFromAst, parseAst } from '@tanstack/router-utils'
 import type { GeneratorResult, ParseAstOptions } from '@tanstack/router-utils'
 
@@ -22,6 +23,11 @@ export type SupportedFunctionPath =
   | babel.NodePath<babel.types.FunctionExpression>
   | babel.NodePath<babel.types.ArrowFunctionExpression>
 
+export type GenerateFunctionIdFn = (opts: {
+  filename: string
+  functionName: string
+}) => string
+
 export type ReplacerFn = (opts: {
   fn: string
   extractedFilename: string
@@ -38,6 +44,7 @@ export type CompileDirectivesOpts = ParseAstOptions & {
   getRuntimeCode?: (opts: {
     directiveFnsById: Record<string, DirectiveFn>
   }) => string
+  generateFunctionId: GenerateFunctionIdFn
   replacer: ReplacerFn
   // devSplitImporter: string
   filename: string
@@ -198,14 +205,6 @@ function findNearestVariableName(
   return nameParts.length > 0 ? nameParts.join('_') : 'anonymous'
 }
 
-function makeFileLocationUrlSafe(location: string): string {
-  return location
-    .replace(/[^a-zA-Z0-9-_]/g, '_') // Replace unsafe chars with underscore
-    .replace(/_{2,}/g, '_') // Collapse multiple underscores
-    .replace(/^_|_$/g, '') // Trim leading/trailing underscores
-    .replace(/_--/g, '--') // Clean up the joiner
-}
-
 function makeIdentifierSafe(identifier: string): string {
   return identifier
     .replace(/[^a-zA-Z0-9_$]/g, '_') // Replace unsafe chars with underscore
@@ -221,6 +220,7 @@ export function findDirectives(
     directive: string
     directiveLabel: string
     replacer?: ReplacerFn
+    generateFunctionId: GenerateFunctionIdFn
     directiveSplitParam: string
     filename: string
     root: string
@@ -460,16 +460,22 @@ export function findDirectives(
       `body.${topParentIndex}.declarations.0.init`,
     ) as SupportedFunctionPath
 
-    const [baseFilename, ..._searchParams] = opts.filename.split('?')
+    const [baseFilename, ..._searchParams] = opts.filename.split('?') as [
+      string,
+      ...Array<string>,
+    ]
     const searchParams = new URLSearchParams(_searchParams.join('&'))
     searchParams.set(opts.directiveSplitParam, '')
 
     const extractedFilename = `${baseFilename}?${searchParams.toString()}`
 
-    const functionId = makeFileLocationUrlSafe(
-      `${baseFilename}--${functionName}`.replace(opts.root, ''),
-    )
-
+    // Relative to have constant functionId regardless of the machine
+    // that we are executing
+    const relativeFilename = path.relative(opts.root, baseFilename)
+    const functionId = opts.generateFunctionId({
+      filename: relativeFilename,
+      functionName: functionName,
+    })
     // If a replacer is provided, replace the function with the replacer
     if (opts.replacer) {
       const replacer = opts.replacer({

packages/directive-functions-plugin/src/index.ts

@@ -2,7 +2,11 @@ import { fileURLToPath, pathToFileURL } from 'node:url'
 
 import { logDiff } from '@tanstack/router-utils'
 import { compileDirectives } from './compilers'
-import type { CompileDirectivesOpts, DirectiveFn } from './compilers'
+import type {
+  CompileDirectivesOpts,
+  DirectiveFn,
+  GenerateFunctionIdFn,
+} from './compilers'
 import type { Plugin } from 'vite'
 
 const debug =
@@ -13,6 +17,7 @@ export type {
   DirectiveFn,
   CompileDirectivesOpts,
   ReplacerFn,
+  GenerateFunctionIdFn,
 } from './compilers'
 
 export type DirectiveFunctionsViteEnvOptions = Pick<
@@ -28,6 +33,7 @@ export type DirectiveFunctionsViteOptions = Pick<
 > &
   DirectiveFunctionsViteEnvOptions & {
     onDirectiveFnsById?: (directiveFnsById: Record<string, DirectiveFn>) => void
+    generateFunctionId: GenerateFunctionIdFn
   }
 
 const createDirectiveRx = (directive: string) =>
@@ -61,6 +67,7 @@ export type DirectiveFunctionsVitePluginEnvOptions = Pick<
     server: DirectiveFunctionsViteEnvOptions & { envName?: string }
   }
   onDirectiveFnsById?: (directiveFnsById: Record<string, DirectiveFn>) => void
+  generateFunctionId: GenerateFunctionIdFn
 }
 
 export function TanStackDirectiveFunctionsPluginEnv(
@@ -131,6 +138,7 @@ function transformCode({
   directive,
   directiveLabel,
   getRuntimeCode,
+  generateFunctionId,
   replacer,
   onDirectiveFnsById,
   root,
@@ -155,6 +163,7 @@ function transformCode({
       directive,
       directiveLabel,
       getRuntimeCode,
+      generateFunctionId,
       replacer,
       code,
       root,

packages/directive-functions-plugin/tests/compiler.test.ts

@@ -3,30 +3,47 @@ import { describe, expect, test } from 'vitest'
 import { compileDirectives } from '../src/compilers'
 import type { CompileDirectivesOpts } from '../src/compilers'
 
+function makeFunctionIdUrlSafe(location: string): string {
+  return location
+    .replace(/[^a-zA-Z0-9-_]/g, '_') // Replace unsafe chars with underscore
+    .replace(/_{2,}/g, '_') // Collapse multiple underscores
+    .replace(/^_|_$/g, '') // Trim leading/trailing underscores
+    .replace(/_--/g, '--') // Clean up the joiner
+}
+
+const generateFunctionId: CompileDirectivesOpts['generateFunctionId'] = (
+  opts,
+) => {
+  return makeFunctionIdUrlSafe(`${opts.filename}--${opts.functionName}`)
+}
+
 const clientConfig: Omit<CompileDirectivesOpts, 'code'> = {
   directive: 'use server',
   directiveLabel: 'Server function',
   root: './test-files',
-  filename: 'test.ts',
+  filename: './test-files/test.ts',
   getRuntimeCode: () => 'import { createClientRpc } from "my-rpc-lib-client"',
+  generateFunctionId,
   replacer: (opts) => `createClientRpc(${JSON.stringify(opts.functionId)})`,
 }
 
 const ssrConfig: Omit<CompileDirectivesOpts, 'code'> = {
   directive: 'use server',
   directiveLabel: 'Server function',
   root: './test-files',
-  filename: 'test.ts',
+  filename: './test-files/test.ts',
   getRuntimeCode: () => 'import { createSsrRpc } from "my-rpc-lib-server"',
+  generateFunctionId,
   replacer: (opts) => `createSsrRpc(${JSON.stringify(opts.functionId)})`,
 }
 
 const serverConfig: Omit<CompileDirectivesOpts, 'code'> = {
   directive: 'use server',
   directiveLabel: 'Server function',
   root: './test-files',
-  filename: 'test.ts',
+  filename: './test-files/test.ts',
   getRuntimeCode: () => 'import { createServerRpc } from "my-rpc-lib-server"',
+  generateFunctionId,
   replacer: (opts) =>
     // On the server build, we need different code for the split function
     // vs any other server functions the split function may reference