[feat]: Add stable IDs for server functions

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

@@ -414,6 +414,55 @@ Server functions can return Server Components - server-rendered React components
 
 Handle request cancellation with `AbortSignal` for long-running operations.
 
+### Manual function IDs
+
+TanStack Start gives every server function an internal ID. By default, that ID is generated from the source file and extracted function name.
+
+Add `.id()` when a function needs to keep the same identity across file moves or variable renames:
+
+```tsx
+import { createServerFn } from '@tanstack/react-start'
+
+export const getUser = createServerFn({ method: 'GET' })
+  .id('get-user')
+  .handler(async () => {
+    return { id: '123' }
+  })
+```
+
+The value must be a static string literal before `.handler()`. It can contain letters, numbers, `_`, and `-`. Manual IDs must be unique. Duplicate manual IDs fail at compile time. If a generated or plugin-generated ID collides with one, the generated ID gets a suffix such as `_1`.
+
+> [!WARNING]
+> IDs are public. They can show up in browser network requests, logs, proxies, and analytics, so keep them short and non-sensitive. Do not include secrets, private file paths, tenant IDs, user IDs, or sensitive business data.
+
+An ID only locates the function. It does not authenticate the caller, authorize the action, validate input, or provide CSRF protection. Put those checks in middleware or in the handler.
+
+A per-function `.id()` takes precedence over `serverFns.generateFunctionId` for that server function.
+
+#### Manual IDs with factories
+
+Set the ID on the server function produced by the factory, not on the reusable factory itself:
+
+```tsx
+const authedServerFn = createServerFn().middleware([authMiddleware])
+
+export const getUser = authedServerFn()
+  .id('get-user')
+  .handler(async () => {
+    return { id: '123' }
+  })
+
+export const updateUser = authedServerFn({ method: 'POST' })
+  .id('update-user')
+  .handler(async () => {
+    return { ok: true }
+  })
+```
+
+Factory calls do not inherit IDs so each function produced by the factory should declare its own ID.
+
+Keep the same `.id()` value to preserve function identity across file moves and variable renames. Keep the same `method` to preserve call compatibility.
+
 ### Function ID generation for production build
 
 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.

packages/start-client-core/src/createServerFn.ts

@@ -97,6 +97,13 @@ export const createServerFn: CreateServerFn<Register> = (options, __opts) => {
 
   const res: ServerFnBuilder<Register, Method, ServerFnStrict> = {
     options: resolvedOptions,
+    id: (id) => {
+      const newOptions = {
+        ...resolvedOptions,
+        id,
+      }
+      return createServerFn(undefined, newOptions) as any
+    },
     middleware: (middleware) => {
       // multiple calls to `middleware()` merge the middlewares with the previously supplied ones
       // this is primarily useful for letting users create their own abstractions on top of `createServerFn`
@@ -216,8 +223,10 @@ export const createServerFn: CreateServerFn<Register> = (options, __opts) => {
     },
   } as ServerFnBuilder<Register, Method, ServerFnStrict>
   const fun = (options?: ServerFnOptions<Method, ServerFnStrict>) => {
+    const inheritedOptions = { ...resolvedOptions }
+    delete inheritedOptions.id
     const newOptions = {
-      ...resolvedOptions,
+      ...inheritedOptions,
       ...options,
     }
     return createServerFn(undefined, newOptions)
@@ -500,6 +509,7 @@ export type ServerFnBaseOptions<
   TInputValidator = unknown,
   TStrict extends ServerFnStrict = true,
 > = {
+  id?: string
   method: TMethod
   strict?: TStrict
   middleware?: Constrain<
@@ -586,6 +596,135 @@ export type AppendMiddlewares<TMiddlewares, TNewMiddlewares> =
       : TMiddlewares
     : TNewMiddlewares
 
+export interface ServerFnId<
+  TRegister,
+  TMethod extends Method,
+  TMiddlewares,
+  TInputValidator,
+  TStrict extends ServerFnStrict,
+> {
+  id: (
+    id: string,
+  ) => ServerFnAfterId<
+    TRegister,
+    TMethod,
+    TMiddlewares,
+    TInputValidator,
+    TStrict
+  >
+}
+
+export interface ServerFnAfterId<
+  TRegister,
+  TMethod extends Method,
+  TMiddlewares,
+  TInputValidator,
+  TStrict extends ServerFnStrict,
+>
+  extends
+    ServerFnWithTypes<
+      TRegister,
+      TMethod,
+      TMiddlewares,
+      TInputValidator,
+      undefined,
+      TStrict
+    >,
+    ServerFnMiddlewareAfterId<
+      TRegister,
+      TMethod,
+      TMiddlewares,
+      TInputValidator,
+      TStrict
+    >,
+    ServerFnValidatorAfterId<TRegister, TMethod, TMiddlewares, TStrict>,
+    ServerFnHandler<
+      TRegister,
+      TMethod,
+      TMiddlewares,
+      TInputValidator,
+      TStrict
+    > {}
+
+export interface ServerFnMiddlewareAfterId<
+  TRegister,
+  TMethod extends Method,
+  TMiddlewares,
+  TInputValidator,
+  TStrict extends ServerFnStrict,
+> {
+  middleware: <const TNewMiddlewares>(
+    middlewares: Constrain<
+      TNewMiddlewares,
+      ReadonlyArray<AnyFunctionMiddleware | AnyRequestMiddleware | AnyServerFn>
+    >,
+  ) => ServerFnAfterId<
+    TRegister,
+    TMethod,
+    AppendMiddlewares<TMiddlewares, TNewMiddlewares>,
+    TInputValidator,
+    TStrict
+  >
+}
+
+export type ValidatorFnAfterId<
+  TRegister,
+  TMethod extends Method,
+  TMiddlewares,
+  TStrict extends ServerFnStrict,
+> = <TInputValidator>(
+  validator: ConstrainValidator<TRegister, TMethod, TInputValidator, TStrict>,
+) => ServerFnAfterIdAfterValidator<
+  TRegister,
+  TMethod,
+  TMiddlewares,
+  TInputValidator,
+  TStrict
+>
+
+export interface ServerFnValidatorAfterId<
+  TRegister,
+  TMethod extends Method,
+  TMiddlewares,
+  TStrict extends ServerFnStrict,
+> {
+  validator: ValidatorFnAfterId<TRegister, TMethod, TMiddlewares, TStrict>
+  // TODO remove upon stable
+  /** @deprecated Use `validator` instead. */
+  inputValidator: ValidatorFnAfterId<TRegister, TMethod, TMiddlewares, TStrict>
+}
+
+export interface ServerFnAfterIdAfterValidator<
+  TRegister,
+  TMethod extends Method,
+  TMiddlewares,
+  TInputValidator,
+  TStrict extends ServerFnStrict,
+>
+  extends
+    ServerFnWithTypes<
+      TRegister,
+      TMethod,
+      TMiddlewares,
+      TInputValidator,
+      undefined,
+      TStrict
+    >,
+    ServerFnMiddlewareAfterId<
+      TRegister,
+      TMethod,
+      TMiddlewares,
+      TInputValidator,
+      TStrict
+    >,
+    ServerFnHandler<
+      TRegister,
+      TMethod,
+      TMiddlewares,
+      TInputValidator,
+      TStrict
+    > {}
+
 export interface ServerFnMiddleware<
   TRegister,
   TMethod extends Method,
@@ -625,6 +764,7 @@ export interface ServerFnAfterMiddleware<
     >,
     ServerFnMiddleware<TRegister, TMethod, TMiddlewares, undefined, TStrict>,
     ServerFnValidator<TRegister, TMethod, TMiddlewares, TStrict>,
+    ServerFnId<TRegister, TMethod, TMiddlewares, TInputValidator, TStrict>,
     ServerFnHandler<
       TRegister,
       TMethod,
@@ -696,13 +836,8 @@ export interface ServerFnAfterValidator<
       TInputValidator,
       TStrict
     >,
-    ServerFnHandler<
-      TRegister,
-      TMethod,
-      TMiddlewares,
-      TInputValidator,
-      TStrict
-    > {}
+    ServerFnHandler<TRegister, TMethod, TMiddlewares, TInputValidator, TStrict>,
+    ServerFnId<TRegister, TMethod, TMiddlewares, TInputValidator, TStrict> {}
 
 export interface ServerFnAfterTyper<
   TRegister,
@@ -764,6 +899,7 @@ export interface ServerFnBuilder<
     >,
     ServerFnMiddleware<TRegister, TMethod, undefined, undefined, TStrict>,
     ServerFnValidator<TRegister, TMethod, undefined, TStrict>,
+    ServerFnId<TRegister, TMethod, undefined, undefined, TStrict>,
     ServerFnHandler<TRegister, TMethod, undefined, undefined, TStrict> {
   <
     TNewMethod extends Method = TMethod,

packages/start-client-core/src/tests/createServerFn.test-d.ts

@@ -578,6 +578,68 @@ test('createServerFn strict false factory preserves strictness', () => {
   >()
 })
 
+test('createServerFn id can be set before handler', () => {
+  const builder = createServerFn().id('get-user')
+
+  expectTypeOf(builder).toHaveProperty('handler')
+  expectTypeOf(builder).toHaveProperty('middleware')
+  expectTypeOf(builder).toHaveProperty('validator')
+  expectTypeOf(builder).not.toHaveProperty('id')
+
+  const fn = builder.handler(() => ({}))
+
+  expectTypeOf(fn).not.toHaveProperty('id')
+
+  const builderAfterMiddleware = createServerFn()
+    .id('list-users')
+    .middleware([])
+
+  expectTypeOf(builderAfterMiddleware).toHaveProperty('handler')
+  expectTypeOf(builderAfterMiddleware).not.toHaveProperty('id')
+
+  const builderWithIdAfterMiddleware = createServerFn()
+    .middleware([])
+    .id('update-user')
+
+  expectTypeOf(builderWithIdAfterMiddleware).toHaveProperty('handler')
+  expectTypeOf(builderWithIdAfterMiddleware).not.toHaveProperty('id')
+
+  const builderWithIdAfterValidator = createServerFn()
+    .validator((input: { id: string }) => input)
+    .id('delete-user')
+
+  expectTypeOf(builderWithIdAfterValidator).toHaveProperty('handler')
+  expectTypeOf(builderWithIdAfterValidator).not.toHaveProperty('id')
+})
+
+test('createServerFn factory children set their own ids', () => {
+  const middleware = createMiddleware({ type: 'function' }).server(
+    ({ next }) => {
+      return next({ context: { user: 'alice' } as const })
+    },
+  )
+
+  const createAuthedServerFn = createServerFn({ method: 'POST' }).middleware([
+    middleware,
+  ])
+
+  const builder = createAuthedServerFn().id('save-user')
+
+  expectTypeOf(builder).toHaveProperty('handler')
+  expectTypeOf(builder).not.toHaveProperty('id')
+
+  builder.handler((options) => {
+    expectTypeOf(options).toEqualTypeOf<{
+      context: {
+        readonly user: 'alice'
+      }
+      data: undefined
+      method: 'POST'
+      serverFnMeta: ServerFnMeta
+    }>()
+  })
+})
+
 test('createServerFn strict input false can validate function', () => {
   const fn = createServerFn({ strict: { input: false } })
     .validator((input: { func: () => 'input' }) => ({