fix(types): fix the type definitions for factory models and config, a… (#408)

* fix(types): fix the type definitions for factory models and config, and add InferRestArgs utility type

* refactor(types): simplify type definitions for factory models by removing unused InferConfig type

* fix(types): handle optional model param, restore explicit return type, add regression test

Review follow-ups for #408:
- InferRestArgs now matches [any?, ...] so factories with an optional
  first parameter don't silently drop their trailing args
- Extract the extended signature into a named ExtendedFactory type and
  restore an explicit return type on extendAdapter (no cast needed)
- Add a regression test for 3-parameter factories like
  createAnthropicChat(model, apiKey, config?) (#407)
- Add a patch changeset for @tanstack/ai

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* refactor(types): remove all any from extend-adapter type utilities

Replace any-typed function shapes with a sound AnyAdapterFactory
constraint built on never params / unknown return (parameters are
contravariant, so never accepts every factory). The deliberate
model-union widening now lives in an overload signature instead of an
unchecked assignment, so no cast or any remains anywhere in the file.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* test(types): assert invalid model names are rejected for 3-param factories

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: liyown

.changeset/fix-extend-adapter-rest-args.md

@@ -0,0 +1,5 @@
+---
+'@tanstack/ai': patch
+---
+
+Fix `extendAdapter` dropping required parameters after the model (e.g. `apiKey` in `createAnthropicChat`). All factory parameters after the model are now preserved, including labels and optionality.

packages/ai/src/extend-adapter.ts

@@ -144,39 +144,59 @@ type ExtractCustomModelNames<TDefs extends ReadonlyArray<ExtendedModelDef>> =
 // Factory Type Inference
 // ===========================
 
+/**
+ * The widest factory shape `extendAdapter` accepts: any function taking a
+ * model as its first parameter. Parameters are contravariant, so `never`
+ * params and an `unknown` return accept every factory without resorting
+ * to `any`.
+ */
+type AnyAdapterFactory = (model: never, ...args: Array<never>) => unknown
+
 /**
  * Infer the model parameter type from an adapter factory function.
  * For generic functions like `<T extends Union>(model: T)`, this gets `T` which
  * TypeScript treats as the constraint union when used in parameter position.
  */
 type InferFactoryModels<TFactory> = TFactory extends (
   model: infer TModel,
-  ...args: Array<any>
-) => any
+  ...args: Array<never>
+) => unknown
   ? TModel extends string
     ? TModel
     : string
   : string
 
-/**
- * Infer the config parameter type from an adapter factory function.
- */
-type InferConfig<TFactory> = TFactory extends (
-  model: any,
-  config?: infer TConfig,
-) => any
-  ? TConfig
-  : undefined
-
 /**
  * Infer the adapter return type from a factory function.
  */
 type InferAdapterReturn<TFactory> = TFactory extends (
-  ...args: Array<any>
+  ...args: Array<never>
 ) => infer TReturn
   ? TReturn
   : never
 
+/**
+ * Extracts all parameter types after the model parameter from a factory,
+ * preserving labels and optionality (e.g. `[apiKey: string, config?: C]`).
+ * Note: overloaded factories resolve against their last overload (a
+ * `Parameters` limitation).
+ */
+type InferRestArgs<TFactory extends AnyAdapterFactory> =
+  Parameters<TFactory> extends [unknown?, ...infer TRest] ? TRest : []
+
+/**
+ * The factory signature produced by `extendAdapter`: accepts both original
+ * and custom model names while preserving all remaining parameters and the
+ * return type of the original factory.
+ */
+type ExtendedFactory<
+  TFactory extends AnyAdapterFactory,
+  TDefs extends ReadonlyArray<ExtendedModelDef>,
+> = (
+  model: InferFactoryModels<TFactory> | ExtractCustomModelNames<TDefs>,
+  ...args: InferRestArgs<TFactory>
+) => InferAdapterReturn<TFactory>
+
 // ===========================
 // extendAdapter Function
 // ===========================
@@ -225,19 +245,17 @@ type InferAdapterReturn<TFactory> = TFactory extends (
  * ```
  */
 export function extendAdapter<
-  TFactory extends (...args: Array<any>) => any,
+  TFactory extends AnyAdapterFactory,
   const TDefs extends ReadonlyArray<ExtendedModelDef>,
->(
-  factory: TFactory,
-  _customModels: TDefs,
-): (
-  model: InferFactoryModels<TFactory> | ExtractCustomModelNames<TDefs>,
-  ...args: InferConfig<TFactory> extends undefined
-    ? []
-    : [config?: InferConfig<TFactory>]
-) => InferAdapterReturn<TFactory> {
+>(factory: TFactory, _customModels: TDefs): ExtendedFactory<TFactory, TDefs>
+// The implementation signature stays at the honest `AnyAdapterFactory` width;
+// the overload above performs the deliberate model-union widening.
+export function extendAdapter(
+  factory: AnyAdapterFactory,
+  _customModels: ReadonlyArray<ExtendedModelDef>,
+): AnyAdapterFactory {
   // At runtime, we simply pass through to the original factory.
   // The _customModels parameter is only used for type inference.
   // No runtime validation - users are trusted to pass valid model names.
-  return factory as any
+  return factory
 }

packages/ai/tests/extend-adapter.test.ts

@@ -243,6 +243,38 @@ describe('extendAdapter', () => {
     })
   })
 
+  describe('Factories with required args after model (#407)', () => {
+    // Mimics createAnthropicChat(model, apiKey, config?)
+    function mockChat<TModel extends MockModel>(
+      model: TModel,
+      apiKey: string,
+      config?: MockAdapterConfig,
+    ): MockTextAdapter<TModel> {
+      void apiKey
+      return new MockTextAdapter(model, config)
+    }
+
+    it('should preserve required apiKey and optional config parameters', () => {
+      const extendedMock = extendAdapter(mockChat, customModels)
+
+      expectTypeOf(extendedMock).parameter(1).toEqualTypeOf<string>()
+
+      const adapter = extendedMock('my-fine-tuned-model', 'sk-test', {
+        baseURL: 'https://custom.api.com',
+      })
+      expect(adapter.model).toBe('my-fine-tuned-model')
+
+      // config stays optional
+      void extendedMock('mock-gpt-4', 'sk-test')
+
+      // @ts-expect-error - apiKey is required
+      void extendedMock('mock-gpt-4')
+
+      // @ts-expect-error - invalid model names still rejected
+      void extendedMock('not-a-model', 'sk-test')
+    })
+  })
+
   describe('Empty custom models', () => {
     it('should work with empty custom models array', () => {
       const extendedMock = extendAdapter(mockText, [] as const)