diff --git a/packages/typescript/ai/src/extend-adapter.ts b/packages/typescript/ai/src/extend-adapter.ts
index 8547ebef..9e4f69b6 100644
--- a/packages/typescript/ai/src/extend-adapter.ts
+++ b/packages/typescript/ai/src/extend-adapter.ts
@@ -97,15 +97,6 @@ type InferFactoryModels<TFactory> = TFactory extends (
     : 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.
@@ -116,6 +107,12 @@ type InferAdapterReturn<TFactory> = TFactory extends (
   ? TReturn
   : never
 
+/**
+ * Extracts all parameter types after the first parameter from a function.
+ */
+type InferRestArgs<TFactory extends (...args: any) => any> =
+  Parameters<TFactory> extends [any, ...infer Rest] ? Rest : []
+
 // ===========================
 // extendAdapter Function
 // ===========================
@@ -164,19 +161,17 @@ type InferAdapterReturn<TFactory> = TFactory extends (
  * ```
  */
 export function extendAdapter<
-  TFactory extends (...args: Array<any>) => any,
+  TFactory extends (model: any, ...args: Array<any>) => any,
   const TDefs extends ReadonlyArray<ExtendedModelDef>,
 >(
   factory: TFactory,
   _customModels: TDefs,
-): (
-  model: InferFactoryModels<TFactory> | ExtractCustomModelNames<TDefs>,
-  ...args: InferConfig<TFactory> extends undefined
-    ? []
-    : [config?: InferConfig<TFactory>]
-) => InferAdapterReturn<TFactory> {
+) {
   // 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 as unknown as (
+    model: InferFactoryModels<TFactory> | ExtractCustomModelNames<TDefs>,
+    ...args: InferRestArgs<TFactory>
+  ) => InferAdapterReturn<TFactory>
+}
\ No newline at end of file