diff --git a/packages/core/core/prompt/prepare-tools-and-tool-choice.ts b/packages/core/core/prompt/prepare-tools-and-tool-choice.ts
index 50042be9156..31e894f5b8c 100644
--- a/packages/core/core/prompt/prepare-tools-and-tool-choice.ts
+++ b/packages/core/core/prompt/prepare-tools-and-tool-choice.ts
@@ -34,7 +34,7 @@ export function prepareToolsAndToolChoice<
       parameters: asSchema(tool.parameters).jsonSchema,
     })),
     toolChoice:
-      toolChoice == null
+      toolChoice === undefined
         ? { type: 'auto' }
         : typeof toolChoice === 'string'
         ? { type: toolChoice }
diff --git a/packages/core/core/types/language-model.ts b/packages/core/core/types/language-model.ts
index a7d490a0beb..ba69814c622 100644
--- a/packages/core/core/types/language-model.ts
+++ b/packages/core/core/types/language-model.ts
@@ -44,6 +44,7 @@ Tool choice for the generation. It supports the following settings:
  */
 // TODO rename to ToolChoice once shared/types.ts ToolChoice is gone
 export type CoreToolChoice<TOOLS extends Record<string, unknown>> =
+  | null
   | 'auto'
   | 'none'
   | 'required'