diff --git a/packages/ai/src/protocols/utils/tool-stream.ts b/packages/ai/src/protocols/utils/tool-stream.ts
index 2640077411c..7f15395693d 100644
--- a/packages/ai/src/protocols/utils/tool-stream.ts
+++ b/packages/ai/src/protocols/utils/tool-stream.ts
@@ -60,14 +60,21 @@ const inputStart = (tool: PendingTool) =>
     providerMetadata: tool.providerMetadata,
   })
 
-const inputDelta = (tool: PendingTool, text: string) =>
-  LLMEvent.toolInputDelta({
-    id: tool.id,
-    name: tool.name,
-    namespace: tool.namespace,
-    text,
-    input: Option.getOrElse(parsePartialInput(tool.input), () => ({})),
-  })
+const inputDelta = (tool: PendingTool, text: string): LLMEvent => {
+  const raw = tool.input
+  let parsed: unknown
+  return {
+    ...LLMEvent.toolInputDelta({
+      id: tool.id,
+      name: tool.name,
+      namespace: tool.namespace,
+      text,
+    }),
+    get input() {
+      return (parsed ??= Option.getOrElse(parsePartialInput(raw), () => ({})))
+    },
+  }
+}
 
 const toolCall = (route: string, tool: PendingTool, inputOverride?: string) => {
   const raw = inputOverride ?? tool.input