diff --git a/open-sse/translator/response/openai-responses.ts b/open-sse/translator/response/openai-responses.ts
index 96aa862424..b22b9f5f46 100644
--- a/open-sse/translator/response/openai-responses.ts
+++ b/open-sse/translator/response/openai-responses.ts
@@ -464,6 +464,9 @@ export function openaiResponsesToOpenAIResponse(chunk, state) {
   }
 
   // Function call arguments delta
+  // NOTE: Do NOT include `id` or `type` here - only first chunk (response.output_item.added)
+  // should have them. Including `id` on every chunk causes openai-to-claude.ts to emit
+  // a new content_block_start for each delta, breaking Claude Code ACP sessions.
   if (eventType === "response.function_call_arguments.delta") {
     const argsDelta = data.delta || "";
     if (!argsDelta) return null;
@@ -480,8 +483,6 @@ export function openaiResponsesToOpenAIResponse(chunk, state) {
             tool_calls: [
               {
                 index: state.toolCallIndex,
-                id: state.currentToolCallId,
-                type: "function",
                 function: { arguments: argsDelta },
               },
             ],