fix(ai): emit missing responses toolcall delta closes #2745

packages/ai/CHANGELOG.md

@@ -5,6 +5,7 @@
 ### Fixed
 
 - Fixed Anthropic context overflow detection to recognize HTTP 413 `request_too_large` errors, so callers can trigger compaction and retry instead of getting stuck on repeated oversized-image requests ([#2734](https://github.com/badlogic/pi-mono/issues/2734))
+- Fixed OpenAI Responses tool-call streaming to emit a `toolcall_delta` when function call arguments arrive only in `response.function_call_arguments.done`, and to emit only the missing suffix when `.done` extends earlier streamed arguments ([#2745](https://github.com/badlogic/pi-mono/issues/2745))
 
 ## [0.64.0] - 2026-03-29
 

packages/ai/src/providers/openai-responses-shared.ts

@@ -408,8 +408,21 @@ export async function processResponsesStream<TApi extends Api>(
 			}
 		} else if (event.type === "response.function_call_arguments.done") {
 			if (currentItem?.type === "function_call" && currentBlock?.type === "toolCall") {
+				const previousPartialJson = currentBlock.partialJson;
 				currentBlock.partialJson = event.arguments;
 				currentBlock.arguments = parseStreamingJson(currentBlock.partialJson);
+
+				if (event.arguments.startsWith(previousPartialJson)) {
+					const delta = event.arguments.slice(previousPartialJson.length);
+					if (delta.length > 0) {
+						stream.push({
+							type: "toolcall_delta",
+							contentIndex: blockIndex(),
+							delta,
+							partial: output,
+						});
+					}
+				}
 			}
 		} else if (event.type === "response.output_item.done") {
 			const item = event.item;