From a21aa370e789db6f634cf8e2512fd154b0acd6c5 Mon Sep 17 00:00:00 2001
From: Henry Su <henrysu4707@gmail.com>
Date: Fri, 24 Jul 2026 22:40:00 -0500
Subject: [PATCH] fix(responses): safely handle stream deltas on uninitialized
 properties

---
 src/lib/responses/ResponseAccumulator.ts | 16 +++---
 tests/lib/ResponseAccumulator.test.ts    | 62 ++++++++++++++++++++++++
 2 files changed, 70 insertions(+), 8 deletions(-)

diff --git a/src/lib/responses/ResponseAccumulator.ts b/src/lib/responses/ResponseAccumulator.ts
index f3dee49f0..698a8f914 100644
--- a/src/lib/responses/ResponseAccumulator.ts
+++ b/src/lib/responses/ResponseAccumulator.ts
@@ -90,8 +90,8 @@ export function accumulateResponse(
         if (content.type !== 'output_text') {
           throw new OpenAIError(`expected content to be 'output_text', got ${content.type}`);
         }
-        content.text += event.delta;
-        snapshot.output_text += event.delta;
+        content.text = (content.text ?? '') + event.delta;
+        snapshot.output_text = (snapshot.output_text ?? '') + event.delta;
       }
       break;
     }
@@ -127,7 +127,7 @@ export function accumulateResponse(
         if (content.type !== 'refusal') {
           throw new OpenAIError(`expected content to be 'refusal', got ${content.type}`);
         }
-        content.refusal += event.delta;
+        content.refusal = (content.refusal ?? '') + event.delta;
       }
       break;
     }
@@ -145,7 +145,7 @@ export function accumulateResponse(
     case 'response.function_call_arguments.delta': {
       const output = getOutput(snapshot, event.output_index);
       if (output.type === 'function_call') {
-        output.arguments += event.delta;
+        output.arguments = (output.arguments ?? '') + event.delta;
       }
       break;
     }
@@ -166,7 +166,7 @@ export function accumulateResponse(
         if (content.type !== 'reasoning_text') {
           throw new OpenAIError(`expected content to be 'reasoning_text', got ${content.type}`);
         }
-        content.text += event.delta;
+        content.text = (content.text ?? '') + event.delta;
       }
       break;
     }
@@ -203,7 +203,7 @@ export function accumulateResponse(
       const output = getOutput(snapshot, event.output_index);
       if (output.type === 'reasoning') {
         const part = getContent(output.summary, event.summary_index);
-        part.text += event.delta;
+        part.text = (part.text ?? '') + event.delta;
       }
       break;
     }
@@ -218,7 +218,7 @@ export function accumulateResponse(
     case 'response.custom_tool_call_input.delta': {
       const output = getOutput(snapshot, event.output_index);
       if (output.type === 'custom_tool_call') {
-        output.input += event.delta;
+        output.input = (output.input ?? '') + event.delta;
       }
       break;
     }
@@ -232,7 +232,7 @@ export function accumulateResponse(
     case 'response.mcp_call_arguments.delta': {
       const output = getOutput(snapshot, event.output_index);
       if (output.type === 'mcp_call') {
-        output.arguments += event.delta;
+        output.arguments = (output.arguments ?? '') + event.delta;
       }
       break;
     }
diff --git a/tests/lib/ResponseAccumulator.test.ts b/tests/lib/ResponseAccumulator.test.ts
index 0fbcb111a..df83bd10f 100644
--- a/tests/lib/ResponseAccumulator.test.ts
+++ b/tests/lib/ResponseAccumulator.test.ts
@@ -222,6 +222,68 @@ describe('ResponseAccumulator', () => {
       content: [{ type: 'refusal', refusal: 'I cannot help with that.' }],
     });
   });
+
+  it('safely accumulates deltas when initial string fields are uninitialized', () => {
+    const snapshot = accumulateEvents([
+      { type: 'response.created', sequence_number: 0, response: makeResponse() },
+      {
+        type: 'response.output_item.added',
+        sequence_number: 1,
+        output_index: 0,
+        item: {
+          id: 'fc_1',
+          type: 'function_call',
+          call_id: 'call_1',
+          name: 'get_weather',
+          status: 'in_progress',
+        } as any,
+      },
+      {
+        type: 'response.function_call_arguments.delta',
+        sequence_number: 2,
+        item_id: 'fc_1',
+        output_index: 0,
+        delta: '{"city": "Paris"}',
+      },
+      {
+        type: 'response.output_item.added',
+        sequence_number: 3,
+        output_index: 1,
+        item: {
+          id: 'msg_123',
+          type: 'message',
+          role: 'assistant',
+          status: 'in_progress',
+          content: [],
+        },
+      },
+      {
+        type: 'response.content_part.added',
+        sequence_number: 4,
+        item_id: 'msg_123',
+        output_index: 1,
+        content_index: 0,
+        part: { type: 'refusal' } as any,
+      },
+      {
+        type: 'response.refusal.delta',
+        sequence_number: 5,
+        item_id: 'msg_123',
+        output_index: 1,
+        content_index: 0,
+        delta: 'Permission denied',
+      },
+    ]);
+
+    expect(snapshot.output[0]).toMatchObject({
+      type: 'function_call',
+      arguments: '{"city": "Paris"}',
+    });
+    expect(snapshot.output[1]).toMatchObject({
+      type: 'message',
+      content: [{ type: 'refusal', refusal: 'Permission denied' }],
+    });
+  });
 });
 
 function accumulateEvents(events: ResponseStreamEvent[]): Response {