From bf5e69017fdefa7abe10355df45aa082976123cf Mon Sep 17 00:00:00 2001
From: Yarchik <spoko.dev@gmail.com>
Date: Thu, 2 Jul 2026 12:54:38 +0100
Subject: [PATCH] fix(assistants): place array delta entries by index instead
 of appending

---
 src/lib/AssistantStream.ts                   |  2 +-
 tests/streaming/assistants/assistant.test.ts | 15 +++++++++++++++
 2 files changed, 16 insertions(+), 1 deletion(-)

diff --git a/src/lib/AssistantStream.ts b/src/lib/AssistantStream.ts
index f3002582cd..d8a665c000 100644
--- a/src/lib/AssistantStream.ts
+++ b/src/lib/AssistantStream.ts
@@ -686,7 +686,7 @@ export class AssistantStream
 
           const accEntry = accValue[index];
           if (accEntry == null) {
-            accValue.push(deltaEntry);
+            accValue[index] = deltaEntry;
           } else {
             accValue[index] = this.accumulateDelta(accEntry, deltaEntry);
           }
diff --git a/tests/streaming/assistants/assistant.test.ts b/tests/streaming/assistants/assistant.test.ts
index e8db3d5852..5e3547e610 100644
--- a/tests/streaming/assistants/assistant.test.ts
+++ b/tests/streaming/assistants/assistant.test.ts
@@ -29,4 +29,19 @@ describe('assistant tests', () => {
     expect(AssistantStream.accumulateDelta({ a: null }, { a: 'apple' })).toEqual({ a: 'apple' });
     expect(AssistantStream.accumulateDelta({ a: null }, { a: null })).toEqual({ a: null });
   });
+
+  test('array delta accumulation uses the entry index', () => {
+    const acc: any = {
+      tool_calls: [{ index: 0, id: 'call_0', type: 'function', function: { name: 'f0', arguments: '{' } }],
+    };
+    AssistantStream.accumulateDelta(acc, {
+      tool_calls: [{ index: 2, id: 'call_2', type: 'function', function: { name: 'f2', arguments: '{' } }],
+    });
+    AssistantStream.accumulateDelta(acc, {
+      tool_calls: [{ index: 2, function: { arguments: '"x":1}' } }],
+    });
+    const atIndex2 = acc.tool_calls.filter((t: any) => t.index === 2);
+    expect(atIndex2).toHaveLength(1);
+    expect(atIndex2[0].function.arguments).toBe('{"x":1}');
+  });
 });