diff --git a/core/config/types.ts b/core/config/types.ts
index 2500042e887..c696421e10d 100644
--- a/core/config/types.ts
+++ b/core/config/types.ts
@@ -306,6 +306,7 @@ declare global {
   
   export interface ToolCallDelta {
     id?: string;
+    index?: number;
     type?: "function";
     function?: {
       name?: string;
diff --git a/core/index.d.ts b/core/index.d.ts
index bec3e0e0ff8..375741f6341 100644
--- a/core/index.d.ts
+++ b/core/index.d.ts
@@ -364,6 +364,7 @@ export interface ToolCall {
 
 export interface ToolCallDelta {
   id?: string;
+  index?: number;
   type?: "function";
   function?: {
     name?: string;
diff --git a/core/llm/openaiTypeConverters.test.ts b/core/llm/openaiTypeConverters.test.ts
index f597f002262..c3a4ae430d5 100644
--- a/core/llm/openaiTypeConverters.test.ts
+++ b/core/llm/openaiTypeConverters.test.ts
@@ -1,4 +1,8 @@
-import { toResponsesInput, isItemType } from "./openaiTypeConverters";
+import {
+  toResponsesInput,
+  isItemType,
+  fromChatCompletionChunk,
+} from "./openaiTypeConverters";
 import { ChatMessage } from "..";
 import type {
   EasyInputMessage,
@@ -847,4 +851,26 @@ describe("openaiTypeConverters", () => {
       });
     });
   });
+
+  describe("fromChatCompletionChunk", () => {
+    it("should preserve tool_call.index so id-less fragments correlate (issue #13223)", () => {
+      const chunk: any = {
+        choices: [
+          {
+            delta: {
+              tool_calls: [
+                {
+                  index: 1,
+                  function: { name: "tool_b", arguments: '{"target":"B"}' },
+                },
+              ],
+            },
+          },
+        ],
+      };
+      const msg = fromChatCompletionChunk(chunk) as any;
+      expect(msg?.toolCalls?.[0]?.index).toBe(1);
+      expect(msg?.toolCalls?.[0]?.function?.arguments).toBe('{"target":"B"}');
+    });
+  });
 });
diff --git a/core/llm/openaiTypeConverters.ts b/core/llm/openaiTypeConverters.ts
index fb4673e11be..8adb6e1c0ba 100644
--- a/core/llm/openaiTypeConverters.ts
+++ b/core/llm/openaiTypeConverters.ts
@@ -367,6 +367,7 @@ export function fromChatCompletionChunk(
       .filter((tool_call) => !tool_call.type || tool_call.type === "function")
       .map((tool_call) => ({
         id: tool_call.id,
+        index: (tool_call as any).index,
         type: "function" as const,
         function: {
           name: (tool_call as any).function?.name,
diff --git a/gui/src/redux/slices/sessionSlice.ts b/gui/src/redux/slices/sessionSlice.ts
index 8784d0c41dc..1585831bd1f 100644
--- a/gui/src/redux/slices/sessionSlice.ts
+++ b/gui/src/redux/slices/sessionSlice.ts
@@ -113,7 +113,7 @@ export function handleToolCallsInMessage(
  * @param toolCallDelta - The incoming tool call delta from the LLM stream
  * @param toolCallStates - Array of existing tool call states (modified in place)
  */
-function applyToolCallDelta(
+export function applyToolCallDelta(
   toolCallDelta: ToolCallDelta,
   toolCallStates: ToolCallState[],
 ): void {
@@ -127,6 +127,13 @@ function applyToolCallDelta(
     existingStateIndex = toolCallStates.findIndex(
       (state) => state.toolCallId === toolCallDelta.id,
     );
+  } else if (toolCallDelta.index !== undefined) {
+    // No ID but a stream index is present (OpenAI parallel tool calls):
+    // initial fragments carry both, continuation fragments carry index only.
+    // Match by the position in the toolCalls array so one call's args
+    // are never applied to another call.
+    existingStateIndex =
+      toolCallDelta.index < toolCallStates.length ? toolCallDelta.index : -1;
   } else {
     // No ID in delta (common in OpenAI streaming fragments)
     // Strategy: Update the most recently added tool call that's still being generated