diff --git a/packages/agent-core-v2/src/agent/loop/loopService.ts b/packages/agent-core-v2/src/agent/loop/loopService.ts
index cfc901e12..7a587f850 100644
--- a/packages/agent-core-v2/src/agent/loop/loopService.ts
+++ b/packages/agent-core-v2/src/agent/loop/loopService.ts
@@ -1,35 +1,29 @@
+import { randomUUID } from 'node:crypto';
+
+import type { AgentEvent } from '@moonshot-ai/protocol';
+
 import { InstantiationType } from '#/_base/di/extensions';
-import {  LifecycleScope, registerScopedService } from '#/_base/di/scope';
-
-import {
-  APIContextOverflowError,
-  createToolMessage,
-  isToolCall,
-  isToolCallPart,
-  type ContentPart,
-  type StreamedMessagePart,
-  type TokenUsage
-} from '#/app/llmProtocol';
-
+import { LifecycleScope, registerScopedService } from '#/_base/di/scope';
 import { IAgentContextSizeService } from '#/agent/contextSize';
 import { IAgentExternalHooksService } from '#/agent/externalHooks';
-import {
-  IAgentLLMRequesterService,
-  type LLMRequestFinish,
-} from '#/agent/llmRequester';
+import { IAgentLLMRequesterService, type LLMRequestFinish } from '#/agent/llmRequester';
 import { IAgentProfileService } from '#/agent/profile';
 import { IAgentRecordService } from '#/agent/record';
 import type { ToolResult } from '#/agent/tool';
 import { IAgentToolExecutorService } from '#/agent/toolExecutor';
 import { IConfigService } from '#/app/config';
-import { ILogService } from '#/app/log';
 import {
-  ErrorCodes,
-  isKimiError
-} from "#/errors";
+  APIContextOverflowError,
+  createToolMessage,
+  type ContentPart,
+  type StreamedMessagePart,
+  type TokenUsage,
+} from '#/app/llmProtocol';
+import { ILogService } from '#/app/log';
+import { ErrorCodes, isKimiError } from '#/errors';
 import { OrderedHookSlot } from '#/hooks';
-import type { AgentEvent } from '@moonshot-ai/protocol';
-import { randomUUID } from 'node:crypto';
+
+import { IAgentContextMemoryService, newMessageId, type ContextMessage } from '../contextMemory';
 import { LOOP_CONTROL_SECTION, type LoopControl } from './configSection';
 import {
   createMaxStepsExceededError,
@@ -44,7 +38,6 @@ import type {
   LoopTurnStopReason,
   TurnResult,
 } from './types';
-import { IAgentContextMemoryService, newMessageId, type ContextMessage } from '../contextMemory';
 
 const TOOL_ERROR_STATUS = '<system>ERROR: Tool execution failed.</system>';
 const TOOL_EMPTY_STATUS = '<system>Tool output is empty.</system>';
@@ -132,9 +125,7 @@ export class AgentLoopService implements IAgentLoopService {
           return { stopReason: 'aborted', steps };
         }
 
-        const reason: LoopInterruptReason = isMaxStepsExceededError(error)
-          ? 'max_steps'
-          : 'error';
+        const reason: LoopInterruptReason = isMaxStepsExceededError(error) ? 'max_steps' : 'error';
         this.emitStepInterrupted(turnId, activeStep, reason, errorMessage(error));
 
         if (isContextOverflowError(error)) {
@@ -168,7 +159,7 @@ export class AgentLoopService implements IAgentLoopService {
 
     emit({ type: 'turn.step.started', turnId, step: currentStep, stepId: stepUuid });
 
-    const emitToolCallDelta = createToolCallDeltaHandler(emit, turnId);
+    const emitStreamPart = this.createStreamPartHandler(turnId);
     const response = await this.llmRequester.request(
       {
         requestLogFields: { turnStep },
@@ -192,9 +183,7 @@ export class AgentLoopService implements IAgentLoopService {
         },
         usageContext: { type: 'turn', turnId },
       },
-      (part) => {
-        this.emitStreamPart(turnId, emitToolCallDelta, part);
-      },
+      emitStreamPart,
       signal,
     );
 
@@ -283,33 +272,6 @@ export class AgentLoopService implements IAgentLoopService {
     this.contextSize.measured(this.context.get().length, tokens);
   }
 
-  private emitStreamPart(
-    turnId: number,
-    emitToolCallDelta: (part: StreamedMessagePart) => void,
-    part: StreamedMessagePart,
-  ): void {
-    switch (part.type) {
-      case 'text':
-        this.record.signal({ type: 'assistant.delta', turnId, delta: part.text });
-        return;
-      case 'think':
-        this.record.signal({ type: 'thinking.delta', turnId, delta: part.think });
-        return;
-      case 'image_url':
-      case 'audio_url':
-      case 'video_url':
-        return;
-      case 'function':
-      case 'tool_call_part':
-        emitToolCallDelta(part);
-        return;
-      default: {
-        const _exhaustive: never = part;
-        return _exhaustive;
-      }
-    }
-  }
-
   private emitStepCompleted(
     turnId: number,
     step: number,
@@ -356,6 +318,61 @@ export class AgentLoopService implements IAgentLoopService {
       message,
     });
   }
+
+  private createStreamPartHandler(turnId: number): (part: StreamedMessagePart) => void {
+    // Maps a tool call's streaming index to its identity so that interleaved
+    // argument deltas from parallel tool calls can be routed to the right call.
+    // Each provider emits a `function` header before any of its `tool_call_part`
+    // deltas, and a delta's `index` always matches a previously-seen header's
+    // `_streamIndex`. The `undefined` key doubles as the single-call fallback
+    // for providers that stream without indices: those streams never mix indexed
+    // and unindexed parts, so the most recent unindexed header is always the
+    // target.
+    const callsByIndex = new Map<number | string | undefined, { id: string; name: string }>();
+
+    return (part) => {
+      switch (part.type) {
+        case 'text':
+          this.record.signal({ type: 'assistant.delta', turnId, delta: part.text });
+          return;
+        case 'think':
+          this.record.signal({ type: 'thinking.delta', turnId, delta: part.think });
+          return;
+        case 'image_url':
+        case 'audio_url':
+        case 'video_url':
+          return;
+        case 'function': {
+          callsByIndex.set(part._streamIndex, { id: part.id, name: part.name });
+          this.record.signal({
+            type: 'tool.call.delta',
+            turnId,
+            toolCallId: part.id,
+            name: part.name,
+            argumentsPart: part.arguments ?? undefined,
+          });
+          return;
+        }
+        case 'tool_call_part': {
+          if (part.argumentsPart === null) return;
+          const toolCall = callsByIndex.get(part.index);
+          if (toolCall === undefined) return;
+          this.record.signal({
+            type: 'tool.call.delta',
+            turnId,
+            toolCallId: toolCall.id,
+            name: toolCall.name,
+            argumentsPart: part.argumentsPart,
+          });
+          return;
+        }
+        default: {
+          const _exhaustive: never = part;
+          return _exhaustive;
+        }
+      }
+    };
+  }
 }
 
 function isContextOverflowError(error: unknown): boolean {
@@ -419,41 +436,6 @@ function deriveStepStopReason(response: LLMRequestFinish): LoopStepStopReason {
   }
 }
 
-function createToolCallDeltaHandler(
-  emit: (event: AgentEvent) => void,
-  turnId: number,
-): (part: StreamedMessagePart) => void {
-  const callsByIndex = new Map<number | string, { id: string; name: string }>();
-  let lastToolCall: { id: string; name: string } | undefined;
-
-  return (part) => {
-    if (isToolCall(part)) {
-      lastToolCall = { id: part.id, name: part.name };
-      if (part._streamIndex !== undefined) {
-        callsByIndex.set(part._streamIndex, lastToolCall);
-      }
-      emit({
-        type: 'tool.call.delta',
-        turnId,
-        toolCallId: part.id,
-        name: part.name,
-        argumentsPart: part.arguments ?? undefined,
-      });
-      return;
-    }
-    if (!isToolCallPart(part) || part.argumentsPart === null) return;
-    const toolCall = part.index !== undefined ? callsByIndex.get(part.index) : lastToolCall;
-    if (toolCall === undefined) return;
-    emit({
-      type: 'tool.call.delta',
-      turnId,
-      toolCallId: toolCall.id,
-      name: toolCall.name,
-      argumentsPart: part.argumentsPart,
-    });
-  };
-}
-
 registerScopedService(
   LifecycleScope.Agent,
   IAgentLoopService,