feat(react): add loop detection to prevent repeated identical tool calls

U1: Sliding window hash detection in ReAct loop. When the same tool is
called with identical arguments >= threshold times (default 2), injects
a correction message first, then raises LoopDetectedError if the LLM
doesn't change strategy. Covers both _execute_loop and execute_stream.

src/agentkit/core/exceptions.py

@@ -60,6 +60,22 @@ class TaskCancelledError(AgentFrameworkError):
         super().__init__(f"Task {task_id} was cancelled")
 
 
+class LoopDetectedError(AgentFrameworkError):
+    """ReAct 循环检测异常 — LLM 重复调用相同工具+参数且纠正后未改变策略。
+
+    ponytail: 滑动窗口 hash 检测，窗口大小和阈值可配置。
+    升级路径：可引入语义相似度检测（embedding 距离）替代精确 hash。
+    """
+
+    def __init__(self, tool_name: str, repetitions: int):
+        self.tool_name = tool_name
+        self.repetitions = repetitions
+        super().__init__(
+            f"Loop detected: tool '{tool_name}' called {repetitions} times "
+            f"with identical arguments after correction"
+        )
+
+
 class NoAvailableAgentError(AgentFrameworkError):
     def __init__(self, task_type: str):
         self.task_type = task_type

src/agentkit/core/react.py

@@ -9,11 +9,12 @@ import json
 import logging
 import re
 import time
+from collections import Counter, deque
 from dataclasses import dataclass, field
 from datetime import datetime, timezone
 from typing import TYPE_CHECKING, Any
 
-from agentkit.core.exceptions import TaskCancelledError, TaskTimeoutError
+from agentkit.core.exceptions import LoopDetectedError, TaskCancelledError, TaskTimeoutError
 from agentkit.core.protocol import CancellationToken
 from agentkit.llm.gateway import LLMGateway
 from agentkit.llm.protocol import LLMResponse
@@ -187,6 +188,12 @@ class ReActEngine:
 
             self._compressor = ContextCompressor(llm_gateway=llm_gateway, keep_recent=10)
 
+        # Loop detection: sliding window of (tool_name, args_hash) to catch
+        # repeated identical tool calls. ponytail: hash-based, not semantic.
+        self._loop_window: deque[str] = deque(maxlen=5)
+        self._loop_threshold: int = 2
+        self._loop_corrected: bool = False
+
     def reset(self) -> None:
         """Reset internal state for reuse across conversations.
 
@@ -196,7 +203,31 @@ class ReActEngine:
         # ReActEngine is stateless between calls — conversation history,
         # step counts, and trajectory are local to each execute call.
         # This method exists for API clarity and future stateful extensions.
-        pass
+        self._loop_window.clear()
+        self._loop_corrected = False
+
+    def _check_tool_loop(self, tool_calls: list[Any]) -> str | None:
+        """检测重复工具调用模式。
+
+        将当前步的工具调用 hash 加入滑动窗口，若同一 hash 在窗口内出现
+        >= threshold 次，返回对应的 tool_name；否则返回 None。
+
+        ponytail: 精确 hash 匹配，不做语义相似度。
+        """
+        for tc in tool_calls:
+            args_str = json.dumps(tc.arguments, sort_keys=True, default=str)
+            h = hash(f"{tc.name}:{args_str}")
+            self._loop_window.append(str(h))
+
+        counts = Counter(self._loop_window)
+        for h, count in counts.items():
+            if count >= self._loop_threshold:
+                # Find the tool name for this hash
+                for tc in tool_calls:
+                    args_str = json.dumps(tc.arguments, sort_keys=True, default=str)
+                    if str(hash(f"{tc.name}:{args_str}")) == h:
+                        return tc.name
+        return None
 
     async def execute(
         self,
@@ -407,6 +438,33 @@ class ReActEngine:
 
                 # 检查是否有 Function Calling 的 tool_calls
                 if response.has_tool_calls:
+                    # 循环检测：检查是否重复调用相同工具+参数
+                    looped_tool = self._check_tool_loop(response.tool_calls)
+                    if looped_tool is not None:
+                        if not self._loop_corrected:
+                            # 第一次检测：注入纠正消息，给 LLM 改变策略的机会
+                            logger.warning(
+                                f"Loop detected: tool '{looped_tool}' repeated, "
+                                f"injecting correction at step {step}"
+                            )
+                            correction_msg = {
+                                "role": "user",
+                                "content": (
+                                    f"You are repeatedly calling tool '{looped_tool}' "
+                                    f"with the same arguments. This indicates a loop. "
+                                    f"Please change your strategy or provide a final answer."
+                                ),
+                            }
+                            conversation.append(correction_msg)
+                            self._loop_corrected = True
+                            continue
+                        else:
+                            # 第二次检测：纠正后仍未改变，强制中断
+                            raise LoopDetectedError(
+                                tool_name=looped_tool,
+                                repetitions=self._loop_threshold + 1,
+                            )
+
                     # 记录 LLM 调用步骤
                     if trace_recorder is not None:
                         trace_recorder.record_step(
@@ -1014,14 +1072,41 @@ class ReActEngine:
                 total_tokens += step_tokens
 
                 if response.has_tool_calls:
-                    # 记录 LLM 调用步骤
-                    if trace_recorder is not None:
-                        trace_recorder.record_step(
-                            step=step,
-                            action="llm_call",
-                            duration_ms=llm_duration_ms,
-                            tokens_used=step_tokens,
+                    # 循环检测：检查是否重复调用相同工具+参数
+                    looped_tool = self._check_tool_loop(response.tool_calls)
+                    if looped_tool is not None:
+                        if not self._loop_corrected:
+                            logger.warning(
+                                f"Loop detected (stream): tool '{looped_tool}' repeated, "
+                                f"injecting correction at step {step}"
                             )
+                            correction_msg = {
+                                "role": "user",
+                                "content": (
+                                    f"You are repeatedly calling tool '{looped_tool}' "
+                                    f"with the same arguments. This indicates a loop. "
+                                    f"Please change your strategy or provide a final answer."
+                                ),
+                            }
+                            conversation.append(correction_msg)
+                            self._loop_corrected = True
+                            yield ReActEvent(
+                                event_type="step",
+                                step=step,
+                                data={
+                                    "message": f"Loop detected: tool '{looped_tool}' repeated. Correction injected.",
+                                    "loop_detected": True,
+                                    "tool_name": looped_tool,
+                                },
+                            )
+                            continue
+                        else:
+                            raise LoopDetectedError(
+                                tool_name=looped_tool,
+                                repetitions=self._loop_threshold + 1,
+                            )
+
+                    # 记录 LLM 调用步骤
 
                     # Record assistant message
                     assistant_msg: dict[str, Any] = {

tests/unit/test_react_engine.py

@@ -165,12 +165,15 @@ class TestReActMaxSteps:
 
         tool = FakeTool(name="search", result={"results": ["data"]})
 
-        # LLM 一直返回 tool_calls，不会给出 final answer
-        always_tool_response = make_response(
+        # LLM 一直返回 tool_calls（参数递增以避免循环检测），不会给出 final answer
+        responses = [
+            make_response(
                 content="Thinking...",
-            tool_calls=[ToolCall(id="tc_loop", name="search", arguments={"query": "more"})],
+                tool_calls=[ToolCall(id=f"tc_{i}", name="search", arguments={"query": f"attempt_{i}"})],
             )
-        gateway = make_mock_gateway([always_tool_response] * 20)
+            for i in range(20)
+        ]
+        gateway = make_mock_gateway(responses)
         engine = ReActEngine(llm_gateway=gateway, max_steps=3)
 
         result = await engine.execute(
@@ -653,3 +656,123 @@ class TestReActCancellation:
         )
         assert result.output == "Answer"
         assert result.status == "success"
+
+
+class TestLoopDetection:
+    """循环检测：ReAct 循环内滑动窗口 hash 检测重复工具调用"""
+
+    async def test_normal_different_tools_no_detection(self):
+        """不同工具调用不触发检测"""
+        from agentkit.core.react import ReActEngine
+
+        tool1 = FakeTool(name="search", result={"results": ["a"]})
+        tool2 = FakeTool(name="calculator", result={"value": 42})
+        gateway = make_mock_gateway([
+            make_response(
+                tool_calls=[ToolCall(id="tc_1", name="search", arguments={"q": "test"})],
+            ),
+            make_response(
+                tool_calls=[ToolCall(id="tc_2", name="calculator", arguments={"expr": "6*7"})],
+            ),
+            make_response(content="Done"),
+        ])
+        engine = ReActEngine(llm_gateway=gateway)
+
+        result = await engine.execute(
+            messages=[{"role": "user", "content": "Search and calculate"}],
+            tools=[tool1, tool2],
+        )
+        assert result.status == "success"
+        assert result.total_steps == 3
+
+    async def test_same_tool_different_args_no_detection(self):
+        """相同工具不同参数不触发检测"""
+        from agentkit.core.react import ReActEngine
+
+        tool = FakeTool(name="search", result={"results": []})
+        gateway = make_mock_gateway([
+            make_response(
+                tool_calls=[ToolCall(id="tc_1", name="search", arguments={"q": "hello"})],
+            ),
+            make_response(
+                tool_calls=[ToolCall(id="tc_2", name="search", arguments={"q": "world"})],
+            ),
+            make_response(content="Done"),
+        ])
+        engine = ReActEngine(llm_gateway=gateway)
+
+        result = await engine.execute(
+            messages=[{"role": "user", "content": "Search twice"}],
+            tools=[tool],
+        )
+        assert result.status == "success"
+        assert result.total_steps == 3
+
+    async def test_loop_detected_injects_correction_then_raises(self):
+        """连续重复调用相同工具+参数：第一次注入纠正，第二次抛 LoopDetectedError"""
+        from agentkit.core.react import ReActEngine
+        from agentkit.core.exceptions import LoopDetectedError
+
+        tool = FakeTool(name="search", result={"results": []})
+        # Step 1: tool call (executed, window=[hash])
+        # Step 2: same tool call (detected, correction injected, continue)
+        # Step 3: same tool call again (detected, already corrected → raise)
+        gateway = make_mock_gateway([
+            make_response(
+                tool_calls=[ToolCall(id="tc_1", name="search", arguments={"q": "test"})],
+            ),
+            make_response(
+                tool_calls=[ToolCall(id="tc_2", name="search", arguments={"q": "test"})],
+            ),
+            make_response(
+                tool_calls=[ToolCall(id="tc_3", name="search", arguments={"q": "test"})],
+            ),
+        ])
+        engine = ReActEngine(llm_gateway=gateway, max_steps=10)
+
+        with pytest.raises(LoopDetectedError) as exc_info:
+            await engine.execute(
+                messages=[{"role": "user", "content": "Search"}],
+                tools=[tool],
+            )
+        assert "search" in str(exc_info.value)
+
+    async def test_loop_correction_allows_recovery(self):
+        """循环检测注入纠正后，LLM 改变策略则正常完成"""
+        from agentkit.core.react import ReActEngine
+
+        tool = FakeTool(name="search", result={"results": []})
+        # Step 1: tool call (executed)
+        # Step 2: same tool call (detected, correction injected)
+        # Step 3: LLM changes strategy → final answer
+        gateway = make_mock_gateway([
+            make_response(
+                tool_calls=[ToolCall(id="tc_1", name="search", arguments={"q": "test"})],
+            ),
+            make_response(
+                tool_calls=[ToolCall(id="tc_2", name="search", arguments={"q": "test"})],
+            ),
+            make_response(content="I found the answer after changing strategy"),
+        ])
+        engine = ReActEngine(llm_gateway=gateway, max_steps=10)
+
+        result = await engine.execute(
+            messages=[{"role": "user", "content": "Search"}],
+            tools=[tool],
+        )
+        assert result.status == "success"
+        assert "changing strategy" in result.output
+
+    async def test_reset_clears_loop_state(self):
+        """reset() 清除循环检测状态"""
+        from agentkit.core.react import ReActEngine
+
+        gateway = make_mock_gateway([make_response(content="Done")])
+        engine = ReActEngine(llm_gateway=gateway)
+        engine._loop_window.append("some_hash")
+        engine._loop_corrected = True
+
+        engine.reset()
+
+        assert len(engine._loop_window) == 0
+        assert engine._loop_corrected is False