feat: tool fixation detection — break repetitive loops (#886)

Marathon sessions show tool fixation: agent latches onto one tool
and calls it repeatedly. Observed streaks of 8-25 identical calls.

New agent/tool_fixation_detector.py:
- ToolFixationDetector: tracks consecutive tool calls
- record(tool_name): returns nudge prompt when threshold reached
- Default threshold: 5 consecutive calls (configurable via
  TOOL_FIXATION_THRESHOLD env var)
- Nudge prompt explains the fixation and suggests alternatives:
  1. Read error carefully
  2. Try different tool
  3. Ask user for clarification
  4. Check if task is complete
- get_streak_info(): current streak state
- format_report(): human-readable fixation events
- Singleton via get_fixation_detector()

Config:
- TOOL_FIXATION_THRESHOLD (default: 5)
- TOOL_FIXATION_WINDOW (default: 10)

Tests: tests/test_tool_fixation_detector.py (9 tests)

Closes #886

agent/tool_fixation_detector.py

@@ -0,0 +1,156 @@
+"""Tool fixation detection — break repetitive tool calling loops.
+
+Detects when the agent latches onto one tool and calls it repeatedly
+without making progress. Injects a nudge prompt to break the loop.
+
+Usage:
+    from agent.tool_fixation_detector import ToolFixationDetector
+    detector = ToolFixationDetector()
+    nudge = detector.record("execute_code")
+    if nudge:
+        # Inject nudge into conversation
+        messages.append({"role": "system", "content": nudge})
+"""
+
+from __future__ import annotations
+
+import os
+from dataclasses import dataclass, field
+from typing import Dict, List, Optional
+
+
+# Default thresholds
+_DEFAULT_THRESHOLD = int(os.getenv("TOOL_FIXATION_THRESHOLD", "5"))
+_DEFAULT_WINDOW = int(os.getenv("TOOL_FIXATION_WINDOW", "10"))
+
+
+@dataclass
+class FixationEvent:
+    """Record of a fixation detection."""
+    tool_name: str
+    streak_length: int
+    threshold: int
+    nudge_sent: bool = False
+
+
+class ToolFixationDetector:
+    """Detects and breaks tool fixation loops.
+
+    Tracks the sequence of tool calls and detects when the same tool
+    is called N times consecutively. When detected, returns a nudge
+    prompt to inject into the conversation.
+    """
+
+    def __init__(self, threshold: int = 0, window: int = 0):
+        self.threshold = threshold or _DEFAULT_THRESHOLD
+        self.window = window or _DEFAULT_WINDOW
+        self._history: List[str] = []
+        self._current_streak: str = ""
+        self._streak_count: int = 0
+        self._nudges_sent: int = 0
+        self._events: List[FixationEvent] = []
+
+    @property
+    def nudges_sent(self) -> int:
+        return self._nudges_sent
+
+    @property
+    def events(self) -> List[FixationEvent]:
+        return list(self._events)
+
+    def record(self, tool_name: str) -> Optional[str]:
+        """Record a tool call and return nudge prompt if fixation detected.
+
+        Args:
+            tool_name: Name of the tool that was called.
+
+        Returns:
+            Nudge prompt string if fixation detected, None otherwise.
+        """
+        self._history.append(tool_name)
+
+        # Trim history to window
+        if len(self._history) > self.window:
+            self._history = self._history[-self.window:]
+
+        # Update streak
+        if tool_name == self._current_streak:
+            self._streak_count += 1
+        else:
+            self._current_streak = tool_name
+            self._streak_count = 1
+
+        # Check for fixation
+        if self._streak_count >= self.threshold:
+            event = FixationEvent(
+                tool_name=tool_name,
+                streak_length=self._streak_count,
+                threshold=self.threshold,
+                nudge_sent=True,
+            )
+            self._events.append(event)
+            self._nudges_sent += 1
+
+            return self._build_nudge(tool_name, self._streak_count)
+
+        return None
+
+    def _build_nudge(self, tool_name: str, count: int) -> str:
+        """Build a nudge prompt to break the fixation loop."""
+        return (
+            f"[SYSTEM: You have called `{tool_name}` {count} times in a row "
+            f"without switching tools. This suggests a fixation loop. "
+            f"Consider:\n"
+            f"1. Is the tool returning an error? Read the error carefully.\n"
+            f"2. Is there a different tool that could help?\n"
+            f"3. Should you ask the user for clarification?\n"
+            f"4. Is the task actually complete?\n"
+            f"Break the loop by trying a different approach.]"
+        )
+
+    def reset(self) -> None:
+        """Reset the detector state."""
+        self._history.clear()
+        self._current_streak = ""
+        self._streak_count = 0
+
+    def get_streak_info(self) -> dict:
+        """Get current streak information."""
+        return {
+            "current_tool": self._current_streak,
+            "streak_count": self._streak_count,
+            "threshold": self.threshold,
+            "at_threshold": self._streak_count >= self.threshold,
+            "nudges_sent": self._nudges_sent,
+        }
+
+    def format_report(self) -> str:
+        """Format fixation events as a report."""
+        if not self._events:
+            return "No tool fixation detected."
+
+        lines = [
+            f"Tool Fixation Report ({len(self._events)} events)",
+            "=" * 40,
+        ]
+        for e in self._events:
+            lines.append(f"  {e.tool_name}: {e.streak_length} consecutive calls (threshold: {e.threshold})")
+        return "\n".join(lines)
+
+
+# Singleton
+_detector: Optional[ToolFixationDetector] = None
+
+
+def get_fixation_detector() -> ToolFixationDetector:
+    """Get or create the singleton detector."""
+    global _detector
+    if _detector is None:
+        _detector = ToolFixationDetector()
+    return _detector
+
+
+def reset_fixation_detector() -> None:
+    """Reset the singleton."""
+    global _detector
+    _detector = None

tests/test_tool_fixation_detector.py

@@ -0,0 +1,76 @@
+"""Tests for tool fixation detection."""
+
+import pytest
+import sys
+from pathlib import Path
+
+sys.path.insert(0, str(Path(__file__).resolve().parent.parent))
+
+from agent.tool_fixation_detector import ToolFixationDetector, get_fixation_detector
+
+
+class TestFixationDetection:
+    def test_no_fixation_below_threshold(self):
+        d = ToolFixationDetector(threshold=5)
+        for i in range(4):
+            assert d.record("execute_code") is None
+
+    def test_fixation_at_threshold(self):
+        d = ToolFixationDetector(threshold=3)
+        d.record("execute_code")
+        d.record("execute_code")
+        nudge = d.record("execute_code")
+        assert nudge is not None
+        assert "execute_code" in nudge
+        assert "3 times" in nudge
+
+    def test_fixation_above_threshold(self):
+        d = ToolFixationDetector(threshold=3)
+        d.record("execute_code")
+        d.record("execute_code")
+        d.record("execute_code")  # threshold hit
+        nudge = d.record("execute_code")  # still nudging
+        assert nudge is not None
+
+    def test_streak_resets_on_different_tool(self):
+        d = ToolFixationDetector(threshold=3)
+        d.record("execute_code")
+        d.record("execute_code")
+        d.record("terminal")  # breaks streak
+        assert d._streak_count == 1
+        assert d._current_streak == "terminal"
+
+    def test_nudges_sent_counter(self):
+        d = ToolFixationDetector(threshold=2)
+        d.record("a")
+        d.record("a")  # nudge 1
+        d.record("a")  # nudge 2
+        assert d.nudges_sent == 2
+
+    def test_events_recorded(self):
+        d = ToolFixationDetector(threshold=2)
+        d.record("x")
+        d.record("x")
+        assert len(d.events) == 1
+        assert d.events[0].tool_name == "x"
+        assert d.events[0].streak_length == 2
+
+    def test_report(self):
+        d = ToolFixationDetector(threshold=2)
+        d.record("x")
+        d.record("x")
+        report = d.format_report()
+        assert "x" in report
+
+    def test_reset(self):
+        d = ToolFixationDetector(threshold=2)
+        d.record("x")
+        d.record("x")
+        d.reset()
+        assert d._streak_count == 0
+        assert d._current_streak == ""
+
+    def test_singleton(self):
+        d1 = get_fixation_detector()
+        d2 = get_fixation_detector()
+        assert d1 is d2