feat: add agent loop protection - max iterations and tool result truncation

- Add max-tool-iterations config (default 16) to prevent runaway agent loops
- Add max-tool-result-chars config (default 8000) to truncate oversized tool results
- Both settings are configurable in pipeline UI under Local Agent settings
- Logs warnings when limits are hit for debugging

Closes #2051

src/langbot/pkg/provider/runners/localagent.py

@@ -132,6 +132,12 @@ class LocalAgentRunner(runner.RequestRunner):
         """Run request"""
         pending_tool_calls = []
 
+        # Agent loop protection config
+        agent_config = query.pipeline_config['ai']['local-agent']
+        max_tool_iterations = agent_config.get('max-tool-iterations', 16)
+        max_tool_result_chars = agent_config.get('max-tool-result-chars', 8000)
+        iteration_count = 0
+
         # Get knowledge bases list (new field)
         kb_uuids = query.pipeline_config['ai']['local-agent'].get('knowledge-bases', [])
 
@@ -295,6 +301,14 @@ class LocalAgentRunner(runner.RequestRunner):
         # Once a model succeeds, commit to it for the tool call loop
         # (no fallback mid-conversation — different models may interpret tool results differently)
         while pending_tool_calls:
+            iteration_count += 1
+            if iteration_count > max_tool_iterations:
+                self.ap.logger.warning(
+                    f'localagent: query={query.query_id} agent loop exceeded max iterations ({max_tool_iterations}), '
+                    f'forcing termination'
+                )
+                break
+
             for tool_call in pending_tool_calls:
                 try:
                     func = tool_call.function
@@ -317,6 +331,14 @@ class LocalAgentRunner(runner.RequestRunner):
                     else:
                         tool_content = json.dumps(func_ret, ensure_ascii=False)
 
+                    # Truncate oversized tool results to prevent context overflow
+                    if isinstance(tool_content, str) and len(tool_content) > max_tool_result_chars:
+                        self.ap.logger.warning(
+                            f'localagent: tool {func.name} returned {len(tool_content)} chars, '
+                            f'truncating to {max_tool_result_chars}'
+                        )
+                        tool_content = tool_content[:max_tool_result_chars] + '\n...[result truncated]'
+
                     if is_stream:
                         msg = provider_message.MessageChunk(
                             role='tool',

src/langbot/templates/metadata/pipeline/ai.yaml

@@ -93,6 +93,26 @@ stages:
         type: knowledge-base-multi-selector
         required: false
         default: []
+      - name: max-tool-iterations
+        label:
+          en_US: Max Tool Iterations
+          zh_Hans: 最大工具调用轮次
+        description:
+          en_US: Maximum number of tool call iterations in a single agent loop to prevent runaway loops
+          zh_Hans: 单次 Agent 循环中工具调用的最大轮次，防止无限循环
+        type: integer
+        required: false
+        default: 16
+      - name: max-tool-result-chars
+        label:
+          en_US: Max Tool Result Length
+          zh_Hans: 工具返回最大字符数
+        description:
+          en_US: Maximum character length of a single tool call result, longer results will be truncated
+          zh_Hans: 单次工具调用返回结果的最大字符数，超出部分将被截断
+        type: integer
+        required: false
+        default: 8000
   - name: tbox-app-api
     label:
       en_US: Tbox App API