fix(agent): add retry hint to tool exceptions

tests/agent/test_runner_errors.py

@@ -52,7 +52,31 @@ async def test_runner_returns_tool_exception_to_model_for_recovery():
         {"name": "list_dir", "status": "error", "detail": "boom"}
     ]
     tool_message = next(message for message in result.messages if message.get("role") == "tool")
+    retry_hint = "[Analyze the error above and try a different approach.]"
     assert "Error: RuntimeError: boom" in tool_message["content"]
+    assert tool_message["content"].count(retry_hint) == 1
+
+
+@pytest.mark.asyncio
+async def test_tool_execution_does_not_duplicate_existing_retry_hint():
+    retry_hint = "\n\n[Analyze the error above and try a different approach.]"
+    tools = SimpleNamespace(
+        execute=AsyncMock(return_value=ToolResult.error("Error: boom" + retry_hint)),
+    )
+
+    results, events = await execute_tool_calls(
+        tools,
+        [ToolCallRequest(id="call_1", name="list_dir", arguments={})],
+        concurrent=False,
+        external_lookup_counts={},
+        workspace_violation_counts={},
+        hook=AgentHook(),
+        context=AgentHookContext(iteration=0, messages=[]),
+    )
+
+    assert results == ["Error: boom" + retry_hint]
+    assert results[0].count(retry_hint) == 1
+    assert events[0]["status"] == "error"
 
 
 @pytest.mark.asyncio