循环主体在 PregelLoop。step 初始为 0，每个超步结束 +1：

        self.step = 0                    # 循环起点
        ...
        if not exiting:
            # increment step
            self.step += 1               # 每个超步收尾时 +1

stop（上限）在循环启动时按 recursion_limit 算出：

        self.step = self.checkpoint_metadata["step"] + 1          # 从存档点续跑时的起始步
        self.stop = self.step + self.config["recursion_limit"] + 1  # ★上限 = 起始 + 限额 + 1

每个超步的入口 tick() 第一件事就是查账：

        # check if iteration limit is reached
        if self.step > self.stop:
            self.status = "out_of_steps"      # ★不直接抛异常，先打个"没步数了"的标记
            return False                       # 返回 False：告诉主循环"别再 tick 了"

循环退出后，外层检查状态，若是 "out_of_steps" 就拼错误信息、抛异常：

            if loop.status == "out_of_steps":
                msg = create_error_message(
                    message=(
                        f"Recursion limit of {config['recursion_limit']} reached "
                        "without hitting a stop condition. You can increase the "
                        "limit by setting the `recursion_limit` config key."
                    ),
                    error_code=ErrorCode.GRAPH_RECURSION_LIMIT,
                )
                raise GraphRecursionError(msg)

异常类本身很简单，但 docstring 直接教你怎么解决：

class GraphRecursionError(RecursionError):
    """Raised when the graph has exhausted the maximum number of steps.
    This prevents infinite loops. To increase the maximum number of steps,
    run your graph with a config specifying a higher `recursion_limit`.
    Examples:
        graph.invoke(
            {"messages": [("user", "Hello, world!")]},
            {"recursion_limit": 1000},          # ← 第二个位置参数就是 config
        )
    """

有时你想让节点主动在快没步数时收尾（比如"再没步数就直接给个兜底答案"）。LangGraph 提供两个托管值（还记得 D05 说的 managed 字段吗）让节点读到步数信息：

class IsLastStepManager(ManagedValue[bool]):
    @staticmethod
    def get(scratchpad):
        return scratchpad.step == scratchpad.stop - 1     # 是不是最后一步

IsLastStep = Annotated[bool, IsLastStepManager]

class RemainingStepsManager(ManagedValue[int]):
    @staticmethod
    def get(scratchpad):
        return scratchpad.stop - scratchpad.step          # 还剩几步

RemainingSteps = Annotated[int, RemainingStepsManager]

✋ 10 分钟动手

# 1. 看 step/stop 记账
sed -n '606,609p' libs/langgraph/langgraph/pregel/_loop.py
sed -n '1700,1701p' libs/langgraph/langgraph/pregel/_loop.py

# 2. 看超限报错组织
sed -n '3002,3011p' libs/langgraph/langgraph/pregel/main.py

# 3. 看异常类与托管值
sed -n '67,87p' libs/langgraph/langgraph/errors.py
cat libs/langgraph/langgraph/managed/is_last_step.py

# 4. 亲手制造一个死循环，观察 GraphRecursionError
python3 -c "
from langgraph.graph import StateGraph, START, END
from typing_extensions import TypedDict
class S(TypedDict): n:int
def loop(s): return {'n': s['n']+1}
g=StateGraph(S); g.add_node('a', loop)
g.add_edge(START,'a'); g.add_edge('a','a')   # a→a 永远循环，没有 END 出口
try:
    g.compile().invoke({'n':0}, {'recursion_limit': 5})
except Exception as e:
    print(type(e).__name__)   # GraphRecursionError
"