它们的定义，简单到只有两行有效代码：

END = sys.intern("__end__")
"""The last (maybe virtual) node in graph-style Pregel."""
START = sys.intern("__start__")
"""The first (maybe virtual) node in graph-style Pregel."""

同文件还定义了两个流式相关标签，也用 sys.intern：

TAG_NOSTREAM = sys.intern("nostream")
TAG_HIDDEN = sys.intern("langsmith:hidden")

你可能见过两种写入口的写法。它们其实是同一件事：

    def set_entry_point(self, key: str) -> Self:
        """Specifies the first node to be called in the graph.
        Equivalent to calling `add_edge(START, key)`."""
        return self.add_edge(START, key)                # ★就是加一条 START→key 的边

    def set_finish_point(self, key: str) -> Self:
        """Marks a node as a finish point of the graph."""
        return self.add_edge(key, END)                  # ★就是加一条 key→END 的边

连"条件入口"也是一层糖——包在 add_conditional_edges(START, ...) 上：

    def set_conditional_entry_point(self, path, path_map=None) -> Self:
        return self.add_conditional_edges(START, path, path_map)

既然 START/END 是"节点"，就得防止你把它们用反。add_edge 开头几行就是这些护栏：

        if isinstance(start_key, str):
            if start_key == END:
                raise ValueError("END cannot be a start node")     # END 不能当起点
            if end_key == START:
                raise ValueError("START cannot be an end node")     # START 不能当终点
            ...
            self.edges.add((start_key, end_key))                    # 普通边进 edges
            return self

汇合边（多起点）那一支也有对称的校验，还多查"节点必须先 add_node"：

        for start in start_key:
            if start == END:
                raise ValueError("END cannot be a start node")
            if start not in self.nodes:
                raise ValueError(f"Need to add_node `{start}` first")   # 起点得是已注册节点
        if end_key == START:
            raise ValueError("START cannot be an end node")
        if end_key != END and end_key not in self.nodes:
            raise ValueError(f"Need to add_node `{end_key}` first")
        self.waiting_edges.add((tuple(start_key), end_key))

compile 会调 validate。它先把所有边的起点汇成 all_sources，再检查 START 在不在里面：

    def validate(self, interrupt=None):
        all_sources = {src for src, _ in self._all_edges}       # 所有边的起点集合
        for start, branches in self.branches.items():
            all_sources.add(start)                              # 条件边的源也算
        for name, spec in self.nodes.items():
            if spec.ends:
                all_sources.add(name)
        # validate sources
        for source in all_sources:
            if source not in self.nodes and source != START:
                raise ValueError(f"Found edge starting at unknown node '{source}'")
        if START not in all_sources:                            # ★没有任何边从 START 出发
            raise ValueError(
                "Graph must have an entrypoint: add at least one edge from START to another node")

出口侧则校验所有边的终点都是已知节点或 END：

        all_targets = {end for _, end in self._all_edges}
        ...
        for target in all_targets:
            if target not in self.nodes and target != END:
                raise ValueError(f"Found edge ending at unknown node `{target}`")
        ...
        self.compiled = True
        return self

🗺️ 阶段 3「控制流」五天回顾