先认识树的节点。打开 tracers/schemas.py 你会看到一个惊喜——它几乎是空的：

# libs/core/langchain_core/tracers/schemas.py:1
"""Schemas for tracers."""

from __future__ import annotations

from langsmith import RunTree          # ★直接从 langsmith SDK 导入

# Begin V2 API Schemas

Run = RunTree  # For backwards compatibility   # ← schemas.py:10

__all__ = [
    "Run",
]

拼树的骨架在 _TracerCore（libs/core/langchain_core/tracers/core.py:40，维护 run_map）+ BaseTracer（tracers/base.py:33）：

# libs/core/langchain_core/tracers/base.py:33
class BaseTracer(_TracerCore, BaseCallbackHandler, ABC):   # ★本质：一个回调 handler
    """Base interface for tracers."""

    @abstractmethod
    def _persist_run(self, run: Run) -> None:
        """Persist a run."""                    # 子类决定"存到哪"：打印/存内存/发 LangSmith

    def _start_trace(self, run: Run) -> None:   # base.py:40 每个 on_*_start 都会走到
        super()._start_trace(run)               # → _TracerCore：挂到父节点 + 登记进 run_map
        self._on_run_create(run)

    def _end_trace(self, run: Run) -> None:     # base.py:45 每个 on_*_end 都会走到
        if not run.parent_run_id:
            self._persist_run(run)              # ① ★只有根 Run 结束才"整树持久化"
        self.run_map.pop(str(run.id))           # ② 本节点出场，从"在飞航班表"删掉
        ...
        self._on_run_update(run)                # ③ 给子类的"节点更新"钩子

真正连接平台的子类 LangChainTracer（libs/core/langchain_core/tracers/langchain.py:134）：

# libs/core/langchain_core/tracers/langchain.py:134
class LangChainTracer(BaseTracer):
    """Implementation of the SharedTracer that POSTS to the LangChain endpoint."""

    run_inline = True                             # ★要求同步执行，保证事件顺序

    def __init__(self, example_id=None, project_name=None, client=None, tags=None, ...):
        super().__init__(**kwargs)
        self.project_name = project_name or ls_utils.get_tracer_project()  # 默认项目名
        self.client = client or get_client()      # langsmith.Client：真正发 HTTP 的人

# langchain.py:220（节选）
def _start_trace(self, run: Run) -> None:
    if self.project_name:
        run.session_name = self.project_name      # 归属到哪个项目
    super()._start_trace(run)                     # 正常拼树
    if get_tracing_context().get("enabled") is False:
        run.extra["__disabled"] = True            # 上下文里临时关了追踪 → 打标记不上报

# langchain.py:325
def _persist_run_single(self, run: Run) -> None:
    """Persist a run."""
    if run.extra.get("__disabled"):
        return
    run.extra["runtime"] = get_runtime_environment()  # 附带 py 版本/平台等运行环境
    run.tags = self._get_tags(run)
    run.post()                                    # ★★RunTree.post()：POST 给 LangSmith

用户体感最神奇的一点：代码一行不改，设 LANGSMITH_TRACING=true + LANGSMITH_API_KEY=... 就全量追踪。拼图的最后一块在 Day17 看过的 _configure（libs/core/langchain_core/callbacks/manager.py:2390）里：

# libs/core/langchain_core/callbacks/manager.py:2497
tracing_v2_enabled_ = _tracing_v2_is_enabled()        # ← tracers/context.py:132
#   其内部：ls_utils.tracing_is_enabled()——读 LANGSMITH_TRACING 等环境变量

# libs/core/langchain_core/callbacks/manager.py:2524
if tracing_v2_enabled_ and not any(
    isinstance(handler, LangChainTracer)              # 已挂过就不重复挂
    for handler in callback_manager.handlers
):
    ...
    handler = LangChainTracer(                        # ★悄悄挂上记录员
        project_name=tracer_project,                  #   LANGSMITH_PROJECT 或 "default"
        client=..., tags=tracing_tags, metadata=tracing_metadata,
    )
    callback_manager.add_handler(handler)