fix(tracing): support LANGCHAIN_* env fallback for LangSmith config (#1065)

* fix(tracing): support LANGCHAIN_* env fallback for LangSmith config

- add backward-compatible env parsing in tracing_config.py
- support fallback keys:
   LANGCHAIN_TRACING_V2 / LANGCHAIN_TRACING
   LANGCHAIN_API_KEY
   LANGCHAIN_PROJECT
   LANGCHAIN_ENDPOINT
- keep LANGSMITH_* as preferred source when both are present
- add regression tests in test_tracing_config.py

* fix(tracing): correct LANGSMITH_* precedence over LANGCHAIN_* for enabled flag (#1067)

* Initial plan

* fix(tracing): use first-present-wins logic for enabled flag, add precedence docs and test

Co-authored-by: WillemJiang <219644+WillemJiang@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: WillemJiang <219644+WillemJiang@users.noreply.github.com>

---------

Co-authored-by: Copilot <198982749+Copilot@users.noreply.github.com>

backend/src/config/tracing_config.py

@@ -25,8 +25,48 @@ class TracingConfig(BaseModel):
 _tracing_config: TracingConfig | None = None
 
 
+_TRUTHY_VALUES = {"1", "true", "yes", "on"}
+
+
+def _env_flag_preferred(*names: str) -> bool:
+    """Return the boolean value of the first env var that is present and non-empty.
+
+    Accepted truthy values (case-insensitive): ``1``, ``true``, ``yes``, ``on``.
+    Any other non-empty value is treated as falsy.  If none of the named
+    variables is set, returns ``False``.
+    """
+    for name in names:
+        value = os.environ.get(name)
+        if value is not None and value.strip():
+            return value.strip().lower() in _TRUTHY_VALUES
+    return False
+
+
+def _first_env_value(*names: str) -> str | None:
+    """Return the first non-empty environment value from candidate names."""
+    for name in names:
+        value = os.environ.get(name)
+        if value and value.strip():
+            return value.strip()
+    return None
+
+
 def get_tracing_config() -> TracingConfig:
     """Get the current tracing configuration from environment variables.
+
+    ``LANGSMITH_*`` variables take precedence over their legacy ``LANGCHAIN_*``
+    counterparts.  For boolean flags (``enabled``), the *first* variable that is
+    present and non-empty in the priority list is the sole authority – its value
+    is parsed and returned without consulting the remaining candidates.  Accepted
+    truthy values are ``1``, ``true``, ``yes``, and ``on`` (case-insensitive);
+    any other non-empty value is treated as falsy.
+
+    Priority order:
+        enabled  : LANGSMITH_TRACING > LANGCHAIN_TRACING_V2 > LANGCHAIN_TRACING
+        api_key  : LANGSMITH_API_KEY  > LANGCHAIN_API_KEY
+        project  : LANGSMITH_PROJECT  > LANGCHAIN_PROJECT   (default: "deer-flow")
+        endpoint : LANGSMITH_ENDPOINT > LANGCHAIN_ENDPOINT  (default: https://api.smith.langchain.com)
+
     Returns:
         TracingConfig with current settings.
     """
@@ -37,10 +77,11 @@ def get_tracing_config() -> TracingConfig:
         if _tracing_config is not None:  # Double-check after acquiring lock
             return _tracing_config
         _tracing_config = TracingConfig(
-            enabled=os.environ.get("LANGSMITH_TRACING", "").lower() == "true",
-            api_key=os.environ.get("LANGSMITH_API_KEY"),
-            project=os.environ.get("LANGSMITH_PROJECT", "deer-flow"),
-            endpoint=os.environ.get("LANGSMITH_ENDPOINT", "https://api.smith.langchain.com"),
+            # Keep compatibility with both legacy LANGCHAIN_* and newer LANGSMITH_* variables.
+            enabled=_env_flag_preferred("LANGSMITH_TRACING", "LANGCHAIN_TRACING_V2", "LANGCHAIN_TRACING"),
+            api_key=_first_env_value("LANGSMITH_API_KEY", "LANGCHAIN_API_KEY"),
+            project=_first_env_value("LANGSMITH_PROJECT", "LANGCHAIN_PROJECT") or "deer-flow",
+            endpoint=_first_env_value("LANGSMITH_ENDPOINT", "LANGCHAIN_ENDPOINT") or "https://api.smith.langchain.com",
         )
         return _tracing_config