diff --git a/python/langsmith/async_client.py b/python/langsmith/async_client.py
index 1dc6f7119..b65d77637 100644
--- a/python/langsmith/async_client.py
+++ b/python/langsmith/async_client.py
@@ -52,8 +52,10 @@ def __init__(
             "Content-Type": "application/json",
         }
         api_key = ls_utils.get_api_key(api_key)
+        api_url = ls_utils.get_api_url(api_url)
         if api_key:
             _headers[ls_client.X_API_KEY] = api_key
+        ls_client._validate_api_key_if_hosted(api_url, api_key)
 
         if isinstance(timeout_ms, int):
             timeout_: Union[Tuple, float] = (timeout_ms / 1000, None, None, None)
@@ -62,7 +64,7 @@ def __init__(
         else:
             timeout_ = 10
         self._client = httpx.AsyncClient(
-            base_url=ls_utils.get_api_url(api_url), headers=_headers, timeout=timeout_
+            base_url=api_url, headers=_headers, timeout=timeout_
         )
 
     async def __aenter__(self) -> "AsyncClient":
diff --git a/python/langsmith/client.py b/python/langsmith/client.py
index 3d6e7f134..d91036522 100644
--- a/python/langsmith/client.py
+++ b/python/langsmith/client.py
@@ -330,8 +330,9 @@ def _validate_api_key_if_hosted(api_url: str, api_key: Optional[str]) -> None:
     # If the domain is langchain.com, raise error if no api_key
     if not api_key:
         if _is_langchain_hosted(api_url):
-            raise ls_utils.LangSmithUserError(
-                "API key must be provided when using hosted LangSmith API"
+            warnings.warn(
+                "API key must be provided when using hosted LangSmith API",
+                ls_utils.LangSmithMissingAPIKeyWarning,
             )
 
 
diff --git a/python/langsmith/utils.py b/python/langsmith/utils.py
index c28b23c38..8fce4106d 100644
--- a/python/langsmith/utils.py
+++ b/python/langsmith/utils.py
@@ -74,6 +74,17 @@ class LangSmithConnectionError(LangSmithError):
     """Couldn't connect to the LangSmith API."""
 
 
+## Warning classes
+
+
+class LangSmithWarning(UserWarning):
+    """Base class for warnings."""
+
+
+class LangSmithMissingAPIKeyWarning(LangSmithWarning):
+    """Warning for missing API key."""
+
+
 def tracing_is_enabled(ctx: Optional[dict] = None) -> bool:
     """Return True if tracing is enabled."""
     from langsmith.run_helpers import get_current_run_tree, get_tracing_context
diff --git a/python/pyproject.toml b/python/pyproject.toml
index ff36e6f2b..fd0149ae8 100644
--- a/python/pyproject.toml
+++ b/python/pyproject.toml
@@ -1,6 +1,6 @@
 [tool.poetry]
 name = "langsmith"
-version = "0.1.105"
+version = "0.1.106"
 description = "Client library to connect to the LangSmith LLM Tracing and Evaluation Platform."
 authors = ["LangChain <support@langchain.dev>"]
 license = "MIT"
diff --git a/python/tests/unit_tests/test_client.py b/python/tests/unit_tests/test_client.py
index ed8159804..24a066fe0 100644
--- a/python/tests/unit_tests/test_client.py
+++ b/python/tests/unit_tests/test_client.py
@@ -10,11 +10,12 @@
 import threading
 import time
 import uuid
+import warnings
 import weakref
 from datetime import datetime, timezone
 from enum import Enum
 from io import BytesIO
-from typing import Any, NamedTuple, Optional
+from typing import Any, NamedTuple, Optional, Type, Union
 from unittest import mock
 from unittest.mock import MagicMock, patch
 
@@ -28,7 +29,7 @@
 
 import langsmith.env as ls_env
 import langsmith.utils as ls_utils
-from langsmith import EvaluationResult, run_trees
+from langsmith import AsyncClient, EvaluationResult, run_trees
 from langsmith import schemas as ls_schemas
 from langsmith.client import (
     Client,
@@ -54,16 +55,6 @@ def test__is_langchain_hosted() -> None:
     assert _is_langchain_hosted("https://dev.api.smith.langchain.com")
 
 
-def test_validate_api_key_if_hosted(monkeypatch: pytest.MonkeyPatch) -> None:
-    monkeypatch.delenv("LANGCHAIN_API_KEY", raising=False)
-    monkeypatch.delenv("LANGSMITH_API_KEY", raising=False)
-    with pytest.raises(ls_utils.LangSmithUserError, match="API key must be provided"):
-        Client(api_url="https://api.smith.langchain.com")
-    client = Client(api_url="http://localhost:1984")
-    assert client.api_url == "http://localhost:1984"
-    assert client.api_key is None
-
-
 def test_validate_api_url(monkeypatch: pytest.MonkeyPatch) -> None:
     # Scenario 1: Both LANGCHAIN_ENDPOINT and LANGSMITH_ENDPOINT
     # are set, but api_url is not
@@ -1080,3 +1071,17 @@ def test_select_eval_results():
         assert client._select_eval_results(input_) == [
             expected2,
         ]
+
+
+@pytest.mark.parametrize("client_cls", [Client, AsyncClient])
+def test_validate_api_key_if_hosted(
+    monkeypatch: pytest.MonkeyPatch, client_cls: Union[Type[Client], Type[AsyncClient]]
+) -> None:
+    monkeypatch.delenv("LANGCHAIN_API_KEY", raising=False)
+    monkeypatch.delenv("LANGSMITH_API_KEY", raising=False)
+    with pytest.warns(ls_utils.LangSmithMissingAPIKeyWarning):
+        client_cls(api_url="https://api.smith.langchain.com")
+    with warnings.catch_warnings():
+        # Check no warning is raised here.
+        warnings.simplefilter("error")
+        client_cls(api_url="http://localhost:1984")