diff --git a/pyproject.toml b/pyproject.toml
index 2c79c9d7..9fcb51fb 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -13,6 +13,7 @@ dependencies = [
     "typing-extensions>=4.5, <5",
     "anyio>=3.5.0, <4",
     "distro>=1.7.0, <2",
+    "sniffio",
     "tokenizers >= 0.13.0"
 ]
 requires-python = ">= 3.7"
diff --git a/src/anthropic/_client.py b/src/anthropic/_client.py
index 8520f62f..33648605 100644
--- a/src/anthropic/_client.py
+++ b/src/anthropic/_client.py
@@ -23,7 +23,7 @@
     AsyncTransport,
     RequestOptions,
 )
-from ._utils import is_given
+from ._utils import is_given, get_async_library
 from ._version import __version__
 from ._streaming import Stream as Stream
 from ._streaming import AsyncStream as AsyncStream
@@ -160,6 +160,7 @@ def _bearer_auth(self) -> dict[str, str]:
     def default_headers(self) -> dict[str, str | Omit]:
         return {
             **super().default_headers,
+            "X-Stainless-Async": "false",
             "anthropic-version": "2023-06-01",
             **self._custom_headers,
         }
@@ -420,6 +421,7 @@ def _bearer_auth(self) -> dict[str, str]:
     def default_headers(self) -> dict[str, str | Omit]:
         return {
             **super().default_headers,
+            "X-Stainless-Async": f"async:{get_async_library()}",
             "anthropic-version": "2023-06-01",
             **self._custom_headers,
         }
diff --git a/src/anthropic/_utils/__init__.py b/src/anthropic/_utils/__init__.py
index d3397212..400ca9b8 100644
--- a/src/anthropic/_utils/__init__.py
+++ b/src/anthropic/_utils/__init__.py
@@ -25,6 +25,7 @@
 from ._utils import deepcopy_minimal as deepcopy_minimal
 from ._utils import extract_type_arg as extract_type_arg
 from ._utils import is_required_type as is_required_type
+from ._utils import get_async_library as get_async_library
 from ._utils import is_annotated_type as is_annotated_type
 from ._utils import maybe_coerce_float as maybe_coerce_float
 from ._utils import get_required_header as get_required_header
diff --git a/src/anthropic/_utils/_utils.py b/src/anthropic/_utils/_utils.py
index 4b51dcb2..d2bfc91a 100644
--- a/src/anthropic/_utils/_utils.py
+++ b/src/anthropic/_utils/_utils.py
@@ -18,6 +18,8 @@
 from pathlib import Path
 from typing_extensions import Required, Annotated, TypeGuard, get_args, get_origin
 
+import sniffio
+
 from .._types import Headers, NotGiven, FileTypes, NotGivenOr, HeadersLike
 from .._compat import is_union as _is_union
 from .._compat import parse_date as parse_date
@@ -406,3 +408,10 @@ def get_required_header(headers: HeadersLike, header: str) -> str:
             return value
 
     raise ValueError(f"Could not find {header} header")
+
+
+def get_async_library() -> str:
+    try:
+        return sniffio.current_async_library()
+    except Exception:
+        return "false"