diff --git a/httpcore/_backends/auto.py b/httpcore/_backends/auto.py
index 3ac05f4d..9b362764 100644
--- a/httpcore/_backends/auto.py
+++ b/httpcore/_backends/auto.py
@@ -1,14 +1,14 @@
 import typing
 from typing import Optional
 
-from .._synchronization import current_async_library
+from .._synchronization import current_async_backend
 from .base import SOCKET_OPTION, AsyncNetworkBackend, AsyncNetworkStream
 
 
 class AutoBackend(AsyncNetworkBackend):
     async def _init_backend(self) -> None:
         if not (hasattr(self, "_backend")):
-            backend = current_async_library()
+            backend = current_async_backend()
             if backend == "trio":
                 from .trio import TrioBackend
 
diff --git a/httpcore/_synchronization.py b/httpcore/_synchronization.py
index 3dc4cf85..2d86f8b8 100644
--- a/httpcore/_synchronization.py
+++ b/httpcore/_synchronization.py
@@ -1,5 +1,4 @@
 import asyncio
-import os
 import threading
 from types import TracebackType
 from typing import (
@@ -31,7 +30,6 @@
 
 
 AsyncBackend = Literal["asyncio", "trio"]
-AsyncLibrary = Literal["asyncio", "trio", "anyio"]
 
 
 def current_async_backend() -> AsyncBackend:
@@ -47,6 +45,11 @@ def current_async_backend() -> AsyncBackend:
     if environment not in ("asyncio", "trio"):  # pragma: nocover
         raise RuntimeError("Running under an unsupported async environment.")
 
+    if environment == "asyncio" and anyio is None:  # pragma: nocover
+        raise RuntimeError(
+            "Running with asyncio requires installation of 'httpcore[asyncio]'."
+        )
+
     if environment == "trio" and trio is None:  # pragma: nocover
         raise RuntimeError(
             "Running with trio requires installation of 'httpcore[trio]'."
@@ -55,18 +58,6 @@ def current_async_backend() -> AsyncBackend:
     return environment
 
 
-def current_async_library() -> AsyncLibrary:
-    if current_async_backend() == "trio":
-        return "trio"
-
-    if anyio is not None:
-        anyio_env = os.environ.get("HTTPCORE_PREFER_ANYIO", "true").lower()
-        if anyio_env in ("true", "1"):
-            return "anyio"
-
-    return "asyncio"
-
-
 class _LockProto(Protocol):
     async def acquire(self) -> Any: ...
     def release(self) -> None: ...
diff --git a/tests/test_synchronization.py b/tests/test_synchronization.py
deleted file mode 100644
index d74e58e9..00000000
--- a/tests/test_synchronization.py
+++ /dev/null
@@ -1,42 +0,0 @@
-import os
-import typing
-from typing import Generator, List
-
-import pytest
-
-from httpcore import AnyIOBackend, TrioBackend
-from httpcore._backends.auto import AutoBackend
-from httpcore._synchronization import AsyncLibrary, current_async_library
-
-
-@pytest.fixture(scope="session", autouse=True)
-def check_tested_async_libraries() -> Generator[List[str], None, None]:
-    # Ensure tests cover all supported async variants
-    async_libraries: List[str] = []
-    yield async_libraries
-    expected = sorted(["asyncio", "anyio", "trio"])
-    assert sorted(async_libraries) == expected
-    assert sorted(typing.get_args(AsyncLibrary)) == expected
-
-
-@pytest.mark.anyio
-async def test_current_async_library(anyio_backend, check_tested_async_libraries):
-    current = current_async_library()
-    check_tested_async_libraries.append(current)
-
-    backend_name, _ = anyio_backend
-    auto_backend = AutoBackend()
-    await auto_backend._init_backend()
-
-    if backend_name == "trio":
-        assert current == "trio"
-        assert isinstance(auto_backend._backend, TrioBackend)
-    else:
-        assert backend_name == "asyncio"
-        if os.environ["HTTPCORE_PREFER_ANYIO"] == "1":
-            assert current == "anyio"
-            assert isinstance(auto_backend._backend, AnyIOBackend)
-        else:
-            assert os.environ["HTTPCORE_PREFER_ANYIO"] == "0"
-            assert current == "asyncio"
-            assert isinstance(auto_backend._backend, AnyIOBackend)