diff --git a/pyproject.toml b/pyproject.toml
index a9ef5bec90..25afec03db 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -68,6 +68,7 @@ dev-dependencies = [
     "types-pyaudio > 0",
     "trio >=0.22.2",
     "nest_asyncio==1.6.0",
+    "pytest-xdist>=3.6.1",
 ]
 
 [tool.rye.scripts]
@@ -139,7 +140,7 @@ replacement = '[\1](https://github.com/openai/openai-python/tree/main/\g<2>)'
 
 [tool.pytest.ini_options]
 testpaths = ["tests"]
-addopts = "--tb=short"
+addopts = "--tb=short -n auto"
 xfail_strict = true
 asyncio_mode = "auto"
 asyncio_default_fixture_loop_scope = "session"
diff --git a/requirements-dev.lock b/requirements-dev.lock
index 9875a2b860..787c15be6a 100644
--- a/requirements-dev.lock
+++ b/requirements-dev.lock
@@ -54,6 +54,8 @@ exceptiongroup==1.2.2
     # via anyio
     # via pytest
     # via trio
+execnet==2.1.1
+    # via pytest-xdist
 executing==2.1.0
     # via inline-snapshot
 filelock==3.12.4
@@ -129,7 +131,9 @@ pyjwt==2.8.0
 pyright==1.1.399
 pytest==8.3.3
     # via pytest-asyncio
+    # via pytest-xdist
 pytest-asyncio==0.24.0
+pytest-xdist==3.7.0
 python-dateutil==2.8.2
     # via pandas
     # via time-machine
diff --git a/src/openai/_base_client.py b/src/openai/_base_client.py
index 44b3603008..2f87d23aaa 100644
--- a/src/openai/_base_client.py
+++ b/src/openai/_base_client.py
@@ -1088,7 +1088,14 @@ def _process_response(
 
         origin = get_origin(cast_to) or cast_to
 
-        if inspect.isclass(origin) and issubclass(origin, BaseAPIResponse):
+        if (
+            inspect.isclass(origin)
+            and issubclass(origin, BaseAPIResponse)
+            # we only want to actually return the custom BaseAPIResponse class if we're
+            # returning the raw response, or if we're not streaming SSE, as if we're streaming
+            # SSE then `cast_to` doesn't actively reflect the type we need to parse into
+            and (not stream or bool(response.request.headers.get(RAW_RESPONSE_HEADER)))
+        ):
             if not issubclass(origin, APIResponse):
                 raise TypeError(f"API Response types must subclass {APIResponse}; Received {origin}")
 
@@ -1606,7 +1613,14 @@ async def _process_response(
 
         origin = get_origin(cast_to) or cast_to
 
-        if inspect.isclass(origin) and issubclass(origin, BaseAPIResponse):
+        if (
+            inspect.isclass(origin)
+            and issubclass(origin, BaseAPIResponse)
+            # we only want to actually return the custom BaseAPIResponse class if we're
+            # returning the raw response, or if we're not streaming SSE, as if we're streaming
+            # SSE then `cast_to` doesn't actively reflect the type we need to parse into
+            and (not stream or bool(response.request.headers.get(RAW_RESPONSE_HEADER)))
+        ):
             if not issubclass(origin, AsyncAPIResponse):
                 raise TypeError(f"API Response types must subclass {AsyncAPIResponse}; Received {origin}")