diff --git a/src/agents/models/openai_provider.py b/src/agents/models/openai_provider.py
index 59a04071..dd4b888c 100644
--- a/src/agents/models/openai_provider.py
+++ b/src/agents/models/openai_provider.py
@@ -7,6 +7,7 @@ import weakref
 import httpx
 from openai import AsyncOpenAI, DefaultAsyncHttpxClient
 
+from ..exceptions import UserError
 from . import _openai_shared
 from .default_models import get_default_model
 from .interface import Model, ModelProvider
@@ -86,10 +87,11 @@ class OpenAIProvider(ModelProvider):
                 chunk semantics are not reliable enough for incremental processing.
         """
         if openai_client is not None:
-            assert api_key is None and base_url is None and websocket_base_url is None, (
-                "Don't provide api_key, base_url, or websocket_base_url if you provide "
-                "openai_client"
-            )
+            if api_key is not None or base_url is not None or websocket_base_url is not None:
+                raise UserError(
+                    "Don't provide api_key, base_url, or websocket_base_url if you provide "
+                    "openai_client"
+                )
             self._client: AsyncOpenAI | None = openai_client
         else:
             self._client = None
diff --git a/src/agents/voice/models/openai_model_provider.py b/src/agents/voice/models/openai_model_provider.py
index 31482570..b992f9b4 100644
--- a/src/agents/voice/models/openai_model_provider.py
+++ b/src/agents/voice/models/openai_model_provider.py
@@ -3,6 +3,7 @@ from __future__ import annotations
 import httpx
 from openai import AsyncOpenAI, DefaultAsyncHttpxClient
 
+from ...exceptions import UserError
 from ...models import _openai_shared
 from ...models.openai_agent_registration import (
     OpenAIAgentRegistrationConfig,
@@ -56,9 +57,8 @@ class OpenAIVoiceModelProvider(VoiceModelProvider):
             agent_registration: Optional agent registration configuration.
         """
         if openai_client is not None:
-            assert api_key is None and base_url is None, (
-                "Don't provide api_key or base_url if you provide openai_client"
-            )
+            if api_key is not None or base_url is not None:
+                raise UserError("Don't provide api_key or base_url if you provide openai_client")
             self._client: AsyncOpenAI | None = openai_client
         else:
             self._client = None
diff --git a/tests/test_config.py b/tests/test_config.py
index debb6c16..0eefc367 100644
--- a/tests/test_config.py
+++ b/tests/test_config.py
@@ -7,6 +7,7 @@ import openai
 import pytest
 
 from agents import (
+    UserError,
     responses_websocket_session,
     set_default_openai_api,
     set_default_openai_client,
@@ -97,6 +98,27 @@ def test_set_default_openai_responses_transport_rejects_invalid_value():
         set_default_openai_responses_transport("ws")  # type: ignore[arg-type]
 
 
+@pytest.mark.parametrize(
+    "conflicting_kwargs",
+    [
+        {"api_key": "other_key"},
+        {"base_url": "https://example.com"},
+        {"websocket_base_url": "wss://example.com"},
+        {
+            "api_key": "other_key",
+            "base_url": "https://example.com",
+            "websocket_base_url": "wss://example.com",
+        },
+    ],
+)
+def test_openai_provider_rejects_client_with_conflicting_args(conflicting_kwargs):
+    # Regression test for #3808: this validation used a bare `assert`, which is
+    # stripped under `python -O`, silently ignoring the conflicting arguments.
+    client = openai.AsyncOpenAI(api_key="test_key")
+    with pytest.raises(UserError, match="Don't provide"):
+        OpenAIProvider(openai_client=client, **conflicting_kwargs)
+
+
 def test_openai_provider_transport_override_beats_default():
     set_default_openai_api("responses")
     set_default_openai_responses_transport("websocket")
diff --git a/tests/voice/test_openai_model_provider.py b/tests/voice/test_openai_model_provider.py
new file mode 100644
index 00000000..2d9de3ae
--- /dev/null
+++ b/tests/voice/test_openai_model_provider.py
@@ -0,0 +1,29 @@
+# Tests for the OpenAI voice model provider (OpenAIVoiceModelProvider).
+
+import openai
+import pytest
+
+from agents.exceptions import UserError
+from agents.voice.models.openai_model_provider import OpenAIVoiceModelProvider
+
+
+@pytest.mark.parametrize(
+    "conflicting_kwargs",
+    [
+        {"api_key": "other_key"},
+        {"base_url": "https://example.com"},
+        {"api_key": "other_key", "base_url": "https://example.com"},
+    ],
+)
+def test_voice_provider_rejects_client_with_conflicting_args(conflicting_kwargs):
+    # Regression test for #3808: this validation used a bare `assert`, which is
+    # stripped under `python -O`, silently ignoring the conflicting arguments.
+    client = openai.AsyncOpenAI(api_key="test_key")
+    with pytest.raises(UserError, match="Don't provide"):
+        OpenAIVoiceModelProvider(openai_client=client, **conflicting_kwargs)
+
+
+def test_voice_provider_accepts_client_without_conflicting_args():
+    client = openai.AsyncOpenAI(api_key="test_key")
+    provider = OpenAIVoiceModelProvider(openai_client=client)
+    assert provider._get_client() is client