diff --git a/litellm/main.py b/litellm/main.py
index 0bca4a7350e..6f8310a9d8a 100644
--- a/litellm/main.py
+++ b/litellm/main.py
@@ -26,6 +26,7 @@ from copy import deepcopy
 from functools import partial
 from types import MappingProxyType
 from typing import TYPE_CHECKING, Any, Final, Literal, Optional, Protocol, Union, cast, get_args
+from urllib.parse import urlsplit
 
 from litellm._logging import _redact_string
 from litellm._uuid import uuid
@@ -984,6 +985,12 @@ def mock_completion(
 
 
 _OPENAI_DEFAULT_API_BASE: Final = "https://api.openai.com/v1"
+_OPENAI_API_HOST: Final = "api.openai.com"
+
+
+def _is_openai_backed_api_base(api_base: str) -> bool:
+    hostname: Final = urlsplit(api_base).hostname
+    return hostname is not None and (hostname == _OPENAI_API_HOST or hostname.endswith(f".{_OPENAI_API_HOST}"))
 
 
 def _resolve_openai_api_base(api_base: str | None) -> str:
@@ -1053,7 +1060,7 @@ def responses_api_bridge_check(
     #   natively by Chat Completions with reasoning on, so custom-only requests stay on
     #   chat and keep their native custom tool_call response shape.
     # - The UNSET-effort arm only fires against endpoints known to enforce that
-    #   constraint (the default OpenAI endpoint, or Azure OpenAI where api_base is
+    #   constraint (any api.openai.com host, or Azure OpenAI where api_base is
     #   always set): chat-only OpenAI-compatible backends registered under the openai
     #   provider with a custom api_base and gpt-5.4+ model names serve tools without
     #   reasoning fine and have no /responses route, so they keep pre-existing
@@ -1068,14 +1075,15 @@ def responses_api_bridge_check(
         reasoning_active = reasoning_effort.get("effort") != "none" or reasoning_effort.get("summary") is not None
     else:
         reasoning_active = reasoning_effort != "none"
-    # The reasoning+tools constraint is enforced only by the real OpenAI endpoint (and Azure OpenAI).
-    # Resolve the effective base arg>global>env>default exactly as the chat handler does, so a custom
-    # base set via litellm.api_base or OPENAI_BASE_URL/OPENAI_API_BASE isn't misread as the default and
-    # bridged to a /responses route it lacks. A whitespace-only base collapses to the default too.
-    resolved_api_base: Final = _resolve_openai_api_base(api_base)
-    on_constraint_enforcing_endpoint: Final = custom_llm_provider == "azure" or resolved_api_base.strip() in (
-        "",
-        _OPENAI_DEFAULT_API_BASE,
+    # The reasoning+tools constraint is enforced by the real OpenAI backend behind any api.openai.com
+    # host (the default URL or a PrivateLink hostname such as <region>.privatelink.api.openai.com) and
+    # by Azure OpenAI. Resolve the effective base arg>global>env>default exactly as the chat handler
+    # does, so a custom base set via litellm.api_base or OPENAI_BASE_URL/OPENAI_API_BASE isn't misread
+    # as the default and bridged to a /responses route it lacks. A whitespace-only base collapses to
+    # the default too.
+    resolved_api_base: Final = _resolve_openai_api_base(api_base).strip()
+    on_constraint_enforcing_endpoint: Final = (
+        custom_llm_provider == "azure" or resolved_api_base == "" or _is_openai_backed_api_base(resolved_api_base)
     )
     if (
         custom_llm_provider in ("openai", "azure")
diff --git a/tests/test_litellm/test_main.py b/tests/test_litellm/test_main.py
index 3c8bf142835..4cba5048e5d 100644
--- a/tests/test_litellm/test_main.py
+++ b/tests/test_litellm/test_main.py
@@ -1133,6 +1133,82 @@ def test_responses_api_bridge_check_custom_api_base_via_env_with_unset_effort_st
     assert model_info.get("mode") != "responses"
 
 
+@pytest.mark.parametrize(
+    "api_base",
+    [
+        "https://southcentralus.privatelink.api.openai.com/v1",
+        "https://privatelink.corp.api.openai.com/v1",
+        "https://api.openai.com:443/v1",
+        "https://api.openai.com/v1/",
+        "HTTPS://API.OPENAI.COM/v1",
+    ],
+)
+def test_responses_api_bridge_check_openai_backed_custom_api_base_with_unset_effort_routes_to_responses(api_base):
+    """
+    A custom api_base whose host is api.openai.com or a subdomain of it (a PrivateLink hostname, a
+    port-qualified or trailing-slash default) still reaches the real OpenAI backend, which rejects
+    function tools with reasoning on Chat Completions, so the unset-effort arm must bridge exactly as
+    it does for the literal default URL. Regression guard for GH #39353.
+    """
+    from litellm.main import responses_api_bridge_check
+
+    model_info, model = responses_api_bridge_check(
+        model="gpt-5.6",
+        custom_llm_provider="openai",
+        tools=[{"type": "function", "function": {"name": "get_capital"}}],
+        reasoning_effort=None,
+        api_base=api_base,
+        )
+
+    assert model == "gpt-5.6"
+    assert model_info.get("mode") == "responses"
+
+
+@pytest.mark.parametrize(
+    "api_base",
+    [
+        "https://api.openai.com.evil.example/v1",
+        "https://notapi.openai.com/v1",
+        "https://gateway.example/v1?upstream=api.openai.com",
+        "https://openai.internal.example/api.openai.com/v1",
+    ],
+)
+def test_responses_api_bridge_check_lookalike_custom_api_base_with_unset_effort_stays_chat(api_base):
+    """Only the host decides: api.openai.com appearing elsewhere in the URL is still a foreign backend."""
+    from litellm.main import responses_api_bridge_check
+
+    model_info, model = responses_api_bridge_check(
+        model="gpt-5.6",
+        custom_llm_provider="openai",
+        tools=[{"type": "function", "function": {"name": "get_capital"}}],
+        reasoning_effort=None,
+        api_base=api_base,
+        )
+
+    assert model == "gpt-5.6"
+    assert model_info.get("mode") != "responses"
+
+
+def test_responses_api_bridge_check_privatelink_api_base_via_env_with_unset_effort_routes_to_responses(monkeypatch):
+    """A PrivateLink base set through OPENAI_BASE_URL resolves the way the chat handler's does and still bridges."""
+    import litellm
+    from litellm.main import responses_api_bridge_check
+
+    monkeypatch.setattr(litellm, "api_base", None)
+    monkeypatch.delenv("OPENAI_API_BASE", raising=False)
+    monkeypatch.setenv("OPENAI_BASE_URL", "https://southcentralus.privatelink.api.openai.com/v1")
+    model_info, model = responses_api_bridge_check(
+        model="gpt-5.6",
+        custom_llm_provider="openai",
+        tools=[{"type": "function", "function": {"name": "get_capital"}}],
+        reasoning_effort=None,
+        api_base=None,
+        )
+
+    assert model == "gpt-5.6"
+    assert model_info.get("mode") == "responses"
+
+
 def test_responses_api_bridge_check_custom_api_base_with_explicit_effort_still_routes():
     """Explicit reasoning_effort keeps its pre-existing bridging behavior on any api_base."""
     from litellm.main import responses_api_bridge_check