diff --git a/litellm/main.py b/litellm/main.py
index 4aa6bf9a19b..fbb43dd41fa 100644
--- a/litellm/main.py
+++ b/litellm/main.py
@@ -979,6 +979,23 @@ def mock_completion(
         raise Exception("Mock completion response failed - {}".format(e))
 
 
+_OPENAI_DEFAULT_API_BASE = "https://api.openai.com/v1"
+
+
+def _resolve_openai_api_base(api_base: str | None) -> str:
+    """Effective OpenAI base a chat request will hit: arg > global > env > default. The bridge gate
+    and the ``_complete_custom_openai`` chat handler MUST resolve this identically, or a custom base
+    set via ``litellm.api_base`` or ``OPENAI_BASE_URL``/``OPENAI_API_BASE`` is invisible to the gate,
+    which then misreads it as the default OpenAI endpoint and bridges a request the backend can't serve."""
+    return (
+        api_base
+        or litellm.api_base
+        or get_secret_str("OPENAI_BASE_URL")
+        or get_secret_str("OPENAI_API_BASE")
+        or _OPENAI_DEFAULT_API_BASE
+    )
+
+
 def responses_api_bridge_check(
     model: str,
     custom_llm_provider: str,
@@ -1047,10 +1064,15 @@ 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"
-    # A blank api_base (None, "", or whitespace) is not a custom endpoint: it resolves
-    # to the default OpenAI base downstream, which does enforce the reasoning+tools
-    # constraint. Azure always targets an OpenAI-constraint endpoint regardless.
-    on_constraint_enforcing_endpoint = custom_llm_provider == "azure" or not (api_base and api_base.strip())
+    # 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 = _resolve_openai_api_base(api_base)
+    on_constraint_enforcing_endpoint = custom_llm_provider == "azure" or resolved_api_base.strip() in (
+        "",
+        _OPENAI_DEFAULT_API_BASE,
+    )
     if (
         custom_llm_provider in ("openai", "azure")
         and model_info.get("mode") != "responses"
@@ -2396,13 +2418,8 @@ def _complete_custom_openai(
     stream = ctx.stream
     timeout = ctx.timeout
 
-    api_base = (
-        api_base  # for deepinfra/perplexity/anyscale/groq/friendliai we check in get_llm_provider and pass in the api base from there
-        or litellm.api_base
-        or get_secret("OPENAI_BASE_URL")
-        or get_secret("OPENAI_API_BASE")
-        or "https://api.openai.com/v1"
-    )
+    # for deepinfra/perplexity/anyscale/groq/friendliai we check in get_llm_provider and pass in the api base from there
+    api_base = _resolve_openai_api_base(api_base)
     organization = (
         organization
         or litellm.organization
diff --git a/tests/test_litellm/test_main.py b/tests/test_litellm/test_main.py
index 057d11e1ecd..9e160370048 100644
--- a/tests/test_litellm/test_main.py
+++ b/tests/test_litellm/test_main.py
@@ -1043,6 +1043,58 @@ def test_responses_api_bridge_check_custom_api_base_with_unset_effort_stays_chat
     assert model_info.get("mode") != "responses"
 
 
+def test_responses_api_bridge_check_custom_api_base_via_global_with_unset_effort_stays_chat(monkeypatch):
+    """
+    A custom base set through the litellm.api_base global (not the call arg) is resolved the
+    same way the chat handler resolves it, so the unset-effort arm must not reroute a chat-only
+    backend to a /responses route it lacks. Regression guard: the gate previously inspected only
+    the call-level api_base and bridged these requests.
+    """
+    import litellm
+    from litellm.main import responses_api_bridge_check
+
+    monkeypatch.setattr(litellm, "api_base", "http://vllm.internal:8000/v1")
+    with patch("litellm.main._get_model_info_helper") as mock_get_model_info:
+        mock_get_model_info.return_value = {"max_tokens": 128000}
+        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"
+
+
+@pytest.mark.parametrize("env_var", ["OPENAI_BASE_URL", "OPENAI_API_BASE"])
+def test_responses_api_bridge_check_custom_api_base_via_env_with_unset_effort_stays_chat(monkeypatch, env_var):
+    """
+    A custom base set via OPENAI_BASE_URL/OPENAI_API_BASE env is resolved identically to the chat
+    handler, so the unset-effort arm leaves the request on chat instead of bridging it.
+    """
+    import litellm
+    from litellm.main import responses_api_bridge_check
+
+    monkeypatch.setattr(litellm, "api_base", None)
+    monkeypatch.delenv("OPENAI_BASE_URL", raising=False)
+    monkeypatch.delenv("OPENAI_API_BASE", raising=False)
+    monkeypatch.setenv(env_var, "http://vllm.internal:8000/v1")
+    with patch("litellm.main._get_model_info_helper") as mock_get_model_info:
+        mock_get_model_info.return_value = {"max_tokens": 128000}
+        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