diff --git a/agent/auxiliary_client.py b/agent/auxiliary_client.py
index 5532d5eab1f22..aa4b59292fd67 100644
--- a/agent/auxiliary_client.py
+++ b/agent/auxiliary_client.py
@@ -6281,8 +6281,18 @@ def _wrap_if_needed(client_obj, final_model_str: str, base_url_str: str = "",
     if provider == "custom":
         custom_base = ""
         custom_key = ""
+        # Base passed to _wrap_if_needed for the Anthropic-wrap decision.  It
+        # normally equals custom_base, but anthropic_messages talks to the
+        # /anthropic surface directly, so it must keep the raw /anthropic base
+        # while the plain OpenAI client (created from custom_base below, and the
+        # OpenAI-wire fallback taken when the anthropic SDK is unavailable) still
+        # uses the /v1-rewritten base so it never lands on
+        # /anthropic/chat/completions.  Empty means "use custom_base". See #16254.
+        wrap_base = ""
         if explicit_base_url:
             custom_base = _to_openai_base_url(explicit_base_url).strip()
+            if api_mode == "anthropic_messages":
+                wrap_base = (explicit_base_url or "").strip().rstrip("/")
             custom_key = (
                 (explicit_api_key or "").strip()
                 or _scoped_key_env("OPENAI_API_KEY")
@@ -6339,7 +6349,7 @@ def _wrap_if_needed(client_obj, final_model_str: str, base_url_str: str = "",
             if _merged_custom:
                 extra["default_headers"] = _merged_custom
             client = _create_openai_client(api_key=custom_key, base_url=_clean_base, **extra)
-            client = _wrap_if_needed(client, final_model, custom_base, custom_key)
+            client = _wrap_if_needed(client, final_model, wrap_base or custom_base, custom_key)
             return (_to_async_client(client, final_model, is_vision=is_vision) if async_mode
                     else (client, final_model))
         # Try custom first, then API-key providers (Codex excluded here:
diff --git a/tests/agent/test_auxiliary_explicit_base_anthropic.py b/tests/agent/test_auxiliary_explicit_base_anthropic.py
new file mode 100644
index 0000000000000..f46fdec25f91d
--- /dev/null
+++ b/tests/agent/test_auxiliary_explicit_base_anthropic.py
@@ -0,0 +1,118 @@
+"""Tests for resolve_provider_client's ``custom`` + ``explicit_base_url`` branch
+when the endpoint speaks Anthropic Messages.
+
+When the main provider is ``custom`` and its ``base_url`` ends in ``/anthropic``
+(a proxied Anthropic gateway — MiniMax, Zhipu GLM, LiteLLM, or a self-hosted
+LLM proxy), auxiliary tasks reach ``resolve_provider_client("custom",
+explicit_base_url=..., api_mode="anthropic_messages")`` — directly for a
+per-task ``auxiliary.<task>`` override, or via ``_resolve_auto`` Step 1 which
+forwards the main runtime's ``api_mode``.
+
+The bug (issue #16254): this branch called ``_to_openai_base_url()``
+unconditionally, stripping the ``/anthropic`` tail to ``/v1`` even for
+``api_mode=anthropic_messages``.  The Anthropic wrapper then never saw the real
+``/anthropic`` path, so every side task (title generation, compression, vision,
+web_extract, session_search) hit ``.../v1/chat/completions`` on a Messages-only
+endpoint and failed.  The sibling named-custom-provider branch already guarded
+the rewrite on ``api_mode``; this makes the explicit-base branch consistent.
+"""
+
+from __future__ import annotations
+
+from unittest.mock import MagicMock, patch
+
+import pytest
+
+
+@pytest.fixture(autouse=True)
+def _clean_env(monkeypatch):
+    for key in (
+        "OPENAI_API_KEY", "OPENAI_BASE_URL",
+        "ANTHROPIC_API_KEY", "ANTHROPIC_TOKEN",
+    ):
+        monkeypatch.delenv(key, raising=False)
+
+
+_ANTHROPIC_BASE = "https://gateway.example.com/proxy/anthropic"
+
+
+def _client_base_url(client) -> str:
+    for chain in (("base_url",), ("_real_client", "base_url"), ("_client", "base_url")):
+        obj = client
+        try:
+            for attr in chain:
+                obj = getattr(obj, attr)
+            return str(obj)
+        except AttributeError:
+            continue
+    return ""
+
+
+def test_explicit_base_anthropic_messages_keeps_anthropic_path():
+    """api_mode=anthropic_messages must build the Anthropic wrapper on the raw
+    ``/anthropic`` base — not the ``/v1``-rewritten one."""
+    from agent.auxiliary_client import resolve_provider_client, AnthropicAuxiliaryClient
+
+    fake_anthropic = MagicMock(name="anthropic_sdk_client")
+    with patch(
+        "agent.anthropic_adapter.build_anthropic_client",
+        return_value=fake_anthropic,
+    ) as mock_build:
+        client, model = resolve_provider_client(
+            "custom",
+            model="claude-opus-4-8",
+            explicit_base_url=_ANTHROPIC_BASE,
+            explicit_api_key="k",
+            api_mode="anthropic_messages",
+        )
+
+    assert isinstance(client, AnthropicAuxiliaryClient), (
+        "custom endpoint with api_mode=anthropic_messages must return the native "
+        f"Anthropic wrapper, got {type(client).__name__}"
+    )
+    # The wrapper — and the Anthropic SDK client it was built from — must keep
+    # the /anthropic path, NOT the /v1-rewritten one.
+    mock_build.assert_called_once_with("k", _ANTHROPIC_BASE)
+    assert client.base_url == _ANTHROPIC_BASE
+    assert model == "claude-opus-4-8"
+
+
+def test_explicit_base_anthropic_messages_openai_fallback_uses_v1():
+    """When the anthropic SDK is unavailable, _maybe_wrap_anthropic returns the
+    plain OpenAI client — which must be on the /v1 base, never /anthropic."""
+    from agent.auxiliary_client import resolve_provider_client, AnthropicAuxiliaryClient
+
+    with patch(
+        "agent.anthropic_adapter.build_anthropic_client",
+        side_effect=ImportError("anthropic package not installed"),
+    ):
+        client, model = resolve_provider_client(
+            "custom",
+            model="claude-opus-4-8",
+            explicit_base_url=_ANTHROPIC_BASE,
+            explicit_api_key="k",
+            api_mode="anthropic_messages",
+        )
+
+    assert client is not None
+    assert not isinstance(client, AnthropicAuxiliaryClient)
+    # /anthropic → /v1 so the OpenAI SDK never hits /anthropic/chat/completions.
+    assert _client_base_url(client).rstrip("/").endswith("/proxy/v1")
+
+
+def test_explicit_base_without_anthropic_mode_preserves_v1_rewrite():
+    """Regression: with no anthropic_messages api_mode, the /anthropic → /v1
+    OpenAI-wire rewrite is preserved (fix is scoped, no behavior change)."""
+    from agent.auxiliary_client import resolve_provider_client, AnthropicAuxiliaryClient
+
+    client, model = resolve_provider_client(
+        "custom",
+        model="my-model",
+        explicit_base_url=_ANTHROPIC_BASE,
+        explicit_api_key="k",
+        api_mode="chat_completions",
+    )
+
+    assert client is not None
+    assert not isinstance(client, AnthropicAuxiliaryClient)
+    assert _client_base_url(client).rstrip("/").endswith("/proxy/v1")