fix(#4022): preserve different-endpoint config base_url override (Codex CORE)

Codex caught a regression: the contributor's _runtime_preferred_base_url preferred
the runtime base_url for ALL non-custom providers, which would clobber an explicit
providers.<id>.base_url override pointing at a different host/port (LM Studio at a
LAN IP, an OpenRouter mirror). Now only prefer the runtime URL when it's the SAME
scheme+host+port as the configured one (the #3895 /v1-dedup case is path-only on
the same host); a genuinely different configured endpoint wins. Added
_same_base_url_endpoint() + 2 regression tests (different-endpoint preserved,
same-endpoint normalization).

api/streaming.py

@@ -237,17 +237,58 @@ def _resolve_custom_provider_runtime_overrides(
     return resolved_provider, resolved_api_key, resolved_base_url
 
 
+def _same_base_url_endpoint(url_a: str, url_b: str) -> bool:
+    """True if two base URLs point at the same scheme+host+port endpoint.
+
+    Used to decide whether a runtime base_url is just a normalized form of the
+    configured one (e.g. OpenCode-Go's ``/v1`` de-duplication on the same host)
+    versus a genuinely different endpoint (an explicit ``providers.<id>.base_url``
+    override at a different host/port that must be preserved). Path/query are
+    intentionally ignored — the normalization #3895 fixes is path-only.
+    """
+    from urllib.parse import urlsplit
+    try:
+        a = urlsplit((url_a or "").strip())
+        b = urlsplit((url_b or "").strip())
+    except Exception:
+        return False
+    _default_port = {"http": 80, "https": 443}
+    a_host = (a.hostname or "").lower()
+    b_host = (b.hostname or "").lower()
+    a_scheme = (a.scheme or "").lower()
+    b_scheme = (b.scheme or "").lower()
+    a_port = a.port or _default_port.get(a_scheme)
+    b_port = b.port or _default_port.get(b_scheme)
+    return bool(a_host) and a_host == b_host and a_scheme == b_scheme and a_port == b_port
+
+
 def _runtime_preferred_base_url(
     runtime_provider: dict | None,
     resolved_provider: str | None,
     configured_base_url: str | None,
 ) -> str | None:
-    """Prefer runtime-normalized URLs except for named custom endpoints."""
+    """Prefer the runtime-normalized base_url, but never override an explicit
+    configured endpoint that points somewhere genuinely different.
+
+    The #3895 bug was that WebUI used the *configured* base_url (which can carry a
+    duplicated ``/v1``) instead of the runtime provider's per-model-normalized
+    base_url, 404ing OpenCode-Go. But blindly preferring the runtime URL would
+    clobber a legitimate ``providers.<id>.base_url`` override (e.g. LM Studio at a
+    LAN IP, an OpenRouter mirror). So:
+      - no runtime URL            -> keep configured
+      - no configured URL         -> use runtime (all we have)
+      - named ``custom:`` endpoint -> configured wins (then runtime as fallback)
+      - same scheme+host+port     -> runtime wins (it's the normalized/corrected
+                                      form of the same endpoint — the #3895 case)
+      - different endpoint        -> configured override wins (no regression)
+    """
     runtime_base_url = None
     if isinstance(runtime_provider, dict):
         runtime_base_url = runtime_provider.get("base_url")
     if not runtime_base_url:
         return configured_base_url
+    if not configured_base_url:
+        return runtime_base_url
 
     provider_id = str(
         resolved_provider
@@ -256,7 +297,12 @@ def _runtime_preferred_base_url(
     ).strip().lower()
     if provider_id.startswith("custom:"):
         return configured_base_url or runtime_base_url
-    return runtime_base_url
+
+    # An explicit configured override at a DIFFERENT endpoint must be preserved;
+    # only prefer the runtime URL when it's the same endpoint (path-normalized).
+    if _same_base_url_endpoint(configured_base_url, runtime_base_url):
+        return runtime_base_url
+    return configured_base_url
 
 
 def _is_fallback_lifecycle_message(kind: str, message: str) -> bool:

tests/test_issue3895_opencode_go_model_picker.py

@@ -35,6 +35,40 @@ def test_runtime_preferred_base_url_uses_runtime_for_custom_provider_without_con
     ) == "https://runtime.example.com"
 
 
+def test_runtime_preferred_base_url_preserves_different_endpoint_config_override():
+    """Codex CORE regression guard: an explicit providers.<id>.base_url override
+    at a DIFFERENT host/port must NOT be clobbered by the runtime default. Only a
+    same-endpoint runtime URL (the #3895 /v1-dedup case) should win."""
+    # LM Studio pinned at a LAN IP — must be preserved over the runtime localhost default.
+    assert streaming._runtime_preferred_base_url(
+        {"provider": "lmstudio", "base_url": "http://localhost:1234/v1"},
+        "lmstudio",
+        "http://10.0.0.5:1234/v1",
+    ) == "http://10.0.0.5:1234/v1"
+    # OpenRouter mirror override preserved over the runtime canonical host.
+    assert streaming._runtime_preferred_base_url(
+        {"provider": "openrouter", "base_url": "https://openrouter.ai/api/v1"},
+        "openrouter",
+        "https://my-mirror.example.com/api/v1",
+    ) == "https://my-mirror.example.com/api/v1"
+
+
+def test_runtime_preferred_base_url_prefers_runtime_for_same_endpoint_path_normalization():
+    """The #3895 case: same scheme+host+port, runtime just strips a duplicated
+    path segment — runtime (corrected) form wins."""
+    assert streaming._runtime_preferred_base_url(
+        {"provider": "opencode-go", "base_url": "https://opencode.example.com/zen/go"},
+        "opencode-go",
+        "https://opencode.example.com/zen/go/v1",
+    ) == "https://opencode.example.com/zen/go"
+    # Same host different port = different endpoint → configured wins.
+    assert streaming._runtime_preferred_base_url(
+        {"provider": "someprov", "base_url": "https://host.example.com:8080/v1"},
+        "someprov",
+        "https://host.example.com:9090/v1",
+    ) == "https://host.example.com:9090/v1"
+
+
 def test_streaming_passes_target_model_and_prefers_runtime_base_url(monkeypatch):
     captured = {}