fix: prefer active provider for default model overlap

api/config.py

@@ -1517,18 +1517,36 @@ def resolve_model_provider(model_id: str) -> tuple:
 
     # Custom providers declared in config.yaml should win over slash-based
     # OpenRouter heuristics. Their model IDs commonly contain '/' too.
-    custom_providers = cfg.get("custom_providers", [])
-    if isinstance(custom_providers, list):
+    # However, when the active provider is an explicit non-custom provider and
+    # the requested model_id is the configured default model, that active
+    # provider takes precedence over overlapping custom_providers[] entries.
+    # Otherwise WebUI routes to custom:<name> instead of the intended endpoint
+    # and can surface a 401 from the wrong provider (#1922).
+    # For all other cases, preserve custom_providers[] routing for explicitly
+    # selected custom provider models.
+    _is_explicit_non_custom_provider = (
+        config_provider is not None
+        and config_provider != 'custom'
+        and not config_provider.startswith('custom:')
+    )
+    _default_model = model_cfg.get('default') if isinstance(model_cfg, dict) else None
+    _skip_custom_providers = (
+        _is_explicit_non_custom_provider
+        and _default_model is not None
+        and model_id == _default_model
+    )
+    custom_providers = cfg.get('custom_providers', [])
+    if isinstance(custom_providers, list) and not _skip_custom_providers:
         for entry in custom_providers:
             if not isinstance(entry, dict):
                 continue
-            entry_model = (entry.get("model") or "").strip()
-            entry_name = (entry.get("name") or "").strip()
-            entry_base_url = (entry.get("base_url") or "").strip()
+            entry_model = (entry.get('model') or '').strip()
+            entry_name = (entry.get('name') or '').strip()
+            entry_base_url = (entry.get('base_url') or '').strip()
             entry_model_ids = set()
             if entry_model:
                 entry_model_ids.add(entry_model)
-            entry_models = entry.get("models")
+            entry_models = entry.get('models')
             if isinstance(entry_models, dict):
                 entry_model_ids.update(
                     key.strip()
@@ -1536,7 +1554,7 @@ def resolve_model_provider(model_id: str) -> tuple:
                     if isinstance(key, str) and key.strip()
                 )
             if entry_name and model_id in entry_model_ids:
-                provider_hint = "custom:" + entry_name.lower().replace(" ", "-")
+                provider_hint = 'custom:' + entry_name.lower().replace(' ', '-')
                 return model_id, provider_hint, entry_base_url or None
 
     # @provider:model format — explicit provider hint from the dropdown.

tests/test_model_resolver.py

@@ -179,6 +179,58 @@ def test_custom_provider_models_dict_routes_to_named_custom_provider():
     assert base_url == 'http://127.0.0.1:8080/v1'
 
 
+# ── Issue #1922: default model shadowed by overlapping custom_providers[] ──
+
+def test_default_model_not_shadowed_by_overlapping_custom_provider():
+    r'''Regression test for #1922.
+
+    When the active provider is an explicit non-custom provider (e.g. ai-gateway,
+    openrouter, xiaomi) AND the requested model_id matches the configured default
+    model, the active provider's base_url must take precedence over an overlapping
+    custom_providers[] entry. Otherwise the WebUI routes to 'custom:<name>' with
+    the wrong endpoint, causing 401 errors.
+
+    This test mirrors the reported scenario:
+      - provider: ai-gateway
+      - base_url: https://api.ai-gateway.example/v1
+      - default: gpt-5.4
+      - An overlapping custom_providers[] entry with the same default model
+
+    Expected: active provider (ai-gateway) wins over custom provider.
+    '''
+    model, provider, base_url = _resolve_with_config(
+        'gpt-5.4',
+        provider='ai-gateway',
+        base_url='https://api.ai-gateway.example/v1',
+        default='gpt-5.4',
+        custom_providers=[{
+            'name': 'My Custom Endpoint',
+            'base_url': 'http://localhost:8080/v1',
+            'model': 'gpt-5.4',
+        }],
+    )
+    assert model == 'gpt-5.4', f'Expected model=gpt-5.4, got {model!r}'
+    assert provider == 'ai-gateway', f'Expected provider=ai-gateway, got {provider!r}'
+    assert base_url == 'https://api.ai-gateway.example/v1', f'Expected base_url from active provider, got {base_url!r}'
+
+
+def test_default_model_shadowed_with_xiaomi_provider():
+    r'''Same regression test with provider=xiaomi instead of ai-gateway.'''
+    model, provider, base_url = _resolve_with_config(
+        'deepseek-v4-flash',
+        provider='xiaomi',
+        default='deepseek-v4-flash',
+        custom_providers=[{
+            'name': 'LiteLLM Proxy',
+            'base_url': 'http://127.0.0.1:8080/v1',
+            'model': 'deepseek-v4-flash',
+        }],
+    )
+    assert model == 'deepseek-v4-flash'
+    assert provider == 'xiaomi'
+    assert base_url is None  # xiaomi has no config base_url in this test
+
+
 # ── get_available_models() @provider: hint behaviour ──────────────────────