diff --git a/api/config.py b/api/config.py
index edd01c08..2b48872b 100644
--- a/api/config.py
+++ b/api/config.py
@@ -3605,7 +3605,12 @@ def set_hermes_default_model(model_id: str, provider: str | None = None, advance
         elif persisted_provider != previous_provider:
             if persisted_provider == "openai":
                 model_cfg["base_url"] = "https://api.openai.com/v1"
-            elif not persisted_provider.startswith("custom:"):
+            else:
+                # Provider changed and we have no resolved URL for the new one.
+                # Drop the previous provider's base_url so New Chat doesn't route
+                # to the old endpoint — this MUST also cover custom:* providers
+                # (a different custom provider has a different URL); leaving the
+                # stale base_url sent requests to the wrong host (#4728).
                 model_cfg.pop("base_url", None)
 
         _apply_advanced_model_options(model_cfg, advanced)
diff --git a/static/sessions.js b/static/sessions.js
index 636ef98d..25fc5a33 100644
--- a/static/sessions.js
+++ b/static/sessions.js
@@ -886,7 +886,9 @@ async function newSession(flash, options={}){
     if(!hasLoadedSession&&explicitModelOverride&&explicitModelOverride.model){
       newModelState=explicitModelOverride;
       consumedExplicitModelOverride=true;
-    }else if(hasLoadedSession&&window._defaultModel){
+    }else if(window._defaultModel){
+      // Configured default wins over stale picker/persisted state even with no
+      // loaded session (deleting the last session left S.session null + stale picker) (#4728).
       newModelState={model:window._defaultModel,model_provider:null};
       usingConfiguredDefault=true;
     }else if(modelSelForNew&&modelSelForNew.value&&typeof _modelStateForSelect==='function'){
diff --git a/tests/test_issue2518_active_provider_fallback.py b/tests/test_issue2518_active_provider_fallback.py
index 2763484b..e61608ba 100644
--- a/tests/test_issue2518_active_provider_fallback.py
+++ b/tests/test_issue2518_active_provider_fallback.py
@@ -214,7 +214,7 @@ def _provider_assignment_in_new_session() -> str:
     src = _read("static/sessions.js")
     idx = src.find("async function newSession(flash, options={}){")
     assert idx != -1, "newSession() must be defined in static/sessions.js"
-    body = src[idx : idx + 6000]
+    body = src[idx : idx + 7000]
     guard_start = body.find("const _bareModel")
     assert guard_start != -1, (
         "newSession() must declare a 'const _bareModel' guard for the "
diff --git a/tests/test_new_chat_default_model_frontend.py b/tests/test_new_chat_default_model_frontend.py
index 714e1f0d..6ce232d9 100644
--- a/tests/test_new_chat_default_model_frontend.py
+++ b/tests/test_new_chat_default_model_frontend.py
@@ -98,7 +98,7 @@ def test_new_session_posts_picker_model_before_server_default():
     fn = _new_session_function()
     assert "reqBody.model=newModelState.model" in fn
     assert "explicitModelOverride" in fn
-    assert "hasLoadedSession&&window._defaultModel" in fn
+    assert "}else if(window._defaultModel){" in fn
     assert "modelSelForNew&&modelSelForNew.value&&typeof _modelStateForSelect==='function'" in fn
     provider_assignment = fn[fn.index("reqBody.model_provider="):].split(";", 1)[0]
     assert "newModelState.model_provider" in provider_assignment
@@ -106,10 +106,10 @@ def test_new_session_posts_picker_model_before_server_default():
     assert "window._activeProvider" in fn
     assert "S.session&&S.session.model_provider" in fn
     pos_override = fn.index("explicitModelOverride")
-    pos_default = fn.index("hasLoadedSession&&window._defaultModel")
+    pos_default = fn.index("}else if(window._defaultModel){")
     pos_legacy = fn.index("modelSelForNew&&modelSelForNew.value&&typeof _modelStateForSelect==='function'")
     assert pos_override < pos_default < pos_legacy, (
-        "newSession() must prefer the empty-composer override first, then the configured default for loaded-session flows, then legacy picker state"
+        "newSession() must prefer the empty-composer override first, then the configured default, then legacy picker state"
     )
     assert "_familyMismatch" in fn
     assert "_readPersistedModelState" in fn
@@ -140,8 +140,8 @@ def test_new_chat_prefers_explicit_empty_composer_override_before_configured_def
     assert "usingConfiguredDefault" in fn
     assert "_clearEmptyComposerModelOverride" in fn
     assert "newModelState={model:window._defaultModel,model_provider:null};" in fn
-    assert fn.index("explicitModelOverride") < fn.index("hasLoadedSession&&window._defaultModel") < fn.index("_modelStateForSelect"), (
-        "newSession() must prefer the empty-composer override first, then the configured default for loaded-session flows, then legacy picker state"
+    assert fn.index("explicitModelOverride") < fn.index("}else if(window._defaultModel){") < fn.index("_modelStateForSelect"), (
+        "newSession() must prefer the empty-composer override first, then the configured default, then legacy picker state"
     )
 
 
diff --git a/tests/test_provider_mismatch.py b/tests/test_provider_mismatch.py
index e146dfcb..0c02452d 100644
--- a/tests/test_provider_mismatch.py
+++ b/tests/test_provider_mismatch.py
@@ -380,6 +380,48 @@ def test_default_model_save_persists_codex_provider_for_qualified_model(tmp_path
     assert saved["model"].get("base_url") != "https://openrouter.ai/api/v1"
 
 
+def test_default_model_save_clears_stale_custom_base_url_on_provider_change(tmp_path, monkeypatch):
+    """Switching the main default from one custom provider to another must drop
+    the previous provider's base_url so New Chat doesn't route to the old
+    endpoint (#4728). Previously custom:* was exempted from the base_url clear,
+    so the stale URL lingered."""
+    import yaml
+    import api.config as config
+
+    config_file = tmp_path / "config.yaml"
+    config_file.write_text(
+        "model:\n"
+        "  provider: custom:old-local\n"
+        "  default: old-model\n"
+        "  base_url: http://old.local/v1\n",
+        encoding="utf-8",
+    )
+    old_cfg = dict(config.cfg)
+    old_mtime = config._cfg_mtime
+    monkeypatch.setattr(config, "_get_config_path", lambda: config_file)
+    config.cfg["model"] = {
+        "provider": "custom:old-local",
+        "default": "old-model",
+        "base_url": "http://old.local/v1",
+    }
+    config._cfg_mtime = config_file.stat().st_mtime
+    try:
+        result = config.set_hermes_default_model("new-model", provider="custom:new-local")
+        saved = yaml.safe_load(config_file.read_text(encoding="utf-8"))
+    finally:
+        config.cfg.clear()
+        config.cfg.update(old_cfg)
+        config._cfg_mtime = old_mtime
+        config.invalidate_models_cache()
+
+    assert result["ok"] is True
+    assert saved["model"]["provider"] == "custom:new-local"
+    # The stale base_url from the OLD custom provider must not survive.
+    assert saved["model"].get("base_url") != "http://old.local/v1", (
+        f"stale custom base_url leaked: {saved['model'].get('base_url')!r}"
+    )
+
+
 def test_active_codex_at_provider_session_model_preserved(monkeypatch):
     """@openai-codex:gpt-5.5 session selections must keep their provider hint."""
     import api.routes as routes