fix(desktop): preserve configured base_url on same-provider model switch (#41121)

The desktop model picker calls POST /api/model/set with provider+model only
(no base_url). _apply_main_model_assignment cleared model.base_url for every
non-custom provider, so re-picking a Xiaomi MiMo model wiped a Token Plan
endpoint (https://token-plan-*.xiaomimimo.com/v1) back to the registry default
api.xiaomimimo.com — breaking valid tp- keys with 401s.

Now base_url is cleared only when switching to a different provider (the stale
URL belonged to the old one); same-provider re-assignment preserves it, and an
explicitly supplied base_url is honored for any provider.

tests/hermes_cli/test_web_server.py

@@ -1269,10 +1269,11 @@ class TestWebServerEndpoints:
         assert data.get("gateway_tools", []) == []
 
     def test_apply_main_model_assignment_base_url_and_context_reconcile(self):
-        """The shared main-slot assignment helper must persist base_url only for
-        custom providers, clear stale base_url for hosted ones, and always drop
-        a hardcoded context_length override. Both POST /api/model/set and
-        profile-model writes route through this, so the contract is pinned here."""
+        """The shared main-slot assignment helper must persist a supplied
+        base_url, clear a stale base_url only when switching providers, preserve
+        it on same-provider re-assignment, and always drop a hardcoded
+        context_length override. Both POST /api/model/set and profile-model
+        writes route through this, so the contract is pinned here."""
         from hermes_cli.web_server import _apply_main_model_assignment
 
         # Custom + base_url → persisted; stale context_length dropped.
@@ -1284,16 +1285,39 @@ class TestWebServerEndpoints:
         assert out["base_url"] == "http://127.0.0.1:8000/v1"
         assert "context_length" not in out
 
-        # Hosted provider → stale base_url cleared (no base_url supplied).
+        # Switching providers (custom → openrouter) → stale base_url cleared.
         out = _apply_main_model_assignment(
-            {"base_url": "http://127.0.0.1:8000/v1"}, "openrouter", "anthropic/claude-opus-4.8"
+            {"provider": "custom", "base_url": "http://127.0.0.1:8000/v1"},
+            "openrouter",
+            "anthropic/claude-opus-4.8",
         )
         assert out["provider"] == "openrouter"
         assert out["base_url"] == ""
 
-        # Custom WITHOUT a base_url → don't invent one, clear any stale value.
+        # Same provider, no new base_url → existing custom endpoint preserved.
+        # Regression: picking a different MiMo model under xiaomi must NOT wipe a
+        # Token Plan base_url (https://token-plan-*.xiaomimimo.com/v1).
         out = _apply_main_model_assignment(
-            {"base_url": "http://stale:1/v1"}, "custom", "m"
+            {"provider": "xiaomi", "base_url": "https://token-plan-ams.xiaomimimo.com/v1"},
+            "xiaomi",
+            "mimo-v2.5-pro",
+        )
+        assert out["provider"] == "xiaomi"
+        assert out["default"] == "mimo-v2.5-pro"
+        assert out["base_url"] == "https://token-plan-ams.xiaomimimo.com/v1"
+
+        # A supplied base_url is honored for any provider, not just custom.
+        out = _apply_main_model_assignment(
+            {"provider": "xiaomi"},
+            "xiaomi",
+            "mimo-v2.5",
+            "https://token-plan-cn.xiaomimimo.com/v1",
+        )
+        assert out["base_url"] == "https://token-plan-cn.xiaomimimo.com/v1"
+
+        # Switching providers without a base_url → don't invent one, clear stale.
+        out = _apply_main_model_assignment(
+            {"provider": "openrouter", "base_url": "http://stale:1/v1"}, "custom", "m"
         )
         assert out["base_url"] == ""
 
@@ -1377,6 +1401,34 @@ class TestWebServerEndpoints:
         assert resp.status_code == 200
         assert resp.json()["base_url"] == ""
 
+    def test_set_model_main_same_provider_preserves_base_url(self):
+        """Re-picking a model under the SAME provider must NOT wipe a configured
+        base_url. Regression for the desktop bug where selecting a Xiaomi MiMo
+        model reset a Token Plan endpoint back to the registry default, breaking
+        Token Plan keys (https://token-plan-*.xiaomimimo.com/v1)."""
+        from hermes_cli.config import load_config, save_config
+
+        cfg = load_config()
+        cfg["model"] = {
+            "provider": "xiaomi",
+            "default": "mimo-v2.5-pro",
+            "base_url": "https://token-plan-ams.xiaomimimo.com/v1",
+        }
+        save_config(cfg)
+
+        # Desktop model picker sends provider+model only (no base_url).
+        resp = self.client.post(
+            "/api/model/set",
+            json={"scope": "main", "provider": "xiaomi", "model": "mimo-v2.5"},
+        )
+        assert resp.status_code == 200
+        assert resp.json()["base_url"] == "https://token-plan-ams.xiaomimimo.com/v1"
+
+        model_cfg = load_config().get("model")
+        assert isinstance(model_cfg, dict)
+        assert model_cfg["default"] == "mimo-v2.5"
+        assert model_cfg["base_url"] == "https://token-plan-ams.xiaomimimo.com/v1"
+
     def test_set_model_main_reports_stale_auxiliary_pins(self):
         """Switching the main provider must report auxiliary slots still pinned
         to a *different* provider so the UI can warn the user their helper tasks