diff --git a/CHANGELOG.md b/CHANGELOG.md
index 255366f..8baac29 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,20 @@
 # Hermes Web UI -- Changelog
 
+## [v0.50.142] — 2026-04-22
+
+### Fixed
+- **Stale model no longer shows as "(unavailable)" in the model picker** — users with
+  only `custom_providers` configured (no OpenAI key) were seeing "GPT-5.4 Mini (unavailable)"
+  appear in the picker, visually grouped under their custom provider section and selected
+  as the default model. Two root causes: (1) `_resolve_compatible_session_model()` in
+  `api/routes.py` had a blanket skip for `active_provider == "custom"` that prevented
+  stale cross-provider session models (e.g. `openai/gpt-5.4-mini` from a pre-v0.50 default)
+  from ever being cleaned up. Fixed to only skip normalization when the model's prefix is
+  actually routable by a group in the catalog. (2) `renderSession()` in `static/ui.js`
+  injected a bare `<option>` for unavailable models, which visually inherited the last
+  rendered provider heading in the picker due to missing `<optgroup>` context. Fixed to
+  silently reset to the first available model instead. Closes #829. (#831)
+
 ## [v0.50.141] — 2026-04-22
 
 ### Fixed
@@ -7,7 +22,7 @@
   bfcache was restoring a prior search query into `#sessionSearch` on page restore,
   causing `renderSessionListFromCache()` to silently filter out all sessions (including
   newly created ones). Added `autocomplete="off"` to the search input and an explicit
-  value-clear at boot before the first render. Closes #822. (#829)
+  value-clear at boot before the first render. Closes #822. (#830)
 
 ## [v0.50.140] — 2026-04-22
 
diff --git a/api/routes.py b/api/routes.py
index 38647ca..a014860 100644
--- a/api/routes.py
+++ b/api/routes.py
@@ -208,7 +208,7 @@ def _resolve_compatible_session_model(model_id: str | None) -> tuple[str, bool]:
         return default_model, bool(default_model)
 
     active_provider = _normalize_provider_id(catalog.get("active_provider"))
-    if not active_provider or active_provider in {"custom", "openrouter"}:
+    if not active_provider:
         return model, False
 
     slash = model.find("/")
@@ -223,6 +223,32 @@ def _resolve_compatible_session_model(model_id: str | None) -> tuple[str, bool]:
         return model, False
 
     model_provider = _normalize_provider_id(model[:slash])
+
+    # For custom/openrouter active providers: only skip normalization when the
+    # model's namespace prefix is actually routable by a group in the catalog.
+    # A user who only has custom_providers configured (active_provider="custom")
+    # with a stale session model like "openai/gpt-5.4-mini" would otherwise
+    # never get cleaned up, causing "(unavailable)" to appear in the picker.
+    if active_provider in {"custom", "openrouter"}:
+        # These namespaces are always routable as-is — preserve them.
+        if model_provider in {"", "custom", "openrouter"}:
+            return model, False
+        # Check if any catalog group can actually route this model's prefix.
+        groups = catalog.get("groups") or []
+        routable_provider_ids = {
+            _normalize_provider_id(g.get("provider_id") or "") for g in groups
+        }
+        # openrouter group can route any provider/model namespace
+        has_openrouter_group = any(
+            (g.get("provider_id") or "") == "openrouter" for g in groups
+        )
+        if model_provider in routable_provider_ids or has_openrouter_group:
+            return model, False
+        # Model prefix is not routable — stale cross-provider reference, clear it.
+        if default_model:
+            return default_model, True
+        return model, False
+
     # Skip normalization for models on custom/openrouter namespaces — these are
     # user-controlled and should never be silently replaced.
     if model_provider and model_provider not in {"", "custom", "openrouter"} and model_provider != active_provider and default_model:
diff --git a/static/ui.js b/static/ui.js
index d3f7e9c..be1d210 100644
--- a/static/ui.js
+++ b/static/ui.js
@@ -1209,18 +1209,24 @@ function syncTopbar(){
     currentModel=modelOverride;
   } else {
     const applied=_applyModelToDropdown(currentModel,$('modelSelect'));
-    // If the model isn't in the current provider list, add it as a visually marked
-    // "(unavailable)" entry so the session value is preserved without misleading the user.
-    // Selecting it will still attempt to send (same as before), but the label makes
-    // clear it's a stale model from a previous session.
+    // If the model isn't in the current provider list, silently reset to the
+    // first available model so stale values don't pollute the picker (#829).
     if(!applied && currentModel){
-      const opt=document.createElement('option');
-      opt.value=currentModel;
-      opt.textContent=getModelLabel(currentModel)+t('model_unavailable');
-      opt.style.color='var(--muted, #888)';
-      opt.title=t('model_unavailable_title');
-      $('modelSelect').appendChild(opt);
-      $('modelSelect').value=currentModel;
+      // Stale session model not in the current provider catalog — reset to the
+      // first available model rather than injecting an "(unavailable)" option
+      // that visually appears under the wrong provider group (#829).
+      const modelSel=$('modelSelect');
+      const first=modelSel&&modelSel.querySelector('optgroup > option, option');
+      if(first){
+        modelSel.value=first.value;
+        S.session.model=first.value;
+        // Persist the correction so the session doesn't re-inject on next load.
+        fetch(new URL('api/session/update',location.href).href,{
+          method:'POST',credentials:'include',
+          headers:{'Content-Type':'application/json'},
+          body:JSON.stringify({session_id:S.session.id||S.session.session_id,model:first.value})
+        }).catch(()=>{});
+      }
     }
   }
   if(typeof syncModelChip==='function') syncModelChip();
diff --git a/tests/test_provider_mismatch.py b/tests/test_provider_mismatch.py
index 14c2a8d..b440703 100644
--- a/tests/test_provider_mismatch.py
+++ b/tests/test_provider_mismatch.py
@@ -498,3 +498,133 @@ def test_empty_model_session_does_not_trigger_save(monkeypatch):
         "_normalize_session_model_in_place must not call session.save() when "
         "the session has no stored model — no correction needed, just a fallback."
     )
+
+
+# ── Issue #829: stale cross-provider model on custom_providers-only setup ─────
+
+def test_stale_openai_model_cleared_for_custom_only_provider(monkeypatch):
+    """A stale openai/... session model must be cleared when active provider is
+    'custom' and no catalog group can route the openai prefix (#829)."""
+    import api.routes as routes
+
+    monkeypatch.setattr(
+        routes,
+        "get_available_models",
+        lambda: {
+            "active_provider": "custom",
+            "default_model": "",
+            "groups": [
+                {"provider": "Agent37", "provider_id": "custom:agent37",
+                 "models": [{"id": "agent37/default", "label": "default"}]},
+            ],
+        },
+    )
+
+    effective, changed = routes._resolve_compatible_session_model(
+        "openai/gpt-5.4-mini"
+    )
+
+    # No routable group for openai/ — should clear to default (empty → model itself
+    # only if no default available, which means changed=False when default_model="")
+    # When default_model is empty, we can't clear — preserve and return False
+    assert changed is False
+    assert effective == "openai/gpt-5.4-mini"
+
+
+def test_stale_openai_model_cleared_for_custom_provider_with_default(monkeypatch):
+    """When active_provider='custom', no openrouter group, and default_model is
+    configured, stale openai/... model should be cleared to default (#829)."""
+    import api.routes as routes
+
+    monkeypatch.setattr(
+        routes,
+        "get_available_models",
+        lambda: {
+            "active_provider": "custom",
+            "default_model": "agent37/default",
+            "groups": [
+                {"provider": "Agent37", "provider_id": "custom:agent37",
+                 "models": [{"id": "agent37/default", "label": "default"}]},
+            ],
+        },
+    )
+
+    effective, changed = routes._resolve_compatible_session_model(
+        "openai/gpt-5.4-mini"
+    )
+
+    assert changed is True
+    assert effective == "agent37/default"
+
+
+def test_openrouter_model_preserved_when_openrouter_group_present(monkeypatch):
+    """When active_provider='openrouter' and openrouter group exists,
+    openai/... model IDs must pass through unchanged — they are routable (#829)."""
+    import api.routes as routes
+
+    monkeypatch.setattr(
+        routes,
+        "get_available_models",
+        lambda: {
+            "active_provider": "openrouter",
+            "default_model": "openai/gpt-5.4-mini",
+            "groups": [
+                {"provider": "OpenRouter", "provider_id": "openrouter",
+                 "models": [{"id": "openai/gpt-5.4-mini", "label": "GPT-5.4 Mini"}]},
+            ],
+        },
+    )
+
+    effective, changed = routes._resolve_compatible_session_model(
+        "openai/gpt-5.4-mini"
+    )
+
+    assert changed is False
+    assert effective == "openai/gpt-5.4-mini"
+
+
+def test_custom_namespace_model_always_preserved_on_custom_provider(monkeypatch):
+    """Model IDs with 'custom/' prefix must always pass through unchanged even
+    when active_provider='custom' (#829)."""
+    import api.routes as routes
+
+    monkeypatch.setattr(
+        routes,
+        "get_available_models",
+        lambda: {
+            "active_provider": "custom",
+            "default_model": "agent37/default",
+            "groups": [
+                {"provider": "Agent37", "provider_id": "custom:agent37",
+                 "models": [{"id": "agent37/default", "label": "default"}]},
+            ],
+        },
+    )
+
+    effective, changed = routes._resolve_compatible_session_model(
+        "custom/my-local-llm"
+    )
+
+    assert changed is False
+    assert effective == "custom/my-local-llm"
+
+
+def test_stale_ui_js_does_not_inject_unavailable_option():
+    """renderSession() must no longer inject a bare (unavailable) option into
+    modelSelect when the session model is not in the provider list (#829).
+    It should silently reset to the first available model instead."""
+    import os
+    src = open(os.path.join(os.path.dirname(__file__), "..", "static", "ui.js"),
+               encoding="utf-8").read()
+
+    # The old pattern must be gone — both keys removed from ui.js
+    assert "model_unavailable" not in src and "model_unavailable_title" not in src, (
+        "renderSession() must not inject '(unavailable)' options — "
+        "stale models should be silently reset to the first available model (#829)"
+    )
+
+    # The new silent-reset pattern must be present
+    assert "first.value" in src and "S.session.model=first.value" in src, (
+        "renderSession() must silently reset S.session.model to the first "
+        "available option when the session model is not in the dropdown (#829)"
+    )