diff --git a/backend/apps/agents/agents.py b/backend/apps/agents/agents.py
index 5d912bdc..1ae67167 100644
--- a/backend/apps/agents/agents.py
+++ b/backend/apps/agents/agents.py
@@ -864,7 +864,15 @@ async def list_models():
         if visible:
             result[provider_name] = visible
 
+    # Availability answers "can you use this right now"; the CATALOG answers "does this model still exist".
+    # Only the second one may retire a user's pinned model, or a router bounce silently moves their chat to another vendor (ENG-386).
+    p_catalog: list[str] = [
+        m["value"] for p_models in BUILTIN_MODELS.values() for m in p_models if m.get("value")
+    ]
+    p_catalog_complete = True
+
     # Fetch OpenRouter catalog directly (independent of 9Router) so picker fills the moment a key lands.
+    or_models: list[dict] = []
     if has_openrouter_key:
         try:
             from backend.apps.agents.providers.registry import fetch_openrouter_models
@@ -872,6 +880,10 @@ async def list_models():
         except Exception as e:
             logger.debug(f"OpenRouter catalog fetch failed: {e}")
             or_models = []
+        # A key we could not enumerate means we cannot vouch for the catalog, so nothing may be retired from it this tick.
+        if not or_models:
+            p_catalog_complete = False
+        p_catalog += [m["value"] for m in or_models if m.get("value")]
         if or_models:
             by_vendor: dict[str, list[dict]] = {}
             from backend.apps.agents.providers.registry import (
@@ -941,6 +953,7 @@ async def list_models():
                 "billing_kind": "api_key",
                 "tiers": [3, 3, 1],
             })
+        p_catalog += [e["value"] for e in entries]
         if entries:
             result[cp_name] = entries
 
@@ -954,7 +967,12 @@ async def list_models():
                 hr["billing_kind"] = "free"
             result["Anthropic"] = haiku_rows
 
-    return {"models": result, "notes": notes}
+    return {
+        "models": result,
+        "notes": notes,
+        "known_values": sorted(set(p_catalog)),
+        "catalog_complete": p_catalog_complete,
+    }
 
 
 @agents.router.post("/subscriptions/disconnect")
diff --git a/backend/tests/test_model_catalog_vs_availability.py b/backend/tests/test_model_catalog_vs_availability.py
new file mode 100644
index 00000000..242aea65
--- /dev/null
+++ b/backend/tests/test_model_catalog_vs_availability.py
@@ -0,0 +1,60 @@
+"""A model's ABSENCE from today's picker is not evidence it was retired.
+
+The bug class (ENG-386, seen live on a field install): /models is intersected with available creds
+and with 9Router's in-process provider state, so a router bounce or a provider cooldown drops whole
+vendors out of the payload. The renderer read "not in the list" as "retired", rewrote every session
+pinned to that vendor onto the default model, and PERSISTED it. A chat the user had put on GPT came
+back on Claude, and then collected Anthropic policy blocks the user could not explain.
+
+The seal: the payload carries a catalog that depends on neither creds nor router state. Availability
+answers "can you use this right now"; only the catalog answers "does this still exist", and only the
+catalog may retire a session's model.
+"""
+
+import asyncio
+from unittest.mock import patch
+
+from backend.apps.agents.agents import list_models
+from backend.apps.settings.models import AppSettings, CustomProvider
+
+
+def p_run(cfg: AppSettings, *, router_up: bool = False):
+    with patch("backend.apps.settings.settings.load_settings", return_value=cfg), \
+         patch("backend.apps.nine_router.is_running", return_value=router_up):
+        return asyncio.run(list_models())
+
+
+def test_a_provider_dropout_does_not_empty_the_catalog():
+    """The whole point: with nothing connected, availability collapses but the catalog does not."""
+    result = p_run(AppSettings())
+    available = {m["value"] for rows in result["models"].values() for m in rows}
+    known = set(result["known_values"])
+
+    assert known, "the catalog must never be empty, or the renderer has no way to tell gone from unreachable"
+    assert known - available, "a dropout must leave models known-but-unavailable, which is exactly the state that used to read as retired"
+
+
+def test_subscription_models_survive_a_router_bounce_in_the_catalog():
+    """9Router stamps provider state in-process, so a bounce un-connects every subscription lane."""
+    up = p_run(AppSettings(), router_up=True)
+    down = p_run(AppSettings(), router_up=False)
+    assert set(down["known_values"]) == set(up["known_values"]), "the catalog must not move when the router does"
+
+
+def test_a_custom_providers_models_are_in_the_catalog():
+    cfg = AppSettings(custom_providers=[
+        CustomProvider(name="Ollama Cloud", base_url="https://ollama.com/v1", api_key="x",
+                       models=[{"value": "gpt-oss:120b", "label": "gpt-oss:120b"}]),
+    ])
+    assert "custom/ollama-cloud/gpt-oss:120b" in p_run(cfg)["known_values"]
+
+
+def test_an_unenumerable_provider_marks_the_catalog_incomplete():
+    """A key we could not enumerate means we cannot vouch for the list, so nothing may be retired from it."""
+    cfg = AppSettings(openrouter_api_key="sk-or-broken")
+    with patch("backend.apps.agents.providers.registry.fetch_openrouter_models",
+               side_effect=RuntimeError("network down")):
+        result = p_run(cfg)
+    assert result["catalog_complete"] is False
+
+    assert p_run(AppSettings())["catalog_complete"] is True
diff --git a/frontend/src/app/Main.tsx b/frontend/src/app/Main.tsx
index 218e12d9..79dcb934 100644
--- a/frontend/src/app/Main.tsx
+++ b/frontend/src/app/Main.tsx
@@ -354,6 +354,9 @@ const DefaultModelGuard: React.FC<{ children: React.ReactNode }> = ({ children }
   const settingsLoaded = useAppSelector((s) => s.settings.loaded);
   const byProvider = useAppSelector((s) => s.models.byProvider);
   const modelsLoaded = useAppSelector((s) => s.models.loaded);
+  // Availability is a live fact (a router bounce or a provider cooldown drops models out of /models); the catalog is a static one. Retiring a user's model on availability silently moved whole chats to another vendor, which is how a GPT chat started returning Anthropic policy blocks (ENG-386).
+  const knownValues = useAppSelector((s) => s.models.knownValues);
+  const catalogComplete = useAppSelector((s) => s.models.catalogComplete);
   // Until 9Router answers, /models omits subscription models, so the saved default can look "no longer available" when it's really just not loaded yet. Reconciling then would clobber a real sub user's default down to a fallback (and persist it). Only reconcile against the complete list.
   const nineRouterUp = useAppSelector((s) => s.subscriptions.status?.running === true);
   // A primitive fingerprint, not the sessions map: subscribing the app ROOT to whole sessions re-rendered it on every stream tick; this only changes when some session's MODEL changes.
@@ -372,9 +375,11 @@ const DefaultModelGuard: React.FC<{ children: React.ReactNode }> = ({ children }
     if (pendingRef.current) return;
     if (Object.keys(byProvider).length === 0) return;
 
+    if (!catalogComplete || knownValues.length === 0) return;
     const flat = Object.values(byProvider).flat();
     const currentExists = flat.some((m) => m.value === settings.default_model);
     if (currentExists) return;
+    if (knownValues.includes(settings.default_model)) return;
 
     // Nothing real connected means the synthesized free row is the whole list; persisting it would brand haiku as the user's default forever (ENG-343).
     if (!flat.some((m) => m.billing_kind !== 'free')) return;
@@ -389,7 +394,7 @@ const DefaultModelGuard: React.FC<{ children: React.ReactNode }> = ({ children }
         pendingRef.current = false;
       });
     setSessionSwitch({ toFreeTrial: connectionMode === 'free-trial', runs: freeTrialRemaining ?? null, toLabel: fallback.label });
-  }, [settingsLoaded, modelsLoaded, nineRouterUp, connectionMode, freeTrialRemaining, byProvider, settings, dispatch]);
+  }, [settingsLoaded, modelsLoaded, nineRouterUp, connectionMode, freeTrialRemaining, byProvider, knownValues, catalogComplete, settings, dispatch]);
 
   // Same staleness per session: a session pinned to a now-gone model (e.g. gpt-5.4-api after its key is disconnected) snags on the next send since the send carries that model, so reconcile open sessions to the valid default/fallback and warn once.
   useEffect(() => {
@@ -402,10 +407,13 @@ const DefaultModelGuard: React.FC<{ children: React.ReactNode }> = ({ children }
     if (valid.size === 0) return;
     const fallback = pickFallbackModel(byProvider);
     if (!fallback) return;
+    // A model absent from the catalog is genuinely gone; one merely absent from today's list is a provider we cannot reach this second, and moving the chat off it is never ours to do silently.
+    if (!catalogComplete || knownValues.length === 0) return;
+    const known = new Set(knownValues);
     const target = valid.has(settings.default_model) ? settings.default_model : fallback.value;
     let switched = false;
     for (const sess of Object.values(store.getState().agents.sessions)) {
-      if (sess.model && !valid.has(sess.model)) {
+      if (sess.model && !known.has(sess.model)) {
         // The switch is store-only, and the metadata poll re-hydrates the dead model from disk every
         // few seconds, so re-announcing meant the banner returned forever for anyone holding a chat
         // pinned to a retired model. Fix it every time (the next send must carry a live model), tell
@@ -423,7 +431,7 @@ const DefaultModelGuard: React.FC<{ children: React.ReactNode }> = ({ children }
       const toLabel = flat.find((m) => m.value === target)?.label ?? target;
       setSessionSwitch({ toFreeTrial: connectionMode === 'free-trial', runs: freeTrialRemaining ?? null, toLabel });
     }
-  }, [settingsLoaded, modelsLoaded, nineRouterUp, connectionMode, freeTrialRemaining, byProvider, sessionModelsKey, settings, dispatch]);
+  }, [settingsLoaded, modelsLoaded, nineRouterUp, connectionMode, freeTrialRemaining, byProvider, knownValues, catalogComplete, sessionModelsKey, settings, dispatch]);
 
   return (
     <>
diff --git a/frontend/src/shared/state/modelsSlice.ts b/frontend/src/shared/state/modelsSlice.ts
index 639e3276..29e8e0db 100644
--- a/frontend/src/shared/state/modelsSlice.ts
+++ b/frontend/src/shared/state/modelsSlice.ts
@@ -20,12 +20,18 @@ export interface ModelOption {
 
 interface ModelsState {
   byProvider: Record<string, ModelOption[]>;
+  /** Every model that EXISTS, independent of creds or router state. Availability says "usable now"; only this says "still exists". */
+  knownValues: string[];
+  /** False when a configured provider could not be enumerated, so nothing may be retired from the catalog this tick. */
+  catalogComplete: boolean;
   loaded: boolean;
   failed: boolean;
 }
 
 const initialState: ModelsState = {
   byProvider: {},
+  knownValues: [],
+  catalogComplete: false,
   loaded: false,
   failed: false,
 };
@@ -34,8 +40,11 @@ export const fetchModels = createAsyncThunk('models/fetchModels', async () => {
   const res = await fetch(`${AGENTS_API}/models`);
   if (!res.ok) throw new Error('Failed to fetch models');
   const data = await res.json();
-  const models = data.models || data;
-  return models as Record<string, ModelOption[]>;
+  return {
+    byProvider: (data.models || data) as Record<string, ModelOption[]>,
+    knownValues: (data.known_values ?? []) as string[],
+    catalogComplete: data.catalog_complete !== false,
+  };
 });
 
 const modelsSlice = createSlice({
@@ -45,7 +54,9 @@ const modelsSlice = createSlice({
   extraReducers: (builder) => {
     builder
       .addCase(fetchModels.fulfilled, (state, action) => {
-        state.byProvider = action.payload;
+        state.byProvider = action.payload.byProvider;
+        state.knownValues = action.payload.knownValues;
+        state.catalogComplete = action.payload.catalogComplete;
         state.loaded = true;
       })
       .addCase(fetchModels.rejected, (state) => {