diff --git a/src/components/settings/PostProcessingSettingsApi/usePostProcessProviderState.ts b/src/components/settings/PostProcessingSettingsApi/usePostProcessProviderState.ts
index ab6c33c..676fba0 100644
--- a/src/components/settings/PostProcessingSettingsApi/usePostProcessProviderState.ts
+++ b/src/components/settings/PostProcessingSettingsApi/usePostProcessProviderState.ts
@@ -89,9 +89,31 @@ export const usePostProcessProviderState = (): PostProcessProviderState => {
         }
       }
 
-      void setPostProcessProvider(providerId);
+      await setPostProcessProvider(providerId);
+
+      // Auto-fetch available models for the new provider so the model dropdown
+      // reflects what's actually valid. Without this, a stale model value from
+      // a previous provider/base_url can persist and silently 404 at runtime.
+      // Skip when the provider isn't configured yet (no API key / empty base URL)
+      // to avoid unnecessary backend errors.
+      if (providerId !== APPLE_PROVIDER_ID) {
+        const provider = providers.find((p) => p.id === providerId);
+        const apiKey = settings?.post_process_api_keys?.[providerId] ?? "";
+        const hasBaseUrl = (provider?.base_url ?? "").trim() !== "";
+        const hasApiKey = apiKey.trim() !== "";
+
+        if (provider?.id === "custom" ? hasBaseUrl : hasApiKey) {
+          void fetchPostProcessModels(providerId);
+        }
+      }
     },
-    [selectedProviderId, setPostProcessProvider],
+    [
+      selectedProviderId,
+      setPostProcessProvider,
+      fetchPostProcessModels,
+      providers,
+      settings,
+    ],
   );
 
   const handleBaseUrlChange = useCallback(
diff --git a/src/stores/settingsStore.ts b/src/stores/settingsStore.ts
index 462dbf9..3de95f9 100644
--- a/src/stores/settingsStore.ts
+++ b/src/stores/settingsStore.ts
@@ -374,7 +374,12 @@ export const useSettingsStore = create<SettingsStore>()(
     },
 
     setPostProcessProvider: async (providerId) => {
-      const { settings, setUpdating, refreshSettings } = get();
+      const {
+        settings,
+        setUpdating,
+        refreshSettings,
+        setPostProcessModelOptions,
+      } = get();
       const updateKey = "post_process_provider_id";
       const previousId = settings?.post_process_provider_id ?? null;
 
@@ -388,6 +393,10 @@ export const useSettingsStore = create<SettingsStore>()(
         }));
       }
 
+      // Clear cached model options for the new provider so the dropdown
+      // doesn't show stale models from a previous fetch or base_url.
+      setPostProcessModelOptions(providerId, []);
+
       try {
         await commands.setPostProcessProvider(providerId);
         await refreshSettings();
@@ -436,7 +445,49 @@ export const useSettingsStore = create<SettingsStore>()(
     },
 
     updatePostProcessBaseUrl: async (providerId, baseUrl) => {
-      return get().updatePostProcessSetting("base_url", providerId, baseUrl);
+      const { setUpdating, refreshSettings } = get();
+      const updateKey = `post_process_base_url:${providerId}`;
+
+      setUpdating(updateKey, true);
+
+      try {
+        // Persist the new base URL first.
+        const urlResult = await commands.changePostProcessBaseUrlSetting(
+          providerId,
+          baseUrl,
+        );
+        if (urlResult.status === "error") {
+          console.error("Failed to persist base URL:", urlResult.error);
+          return;
+        }
+
+        // Reset the stored model since the previous value is almost certainly
+        // invalid for the new endpoint (e.g. switching Custom from Groq to
+        // Cerebras). Only proceed if the reset succeeds.
+        const modelResult = await commands.changePostProcessModelSetting(
+          providerId,
+          "",
+        );
+        if (modelResult.status === "error") {
+          console.error("Failed to reset model setting:", modelResult.error);
+          return;
+        }
+
+        // Clear cached model options only after both backend writes succeed.
+        set((state) => ({
+          postProcessModelOptions: {
+            ...state.postProcessModelOptions,
+            [providerId]: [],
+          },
+        }));
+
+        // Single refresh after both backend writes.
+        await refreshSettings();
+      } catch (error) {
+        console.error("Failed to update post-process base URL:", error);
+      } finally {
+        setUpdating(updateKey, false);
+      }
     },
 
     updatePostProcessApiKey: async (providerId, apiKey) => {