From 088030d3847f6af4c22b89f9dd6a495a99a3b211 Mon Sep 17 00:00:00 2001
From: Aleksander Grygier <aleksander.grygier@gmail.com>
Date: Tue, 15 Sep 2026 02:37:16 +0200
Subject: [PATCH] ui : prefetch backend model lists on first selector open

Assisted-by: pi:llama.cpp/DeepSeek-V4.1-Flash
---
 .../ModelsSelectorBackendSwitcher.svelte      | 12 ++-
 .../lib/hooks/use-models-selector.svelte.ts   | 10 ++-
 .../src/lib/stores/backendsModels.svelte.ts   | 78 +++++++++++++++++++
 tools/ui/src/lib/stores/index.ts              |  2 +
 .../ui/src/lib/stores/models/index.svelte.ts  | 20 ++++-
 5 files changed, 118 insertions(+), 4 deletions(-)
 create mode 100644 tools/ui/src/lib/stores/backendsModels.svelte.ts

diff --git a/tools/ui/src/lib/components/app/models/ModelsSelector/ModelsSelectorBackendSwitcher.svelte b/tools/ui/src/lib/components/app/models/ModelsSelector/ModelsSelectorBackendSwitcher.svelte
index 8200072eb1..72fa3873aa 100644
--- a/tools/ui/src/lib/components/app/models/ModelsSelector/ModelsSelectorBackendSwitcher.svelte
+++ b/tools/ui/src/lib/components/app/models/ModelsSelector/ModelsSelectorBackendSwitcher.svelte
@@ -1,5 +1,6 @@
 <script lang="ts">
-	import { Plus } from '@lucide/svelte';
+	import { Loader2, Plus } from '@lucide/svelte';
+	import { backendsModelsStore } from '$lib/stores';
 	import type { Backend } from '$lib/types';
 
 	interface Props {
@@ -14,9 +15,10 @@
 
 <div class="flex items-center gap-1 overflow-x-auto border-b border-border/50 px-2 py-2">
 	{#each backends as backend (backend.id)}
+		{@const state = backendsModelsStore.get(backend.id)}
 		<button
 			class={[
-				'shrink-0 rounded-full px-2.5 py-1 text-xs font-medium whitespace-nowrap transition',
+				'inline-flex shrink-0 items-center gap-1 rounded-full px-2.5 py-1 text-xs font-medium whitespace-nowrap transition',
 				activeId === backend.id
 					? 'bg-muted text-foreground'
 					: 'text-muted-foreground hover:bg-muted/60 hover:text-foreground'
@@ -25,6 +27,12 @@
 			type="button"
 		>
 			{backend.name}
+
+			{#if state.loading}
+				<Loader2 class="h-3 w-3 animate-spin" />
+			{:else if state.error}
+				<span class="h-1.5 w-1.5 rounded-full bg-destructive" title={state.error}></span>
+			{/if}
 		</button>
 	{/each}
 
diff --git a/tools/ui/src/lib/hooks/use-models-selector.svelte.ts b/tools/ui/src/lib/hooks/use-models-selector.svelte.ts
index 1a46115e52..029c76ae56 100644
--- a/tools/ui/src/lib/hooks/use-models-selector.svelte.ts
+++ b/tools/ui/src/lib/hooks/use-models-selector.svelte.ts
@@ -1,6 +1,6 @@
 import { filterModelOptions, groupModelOptions } from '$lib/components/app/navigation/utils';
 import { CHAT_INPUT_FOCUS_SELECTOR } from '$lib/constants';
-import { backendsStore, modelsStore, serverStore } from '$lib/stores';
+import { backendsModelsStore, backendsStore, modelsStore, serverStore } from '$lib/stores';
 import type { Backend } from '$lib/types';
 import type { ModelOption } from '$lib/types/models';
 import { onMount } from 'svelte';
@@ -86,6 +86,7 @@ export function useModelsSelector(opts: UseModelsSelectorOptions): UseModelsSele
 	let searchTerm = $state('');
 	let showModelDialog = $state(false);
 	let infoModelId = $state<string | null>(null);
+	let backendsPrefetched = false;
 
 	const filteredOptions = $derived(filterModelOptions(options, searchTerm));
 	const groupedFilteredOptions = $derived(
@@ -106,6 +107,12 @@ export function useModelsSelector(opts: UseModelsSelectorOptions): UseModelsSele
 	});
 
 	function handleOpenChange(open: boolean) {
+		// first open: prefetch every backend's model list so switching is instant
+		if (open && !backendsPrefetched) {
+			backendsPrefetched = true;
+			void backendsModelsStore.loadAll();
+		}
+
 		if (loading || updating) return;
 
 		if (isRouter) {
@@ -130,6 +137,7 @@ export function useModelsSelector(opts: UseModelsSelectorOptions): UseModelsSele
 		searchTerm = '';
 
 		try {
+			await backendsModelsStore.ensureLoaded(backendId);
 			await modelsStore.switchBackend();
 		} catch (error) {
 			console.error('Failed to switch backend:', error);
diff --git a/tools/ui/src/lib/stores/backendsModels.svelte.ts b/tools/ui/src/lib/stores/backendsModels.svelte.ts
new file mode 100644
index 0000000000..460bb901bb
--- /dev/null
+++ b/tools/ui/src/lib/stores/backendsModels.svelte.ts
@@ -0,0 +1,78 @@
+/**
+ * backendsModelsStore - Per-backend model catalog cache.
+ *
+ * The models selector prefetches every enabled backend's model list the first
+ * time it opens, so switching backends is instant and the switcher can show
+ * load state. The active backend's list still lives in modelsStore, which owns
+ * selection and chat wiring; this cache is the prefetch layer for the others.
+ */
+
+import { BackendsService } from '$lib/services/backends.service';
+import { backendsStore } from '$lib/stores/backends.svelte';
+import type { ModelOption } from '$lib/types/models';
+
+export interface BackendModelsState {
+	error: string | null;
+	loaded: boolean;
+	loading: boolean;
+	models: ModelOption[];
+}
+
+const EMPTY_STATE: BackendModelsState = { error: null, loaded: false, loading: false, models: [] };
+
+class BackendsModelsStore {
+	private states = $state<Record<string, BackendModelsState>>({});
+
+	clear(backendId: string): void {
+		delete this.states[backendId];
+	}
+
+	/**
+	 * Load a backend's models once. Local backends are skipped: their list is
+	 * owned by modelsStore.
+	 */
+	async ensureLoaded(backendId: string): Promise<void> {
+		const backend = backendsStore.enabled.find((candidate) => candidate.id === backendId);
+
+		if (!backend || !backend.baseUrl.trim()) return;
+
+		const state = this.states[backendId];
+
+		if (state?.loaded || state?.loading) return;
+
+		this.states[backendId] = { error: null, loaded: false, loading: true, models: [] };
+
+		const result = await BackendsService.listModels(backend);
+
+		this.states[backendId] = {
+			error: result.error ?? null,
+			loaded: result.ok,
+			loading: false,
+			models: result.models
+		};
+	}
+
+	get(backendId: string): BackendModelsState {
+		return this.states[backendId] ?? EMPTY_STATE;
+	}
+
+	/** Prefetch every enabled external backend, skipping the active one. */
+	async loadAll(): Promise<void> {
+		const enabled = backendsStore.enabled;
+		const ids = new Set(enabled.map((backend) => backend.id));
+
+		for (const id of Object.keys(this.states)) {
+			if (!ids.has(id)) {
+				delete this.states[id];
+			}
+		}
+
+		await Promise.all(
+			enabled
+				.filter((backend) => backend.id !== backendsStore.active.id)
+				.map((backend) => this.ensureLoaded(backend.id))
+		);
+	}
+}
+
+export const backendsModelsStore = new BackendsModelsStore();
diff --git a/tools/ui/src/lib/stores/index.ts b/tools/ui/src/lib/stores/index.ts
index 8dc70da24d..1ab79475be 100644
--- a/tools/ui/src/lib/stores/index.ts
+++ b/tools/ui/src/lib/stores/index.ts
@@ -40,6 +40,8 @@ export { mcpStore } from './mcp/index.svelte';
 // BACKENDS
 export { backendsStore } from './backends.svelte';
 
+export { backendsModelsStore } from './backendsModels.svelte';
+
 // MODELS
 export { modelsStore } from './models/index.svelte';
 
diff --git a/tools/ui/src/lib/stores/models/index.svelte.ts b/tools/ui/src/lib/stores/models/index.svelte.ts
index 189836ec6d..13bb902b77 100644
--- a/tools/ui/src/lib/stores/models/index.svelte.ts
+++ b/tools/ui/src/lib/stores/models/index.svelte.ts
@@ -11,6 +11,8 @@ import { FAVORITE_MODELS_LOCALSTORAGE_KEY } from '$lib/constants';
 import { ServerModelStatus } from '$lib/enums';
 import { ModelsService } from '$lib/services/models.service';
 // direct imports between stores, not via the barrel, to avoid circular deps
+import { backendsStore } from '$lib/stores/backends.svelte';
+import { backendsModelsStore } from '$lib/stores/backendsModels.svelte';
 import { conversationsStore } from '$lib/stores/conversations/index.svelte';
 import { type ModelPropsHost, ModelPropsManager } from '$lib/stores/models/props.svelte';
 import { type ModelStatusHost, ModelStatusManager } from '$lib/stores/models/status.svelte';
@@ -350,11 +352,27 @@ class ModelsStore implements ModelPropsHost, ModelStatusHost {
 	async switchBackend(): Promise<void> {
 		this.status.unsubscribe();
 		this.clearSelection();
-		this.models = [];
 		this.routerModels = [];
 		this.error = null;
 		serverStore.clear();
 
+		// prefer the prefetched list so switching does not refetch
+		const backend = backendsStore.active;
+		const cached = backend.baseUrl.trim() ? backendsModelsStore.get(backend.id) : null;
+
+		if (cached?.loaded) {
+			this.models = cached.models;
+			this.loading = false;
+
+			await serverStore.fetch();
+
+			if (this.models.length > 0) {
+				await this.ensureFirstModelSelected();
+			}
+
+			return;
+		}
+
 		await this.fetch(true);
 	}