fix(desktop): let explicit model switches escape broken config providers (#42241) (#46796)

When a desktop/dashboard session had no agent built yet and the user explicitly
picked a provider in the model picker, config.set('model', ...) would first try
to initialize the agent from the (possibly broken) config default provider —
failing before the user's explicit switch could take effect, trapping them on a
misconfigured default.

config.set now pre-parses the model flags: if an explicit --provider is present
and no agent exists yet, it skips the default-provider agent build and routes
straight through _apply_model_switch with the explicit provider. _apply_model_switch
gained a parsed_flags passthrough (avoids double-parsing) and only falls back to
resolve_runtime_provider(requested=None) when no explicit provider was given.

The desktop hook now sends config.set instead of slash.exec for active-session
model changes, so errors from the selected provider surface to the user instead
of being swallowed.

Co-authored-by: rodboev <rod.boev@gmail.com>

apps/desktop/src/app/session/hooks/use-model-controls.test.tsx

@@ -1,5 +1,5 @@
-import { renderHook } from '@testing-library/react'
 import { QueryClient } from '@tanstack/react-query'
+import { cleanup, render, renderHook } from '@testing-library/react'
 import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest'
 
 import { getGlobalModelInfo } from '@/hermes'
@@ -13,12 +13,51 @@ import {
 
 import { useModelControls } from './use-model-controls'
 
+const setGlobalModel = vi.fn()
+const notifyError = vi.fn()
+
 vi.mock('@/hermes', () => ({
   getGlobalModelInfo: vi.fn(),
-  setGlobalModel: vi.fn()
+  setGlobalModel: (...args: Parameters<typeof setGlobalModel>) => setGlobalModel(...args)
 }))
 
-describe('useModelControls.refreshCurrentModel', () => {
+vi.mock('@/i18n', () => ({
+  useI18n: () => ({
+    t: {
+      desktop: {
+        modelSwitchFailed: 'Model switch failed'
+      }
+    }
+  })
+}))
+
+vi.mock('@/store/notifications', () => ({
+  notifyError: (...args: Parameters<typeof notifyError>) => notifyError(...args)
+}))
+
+type Controls = ReturnType<typeof useModelControls>
+
+function Harness({
+  activeSessionId,
+  onReady,
+  requestGateway
+}: {
+  activeSessionId: string | null
+  onReady: (controls: Controls) => void
+  requestGateway: <T = unknown>(method: string, params?: Record<string, unknown>) => Promise<T>
+}) {
+  const controls = useModelControls({
+    activeSessionId,
+    queryClient: new QueryClient(),
+    requestGateway
+  })
+
+  onReady(controls)
+
+  return null
+}
+
+describe('useModelControls', () => {
   beforeEach(() => {
     $activeSessionId.set(null)
     setCurrentModel('')
@@ -26,6 +65,7 @@ describe('useModelControls.refreshCurrentModel', () => {
   })
 
   afterEach(() => {
+    cleanup()
     vi.restoreAllMocks()
     $activeSessionId.set(null)
     setCurrentModel('')
@@ -74,4 +114,55 @@ describe('useModelControls.refreshCurrentModel', () => {
     expect($currentModel.get()).toBe('deepseek/deepseek-v4-pro')
     expect($currentProvider.get()).toBe('deepseek')
   })
+
+  it('routes active-session picker changes through config.set with an explicit provider', async () => {
+    const requestGateway = vi.fn(async () => ({ key: 'model', value: 'claude-sonnet-4.6' }) as never)
+    let controls!: Controls
+
+    render(
+      <Harness
+        activeSessionId="session-1"
+        onReady={value => (controls = value)}
+        requestGateway={requestGateway}
+      />
+    )
+
+    await expect(
+      controls.selectModel({
+        model: 'claude-sonnet-4.6',
+        persistGlobal: false,
+        provider: 'anthropic'
+      })
+    ).resolves.toBe(true)
+
+    expect(requestGateway).toHaveBeenCalledWith('config.set', {
+      session_id: 'session-1',
+      key: 'model',
+      value: 'claude-sonnet-4.6 --provider anthropic'
+    })
+    expect(requestGateway).not.toHaveBeenCalledWith('slash.exec', expect.anything())
+  })
+
+  it('keeps the global path on setGlobalModel when there is no active session', async () => {
+    setGlobalModel.mockResolvedValue(undefined)
+    let controls!: Controls
+
+    render(
+      <Harness
+        activeSessionId={null}
+        onReady={value => (controls = value)}
+        requestGateway={vi.fn()}
+      />
+    )
+
+    await expect(
+      controls.selectModel({
+        model: 'claude-sonnet-4.6',
+        persistGlobal: false,
+        provider: 'anthropic'
+      })
+    ).resolves.toBe(true)
+
+    expect(setGlobalModel).toHaveBeenCalledWith('anthropic', 'claude-sonnet-4.6')
+  })
 })

apps/desktop/src/app/session/hooks/use-model-controls.ts

@@ -82,9 +82,10 @@ export function useModelControls({ activeSessionId, queryClient, requestGateway
 
       try {
         if (activeSessionId) {
-          await requestGateway('slash.exec', {
+          await requestGateway('config.set', {
             session_id: activeSessionId,
-            command: `/model ${selection.model} --provider ${selection.provider}${selection.persistGlobal ? ' --global' : ''}`
+            key: 'model',
+            value: `${selection.model} --provider ${selection.provider}${selection.persistGlobal ? ' --global' : ''}`
           })
 
           if (selection.persistGlobal) {