🔌 fix: Preserve Ephemeral MCP Selections Across Model Switches (#13697)

The no-spec branch of `useApplyModelSpecEffects` (added in #11796) reset
`ephemeralAgentByConvoId` to null on every `newConversation` call when
model specs are configured. On in-place model/endpoint switches (modular
chat, same conversation or new-chat draft), BadgeRowContext never refills
from localStorage — its init effect only re-runs when the storage suffix
or spec changes — so the MCP selection (and tool toggles) were silently
dropped from subsequent request payloads while the MCP badge kept
displaying them.

Reset now only happens on context transitions (leaving a spec, or moving
to a different conversation key), where a BadgeRowContext refill is
guaranteed; in-place non-spec switches preserve the ephemeral agent.

- Gate the no-spec reset on `prevSpecName` / `prevConvoId`, passed from
  `newConversation` via a snapshot read of the pre-switch conversation
- Add jest coverage for all five branches of the no-spec path
- Add e2e spec asserting `ephemeralAgent.mcp` stays in the chat payload
  after a new-chat model switch and after regenerate on a switched
  conversation (verified failing before the fix, passing after)
- Add non-spec "Mock Provider D" endpoint to the e2e config so tests can
  switch between two real ephemeral endpoints; widen `MockEndpoint` type

client/src/hooks/Agents/__tests__/useApplyModelSpecAgents.spec.tsx

@@ -0,0 +1,147 @@
+import React from 'react';
+import { RecoilRoot, useRecoilValue } from 'recoil';
+import { renderHook, act } from '@testing-library/react';
+import { Constants, EModelEndpoint } from 'librechat-data-provider';
+import type { TEphemeralAgent, TStartupConfig, TModelSpec } from 'librechat-data-provider';
+import { ephemeralAgentByConvoId, useUpdateEphemeralAgent } from '~/store/agents';
+import { useApplyModelSpecEffects } from '../useApplyModelSpecAgents';
+
+const NEW_CONVO = Constants.NEW_CONVO as string;
+
+const Wrapper = ({ children }: { children: React.ReactNode }) => (
+  <RecoilRoot>{children}</RecoilRoot>
+);
+
+const createModelSpec = (name: string): TModelSpec =>
+  ({
+    name,
+    label: name,
+    preset: {
+      endpoint: EModelEndpoint.openAI,
+      model: name,
+    },
+  }) as TModelSpec;
+
+const createStartupConfig = (list: TModelSpec[]): TStartupConfig =>
+  ({
+    modelSpecs: {
+      list,
+      prioritize: false,
+    },
+  }) as TStartupConfig;
+
+const specsConfig = () => createStartupConfig([createModelSpec('test-spec')]);
+
+const useHarness = (conversationId: string) => {
+  const applyModelSpecEffects = useApplyModelSpecEffects();
+  const updateEphemeralAgent = useUpdateEphemeralAgent();
+  const ephemeralAgent = useRecoilValue(ephemeralAgentByConvoId(conversationId));
+  return { applyModelSpecEffects, updateEphemeralAgent, ephemeralAgent };
+};
+
+describe('useApplyModelSpecEffects', () => {
+  it('preserves an existing conversation ephemeral agent on an in-place model switch', () => {
+    const conversationId = 'convo-123';
+    const agent: TEphemeralAgent = { mcp: ['clickhouse'] };
+    const { result } = renderHook(() => useHarness(conversationId), { wrapper: Wrapper });
+
+    act(() => {
+      result.current.updateEphemeralAgent(conversationId, agent);
+    });
+    expect(result.current.ephemeralAgent).toEqual(agent);
+
+    act(() => {
+      result.current.applyModelSpecEffects({
+        convoId: conversationId,
+        specName: null,
+        prevConvoId: conversationId,
+        prevSpecName: null,
+        startupConfig: specsConfig(),
+      });
+    });
+
+    expect(result.current.ephemeralAgent).toEqual(agent);
+  });
+
+  it('preserves a new conversation ephemeral agent on an in-place model switch', () => {
+    const agent: TEphemeralAgent = { mcp: ['clickhouse'] };
+    const { result } = renderHook(() => useHarness(NEW_CONVO), { wrapper: Wrapper });
+
+    act(() => {
+      result.current.updateEphemeralAgent(NEW_CONVO, agent);
+    });
+
+    act(() => {
+      result.current.applyModelSpecEffects({
+        convoId: NEW_CONVO,
+        specName: null,
+        prevConvoId: NEW_CONVO,
+        prevSpecName: null,
+        startupConfig: specsConfig(),
+      });
+    });
+
+    expect(result.current.ephemeralAgent).toEqual(agent);
+  });
+
+  it('resets the ephemeral agent when switching away from a spec', () => {
+    const { result } = renderHook(() => useHarness(NEW_CONVO), { wrapper: Wrapper });
+
+    act(() => {
+      result.current.updateEphemeralAgent(NEW_CONVO, { mcp: ['clickhouse'] });
+    });
+
+    act(() => {
+      result.current.applyModelSpecEffects({
+        convoId: NEW_CONVO,
+        specName: null,
+        prevConvoId: NEW_CONVO,
+        prevSpecName: 'test-spec',
+        startupConfig: specsConfig(),
+      });
+    });
+
+    expect(result.current.ephemeralAgent).toBeNull();
+  });
+
+  it('resets the new conversation ephemeral agent when leaving an existing conversation', () => {
+    const { result } = renderHook(() => useHarness(NEW_CONVO), { wrapper: Wrapper });
+
+    act(() => {
+      result.current.updateEphemeralAgent(NEW_CONVO, { mcp: ['clickhouse'] });
+    });
+
+    act(() => {
+      result.current.applyModelSpecEffects({
+        convoId: NEW_CONVO,
+        specName: null,
+        prevConvoId: 'convo-123',
+        prevSpecName: null,
+        startupConfig: specsConfig(),
+      });
+    });
+
+    expect(result.current.ephemeralAgent).toBeNull();
+  });
+
+  it('leaves the ephemeral agent untouched when no specs are configured', () => {
+    const agent: TEphemeralAgent = { mcp: ['clickhouse'] };
+    const { result } = renderHook(() => useHarness(NEW_CONVO), { wrapper: Wrapper });
+
+    act(() => {
+      result.current.updateEphemeralAgent(NEW_CONVO, agent);
+    });
+
+    act(() => {
+      result.current.applyModelSpecEffects({
+        convoId: NEW_CONVO,
+        specName: null,
+        prevConvoId: 'convo-123',
+        prevSpecName: 'test-spec',
+        startupConfig: {} as TStartupConfig,
+      });
+    });
+
+    expect(result.current.ephemeralAgent).toEqual(agent);
+  });
+});

client/src/hooks/Agents/useApplyModelSpecAgents.ts

@@ -10,7 +10,11 @@ import { getModelSpec, applyModelSpecEphemeralAgent } from '~/utils';
  *
  * When a spec is provided, its tool settings are applied to the ephemeral agent.
  * When no spec is provided but specs are configured, the ephemeral agent is reset
- * to null so BadgeRowContext can apply localStorage defaults (non-spec experience).
+ * to null on context transitions (leaving a spec, or moving to a different
+ * conversation key) so BadgeRowContext refills values from localStorage — both
+ * transitions re-trigger its init effect. In-place switches (same conversation,
+ * non-spec → non-spec) keep the ephemeral agent state (e.g. MCP selections),
+ * since no refill would follow the reset.
  */
 export function useApplyModelSpecEffects() {
   const updateEphemeralAgent = useUpdateEphemeralAgent();
@@ -18,17 +22,25 @@ export function useApplyModelSpecEffects() {
     ({
       convoId,
       specName,
+      prevConvoId,
+      prevSpecName,
       startupConfig,
     }: {
       convoId: string | null;
       specName?: string | null;
+      prevConvoId?: string | null;
+      prevSpecName?: string | null;
       startupConfig?: TStartupConfig;
     }) => {
       if (specName == null || !specName) {
-        if (startupConfig?.modelSpecs?.list?.length) {
-          /** Specs are configured but none selected — reset ephemeral agent to null
-           *  so BadgeRowContext fills all values (tool toggles + MCP) from localStorage. */
-          updateEphemeralAgent((convoId ?? Constants.NEW_CONVO) || Constants.NEW_CONVO, null);
+        if (!startupConfig?.modelSpecs?.list?.length) {
+          return;
+        }
+        const targetId = (convoId ?? Constants.NEW_CONVO) || Constants.NEW_CONVO;
+        const sourceId = (prevConvoId ?? Constants.NEW_CONVO) || Constants.NEW_CONVO;
+        const isContextSwitch = Boolean(prevSpecName) || targetId !== sourceId;
+        if (isContextSwitch) {
+          updateEphemeralAgent(targetId, null);
         }
         return;
       }

client/src/hooks/useNewConvo.ts

@@ -35,6 +35,7 @@ import {
   logger,
 } from '~/utils';
 import { useDeleteFilesMutation, useGetEndpointsQuery, useGetStartupConfig } from '~/data-provider';
+import useGetConversation from './Conversations/useGetConversation';
 import useAssistantListMap from './Assistants/useAssistantListMap';
 import { useResetChatBadges } from './useChatBadges';
 import { useApplyModelSpecEffects } from './Agents';
@@ -46,6 +47,7 @@ const useNewConvo = (index = 0) => {
   const navigate = useNavigate();
   const [searchParams] = useSearchParams();
   const { data: startupConfig } = useGetStartupConfig();
+  const getConversation = useGetConversation(index);
   const applyModelSpecEffects = useApplyModelSpecEffects();
   const clearAllConversations = store.useClearConvoState();
   const defaultPreset = useRecoilValue(store.defaultPreset);
@@ -334,10 +336,13 @@ const useNewConvo = (index = 0) => {
         preset = getModelSpecPreset(defaultModelSpec);
       }
 
+      const prevConversation = getConversation();
       applyModelSpecEffects({
         startupConfig,
         specName: preset?.spec,
         convoId: conversation.conversationId,
+        prevConvoId: prevConversation?.conversationId,
+        prevSpecName: prevConversation?.spec,
       });
 
       if (conversation.conversationId === Constants.NEW_CONVO && !modelsData) {
@@ -384,6 +389,7 @@ const useNewConvo = (index = 0) => {
       startupConfig,
       saveBadgesState,
       endpointsConfig,
+      getConversation,
       pauseGlobalAudio,
       switchToConversation,
       applyModelSpecEffects,