diff --git a/client/src/store/__tests__/families.spec.tsx b/client/src/store/__tests__/families.spec.tsx
new file mode 100644
index 0000000000..610e425a52
--- /dev/null
+++ b/client/src/store/__tests__/families.spec.tsx
@@ -0,0 +1,237 @@
+/* eslint-disable i18next/no-literal-string */
+import React, { useRef } from 'react';
+import { RecoilRoot, useRecoilState } from 'recoil';
+import { Constants } from 'librechat-data-provider';
+import { fireEvent, render, screen, waitFor } from '@testing-library/react';
+import {
+  useNavigate,
+  RouterProvider,
+  useSearchParams,
+  createBrowserRouter,
+} from 'react-router-dom';
+import type { TConversation } from 'librechat-data-provider';
+import families from '../families';
+
+const draftConvo = (overrides: Partial<TConversation> = {}): TConversation =>
+  ({
+    conversationId: Constants.NEW_CONVO,
+    title: 'New Chat',
+    endpoint: 'openAI',
+    model: 'gpt-4o',
+    createdAt: '',
+    updatedAt: '',
+    ...overrides,
+  }) as TConversation;
+
+/**
+ * Each button replays the exact write sequence of a real caller against the
+ * real `conversationByIndex` atom effect and a real browser router, so the
+ * history assertions below cover the mechanism, not a re-implementation:
+ * - `edit-model` — any settings change on the landing (model menu, params panel)
+ * - `chip-switch` — ProjectLandingChip.applyProject (atom write + in-place URL)
+ * - `sidebar-switch` — useNewConvo: setConversation followed by navigate(push)
+ * - `clear-project` — useDeleteProjectMutation scrubbing the active draft
+ */
+function Driver() {
+  const [conversation, setConversation] = useRecoilState(families.conversationByIndex(0));
+  const [searchParams, setSearchParams] = useSearchParams();
+  const navigate = useNavigate();
+  const editCount = useRef(0);
+
+  const editModel = () => {
+    if (!conversation) {
+      return;
+    }
+    editCount.current += 1;
+    setConversation({ ...conversation, model: `edited-${editCount.current}` });
+  };
+
+  const chipSwitch = () => {
+    if (!conversation) {
+      return;
+    }
+    setConversation({ ...conversation, chatProjectId: 'proj-chip' });
+    const nextParams = new URLSearchParams(searchParams);
+    nextParams.set('projectId', 'proj-chip');
+    setSearchParams(nextParams, { replace: true, flushSync: true });
+  };
+
+  const sidebarSwitch = () => {
+    setConversation(draftConvo({ model: 'sidebar-model', chatProjectId: 'proj-side' }));
+    navigate(`/c/${Constants.NEW_CONVO}?projectId=proj-side`);
+  };
+
+  const clearProject = () => {
+    if (!conversation) {
+      return;
+    }
+    setConversation({ ...conversation, chatProjectId: null });
+  };
+
+  return (
+    <div>
+      <span data-testid="convo-id">{conversation?.conversationId ?? 'none'}</span>
+      <span data-testid="convo-model">{conversation?.model ?? 'none'}</span>
+      <button onClick={() => setConversation(draftConvo())}>seed</button>
+      <button onClick={editModel}>edit-model</button>
+      <button onClick={chipSwitch}>chip-switch</button>
+      <button onClick={sidebarSwitch}>sidebar-switch</button>
+      <button onClick={clearProject}>clear-project</button>
+      <button
+        onClick={() =>
+          setConversation(
+            draftConvo({
+              conversationId: 'saved-123',
+              createdAt: '2026-08-16T00:00:00Z',
+              model: 'persisted-model',
+            }),
+          )
+        }
+      >
+        persist
+      </button>
+      <button
+        onClick={() => setConversation(draftConvo({ disableParams: true, model: 'blocked-model' }))}
+      >
+        disabled-edit
+      </button>
+    </div>
+  );
+}
+
+type HistoryOp = { verb: 'push' | 'replace'; url: string };
+
+describe('conversationByIndex URL mirroring', () => {
+  let ops: HistoryOp[] = [];
+
+  const pushes = () => ops.filter((op) => op.verb === 'push');
+  const replaces = () => ops.filter((op) => op.verb === 'replace');
+
+  beforeEach(() => {
+    window.history.replaceState({}, '', `/c/${Constants.NEW_CONVO}`);
+    ops = [];
+    const realPush = window.history.pushState.bind(window.history);
+    const realReplace = window.history.replaceState.bind(window.history);
+    jest.spyOn(window.history, 'pushState').mockImplementation((data, unused, url) => {
+      ops.push({ verb: 'push', url: String(url) });
+      realPush(data, unused, url);
+    });
+    jest.spyOn(window.history, 'replaceState').mockImplementation((data, unused, url) => {
+      ops.push({ verb: 'replace', url: String(url) });
+      realReplace(data, unused, url);
+    });
+  });
+
+  const renderChat = () => {
+    const router = createBrowserRouter([{ path: '/c/:conversationId', element: <Driver /> }]);
+    render(
+      <RecoilRoot>
+        <RouterProvider router={router} />
+      </RecoilRoot>,
+    );
+  };
+
+  const seed = async () => {
+    fireEvent.click(screen.getByRole('button', { name: 'seed' }));
+    await waitFor(() =>
+      expect(screen.getByTestId('convo-id')).toHaveTextContent(
+        new RegExp(`^${Constants.NEW_CONVO}$`),
+      ),
+    );
+    ops.length = 0;
+  };
+
+  it('mirrors draft edits into the URL without minting history entries', async () => {
+    renderChat();
+    await seed();
+    const depthBefore = window.history.length;
+
+    const edit = screen.getByRole('button', { name: 'edit-model' });
+    fireEvent.click(edit);
+    await waitFor(() => expect(window.location.search).toContain('model=edited-1'));
+    fireEvent.click(edit);
+    await waitFor(() => expect(window.location.search).toContain('model=edited-2'));
+    fireEvent.click(edit);
+    await waitFor(() => expect(window.location.search).toContain('model=edited-3'));
+
+    expect(pushes()).toHaveLength(0);
+    expect(replaces()).toHaveLength(3);
+    expect(window.history.length).toBe(depthBefore);
+    expect(window.location.pathname).toBe(`/c/${Constants.NEW_CONVO}`);
+    expect(window.location.search).toContain('endpoint=openAI');
+  });
+
+  it('keeps the landing project switch in place, as ProjectLandingChip intends', async () => {
+    renderChat();
+    await seed();
+    const depthBefore = window.history.length;
+
+    fireEvent.click(screen.getByRole('button', { name: 'chip-switch' }));
+
+    await waitFor(() => expect(window.location.search).toContain('projectId=proj-chip'));
+    expect(pushes()).toHaveLength(0);
+    expect(window.history.length).toBe(depthBefore);
+  });
+
+  it('mirrors onto the entry the sidebar project switch pushed, not its Back target', async () => {
+    renderChat();
+    await seed();
+    const depthBefore = window.history.length;
+
+    fireEvent.click(screen.getByRole('button', { name: 'sidebar-switch' }));
+
+    await waitFor(() => expect(pushes()).toHaveLength(1));
+    await waitFor(() =>
+      expect(replaces().some((op) => op.url.includes('model=sidebar-model'))).toBe(true),
+    );
+
+    const pushIndex = ops.findIndex((op) => op.verb === 'push');
+    const mirrorIndex = ops.findIndex((op) => op.url.includes('model=sidebar-model'));
+    /** Pins Recoil onSet timing relative to the navigate() that follows the
+     * atom write in useNewConvo: the mirror must land after the router's
+     * push — on the new entry — never rewriting the Back target beneath it. */
+    expect(mirrorIndex).toBeGreaterThan(pushIndex);
+
+    expect(pushes()[0].url).toBe(`/c/${Constants.NEW_CONVO}?projectId=proj-side`);
+    expect(window.history.length).toBe(depthBefore + 1);
+    expect(window.location.search).toContain('projectId=proj-side');
+  });
+
+  it('scrubs projectId in place when the active draft loses its project', async () => {
+    renderChat();
+    await seed();
+    fireEvent.click(screen.getByRole('button', { name: 'chip-switch' }));
+    await waitFor(() => expect(window.location.search).toContain('projectId=proj-chip'));
+    ops.length = 0;
+    const depthBefore = window.history.length;
+
+    fireEvent.click(screen.getByRole('button', { name: 'clear-project' }));
+
+    await waitFor(() => expect(window.location.search).not.toContain('projectId'));
+    expect(pushes()).toHaveLength(0);
+    expect(window.history.length).toBe(depthBefore);
+    expect(window.location.search).toContain('model=');
+  });
+
+  it('never touches history once the conversation is persisted', async () => {
+    renderChat();
+    await seed();
+
+    fireEvent.click(screen.getByRole('button', { name: 'persist' }));
+
+    await waitFor(() => expect(screen.getByTestId('convo-id')).toHaveTextContent('saved-123'));
+    expect(ops).toHaveLength(0);
+  });
+
+  it('stays out of the URL entirely when params are disabled', async () => {
+    renderChat();
+    await seed();
+
+    fireEvent.click(screen.getByRole('button', { name: 'disabled-edit' }));
+
+    await waitFor(() =>
+      expect(screen.getByTestId('convo-model')).toHaveTextContent('blocked-model'),
+    );
+    expect(ops).toHaveLength(0);
+  });
+});
diff --git a/client/src/store/families.ts b/client/src/store/families.ts
index 18741a4cd8..122db2c160 100644
--- a/client/src/store/families.ts
+++ b/client/src/store/families.ts
@@ -105,7 +105,11 @@ const conversationByIndex = atomFamily<TConversation | null, string | number>({
           }
           const searchParams = createSearchParams(newParams);
           const url = `${window.location.pathname}?${searchParams.toString()}`;
-          window.history.pushState({}, '', url);
+          /** Mirror, not navigation: Back-worthy entries are minted by real
+           * `navigate()` calls (useNewConvo), and in-place writers like
+           * ProjectLandingChip deliberately replace. Pushing here buried the
+           * Back target under one inert entry per draft edit. */
+          window.history.replaceState({}, '', url);
         }
       });
     },