diff --git a/frontend/src/shell/AppShell.session-mgmt.test.tsx b/frontend/src/shell/AppShell.session-mgmt.test.tsx
index 8f18be4c..1ffdd474 100644
--- a/frontend/src/shell/AppShell.session-mgmt.test.tsx
+++ b/frontend/src/shell/AppShell.session-mgmt.test.tsx
@@ -1,4 +1,4 @@
-import { act, render, screen, waitFor, within } from "@testing-library/react";
+import { act, fireEvent, render, screen, waitFor, within } from "@testing-library/react";
 import userEvent from "@testing-library/user-event";
 import { http, HttpResponse } from "msw";
 import { QueryClient, QueryClientProvider } from "@tanstack/react-query";
@@ -6,6 +6,7 @@ import { server } from "../test/msw";
 import { FakeEventSource } from "../test/fakeEventSource";
 import { AppShell } from "./AppShell";
 import { useSessionStore } from "../store/sessionStore";
+import { ACTIVITY_PANEL_STORAGE_KEY } from "./useActivityPanelResize";
 
 function wrap() {
   const client = new QueryClient({ defaultOptions: { queries: { retry: false } } });
@@ -27,6 +28,7 @@ function deferred() {
 }
 
 beforeEach(() => {
+  localStorage.clear();
   FakeEventSource.instances = [];
   (globalThis as any).EventSource = FakeEventSource;
   (globalThis as any).PointerEvent = MouseEvent;
@@ -779,21 +781,58 @@ test("renaming a group reassigns its members via setSessionGroup", async () => {
 });
 
 
-test("opening Model activity replaces the session rail with a 40/60 activity and chat layout", async () => {
+test("opens an accessible resizable activity split and persists pointer width", async () => {
   server.use(http.post("http://localhost:8787/sessions/:id/resume", () => resumeResult("s1")));
   wrap();
   await userEvent.click(await screen.findByText("Attiva uno"));
   await userEvent.click(await screen.findByRole("button", { name: /resume/i }));
-
-  expect(screen.getByText("Sessions")).toBeInTheDocument();
   await userEvent.click(screen.getByRole("button", { name: /show model activity/i }));
 
   const shell = screen.getByTestId("app-shell");
-  expect(shell).toHaveAttribute("data-activity-layout", "open");
-  expect(screen.getByRole("heading", { name: "Model activity" }).closest("aside")).toHaveClass("w-2/5");
-  expect(screen.queryByText("Sessions")).not.toBeInTheDocument();
+  const separator = screen.getByRole("separator", { name: /resize model activity/i });
+  expect(shell.style.getPropertyValue("--activity-panel-width")).toBe("384px");
+  expect(separator).toHaveAttribute("aria-orientation", "vertical");
+  expect(separator).toHaveAttribute("aria-valuemin", "288");
+  expect(separator).toHaveAttribute("aria-valuenow", "384");
 
-  await userEvent.click(screen.getByRole("button", { name: /hide model activity/i }));
-  expect(shell).toHaveAttribute("data-activity-layout", "closed");
-  expect(screen.getByText("Sessions")).toBeInTheDocument();
+  Object.assign(separator, {
+    setPointerCapture: vi.fn(),
+    releasePointerCapture: vi.fn(),
+    hasPointerCapture: vi.fn(() => true),
+  });
+  fireEvent.pointerDown(separator, { pointerId: 1, clientX: 384 });
+  fireEvent.pointerMove(separator, { pointerId: 1, clientX: 484 });
+  fireEvent.pointerUp(separator, { pointerId: 1, clientX: 484 });
+
+  expect(shell.style.getPropertyValue("--activity-panel-width")).toBe("484px");
+  expect(localStorage.getItem(ACTIVITY_PANEL_STORAGE_KEY)).toBe("484");
+  expect(screen.getByTestId("conversation-column")).toHaveClass("flex-1");
+});
+
+test("resizes the activity split with keyboard and exposes responsive drawer classes", async () => {
+  localStorage.setItem(ACTIVITY_PANEL_STORAGE_KEY, "448");
+  server.use(http.post("http://localhost:8787/sessions/:id/resume", () => resumeResult("s1")));
+  wrap();
+  await userEvent.click(await screen.findByText("Attiva uno"));
+  await userEvent.click(await screen.findByRole("button", { name: /resume/i }));
+  await userEvent.click(screen.getByRole("button", { name: /show model activity/i }));
+
+  const shell = screen.getByTestId("app-shell");
+  const separator = screen.getByRole("separator", { name: /resize model activity/i });
+  expect(shell.style.getPropertyValue("--activity-panel-width")).toBe("448px");
+  fireEvent.keyDown(separator, { key: "ArrowLeft" });
+  expect(shell.style.getPropertyValue("--activity-panel-width")).toBe("432px");
+  fireEvent.keyDown(separator, { key: "ArrowRight", shiftKey: true });
+  expect(shell.style.getPropertyValue("--activity-panel-width")).toBe("480px");
+  fireEvent.keyDown(separator, { key: "Home" });
+  expect(shell.style.getPropertyValue("--activity-panel-width")).toBe("288px");
+
+  const panel = screen.getByRole("heading", { name: "Model activity" }).closest("aside");
+  expect(panel).toHaveClass(
+    "fixed",
+    "w-[min(90vw,24rem)]",
+    "lg:static",
+    "lg:w-[var(--activity-panel-width)]",
+  );
+  expect(separator).toHaveClass("touch-none", "cursor-col-resize");
 });
diff --git a/frontend/src/shell/AppShell.tsx b/frontend/src/shell/AppShell.tsx
index 12963056..83b4a662 100644
--- a/frontend/src/shell/AppShell.tsx
+++ b/frontend/src/shell/AppShell.tsx
@@ -3,6 +3,7 @@ import { useSessionStore } from "../store/sessionStore";
 import { WidgetHost } from "./WidgetHost";
 import { CentralStatus } from "./CentralStatus";
 import { ModelActivityPanel } from "./ModelActivityPanel";
+import { useActivityPanelResize } from "./useActivityPanelResize";
 import { NavSessions } from "./NavSessions";
 import { SessionMenu } from "./SessionMenu";
 import { SessionDocumentsPanel } from "./SessionDocumentsPanel";
@@ -23,6 +24,7 @@ import {
 import type { SessionSummary } from "../api/types";
 import { useQuery, useQueryClient } from "@tanstack/react-query";
 import { useEffect, useMemo, useRef, useState } from "react";
+import type { CSSProperties } from "react";
 
 /**
  * The page is designed to live INSIDE the Omics Portal chrome (its left sidebar
@@ -31,6 +33,10 @@ import { useEffect, useMemo, useRef, useState } from "react";
  * portal's left sidebar, and the workflow phases ride a slim strip above the chat.
  */
 export function AppShell() {
+  const { containerRef, panelWidth, resizing, separatorProps } = useActivityPanelResize();
+  const activityWidthStyle = {
+    "--activity-panel-width": `${panelWidth}px`,
+  } as CSSProperties;
   const [activeSessionId, setActiveSessionId] = useState<string | null>(null);
   const activeSessionIdRef = useRef<string | null>(null);
   const resumeInvocationRef = useRef(0);
@@ -364,9 +370,15 @@ export function AppShell() {
 
   return (
     <div
+      ref={containerRef}
+      style={activityWidthStyle}
       data-testid="app-shell"
       data-activity-layout={showActivity ? "open" : "closed"}
-      className="flex h-screen bg-background text-foreground"
+      data-activity-resizing={resizing}
+      className={[
+        "flex h-screen bg-background text-foreground",
+        resizing ? "select-none cursor-col-resize" : "",
+      ].join(" ")}
     >
       {panelSession && (
         <SessionDocumentsPanel
@@ -376,9 +388,21 @@ export function AppShell() {
         />
       )}
       {showActivity && <ModelActivityPanel onClose={() => setShowActivity(false)} />}
+      {showActivity && (
+        <div className="relative hidden w-0 shrink-0 lg:block">
+          <div
+            {...separatorProps}
+            data-testid="activity-resize-handle"
+            data-dragging={resizing}
+            className="group absolute inset-y-0 left-1/2 z-20 w-3 -translate-x-1/2 touch-none cursor-col-resize outline-none"
+          >
+            <span className="absolute inset-y-0 left-1/2 w-px -translate-x-1/2 bg-border transition-colors group-hover:bg-primary/60 group-focus-visible:bg-primary group-data-[dragging=true]:bg-primary" />
+          </div>
+        </div>
+      )}
 
       {/* Conversation column */}
-      <div className={["flex min-w-0 flex-col", showActivity ? "w-3/5 shrink-0" : "flex-1"].join(" ")}>
+      <div data-testid="conversation-column" className="flex min-w-0 flex-1 flex-col">
         {activeSessionId && (
           <div className="relative shrink-0 border-b border-border/70 bg-background/80 px-6 py-2.5 backdrop-blur">
             {/* Arrow toggle for the left Model-activity panel: → opens it, ← closes it. */}
diff --git a/frontend/src/shell/ModelActivityPanel.tsx b/frontend/src/shell/ModelActivityPanel.tsx
index 2e8609b6..27cbee54 100644
--- a/frontend/src/shell/ModelActivityPanel.tsx
+++ b/frontend/src/shell/ModelActivityPanel.tsx
@@ -70,7 +70,7 @@ export function ModelActivityPanel({ onClose }: { onClose: () => void }) {
   }, [visibleCount, visibleTail]);
 
   return (
-    <aside className="flex w-2/5 shrink-0 flex-col border-r border-border bg-sidebar">
+    <aside className="fixed inset-y-0 left-0 z-30 flex w-[min(90vw,24rem)] shrink-0 flex-col border-r border-border bg-sidebar lg:static lg:z-auto lg:w-[var(--activity-panel-width)] lg:border-r-0">
       <div className="flex items-center justify-between border-b border-border/60 px-3 py-2">
         <h2 className="font-heading text-sm font-semibold text-foreground">Model activity</h2>
         <Button size="icon-sm" variant="ghost" aria-label="Close model activity" onClick={onClose}>
diff --git a/frontend/src/shell/useActivityPanelResize.test.ts b/frontend/src/shell/useActivityPanelResize.test.ts
new file mode 100644
index 00000000..5b8e1f2a
--- /dev/null
+++ b/frontend/src/shell/useActivityPanelResize.test.ts
@@ -0,0 +1,44 @@
+import { describe, expect, test } from "vitest";
+import {
+  ACTIVITY_PANEL_DEFAULT_WIDTH,
+  ACTIVITY_PANEL_STORAGE_KEY,
+  clampActivityPanelWidth,
+  getActivityPanelBounds,
+  loadActivityPanelWidth,
+  resizeWidthFromKey,
+  saveActivityPanelWidth,
+} from "./useActivityPanelResize";
+
+describe("activity panel resize contract", () => {
+  test("computes desktop bounds while preserving 512px centrally", () => {
+    expect(getActivityPanelBounds(1200)).toEqual({ min: 288, max: 576 });
+    expect(getActivityPanelBounds(1024)).toEqual({ min: 288, max: 512 });
+    expect(getActivityPanelBounds(800)).toEqual({ min: 288, max: 288 });
+  });
+
+  test("clamps finite values and falls back for invalid values", () => {
+    const bounds = { min: 288, max: 576 };
+    expect(clampActivityPanelWidth(200, bounds)).toBe(288);
+    expect(clampActivityPanelWidth(420, bounds)).toBe(420);
+    expect(clampActivityPanelWidth(900, bounds)).toBe(576);
+    expect(clampActivityPanelWidth(Number.NaN, bounds)).toBe(ACTIVITY_PANEL_DEFAULT_WIDTH);
+  });
+
+  test("maps keyboard controls to exact clamped increments", () => {
+    const bounds = { min: 288, max: 576 };
+    expect(resizeWidthFromKey(384, "ArrowLeft", false, bounds)).toBe(368);
+    expect(resizeWidthFromKey(384, "ArrowRight", true, bounds)).toBe(432);
+    expect(resizeWidthFromKey(384, "Home", false, bounds)).toBe(288);
+    expect(resizeWidthFromKey(384, "End", false, bounds)).toBe(576);
+    expect(resizeWidthFromKey(384, "Enter", false, bounds)).toBeNull();
+  });
+
+  test("loads, persists, and rejects corrupt browser values", () => {
+    localStorage.clear();
+    expect(loadActivityPanelWidth(localStorage)).toBe(ACTIVITY_PANEL_DEFAULT_WIDTH);
+    localStorage.setItem(ACTIVITY_PANEL_STORAGE_KEY, "corrupt");
+    expect(loadActivityPanelWidth(localStorage)).toBe(ACTIVITY_PANEL_DEFAULT_WIDTH);
+    saveActivityPanelWidth(448, localStorage);
+    expect(loadActivityPanelWidth(localStorage)).toBe(448);
+  });
+});
diff --git a/frontend/src/shell/useActivityPanelResize.ts b/frontend/src/shell/useActivityPanelResize.ts
new file mode 100644
index 00000000..fe78cd9e
--- /dev/null
+++ b/frontend/src/shell/useActivityPanelResize.ts
@@ -0,0 +1,193 @@
+import { useCallback, useLayoutEffect, useRef, useState } from "react";
+import type { KeyboardEventHandler, PointerEventHandler, RefObject } from "react";
+
+export const ACTIVITY_PANEL_DEFAULT_WIDTH = 384;
+export const ACTIVITY_PANEL_MIN_WIDTH = 288;
+export const ACTIVITY_PANEL_MAX_WIDTH = 576;
+export const ACTIVITY_CENTRAL_MIN_WIDTH = 512;
+export const ACTIVITY_PANEL_STORAGE_KEY = "thothii:model-activity-panel-width";
+
+export type ActivityPanelBounds = { min: number; max: number };
+
+export function getActivityPanelBounds(containerWidth: number): ActivityPanelBounds {
+  return {
+    min: ACTIVITY_PANEL_MIN_WIDTH,
+    max: Math.max(
+      ACTIVITY_PANEL_MIN_WIDTH,
+      Math.min(ACTIVITY_PANEL_MAX_WIDTH, containerWidth - ACTIVITY_CENTRAL_MIN_WIDTH),
+    ),
+  };
+}
+
+export function clampActivityPanelWidth(width: number, bounds: ActivityPanelBounds): number {
+  const candidate = Number.isFinite(width) ? width : ACTIVITY_PANEL_DEFAULT_WIDTH;
+  return Math.min(bounds.max, Math.max(bounds.min, candidate));
+}
+
+export function loadActivityPanelWidth(storage: Pick<Storage, "getItem"> | null): number {
+  if (!storage) return ACTIVITY_PANEL_DEFAULT_WIDTH;
+  try {
+    const raw = storage.getItem(ACTIVITY_PANEL_STORAGE_KEY);
+    if (raw === null || raw.trim() === "") return ACTIVITY_PANEL_DEFAULT_WIDTH;
+    const parsed = Number(raw);
+    return Number.isFinite(parsed) ? parsed : ACTIVITY_PANEL_DEFAULT_WIDTH;
+  } catch {
+    return ACTIVITY_PANEL_DEFAULT_WIDTH;
+  }
+}
+
+export function saveActivityPanelWidth(
+  width: number,
+  storage: Pick<Storage, "setItem"> | null,
+): void {
+  if (!storage) return;
+  try {
+    storage.setItem(ACTIVITY_PANEL_STORAGE_KEY, String(Math.round(width)));
+  } catch {
+    // Storage can be unavailable in privacy-restricted browser contexts.
+  }
+}
+
+function getBrowserStorage(): Storage | null {
+  if (typeof window === "undefined") return null;
+  try {
+    return window.localStorage;
+  } catch {
+    return null;
+  }
+}
+
+export function resizeWidthFromKey(
+  width: number,
+  key: string,
+  shiftKey: boolean,
+  bounds: ActivityPanelBounds,
+): number | null {
+  const step = shiftKey ? 48 : 16;
+  if (key === "ArrowLeft") return clampActivityPanelWidth(width - step, bounds);
+  if (key === "ArrowRight") return clampActivityPanelWidth(width + step, bounds);
+  if (key === "Home") return bounds.min;
+  if (key === "End") return bounds.max;
+  return null;
+}
+
+type DragState = { pointerId: number; startX: number; startWidth: number };
+
+export function useActivityPanelResize(): {
+  containerRef: RefObject<HTMLDivElement>;
+  panelWidth: number;
+  resizing: boolean;
+  separatorProps: {
+    role: "separator";
+    tabIndex: 0;
+    "aria-label": string;
+    "aria-orientation": "vertical";
+    "aria-valuemin": number;
+    "aria-valuemax": number;
+    "aria-valuenow": number;
+    onPointerDown: PointerEventHandler<HTMLDivElement>;
+    onPointerMove: PointerEventHandler<HTMLDivElement>;
+    onPointerUp: PointerEventHandler<HTMLDivElement>;
+    onPointerCancel: PointerEventHandler<HTMLDivElement>;
+    onKeyDown: KeyboardEventHandler<HTMLDivElement>;
+  };
+} {
+  const storageRef = useRef<Storage | null>(getBrowserStorage());
+  const initialWidthRef = useRef(loadActivityPanelWidth(storageRef.current));
+  const containerRef = useRef<HTMLDivElement>(null);
+  const widthRef = useRef(initialWidthRef.current);
+  const boundsRef = useRef<ActivityPanelBounds>({
+    min: ACTIVITY_PANEL_MIN_WIDTH,
+    max: ACTIVITY_PANEL_MAX_WIDTH,
+  });
+  const dragRef = useRef<DragState | null>(null);
+  const [panelWidth, setPanelWidth] = useState(initialWidthRef.current);
+  const [bounds, setBounds] = useState(boundsRef.current);
+  const [resizing, setResizing] = useState(false);
+
+  const applyWidth = useCallback((rawWidth: number, persist: boolean) => {
+    const next = clampActivityPanelWidth(rawWidth, boundsRef.current);
+    widthRef.current = next;
+    setPanelWidth(next);
+    if (persist) saveActivityPanelWidth(next, storageRef.current);
+    return next;
+  }, []);
+
+  useLayoutEffect(() => {
+    const container = containerRef.current;
+    if (!container) return;
+    const syncBounds = () => {
+      if (container.clientWidth < ACTIVITY_PANEL_MIN_WIDTH + ACTIVITY_CENTRAL_MIN_WIDTH) return;
+      const nextBounds = getActivityPanelBounds(container.clientWidth);
+      boundsRef.current = nextBounds;
+      setBounds(nextBounds);
+      applyWidth(widthRef.current, true);
+    };
+    syncBounds();
+    const observer = typeof ResizeObserver === "undefined" ? null : new ResizeObserver(syncBounds);
+    observer?.observe(container);
+    window.addEventListener("resize", syncBounds);
+    return () => {
+      observer?.disconnect();
+      window.removeEventListener("resize", syncBounds);
+    };
+  }, [applyWidth]);
+
+  const onPointerDown: PointerEventHandler<HTMLDivElement> = (event) => {
+    dragRef.current = {
+      pointerId: event.pointerId,
+      startX: event.clientX,
+      startWidth: widthRef.current,
+    };
+    event.currentTarget.setPointerCapture?.(event.pointerId);
+    setResizing(true);
+  };
+  const onPointerMove: PointerEventHandler<HTMLDivElement> = (event) => {
+    const drag = dragRef.current;
+    if (!drag || drag.pointerId !== event.pointerId) return;
+    applyWidth(drag.startWidth + event.clientX - drag.startX, false);
+  };
+  const onPointerUp: PointerEventHandler<HTMLDivElement> = (event) => {
+    const drag = dragRef.current;
+    if (!drag || drag.pointerId !== event.pointerId) return;
+    applyWidth(drag.startWidth + event.clientX - drag.startX, true);
+    if (event.currentTarget.hasPointerCapture?.(event.pointerId)) {
+      event.currentTarget.releasePointerCapture?.(event.pointerId);
+    }
+    dragRef.current = null;
+    setResizing(false);
+  };
+  const onPointerCancel: PointerEventHandler<HTMLDivElement> = (event) => {
+    const drag = dragRef.current;
+    if (!drag || drag.pointerId !== event.pointerId) return;
+    applyWidth(drag.startWidth, false);
+    dragRef.current = null;
+    setResizing(false);
+  };
+  const onKeyDown: KeyboardEventHandler<HTMLDivElement> = (event) => {
+    const next = resizeWidthFromKey(widthRef.current, event.key, event.shiftKey, boundsRef.current);
+    if (next === null) return;
+    event.preventDefault();
+    applyWidth(next, true);
+  };
+
+  return {
+    containerRef,
+    panelWidth,
+    resizing,
+    separatorProps: {
+      role: "separator",
+      tabIndex: 0,
+      "aria-label": "Resize model activity",
+      "aria-orientation": "vertical",
+      "aria-valuemin": bounds.min,
+      "aria-valuemax": bounds.max,
+      "aria-valuenow": panelWidth,
+      onPointerDown,
+      onPointerMove,
+      onPointerUp,
+      onPointerCancel,
+      onKeyDown,
+    },
+  };
+}