diff --git a/src/components/clinical-dashboard/use-sidebar-collapsed.ts b/src/components/clinical-dashboard/use-sidebar-collapsed.ts
index ff419375b4..bcc5834089 100644
--- a/src/components/clinical-dashboard/use-sidebar-collapsed.ts
+++ b/src/components/clinical-dashboard/use-sidebar-collapsed.ts
@@ -3,9 +3,23 @@
 import { useCallback } from "react";
 import { createBrowserStore } from "@/lib/client-store-factory";
 
-const storageKey = "clinical-kb-sidebar-collapsed";
+/** localStorage key for an explicit expanded/collapsed pin. */
+export const SIDEBAR_COLLAPSED_STORAGE_KEY = "clinical-kb-sidebar-collapsed";
+
 const changeEvent = "clinical-kb-sidebar-collapsed-change";
 
+/**
+ * Maps a raw stored value to the collapsed preference. Absent key (`null` /
+ * `undefined`) resolves to collapsed so first-run and never-toggled browsers
+ * match the closed-by-default product choice. When a value is present,
+ * collapsed is true only for the explicit `"1"` pin (same ternary the store
+ * used before this helper was extracted). Storage errors are handled by
+ * callers (they pass null / use their own catch path).
+ */
+export function readSidebarCollapsedPreference(storedValue: string | null | undefined): boolean {
+  return storedValue === null || storedValue === undefined ? true : storedValue === "1";
+}
+
 // In-memory fallback when localStorage writes fail (e.g. private browsing mode).
 // Null means no fallback needed; storage is the source of truth.
 let inMemoryFallback: boolean | null = null;
@@ -17,12 +31,15 @@ function getSnapshot() {
     return inMemoryFallback;
   }
   try {
-    const storedValue = window.localStorage.getItem(storageKey);
-    // New users get the labelled (expanded) sidebar: eight icon-only
-    // destinations demand recall/hover; collapsing stays a remembered choice.
-    return storedValue === null ? false : storedValue === "1";
+    const storedValue = window.localStorage.getItem(SIDEBAR_COLLAPSED_STORAGE_KEY);
+    // Collapsed is the default whenever the key is absent. The key is only
+    // written after an explicit toggle, so browsers that never touched the
+    // control (returning users included) also land on collapsed — matching
+    // the already-closed mobile drawer. Explicit "0"/"1" preferences still
+    // win; expanding remains a remembered choice.
+    return readSidebarCollapsedPreference(storedValue);
   } catch {
-    return false;
+    return true;
   }
 }
 
@@ -35,7 +52,7 @@ function subscribe(onChange: () => void) {
   };
 }
 
-const useSidebarCollapsedStore = createBrowserStore(subscribe, getSnapshot, false);
+const useSidebarCollapsedStore = createBrowserStore(subscribe, getSnapshot, true);
 
 /**
  * Desktop sidebar collapse state shared across shells and persisted per
@@ -46,7 +63,7 @@ export function useSidebarCollapsed() {
   const collapsed = useSidebarCollapsedStore();
   const setCollapsed = useCallback((next: boolean) => {
     try {
-      window.localStorage.setItem(storageKey, next ? "1" : "0");
+      window.localStorage.setItem(SIDEBAR_COLLAPSED_STORAGE_KEY, next ? "1" : "0");
       // Storage write succeeded; clear the in-memory fallback so persisted
       // storage remains the source of truth.
       inMemoryFallback = null;
diff --git a/tests/sidebar-collapsed.test.ts b/tests/sidebar-collapsed.test.ts
new file mode 100644
index 0000000000..d144d4994a
--- /dev/null
+++ b/tests/sidebar-collapsed.test.ts
@@ -0,0 +1,32 @@
+import { describe, expect, it } from "vitest";
+import {
+  readSidebarCollapsedPreference,
+  SIDEBAR_COLLAPSED_STORAGE_KEY,
+} from "../src/components/clinical-dashboard/use-sidebar-collapsed";
+
+describe("readSidebarCollapsedPreference", () => {
+  it("defaults to collapsed when the preference key is absent", () => {
+    expect(readSidebarCollapsedPreference(null)).toBe(true);
+    expect(readSidebarCollapsedPreference(undefined)).toBe(true);
+  });
+
+  it("honours an explicit expanded pin", () => {
+    expect(readSidebarCollapsedPreference("0")).toBe(false);
+  });
+
+  it("honours an explicit collapsed pin", () => {
+    expect(readSidebarCollapsedPreference("1")).toBe(true);
+  });
+
+  it('only treats an explicit "1" as collapsed when a value is present', () => {
+    // Preserves the pre-existing ternary: absent → collapsed; otherwise
+    // collapsed iff the stored pin is exactly "1". Stale/garbage pins expand.
+    expect(readSidebarCollapsedPreference("")).toBe(false);
+    expect(readSidebarCollapsedPreference("true")).toBe(false);
+    expect(readSidebarCollapsedPreference("collapsed")).toBe(false);
+  });
+
+  it("pins the storage key used by Playwright seeds and the browser store", () => {
+    expect(SIDEBAR_COLLAPSED_STORAGE_KEY).toBe("clinical-kb-sidebar-collapsed");
+  });
+});
diff --git a/tests/ui-accessibility.spec.ts b/tests/ui-accessibility.spec.ts
index 677d19e9b5..d915ff8891 100644
--- a/tests/ui-accessibility.spec.ts
+++ b/tests/ui-accessibility.spec.ts
@@ -233,12 +233,18 @@ test.describe("Clinical KB accessibility coverage", () => {
     await page.setViewportSize({ width: 1280, height: 800 });
     await mockMinimalDashboardApi(page);
 
+    // New chat lives on both the collapsed rail and the expanded panel. With
+    // collapsed-by-default the expanded panel is unmounted, so scope to the
+    // rail rather than relying on .first() (same hazard the forced-colors
+    // journey below guards against).
+    const railNewChat = page.getByLabel("Clinical Guide collapsed sidebar").getByRole("button", { name: "New chat" });
+
     // Reduced motion → every scripted scroll must be an instant "auto" jump.
     await page.emulateMedia({ reducedMotion: "reduce" });
     await gotoApp(page);
     await expectDashboardUsable(page);
     await resetBehaviours();
-    await page.getByRole("button", { name: "New chat" }).first().click();
+    await railNewChat.click();
     await expect.poll(readBehaviours).not.toHaveLength(0);
     const reduced = await readBehaviours();
     expect(reduced, "reduced motion must not animate scripted scrolls").not.toContain("smooth");
@@ -247,7 +253,7 @@ test.describe("Clinical KB accessibility coverage", () => {
     // No preference → the same action animates smoothly.
     await page.emulateMedia({ reducedMotion: "no-preference" });
     await resetBehaviours();
-    await page.getByRole("button", { name: "New chat" }).first().click();
+    await railNewChat.click();
     await expect.poll(readBehaviours).not.toHaveLength(0);
     expect(await readBehaviours(), "no-preference should animate scripted scrolls").toContain("smooth");
   });
@@ -374,10 +380,14 @@ test.describe("Clinical KB accessibility coverage", () => {
     await page.emulateMedia({ forcedColors: "active" });
     await page.setViewportSize({ width: 1440, height: 1000 });
     await mockMinimalDashboardApi(page);
+    // Target the expanded sidebar's solid --command "New chat" button. With the
+    // collapsed-by-default rail, getByRole("New chat").first() would hit the
+    // icon-rail control (text-muted), not the solid label this regression guards.
+    await page.addInitScript(() => window.localStorage.setItem("clinical-kb-sidebar-collapsed", "0"));
     await gotoApp(page);
     await expectDashboardUsable(page);
 
-    const newChat = page.getByRole("button", { name: "New chat" }).first();
+    const newChat = page.locator("#clinical-tools-sidebar").getByRole("button", { name: "New chat" });
     await expect(newChat).toBeVisible();
     const { canvas, buttonLabelColor, tokenColors } = await newChat.evaluate((button) => {
       const probe = document.createElement("span");
diff --git a/tests/ui-hydration.spec.ts b/tests/ui-hydration.spec.ts
index c9c97b1754..66485701cc 100644
--- a/tests/ui-hydration.spec.ts
+++ b/tests/ui-hydration.spec.ts
@@ -4,9 +4,12 @@ test.describe("React Hydration Safety", () => {
   const scenarios = [
     { name: "dashboard defaults", route: "/", storage: {} },
     {
+      // Seed expanded ("0") so the client preference differs from the
+      // collapsed SSR/server snapshot after the closed-by-default flip —
+      // that is the mismatch/re-render path useSyncExternalStore must handle.
       name: "dashboard persisted theme and sidebar",
       route: "/",
-      storage: { "clinical-kb-theme": "dark", "clinical-kb-sidebar-collapsed": "1" },
+      storage: { "clinical-kb-theme": "dark", "clinical-kb-sidebar-collapsed": "0" },
       themeCookie: "dark",
     },
     {
diff --git a/tests/ui-smoke.spec.ts b/tests/ui-smoke.spec.ts
index f3b6f99b16..7e1cc43544 100644
--- a/tests/ui-smoke.spec.ts
+++ b/tests/ui-smoke.spec.ts
@@ -1164,24 +1164,27 @@ test.describe("Clinical KB UI smoke coverage", () => {
     await expectNoPageHorizontalOverflow(page);
   });
 
-  test("desktop sidebar defaults to the labelled state for new users", async ({ page }) => {
+  test("desktop sidebar defaults to the collapsed state for new users", async ({ page }) => {
     await page.setViewportSize({ width: 1280, height: 900 });
     await mockDemoApi(page);
     await gotoApp(page, "/?mode=answer");
     await waitForDemoDashboardReady(page);
 
-    // No stored preference (PT-10): the labelled navigation remains the default,
-    // so first-run desktop shows the labelled sidebar; collapse is remembered.
-    await expect(page.locator("#clinical-tools-sidebar")).toBeVisible();
-    await expect(page.getByRole("button", { name: "Collapse sidebar" })).toBeVisible();
-    await expect(page.getByRole("button", { name: "Expand sidebar" })).toHaveCount(0);
+    // No stored preference (PT-10): the collapsed icon rail is the default,
+    // so first-run desktop shows the collapsed rail, not the labelled panel;
+    // expanding is remembered. #clinical-tools-sidebar only mounts when
+    // expanded, so its absence (not just hidden) is the collapsed signal.
+    await expect(page.getByLabel("Clinical Guide collapsed sidebar")).toBeVisible();
+    await expect(page.locator("#clinical-tools-sidebar")).toHaveCount(0);
+    await expect(page.getByRole("button", { name: "Expand sidebar" })).toBeVisible();
+    await expect(page.getByRole("button", { name: "Collapse sidebar" })).toHaveCount(0);
   });
 
   test("desktop sidebar mode sync and accessibility affordances stay coherent", async ({ page }) => {
     await page.setViewportSize({ width: 1280, height: 900 });
     await mockDemoApi(page);
-    // This journey exercises the remembered-collapsed rail; new users now
-    // default to the labelled sidebar, so seed the stored preference.
+    // This journey starts from the collapsed rail (now the default for new
+    // users too) and exercises expanding/collapsing it.
     await page.addInitScript(() => window.localStorage.setItem("clinical-kb-sidebar-collapsed", "1"));
     await gotoApp(page, "/?mode=tools");
 
@@ -1226,13 +1229,16 @@ test.describe("Clinical KB UI smoke coverage", () => {
   test("tablet shows icon rail without drawer trigger or expand control @critical", async ({ page }) => {
     await page.setViewportSize({ width: 768, height: 1024 });
     await mockDemoApi(page);
+    // Seed expanded preference so #clinical-tools-sidebar mounts. Without this
+    // seed the panel is absent (count 0) and toBeHidden() would pass vacuously;
+    // we need the remembered-expanded path where the panel exists but stays
+    // display:none below lg while tablet still only presents the icon rail.
+    await page.addInitScript(() => window.localStorage.setItem("clinical-kb-sidebar-collapsed", "0"));
     await gotoApp(page, "/?mode=answer");
     await waitForDemoDashboardReady(page);
 
     await expect(page.getByRole("button", { name: "Open Clinical Guide menu" })).toHaveCount(0);
     await expect(page.getByRole("button", { name: "Expand sidebar" })).toHaveCount(0);
-    // With the labelled default the expanded panel exists in the DOM but stays
-    // display:none below lg; tablet must still only present the icon rail.
     await expect(page.locator("#clinical-tools-sidebar")).toBeHidden();
     await expect(page.getByLabel("Clinical Guide collapsed sidebar")).toBeVisible();
 
@@ -1336,8 +1342,8 @@ test.describe("Clinical KB UI smoke coverage", () => {
   }) => {
     await page.setViewportSize({ width: 1280, height: 900 });
     await mockDemoApi(page);
-    // Exercises both collapsed and expanded account affordances; seed the
-    // remembered-collapsed preference now that new users default to labelled.
+    // Exercises both collapsed and expanded account affordances; seed collapsed
+    // explicitly (also the new-user default) so the journey starts on the rail.
     await page.addInitScript(() => window.localStorage.setItem("clinical-kb-sidebar-collapsed", "1"));
     await gotoApp(page, "/");
     await waitForDemoDashboardReady(page);
@@ -1379,6 +1385,11 @@ test.describe("Clinical KB UI smoke coverage", () => {
     await gotoApp(page, "/");
     await waitForDemoDashboardReady(page);
 
+    // Sidebar defaults to collapsed for new users; expand so the in-rail Settings
+    // control this journey asserts is reachable (same as the account-setup case).
+    await page.getByRole("button", { name: "Expand sidebar" }).click();
+    await expect(page.locator("#clinical-tools-sidebar")).toBeVisible();
+
     const settings = accountSettingsDialog(page);
     await page.locator("#clinical-tools-sidebar").getByRole("button", { name: "Settings", exact: true }).click();
     await expect(settings).toBeVisible();
diff --git a/tests/ui-tools-collapse.spec.ts b/tests/ui-tools-collapse.spec.ts
index 6a1aae73b4..eac1f0056d 100644
--- a/tests/ui-tools-collapse.spec.ts
+++ b/tests/ui-tools-collapse.spec.ts
@@ -9,9 +9,10 @@ async function goto(page: Page, path: string) {
   await expect(page.locator("#main-content").first()).toBeVisible({ timeout: 15_000 });
 }
 
-// The shell's expanded sidebar (now the desktop default) contributes its own
-// "Search recent chats" searchbox, so mockup searches must be scoped to the
-// page content instead of grabbing the first searchbox on the page.
+// The shell's sidebar (expanded when a remembered preference restores it)
+// contributes its own "Search recent chats" searchbox, so mockup searches
+// must be scoped to the page content instead of grabbing the first
+// searchbox on the page.
 function mockupSearch(page: Page) {
   return page.locator("#main-content").getByRole("searchbox").first();
 }
diff --git a/tests/ui-tools.spec.ts b/tests/ui-tools.spec.ts
index 57bb338ce5..23c93c06c4 100644
--- a/tests/ui-tools.spec.ts
+++ b/tests/ui-tools.spec.ts
@@ -556,8 +556,8 @@ test.describe("Clinical KB tools launcher", () => {
 
   test("mode toggle stays global on the services home route", async ({ page }) => {
     await page.setViewportSize({ width: 1280, height: 900 });
-    // Asserts the collapsed rail affordance below; seed the remembered
-    // preference now that new users default to the labelled sidebar.
+    // Asserts the collapsed rail affordance below; explicit for clarity even
+    // though collapsed is now the default for new users too.
     await page.addInitScript(() => window.localStorage.setItem("clinical-kb-sidebar-collapsed", "1"));
     await gotoLauncher(page, "/?mode=answer");