diff --git a/webviews/src/agent-session/acpmux/SessionSidebar.gallery.tsx b/webviews/src/agent-session/acpmux/SessionSidebar.gallery.tsx
index a09812acdff9..506d86d93cbb 100644
--- a/webviews/src/agent-session/acpmux/SessionSidebar.gallery.tsx
+++ b/webviews/src/agent-session/acpmux/SessionSidebar.gallery.tsx
@@ -208,7 +208,7 @@ const entry = componentEntry<Props>({
   pane: true,
   height: 620,
   widths: { narrow: 300, normal: 380, wide: 520 },
-  anchors: [{ selector: ".acpmux-sidebar" }, { selector: ".acpmux-rail" }],
+  anchors: [{ selector: ".acpmux-sidebar" }, { selector: ".acpmux-sidebar-rail" }],
   checks: {
     anchorMovePx: {
       value: 0,
diff --git a/webviews/src/agent-session/acpmux/SessionSidebar.tsx b/webviews/src/agent-session/acpmux/SessionSidebar.tsx
index 4764caa58450..dd7909660f1e 100644
--- a/webviews/src/agent-session/acpmux/SessionSidebar.tsx
+++ b/webviews/src/agent-session/acpmux/SessionSidebar.tsx
@@ -121,7 +121,7 @@ export function SessionSidebar({
   return (
     <OpenSessions.Provider value={openIds}>
       <nav className="acpmux-sidebar" id="acpmux-sidebar" data-list-keyboard aria-label={t("sidebar.label")}>
-        <div className="acpmux-rail" data-list-keyboard>
+        <div className="acpmux-sidebar-rail" data-list-keyboard>
           <RailButton label={t("picker.newChat")} title={t("picker.newChat")} onClick={onNewChat} icon="home" />
 
           <RailButton
diff --git a/webviews/src/agent-session/acpmux/prototype/prototype.css b/webviews/src/agent-session/acpmux/prototype/prototype.css
index 6c67d0139133..9f1d4be9cc88 100644
--- a/webviews/src/agent-session/acpmux/prototype/prototype.css
+++ b/webviews/src/agent-session/acpmux/prototype/prototype.css
@@ -795,7 +795,7 @@ body {
   }
 }
 /* The history layer is the session list alone: the pane's own rail and New chat stay in the pane. */
-.proto-history .acpmux-rail,
+.proto-history .acpmux-sidebar-rail,
 .proto-history .acpmux-sidebar-action,
 .proto-history .acpmux-account {
   display: none;
diff --git a/webviews/src/agent-session/acpmux/styles.css b/webviews/src/agent-session/acpmux/styles.css
index 929402998f89..93b02c7e5c69 100644
--- a/webviews/src/agent-session/acpmux/styles.css
+++ b/webviews/src/agent-session/acpmux/styles.css
@@ -249,7 +249,7 @@
 .acpmux-queue-resume{grid-column:3;justify-self:end;height:22px;padding:0 8px;border:0;border-radius:6px;background:none;color:var(--agent-text);font:inherit;cursor:default}
 .acpmux-queue-resume:hover{background:var(--agent-control-hover)}
 .acpmux-queue-resume:focus-visible{outline:1.5px solid var(--agent-text);outline-offset:-1px}
-.acpmux-sidebar-project>svg{flex:none;color:var(--agent-muted)}.acpmux-sidebar-project>span:not(.acpmux-session-mark){flex:1;min-width:0;overflow:hidden;text-overflow:ellipsis}.acpmux-rail{flex:0 0 48px;display:flex;flex-direction:column;align-items:center;gap:4px;padding:10px 0;box-sizing:border-box}.acpmux-rail-button{position:relative;display:grid;place-items:center;width:34px;height:34px;padding:0;border:0;border-radius:9px;background:none;color:var(--agent-muted);cursor:default}.acpmux-rail-button:hover:not(:disabled){background:color-mix(in srgb,var(--agent-text) 6%,transparent);color:var(--agent-text)}.acpmux-rail-button[aria-current=page]{background:color-mix(in srgb,var(--agent-text) 9%,transparent);color:var(--agent-text)}.acpmux-rail-button:focus-visible,.acpmux-sidebar-action:focus-visible{outline:2px solid color-mix(in srgb,var(--agent-text) 40%,transparent);outline-offset:-2px}.acpmux-rail-button:disabled{opacity:.4}.acpmux-rail-dot{position:absolute;top:6px;right:6px;width:6px;height:6px;border-radius:50%;background:var(--agent-warning,var(--agent-accent))}.acpmux-sidebar-body{flex:1;min-width:0;display:flex;flex-direction:column}.acpmux-sidebar-scroll{flex:1;min-height:0;overflow:auto;padding:10px 8px 16px 0}.acpmux-sidebar-action{display:flex;align-items:center;gap:8px;width:100%;height:30px;margin-bottom:6px;padding:0 9px;border:0;border-radius:8px;background:none;color:var(--agent-text);font:inherit;letter-spacing:inherit;cursor:default}.acpmux-sidebar-action svg{flex:none;color:var(--agent-soft)}.acpmux-sidebar-action:hover{background:color-mix(in srgb,var(--agent-text) 5%,transparent)}.acpmux-sidebar-title{display:flex;align-items:center;height:30px;margin:0 0 6px;padding:0 9px;font-size:13px;font-weight:600}.acpmux-session-row.is-flat{padding-left:9px}.acpmux-session-trailing{flex:none;color:var(--agent-muted);font-size:12px;font-variant-numeric:tabular-nums}.acpmux-account{flex:none;display:flex;align-items:center;gap:8px;height:48px;padding:0 17px 0 9px}.acpmux-avatar{flex:none;display:grid;place-items:center;width:24px;height:24px;border-radius:50%;background:color-mix(in srgb,var(--agent-text) 14%,transparent);color:var(--agent-text);font-size:12px;font-weight:600}.acpmux-account-name{min-width:0;overflow:hidden;text-overflow:ellipsis;white-space:nowrap;font-weight:500}.acpmux-account-detail{flex:none;color:var(--agent-muted)}.acpmux-account-detail::before{content:"· "}.acpmux-hidden-label{position:absolute;width:1px;height:1px;overflow:hidden;clip-path:inset(50%);white-space:nowrap}
+.acpmux-sidebar-project>svg{flex:none;color:var(--agent-muted)}.acpmux-sidebar-project>span:not(.acpmux-session-mark){flex:1;min-width:0;overflow:hidden;text-overflow:ellipsis}.acpmux-sidebar-rail{flex:0 0 48px;display:flex;flex-direction:column;align-items:center;gap:4px;padding:10px 0;box-sizing:border-box}.acpmux-rail-button{position:relative;display:grid;place-items:center;width:34px;height:34px;padding:0;border:0;border-radius:9px;background:none;color:var(--agent-muted);cursor:default}.acpmux-rail-button:hover:not(:disabled){background:color-mix(in srgb,var(--agent-text) 6%,transparent);color:var(--agent-text)}.acpmux-rail-button[aria-current=page]{background:color-mix(in srgb,var(--agent-text) 9%,transparent);color:var(--agent-text)}.acpmux-rail-button:focus-visible,.acpmux-sidebar-action:focus-visible{outline:2px solid color-mix(in srgb,var(--agent-text) 40%,transparent);outline-offset:-2px}.acpmux-rail-button:disabled{opacity:.4}.acpmux-rail-dot{position:absolute;top:6px;right:6px;width:6px;height:6px;border-radius:50%;background:var(--agent-warning,var(--agent-accent))}.acpmux-sidebar-body{flex:1;min-width:0;display:flex;flex-direction:column}.acpmux-sidebar-scroll{flex:1;min-height:0;overflow:auto;padding:10px 8px 16px 0}.acpmux-sidebar-action{display:flex;align-items:center;gap:8px;width:100%;height:30px;margin-bottom:6px;padding:0 9px;border:0;border-radius:8px;background:none;color:var(--agent-text);font:inherit;letter-spacing:inherit;cursor:default}.acpmux-sidebar-action svg{flex:none;color:var(--agent-soft)}.acpmux-sidebar-action:hover{background:color-mix(in srgb,var(--agent-text) 5%,transparent)}.acpmux-sidebar-title{display:flex;align-items:center;height:30px;margin:0 0 6px;padding:0 9px;font-size:13px;font-weight:600}.acpmux-session-row.is-flat{padding-left:9px}.acpmux-session-trailing{flex:none;color:var(--agent-muted);font-size:12px;font-variant-numeric:tabular-nums}.acpmux-account{flex:none;display:flex;align-items:center;gap:8px;height:48px;padding:0 17px 0 9px}.acpmux-avatar{flex:none;display:grid;place-items:center;width:24px;height:24px;border-radius:50%;background:color-mix(in srgb,var(--agent-text) 14%,transparent);color:var(--agent-text);font-size:12px;font-weight:600}.acpmux-account-name{min-width:0;overflow:hidden;text-overflow:ellipsis;white-space:nowrap;font-weight:500}.acpmux-account-detail{flex:none;color:var(--agent-muted)}.acpmux-account-detail::before{content:"· "}.acpmux-hidden-label{position:absolute;width:1px;height:1px;overflow:hidden;clip-path:inset(50%);white-space:nowrap}
 /* New chat (EmptyState.tsx): a centered hero, small and muted, in place of the empty transcript. */
 .acpmux-empty{flex:1;min-height:0;overflow:hidden;display:flex;flex-direction:column;align-items:center;justify-content:center;gap:14px;padding:0 24px 6%;box-sizing:border-box;text-align:center;font-family:system-ui,-apple-system,"Helvetica Neue",sans-serif;-webkit-font-smoothing:antialiased}
 .acpmux-empty-glyph{color:var(--acpmux-composer-placeholder)}
diff --git a/webviews/test/sidebar-rail-css.test.tsx b/webviews/test/sidebar-rail-css.test.tsx
new file mode 100644
index 000000000000..eadcf10703fa
--- /dev/null
+++ b/webviews/test/sidebar-rail-css.test.tsx
@@ -0,0 +1,31 @@
+// The sidebar's icon rail and the chat's session rail are two components; the pane loads both
+// sheets, so the session rail's 240-340px column (summary.css) must not land on the 48px icon rail.
+import { expect, test } from "bun:test";
+import { readFileSync } from "node:fs";
+import { Window } from "happy-dom";
+import { renderToStaticMarkup } from "react-dom/server";
+import { SessionSidebar } from "../src/agent-session/acpmux/SessionSidebar";
+
+const sheet = (path: string) => readFileSync(new URL(path, import.meta.url), "utf8");
+
+test("the sidebar's icon rail keeps its own layout with the session rail's sheet loaded", () => {
+  const markup = renderToStaticMarkup(<SessionSidebar sessions={[]} onSelect={() => {}} />);
+  const window = new Window({ width: 900, height: 600 });
+  const { document } = window;
+  for (const css of [
+    sheet("../src/agent-session/acpmux/styles.css"),
+    sheet("../src/agent-session/acpmux/summary/summary.css"),
+  ]) {
+    const style = document.createElement("style");
+    style.textContent = css;
+    document.head.append(style);
+  }
+  document.body.innerHTML = markup;
+  const rail = document.querySelector("nav.acpmux-sidebar > div")!;
+  expect(rail.querySelector("button")).not.toBeNull();
+  const computed = window.getComputedStyle(rail);
+  // happy-dom drops the session rail's clamp() basis, so check the plain values both rules set.
+  expect([computed.alignItems, computed.paddingLeft]).toEqual(["center", "0px"]);
+  expect(computed.overflowY).not.toBe("auto");
+  window.happyDOM.close();
+});