diff --git a/apps/website/src/components/docs/DocsControlPlane.spec.tsx b/apps/website/src/components/docs/DocsControlPlane.spec.tsx
index 6825b25a6..2ef5dbb4b 100644
--- a/apps/website/src/components/docs/DocsControlPlane.spec.tsx
+++ b/apps/website/src/components/docs/DocsControlPlane.spec.tsx
@@ -3,6 +3,9 @@ import React from 'react';
 import { fireEvent, render, screen, waitFor, within } from '@testing-library/react';
 import { beforeEach, describe, expect, it, vi } from 'vitest';
 import { DocsControlPlane, DocsContextContent } from './DocsControlPlane';
+import { docsConfig } from '../../lib/docs-config';
+
+const LIBRARY_TITLES = docsConfig.filter((l) => l.group === 'library').map((l) => l.title);
 
 const push = vi.fn();
 vi.mock('next/navigation', () => ({
@@ -93,6 +96,113 @@ describe('DocsControlPlane', () => {
     expect(document.getElementById(controlledId)).toBeTruthy();
   });
 
+  it('stacks the adapter title and tagline on separate lines', () => {
+    render(
+      <DocsControlPlane
+        activeLibrary="langgraph"
+        activeSection="guides"
+        activeSlug="streaming"
+        pageTitle="Streaming"
+      />,
+    );
+
+    fireEvent.click(screen.getByRole('button', { name: 'LangGraph' }));
+    const item = screen.getByRole('menuitemradio', { name: /LangGraph/ });
+    const title = item.querySelector('.docs-sidebar-lib-item-title');
+    const tagline = item.querySelector('.docs-sidebar-lib-item-desc');
+    if (!title || !tagline) throw new Error('Expected a title and a tagline');
+
+    // The collision regression: both spans rendered inline on one line, so the
+    // row read as "LangGraphTalk to LangGraph directly".
+    expect(title.textContent).toBe('LangGraph');
+    expect(tagline.textContent).toBe('Talk to LangGraph directly');
+    const wrapper = title.parentElement;
+    if (!wrapper) throw new Error('Expected a text wrapper');
+    expect(wrapper.className).toContain('docs-sidebar-lib-item-text');
+  });
+
+  it('splits the menu into labelled adapter and library groups', () => {
+    render(
+      <DocsControlPlane
+        activeLibrary="langgraph"
+        activeSection="guides"
+        activeSlug="streaming"
+        pageTitle="Streaming"
+      />,
+    );
+
+    fireEvent.click(screen.getByRole('button', { name: 'LangGraph' }));
+    const adapters = screen.getByRole('group', { name: 'Adapters' });
+    const libraries = screen.getByRole('group', { name: 'Libraries' });
+
+    // Adapters are asserted exactly: there are two, and only they carry
+    // taglines. A new library must not quietly land in this group.
+    expect(within(adapters).getAllByRole('menuitemradio').map((i) => i.textContent)).toEqual([
+      'LangGraphTalk to LangGraph directly',
+      'AG-UIAny AG-UI backend',
+    ]);
+    // Libraries are checked against config so adding one does not churn this
+    // test — misclassifying one still fails the assertion above.
+    expect(within(libraries).getAllByRole('menuitemradio').map((i) => i.textContent)).toEqual(
+      LIBRARY_TITLES,
+    );
+  });
+
+  it('renders menu entries as real links with the current one checked', () => {
+    render(
+      <DocsControlPlane
+        activeLibrary="langgraph"
+        activeSection="guides"
+        activeSlug="streaming"
+        pageTitle="Streaming"
+      />,
+    );
+
+    fireEvent.click(screen.getByRole('button', { name: 'LangGraph' }));
+    const agUi = screen.getByRole('menuitemradio', { name: /AG-UI/ });
+    expect(agUi.tagName).toBe('A');
+    expect(agUi.getAttribute('href')).toBe('/docs/ag-ui/getting-started/introduction');
+    expect(agUi.getAttribute('aria-checked')).toBe('false');
+    expect(
+      screen.getByRole('menuitemradio', { name: /LangGraph/ }).getAttribute('aria-checked'),
+    ).toBe('true');
+  });
+
+  it('drops the library row from environment now the picker owns it', () => {
+    render(
+      <DocsControlPlane
+        activeLibrary="langgraph"
+        activeSection="guides"
+        activeSlug="streaming"
+        pageTitle="Streaming"
+      />,
+    );
+
+    fireEvent.click(screen.getByRole('button', { name: 'Environment' }));
+    expect(screen.getByText('Angular')).toBeTruthy();
+    expect(screen.getByText('npm')).toBeTruthy();
+    expect(screen.queryByText('Library')).toBeNull();
+  });
+
+  it('caps the open menu to the space left below the trigger', () => {
+    render(
+      <DocsControlPlane
+        activeLibrary="langgraph"
+        activeSection="guides"
+        activeSlug="streaming"
+        pageTitle="Streaming"
+      />,
+    );
+
+    fireEvent.click(screen.getByRole('button', { name: 'LangGraph' }));
+    const menu = screen.getByRole('menu');
+
+    // A viewport-percentage cap cannot work: the menu opens ~342px down the
+    // pane, so `60vh` still overflows a short window by ~100px. The cap has to
+    // be measured from the trigger's own position.
+    expect(menu.style.maxHeight).toMatch(/^\d+(\.\d+)?px$/);
+  });
+
   it('supports keyboard entry and dismissal for the library menu', () => {
     render(
       <DocsControlPlane
@@ -105,7 +215,15 @@ describe('DocsControlPlane', () => {
 
     const trigger = screen.getByRole('button', { name: 'LangGraph' });
     fireEvent.keyDown(trigger, { key: 'ArrowDown' });
-    const firstItem = screen.getByRole('menuitem', { name: /LangGraph/ });
+    const firstItem = screen.getByRole('menuitemradio', { name: /LangGraph/ });
+    expect(document.activeElement).toBe(firstItem);
+
+    // Traversal must still cross the Adapters/Libraries group boundary.
+    fireEvent.keyDown(firstItem, { key: 'End' });
+    expect(document.activeElement).toBe(
+      screen.getByRole('menuitemradio', { name: LIBRARY_TITLES[LIBRARY_TITLES.length - 1] }),
+    );
+    fireEvent.keyDown(document.activeElement as HTMLElement, { key: 'Home' });
     expect(document.activeElement).toBe(firstItem);
 
     fireEvent.keyDown(firstItem, { key: 'Escape' });
diff --git a/apps/website/src/components/docs/DocsControlPlane.tsx b/apps/website/src/components/docs/DocsControlPlane.tsx
index d2a9ccb08..b4c8b62b7 100644
--- a/apps/website/src/components/docs/DocsControlPlane.tsx
+++ b/apps/website/src/components/docs/DocsControlPlane.tsx
@@ -6,7 +6,6 @@ import {
   Braces,
   Code2,
   ExternalLink,
-  Package,
   Play,
   Search,
 } from 'lucide-react';
@@ -58,7 +57,6 @@ export function DocsContextContent({
     window.setTimeout(dispatchSearch, 0);
   };
   const environmentRows = [
-    { label: 'Library', value: library?.title ?? activeLibrary, icon: <Package size={15} aria-hidden="true" /> },
     { label: 'Framework', value: 'Angular', icon: <Blocks size={15} aria-hidden="true" /> },
     { label: 'Package manager', value: 'npm', icon: <Code2 size={15} aria-hidden="true" /> },
   ];
diff --git a/apps/website/src/components/docs/DocsSidebar.tsx b/apps/website/src/components/docs/DocsSidebar.tsx
index 209bfe4bd..d85d8c406 100644
--- a/apps/website/src/components/docs/DocsSidebar.tsx
+++ b/apps/website/src/components/docs/DocsSidebar.tsx
@@ -9,7 +9,7 @@ import {
   type KeyboardEvent,
 } from 'react';
 import Link from 'next/link';
-import { usePathname, useRouter } from 'next/navigation';
+import { usePathname } from 'next/navigation';
 import {
   Blocks,
   BookOpen,
@@ -24,8 +24,10 @@ import {
 import {
   docsConfig,
   getLibraryConfig,
+  libraryIntroPath,
   specialDocsPages,
   type DocsSection,
+  type LibraryGroup,
   type LibraryId,
 } from '../../lib/docs-config';
 import { LibraryMark } from './LibraryMark';
@@ -39,6 +41,23 @@ export interface DocsNavigationProps {
   onNavigate?: () => void;
 }
 
+/**
+ * Menu entries are `menuitemradio` (single-select from a set), not `menuitem`.
+ * Keyboard traversal reads this — if it drifts from the rendered role, arrow
+ * keys and Escape silently stop finding anything.
+ */
+const MENU_ITEM_SELECTOR = '[role="menuitemradio"]';
+
+/** Breathing room between the open menu and the bottom of the viewport. */
+const MENU_VIEWPORT_GUTTER = 16;
+/** Below this the menu is uselessly short; let the pane scroll instead. */
+const MENU_MIN_HEIGHT = 180;
+
+const LIBRARY_GROUPS: { id: LibraryGroup; label: string }[] = [
+  { id: 'adapter', label: 'Adapters' },
+  { id: 'library', label: 'Libraries' },
+];
+
 function LibraryDropdown({
   activeLibrary,
   onNavigate,
@@ -52,7 +71,6 @@ function LibraryDropdown({
   const menuRef = useRef<HTMLDivElement>(null);
   const initialFocusRef = useRef(0);
   const menuId = useId();
-  const router = useRouter();
 
   useEffect(() => {
     const handler = (event: MouseEvent) => {
@@ -64,10 +82,30 @@ function LibraryDropdown({
 
   useEffect(() => {
     if (!open) return;
-    const items = menuRef.current?.querySelectorAll<HTMLButtonElement>('[role="menuitem"]');
+    const items = menuRef.current?.querySelectorAll<HTMLElement>(MENU_ITEM_SELECTOR);
     items?.[initialFocusRef.current]?.focus();
   }, [open]);
 
+  /**
+   * Cap the menu to the room actually left below the trigger. A CSS `vh` cap
+   * cannot do this: the menu opens ~340px down the pane, so even `60vh` still
+   * overflows a short window by ~100px. Measured from the trigger, it never
+   * runs past the fold — it scrolls internally instead.
+   */
+  useEffect(() => {
+    if (!open) return;
+    const resize = () => {
+      const trigger = triggerRef.current;
+      const menu = menuRef.current;
+      if (!trigger || !menu) return;
+      const available = window.innerHeight - trigger.getBoundingClientRect().bottom - MENU_VIEWPORT_GUTTER;
+      menu.style.maxHeight = `${Math.max(MENU_MIN_HEIGHT, available)}px`;
+    };
+    resize();
+    window.addEventListener('resize', resize);
+    return () => window.removeEventListener('resize', resize);
+  }, [open]);
+
   const openMenu = (initialIndex: number) => {
     initialFocusRef.current = initialIndex;
     setOpen(true);
@@ -80,9 +118,9 @@ function LibraryDropdown({
 
   const onMenuKeyDown = (event: KeyboardEvent<HTMLDivElement>) => {
     const items = Array.from(
-      menuRef.current?.querySelectorAll<HTMLButtonElement>('[role="menuitem"]') ?? [],
+      menuRef.current?.querySelectorAll<HTMLElement>(MENU_ITEM_SELECTOR) ?? [],
     );
-    const current = Math.max(0, items.indexOf(document.activeElement as HTMLButtonElement));
+    const current = Math.max(0, items.indexOf(document.activeElement as HTMLElement));
     const nextIndex =
       event.key === 'Home'
         ? 0
@@ -145,33 +183,45 @@ function LibraryDropdown({
           className="docs-sidebar-lib-menu"
           onKeyDown={onMenuKeyDown}
         >
-          {docsConfig.map((library) => (
-            <button
-              type="button"
-              role="menuitem"
-              tabIndex={-1}
-              key={library.id}
-              onClick={() => {
-                closeMenu();
-                onNavigate?.();
-                router.push(`/docs/${library.id}/getting-started/introduction`);
-              }}
-              className="docs-sidebar-lib-item"
-              data-active={library.id === activeLibrary || undefined}
-            >
-              <span className="docs-sidebar-lib-item-icon">
-                <LibraryMark library={library.id} size={20} />
-              </span>
-              <span className="docs-sidebar-lib-item-text">
-                <span
-                  className="docs-sidebar-lib-item-title"
-                  data-active={library.id === activeLibrary || undefined}
-                >
-                  {library.title}
-                </span>
-                <span className="docs-sidebar-lib-item-desc">{library.description}</span>
+          {LIBRARY_GROUPS.map((group, groupIndex) => (
+            <div role="group" aria-label={group.label} key={group.id}>
+              {groupIndex > 0 ? <span className="docs-sidebar-lib-divider" aria-hidden="true" /> : null}
+              <span className="docs-sidebar-lib-group" aria-hidden="true">
+                {group.label}
               </span>
-            </button>
+              {docsConfig
+                .filter((library) => library.group === group.id)
+                .map((library) => {
+                  const isActive = library.id === activeLibrary;
+                  return (
+                    <Link
+                      role="menuitemradio"
+                      aria-checked={isActive}
+                      tabIndex={-1}
+                      key={library.id}
+                      href={libraryIntroPath(library.id)}
+                      onClick={() => {
+                        closeMenu();
+                        onNavigate?.();
+                      }}
+                      className="docs-sidebar-lib-item"
+                      data-active={isActive || undefined}
+                    >
+                      <span className="docs-sidebar-lib-item-icon">
+                        <LibraryMark library={library.id} size={20} />
+                      </span>
+                      <span className="docs-sidebar-lib-item-text">
+                        <span className="docs-sidebar-lib-item-title" data-active={isActive || undefined}>
+                          {library.title}
+                        </span>
+                        {library.tagline ? (
+                          <span className="docs-sidebar-lib-item-desc">{library.tagline}</span>
+                        ) : null}
+                      </span>
+                    </Link>
+                  );
+                })}
+            </div>
           ))}
         </div>
       ) : null}
diff --git a/apps/website/src/components/shared/Nav.spec.tsx b/apps/website/src/components/shared/Nav.spec.tsx
index e80127b4b..a870ceeeb 100644
--- a/apps/website/src/components/shared/Nav.spec.tsx
+++ b/apps/website/src/components/shared/Nav.spec.tsx
@@ -66,7 +66,7 @@ describe('Docs mobile navigation', () => {
     const libraryTrigger = within(dialog).getByRole('button', { name: 'LangGraph' });
     fireEvent.click(libraryTrigger);
 
-    fireEvent.keyDown(within(dialog).getByRole('menuitem', { name: /LangGraph/ }), {
+    fireEvent.keyDown(within(dialog).getByRole('menuitemradio', { name: /LangGraph/ }), {
       key: 'Escape',
     });
 
diff --git a/apps/website/src/lib/docs-config.ts b/apps/website/src/lib/docs-config.ts
index d802630ab..0fb6e89b0 100644
--- a/apps/website/src/lib/docs-config.ts
+++ b/apps/website/src/lib/docs-config.ts
@@ -27,10 +27,27 @@ export interface DocsSection {
   pages: DocsPage[];
 }
 
+/**
+ * Adapters connect a backend agent runtime; libraries are the companion
+ * packages around them. The picker groups on this.
+ */
+export type LibraryGroup = 'adapter' | 'library';
+
 export interface DocsLibrary {
   id: LibraryId;
   title: string;
+  /**
+   * Long form. Fallback for the page `<meta name="description">` via
+   * {@link resolveDocDescription} — not shown in the picker.
+   */
   description: string;
+  group: LibraryGroup;
+  /**
+   * Shown under the name in the library picker. Adapters only — the companion
+   * libraries are self-describing, and a tagline there is just noise. Keep to
+   * three or four words so picker rows cannot wrap.
+   */
+  tagline?: string;
   /** Optional external live-demo URL, surfaced contextually in docs nav. */
   demoUrl?: string;
   /** Optional label override for the demo link. Defaults to 'Live demo'. */
@@ -59,6 +76,8 @@ export const docsConfig: DocsLibrary[] = [
     id: 'langgraph',
     title: 'LangGraph',
     description: 'LangChain/LangGraph adapter for Angular UI',
+    group: 'adapter',
+    tagline: 'Talk to LangGraph directly',
     sections: [
       {
         title: 'Getting Started',
@@ -116,6 +135,7 @@ export const docsConfig: DocsLibrary[] = [
     id: 'render',
     title: 'Render',
     description: 'Declarative UI rendering from JSON specifications',
+    group: 'library',
     sections: [
       {
         title: 'Getting Started',
@@ -165,6 +185,7 @@ export const docsConfig: DocsLibrary[] = [
     id: 'chat',
     title: 'Chat',
     description: 'Pre-built chat UI components for agent interfaces',
+    group: 'library',
     sections: [
       {
         title: 'Getting Started',
@@ -256,6 +277,8 @@ export const docsConfig: DocsLibrary[] = [
     id: 'ag-ui',
     title: 'AG-UI',
     description: 'Adapter for AG-UI-compatible backends including CrewAI, Mastra, Microsoft AF, AG2, Pydantic AI, and AWS Strands',
+    group: 'adapter',
+    tagline: 'Any AG-UI backend',
     demoUrl: 'https://ag-ui.threadplane.ai',
     sections: [
       {
@@ -314,6 +337,7 @@ export const docsConfig: DocsLibrary[] = [
     id: 'a2ui',
     title: 'A2UI',
     description: 'Protocol types and helpers for agent-driven UI surfaces',
+    group: 'library',
     sections: [
       {
         title: 'Getting Started',
@@ -349,6 +373,7 @@ export const docsConfig: DocsLibrary[] = [
     id: 'middleware',
     title: 'Middleware',
     description: 'Backend helpers for browser-executed client tools',
+    group: 'library',
     sections: [
       {
         title: 'Getting Started',
@@ -397,6 +422,7 @@ export const docsConfig: DocsLibrary[] = [
     id: 'telemetry',
     title: 'Telemetry',
     description: 'Browser and Node telemetry setup, privacy controls, and events',
+    group: 'library',
     sections: [
       {
         title: 'Getting Started',
@@ -431,6 +457,8 @@ export const docsConfig: DocsLibrary[] = [
     id: 'runtimes',
     title: 'Runtimes',
     description: 'Measured AG-UI runtime integrations behind @threadplane/ag-ui',
+    // Reference material *behind* the AG-UI adapter, not an adapter you pick.
+    group: 'library',
     sections: [
       {
         title: 'Getting Started',
diff --git a/apps/website/src/styles/docs-sidebar-styles.spec.ts b/apps/website/src/styles/docs-sidebar-styles.spec.ts
new file mode 100644
index 000000000..37f819f63
--- /dev/null
+++ b/apps/website/src/styles/docs-sidebar-styles.spec.ts
@@ -0,0 +1,42 @@
+import { readFileSync } from 'node:fs';
+import { join } from 'node:path';
+import { describe, expect, it } from 'vitest';
+
+/**
+ * The picker's title and tagline are two sibling spans. They only stack because
+ * `.docs-sidebar-lib-item-text` is a column flex container — there is no other
+ * rule keeping them apart.
+ *
+ * PR #892 moved the JSX off Tailwind onto semantic class names and dropped the
+ * `flex flex-col` utilities without porting them here. Both spans fell back to
+ * `display: inline`, every menu row rendered as one run-on line
+ * ("LangGraphLangChain/LangGraph adapter for Angular UI"), and it shipped to
+ * production unnoticed.
+ *
+ * jsdom does not apply this stylesheet, so the component tests cannot see it.
+ * This is the only guard for that failure mode.
+ */
+const css = readFileSync(join(__dirname, 'docs.css'), 'utf8');
+
+function ruleFor(selector: string): string {
+  const blocks = [...css.matchAll(/([^{}]+)\{([^{}]*)\}/g)]
+    .filter((m) => m[1].split(',').some((s) => s.trim() === selector))
+    .map((m) => m[2]);
+  return blocks.join(';');
+}
+
+describe('docs sidebar library picker styles', () => {
+  it('stacks the menu item title above its tagline', () => {
+    const rule = ruleFor('.docs-sidebar-lib-item-text');
+
+    expect(rule).toMatch(/display:\s*flex/);
+    expect(rule).toMatch(/flex-direction:\s*column/);
+  });
+
+  it('caps the menu height so it cannot run past the fold', () => {
+    const rule = ruleFor('.docs-sidebar-lib-menu');
+
+    expect(rule).toMatch(/max-height:/);
+    expect(rule).toMatch(/overflow-y:\s*auto/);
+  });
+});
diff --git a/apps/website/src/styles/docs.css b/apps/website/src/styles/docs.css
index e3dda6fb4..3b0c9f41b 100644
--- a/apps/website/src/styles/docs.css
+++ b/apps/website/src/styles/docs.css
@@ -783,6 +783,10 @@
   margin-top: 1px;
 }
 .docs-sidebar-lib-item-text {
+  /* The title and tagline are sibling spans; this column is the only thing
+   * stacking them. Losing it renders every row as one run-on line. */
+  display: flex;
+  flex-direction: column;
   min-width: 0;
 }
 .docs-sidebar-lib-item-title {
@@ -1733,16 +1737,38 @@
   inset-inline: 0;
   top: calc(100% + 4px);
   z-index: 30;
-  overflow: hidden;
+  /* The menu lives inside the pane's own scroll container, so an uncapped
+   * height runs straight past the fold. Cap it and scroll internally. */
+  max-height: 60vh;
+  overflow-y: auto;
   border-radius: 9px;
+  padding: 5px;
+}
+.docs-sidebar-lib-group {
+  display: block;
+  padding: 7px 9px 4px;
+  color: var(--color-text-muted);
+  font-family: var(--font-inter);
+  font-size: 10.5px;
+  font-weight: 700;
+  letter-spacing: 0.07em;
+  text-transform: uppercase;
+}
+.docs-sidebar-lib-divider {
+  display: block;
+  height: 1px;
+  margin: 5px 9px;
+  background: var(--color-border);
 }
 .docs-sidebar-lib-item {
   width: 100%;
-  padding: 9px 10px;
+  padding: 7px 9px;
+  border-radius: 7px;
   display: flex;
-  align-items: flex-start;
+  align-items: center;
   gap: 9px;
   text-align: left;
+  text-decoration: none;
 }
 .docs-sidebar-lib-item-title { font-family: var(--font-inter); }
 .docs-sidebar-top-links,
diff --git a/docs/superpowers/specs/2026-08-31-docs-adapter-picker-design.md b/docs/superpowers/specs/2026-08-31-docs-adapter-picker-design.md
new file mode 100644
index 000000000..177a2f573
--- /dev/null
+++ b/docs/superpowers/specs/2026-08-31-docs-adapter-picker-design.md
@@ -0,0 +1,218 @@
+# Docs adapter picker — visual and structural refresh
+
+**Date:** 2026-08-31
+**Scope:** the library picker in the docs control-plane sidebar (`LibraryDropdown`)
+**Status:** approved, ready to implement
+
+## Context
+
+PR #892 ("Unify Docs and Cockpit sidebar control planes") replaced the docs
+sidebar with a control plane: an icon rail, then named sections — **Scope**,
+**Learn**, **Environment**, **Actions**. The library picker moved into **Learn**
+but did not get the same design pass as the rest of the sidebar, and one style
+rule was lost in the migration.
+
+The picker is the control a reader uses to move between `@threadplane/langgraph`
+and `@threadplane/ag-ui`. It should read as the deliberate choice it is, in the
+vocabulary the rest of the sidebar now speaks (Inter, 12–13px, 7–9px radii,
+quiet muted labels).
+
+## Problems
+
+1. **The title and description collide.** `.docs-sidebar-lib-item-text` carries
+   only `min-width: 0`; the `flex flex-col` that stacked the two spans was
+   dropped when #892 moved the JSX off Tailwind onto semantic class names. Both
+   spans compute to `display: inline`, so every row renders as one run-on line:
+   `LangGraphLangChain/LangGraph adapter for Angular UI`.
+   **This is live on threadplane.ai** — production CSS serves
+   `.docs-sidebar-lib-item-text{min-width:0}`.
+
+2. **The menu is clipped.** Measured at a 720px viewport: menu spans y=342→902
+   inside a pane ending at y=720. **182px is off-screen.**
+
+3. **Rows are ragged.** Measured item heights: `66, 66, 66, 138, 66, 66, 90`.
+   AG-UI's 100+ character description is the 138.
+
+4. **The list is undifferentiated.** Seven entries, flat. Two are adapters; five
+   are companion libraries. The picker reads as a library index rather than an
+   adapter choice.
+
+5. **Items are not links.** `<button onClick={router.push}>` — no ⌘-click, no
+   middle-click, no open-in-new-tab.
+
+6. **The library is stated twice.** The picker sits in **Learn**; **Environment**
+   separately lists a `Library — LangGraph` row.
+
+Accessibility is *not* a problem here: #892 added `role="menu"`,
+`aria-expanded`, `aria-haspopup`, `aria-controls`, arrow keys, Home/End, Escape,
+and focus restore. That work stands and is preserved.
+
+## Design
+
+### Placement
+
+The picker **stays in Learn**, directly above the nav it rescopes. Duplication is
+resolved by removing the `Library` row from **Environment**.
+
+Considered and rejected: promoting the picker into **Scope**. Semantically exact
+— changing the library *is* changing scope, and Scope never collapses — but that
+card is a quiet 11px muted readout, and its quietness is load-bearing for the
+sidebar's calm. A 38px bordered control inside it makes one card half-control,
+half-readout. Also rejected: making it the **Environment** `Library` row —
+Environment is collapsed by default and sits at the bottom, so the primary way to
+switch adapters would hide behind a disclosure most readers never open.
+
+### Menu anatomy
+
+Two labelled groups:
+
+- **Adapters** — LangGraph, AG-UI. Mark + name + a 3–4 word tagline. ~46px rows.
+- **Libraries** — Render, Chat, A2UI, Middleware, Telemetry. Mark + name only.
+  ~34px rows.
+
+Rows are uniform *within* a group and deliberately different *between* groups;
+that difference is what makes the two adapters read as the weighted choice.
+Taglines are capped at 3–4 words so rows can never wrap ragged again.
+
+Bare names for the five libraries are intentional: "Chat", "Render", and
+"Telemetry" are self-describing, and a tagline there is the grey noise the
+current design already suffers from.
+
+### Data model — `src/lib/docs-config.ts`
+
+```ts
+export type LibraryGroup = 'adapter' | 'library';
+
+export interface DocsLibrary {
+  id: LibraryId;
+  title: string;
+  /** Long form. Fallback for the page meta description — not shown in the picker. */
+  description: string;
+  group: LibraryGroup;
+  /** Shown in the picker. Adapters only — libraries are self-describing. */
+  tagline?: string;
+  // …unchanged
+}
+```
+
+`DocsLibrary.description` is **kept**, unchanged. An earlier draft of this spec
+removed it as dead data; that was wrong. Besides the picker it is the fallback
+for each page's `<meta name="description">` via `resolveDocDescription()`
+(`src/lib/docs.ts:126`), which also feeds the page's JSON-LD. Deleting it would
+have silently changed search snippets across the docs — the exact budget tuned
+in #880. `tagline` is added *alongside* it: `description` is long-form metadata,
+`tagline` is the short picker string.
+
+Every `description` string stays byte-identical to `main`. Group assignment:
+
+| Library    | Group   | Tagline                      |
+| ---------- | ------- | ---------------------------- |
+| LangGraph  | adapter | Talk to LangGraph directly   |
+| AG-UI      | adapter | Any AG-UI backend            |
+| Render     | library | —                            |
+| Chat       | library | —                            |
+| A2UI       | library | —                            |
+| Middleware | library | —                            |
+| Telemetry  | library | —                            |
+
+### Markup — `src/components/docs/DocsSidebar.tsx`
+
+```tsx
+<div role="menu" aria-labelledby={triggerId}>
+  <div role="group" aria-label="Adapters">
+    <span aria-hidden="true" className="docs-sidebar-lib-group">Adapters</span>
+    <Link
+      href={libraryIntroPath(library.id)}
+      role="menuitemradio"
+      aria-checked={isActive}
+      …
+    >
+```
+
+- **`<Link href>` replaces `<button onClick={router.push}>`.** Restores
+  ⌘-click / middle-click / new-tab. Uses the existing `libraryIntroPath()`
+  helper instead of an inline template string.
+- **`role="menuitemradio"` + `aria-checked`** — single-select from a set, which
+  is what the checkmark means.
+- **`role="group"` + `aria-label`** — makes the visual group labels programmatic
+  rather than decorative.
+
+**Migration hazard:** the keyboard handler queries
+`querySelectorAll<HTMLButtonElement>('[role="menuitem"]')`. Both the selector and
+the element type must change (`[role="menuitemradio"]`, `HTMLElement`) or arrow
+keys, Home/End and Escape silently stop finding items — a failure whose mode is
+silence, so it must be covered by a test that fails before the fix.
+
+### Styles — `src/styles/docs.css`
+
+```css
+.docs-sidebar-lib-item-text {
+  display: flex;
+  flex-direction: column;
+  min-width: 0;
+}
+.docs-sidebar-lib-menu {
+  max-height: 60vh;   /* replaces overflow: hidden — floor only, see below */
+  overflow-y: auto;
+}
+```
+
+**A CSS cap alone is not enough**, and an earlier draft of this spec claimed
+otherwise. The menu opens ~342px down the pane, so at a 600px viewport `60vh`
+(360px) still puts its bottom edge at 702px — **102px past the fold**, measured.
+A viewport percentage cannot account for a large fixed top offset.
+
+So the real cap is measured from the trigger, in an effect that runs on open and
+on resize:
+
+```ts
+const available = window.innerHeight - trigger.getBoundingClientRect().bottom - 16;
+menu.style.maxHeight = `${Math.max(180, available)}px`;
+```
+
+Verified at a 600px viewport: the menu caps to 245px, its bottom lands 12px
+inside the viewport, it scrolls internally, and Telemetry — the last entry —
+stays reachable. The CSS `max-height` remains as a pre-hydration floor.
+
+### Environment — `src/components/docs/DocsControlPlane.tsx`
+
+Remove the `Library` row from `environmentRows`. `Framework` and
+`Package manager` stay.
+
+## Testing
+
+Three existing tests will fail and should:
+
+- `DocsControlPlane.spec.tsx` → `'shows truthful scope and collapsed environment
+  defaults'` — asserts the removed `Library` row.
+- `DocsControlPlane.spec.tsx` → `'supports keyboard entry and dismissal for the
+  library menu'` — assumes button elements and the `[role="menuitem"]` selector.
+- `Nav.spec.tsx` → `'keeps the drawer open when Escape dismisses the nested
+  library menu'` — the mobile drawer renders the same menu, so it asserts the
+  same role.
+
+New coverage:
+
+1. **Title and tagline render on separate lines** — a regression test for the
+   collision. Must fail against current `main`.
+2. **Keyboard navigation still traverses items after the anchor migration** —
+   guards the silent-selector hazard above.
+3. **Groups are labelled** — `role="group"` with accessible names
+   "Adapters" / "Libraries".
+4. **Items are anchors with real hrefs** — guards the ⌘-click regression.
+5. **The menu carries a measured `max-height` when open** — guards the cap,
+   whose absence is invisible until someone opens the picker in a short window.
+6. **A CSS-level guard** (`src/styles/docs-sidebar-styles.spec.ts`) asserting
+   both new rules exist. jsdom does not apply the stylesheet, so no component
+   test can see the collision — this is the only guard for the failure mode that
+   actually reached production.
+
+## Out of scope
+
+Deliberately deferred, both worth doing separately:
+
+- `/docs/choosing-an-adapter` renders without the control plane, so following
+  that link drops the reader into a page with no nav.
+- There is no docs navigation below the `lg` breakpoint.
+
+Neither is about the picker; folding them in would blur what this change is.