diff --git a/gitnexus/src/storage/shared-store.ts b/gitnexus/src/storage/shared-store.ts
new file mode 100644
index 000000000..0b76e13f5
--- /dev/null
+++ b/gitnexus/src/storage/shared-store.ts
@@ -0,0 +1,164 @@
+/**
+ * Shared sibling index store (#3352).
+ *
+ * Linked worktrees of one repository share one store under the GitNexus home:
+ *
+ *   <GITNEXUS_HOME>/stores/<key>/
+ *     caches/                         parse-cache + durable ParsedFile store
+ *     commits/<commit>-<featureKey>/  one immutable graph per commit + settings
+ *     checkouts/<slot>/               one checkout's metadata, membership, and
+ *                                     private graph when it has local edits
+ *
+ * This module only resolves identity and names paths. It never creates,
+ * writes, or deletes anything.
+ *
+ * Membership is decided from the `.git` entry alone (no `git` subprocess), so
+ * the resolver stays cheap on hot paths (hooks, every CLI call). Only tree
+ * roots participate — a subdirectory of a checkout never resolves to a store,
+ * mirroring the `resolveRepoIdentityRoot` gate (#1259). A repository with no
+ * linked worktree keeps its repository-local `.gitnexus`.
+ */
+
+import fs from 'fs';
+import path from 'path';
+import { stripWindowsLongPathPrefix } from '../lib/utils.js';
+import { getGlobalDir } from './global-dir.js';
+import { slotNameForCanonicalPath, STORAGE_PATH_ENV, STORAGE_ROOT_ENV } from './storage-slot.js';
+
+export const SHARED_STORE_ENV = 'GITNEXUS_SHARED_STORE';
+export const STORES_DIR = 'stores';
+
+// Same canonical form as storage-resolver's `storageSlotName`, so a checkout's
+// slot name does not depend on which spelling (symlink, 8.3 name) reached it.
+const slotName = (p: string): string => {
+  const resolved = path.resolve(p);
+  let canonical: string;
+  try {
+    canonical = fs.realpathSync.native(resolved);
+  } catch {
+    canonical = resolved;
+  }
+  return slotNameForCanonicalPath(stripWindowsLongPathPrefix(canonical));
+};
+
+const DISABLED_VALUES = new Set(['off', '0', 'false', 'no']);
+const COMMIT_RE = /^[0-9a-f]{7,64}$/;
+const FEATURE_KEY_RE = /^[0-9a-f]{8,64}$/;
+
+export interface SharedStoreLayout {
+  /** Store key: readable basename plus a hash of the canonical git common dir. */
+  key: string;
+  root: string;
+  cachesDir: string;
+  commitsDir: string;
+  checkoutsDir: string;
+  /** This checkout's slot — the registry `storagePath` for a shared checkout. */
+  checkoutSlot: string;
+}
+
+/** Sharing is off globally, or an explicit storage env override takes precedence. */
+export const isSharedStoreDisabled = (env: NodeJS.ProcessEnv = process.env): boolean => {
+  const value = env[SHARED_STORE_ENV];
+  if (value !== undefined && DISABLED_VALUES.has(value.trim().toLowerCase())) return true;
+  return env[STORAGE_PATH_ENV] !== undefined || env[STORAGE_ROOT_ENV] !== undefined;
+};
+
+const hasLinkedWorktrees = (commonDir: string): boolean => {
+  try {
+    return fs.readdirSync(path.join(commonDir, 'worktrees')).length > 0;
+  } catch {
+    return false;
+  }
+};
+
+/**
+ * Resolve the git common dir for a tree root, or null when `checkoutPath` is
+ * not a tree root (non-git folder or an arbitrary subdirectory).
+ */
+const readCommonDir = (checkoutPath: string): string | null => {
+  const dotGit = path.join(checkoutPath, '.git');
+  let stat: fs.Stats;
+  try {
+    stat = fs.statSync(dotGit);
+  } catch {
+    return null;
+  }
+  if (stat.isDirectory()) return dotGit;
+  if (!stat.isFile()) return null;
+
+  // Linked worktree: `.git` is a file `gitdir: <common>/worktrees/<name>`, and
+  // that per-worktree dir holds a `commondir` file pointing back at <common>.
+  let gitDir: string;
+  try {
+    const match = /^gitdir:\s*(.+?)\s*$/m.exec(fs.readFileSync(dotGit, 'utf-8'));
+    if (!match) return null;
+    gitDir = path.resolve(checkoutPath, match[1]);
+  } catch {
+    return null;
+  }
+  try {
+    const common = fs.readFileSync(path.join(gitDir, 'commondir'), 'utf-8').trim();
+    return path.resolve(gitDir, common);
+  } catch {
+    // Submodules also use a `gitdir:` file but have no `commondir`; they are
+    // standalone repositories, not linked worktrees.
+    return null;
+  }
+};
+
+/**
+ * Store key for a checkout, or null when the checkout does not share.
+ * Main checkout and every linked worktree of one repository get the same key.
+ */
+export const resolveSharedStoreKey = (
+  checkoutPath: string,
+  env: NodeJS.ProcessEnv = process.env,
+): string | null => {
+  if (isSharedStoreDisabled(env)) return null;
+  const commonDir = readCommonDir(path.resolve(checkoutPath));
+  if (!commonDir || !hasLinkedWorktrees(commonDir)) return null;
+  // `<repo>/.git` keys on `<repo>` for a readable name; a bare common dir
+  // (`repo.git`) keys on itself. Both hash the canonical absolute path.
+  const identity = path.basename(commonDir) === '.git' ? path.dirname(commonDir) : commonDir;
+  return slotName(identity);
+};
+
+/** Name every store path for `checkoutPath` under store `key`. */
+export const sharedStoreLayout = (key: string, checkoutPath: string): SharedStoreLayout => {
+  const storesRoot = path.join(getGlobalDir(), STORES_DIR);
+  const root = path.resolve(storesRoot, key);
+  if (path.dirname(root) !== path.resolve(storesRoot)) {
+    throw new Error(`Shared store key escapes the stores directory: ${key}`);
+  }
+  const checkoutsDir = path.join(root, 'checkouts');
+  return {
+    key,
+    root,
+    cachesDir: path.join(root, 'caches'),
+    commitsDir: path.join(root, 'commits'),
+    checkoutsDir,
+    checkoutSlot: path.join(checkoutsDir, slotName(checkoutPath)),
+  };
+};
+
+/** Resolve the full layout for a checkout, or null when it does not share. */
+export const resolveSharedStore = (
+  checkoutPath: string,
+  env: NodeJS.ProcessEnv = process.env,
+): SharedStoreLayout | null => {
+  const key = resolveSharedStoreKey(checkoutPath, env);
+  return key ? sharedStoreLayout(key, checkoutPath) : null;
+};
+
+/** Directory of the immutable graph for one commit and feature key. */
+export const commitGraphDir = (
+  layout: SharedStoreLayout,
+  commit: string,
+  featureKey: string,
+): string => {
+  if (!COMMIT_RE.test(commit)) throw new Error(`Invalid commit id for shared store: ${commit}`);
+  if (!FEATURE_KEY_RE.test(featureKey)) {
+    throw new Error(`Invalid feature key for shared store: ${featureKey}`);
+  }
+  return path.join(layout.commitsDir, `${commit}-${featureKey}`);
+};
diff --git a/gitnexus/src/storage/storage-resolver.ts b/gitnexus/src/storage/storage-resolver.ts
index b44dcc4ad..329759c36 100644
--- a/gitnexus/src/storage/storage-resolver.ts
+++ b/gitnexus/src/storage/storage-resolver.ts
@@ -1,4 +1,3 @@
-import { createHash } from 'node:crypto';
 import fs from 'fs';
 import fsp from 'fs/promises';
 import path from 'path';
@@ -10,11 +9,9 @@ import {
   LEGACY_METADATA_FILE,
   LBUG_DIRECTORY,
 } from './storage-constants.js';
+import { slotNameForCanonicalPath, STORAGE_PATH_ENV, STORAGE_ROOT_ENV } from './storage-slot.js';
 
-export const STORAGE_PATH_ENV = 'GITNEXUS_STORAGE_PATH';
-export const STORAGE_ROOT_ENV = 'GITNEXUS_STORAGE_ROOT';
-
-const STORAGE_SLOT_HASH_LENGTH = 12;
+export { STORAGE_PATH_ENV, STORAGE_ROOT_ENV };
 
 /** File-backend lock sidecars (`index-lock.ts`). Not ownership data. */
 const INDEX_LOCK_ARTIFACTS = new Set(['analyze.lock', 'analyze.lock.guard']);
@@ -211,37 +208,13 @@ const comparablePath = (value: string): string => {
   return process.platform === 'win32' ? canonical.toLowerCase() : canonical;
 };
 
-const sanitizeSlotBasename = (value: string): string => {
-  // Linear: a quantified `/[. ]+$/` on attacker-controlled basenames is
-  // js/polynomial-redos (CodeQL #1056). Cap first, then walk the tail once.
-  const sanitized = value.replace(/[\u0000-\u001f<>:"/\\|?*]/g, '-').slice(0, 80);
-  let end = sanitized.length;
-  while (end > 0) {
-    const code = sanitized.charCodeAt(end - 1);
-    if (code !== 0x20 && code !== 0x2e) break;
-    end--;
-  }
-  const candidate = sanitized.slice(0, end) || 'repository';
-  return /^(con|prn|aux|nul|com[1-9]|lpt[1-9])$/i.test(candidate)
-    ? `repository-${candidate}`
-    : candidate;
-};
-
 /**
  * Stable slot name for one checkout inside a configured external storage root.
  * The canonical absolute path prevents symlink aliases from creating duplicate
  * slots, while the hash keeps same-basename repositories isolated.
  */
-export const storageSlotName = (repoPath: string): string => {
-  const canonical = canonicalRepoPath(repoPath);
-  const identity = process.platform === 'win32' ? canonical.toLowerCase() : canonical;
-  const basename = sanitizeSlotBasename(path.basename(canonical));
-  const digest = createHash('sha256')
-    .update(identity)
-    .digest('hex')
-    .slice(0, STORAGE_SLOT_HASH_LENGTH);
-  return `${basename}-${digest}`;
-};
+export const storageSlotName = (repoPath: string): string =>
+  slotNameForCanonicalPath(canonicalRepoPath(repoPath));
 
 export const defaultStoragePath = (repoPath: string): string =>
   path.join(resolveRepoPath(repoPath), GITNEXUS_DIR);
diff --git a/gitnexus/src/storage/storage-slot.ts b/gitnexus/src/storage/storage-slot.ts
new file mode 100644
index 000000000..063e2fddd
--- /dev/null
+++ b/gitnexus/src/storage/storage-slot.ts
@@ -0,0 +1,46 @@
+/**
+ * Leaf naming primitives for external index slots.
+ *
+ * Kept free of imports from `storage-resolver.ts` and `shared-store.ts` so both
+ * can use them without importing each other (#3352). `storage-resolver.ts`
+ * re-exports everything here, so existing import sites are unchanged.
+ */
+
+import { createHash } from 'node:crypto';
+import path from 'path';
+
+export const STORAGE_PATH_ENV = 'GITNEXUS_STORAGE_PATH';
+export const STORAGE_ROOT_ENV = 'GITNEXUS_STORAGE_ROOT';
+
+const STORAGE_SLOT_HASH_LENGTH = 12;
+
+const sanitizeSlotBasename = (value: string): string => {
+  // Linear: a quantified `/[. ]+$/` on attacker-controlled basenames is
+  // js/polynomial-redos (CodeQL #1056). Cap first, then walk the tail once.
+  const sanitized = value.replace(/[\u0000-\u001f<>:"/\\|?*]/g, '-').slice(0, 80);
+  let end = sanitized.length;
+  while (end > 0) {
+    const code = sanitized.charCodeAt(end - 1);
+    if (code !== 0x20 && code !== 0x2e) break;
+    end--;
+  }
+  const candidate = sanitized.slice(0, end) || 'repository';
+  return /^(con|prn|aux|nul|com[1-9]|lpt[1-9])$/i.test(candidate)
+    ? `repository-${candidate}`
+    : candidate;
+};
+
+/**
+ * Slot name for an already-canonical absolute path: sanitized basename plus a
+ * short hash of the path (case-folded on Windows) so same-basename paths stay
+ * isolated.
+ */
+export const slotNameForCanonicalPath = (canonical: string): string => {
+  const identity = process.platform === 'win32' ? canonical.toLowerCase() : canonical;
+  const basename = sanitizeSlotBasename(path.basename(canonical));
+  const digest = createHash('sha256')
+    .update(identity)
+    .digest('hex')
+    .slice(0, STORAGE_SLOT_HASH_LENGTH);
+  return `${basename}-${digest}`;
+};
diff --git a/gitnexus/test/unit/storage/shared-store.test.ts b/gitnexus/test/unit/storage/shared-store.test.ts
new file mode 100644
index 000000000..da110ad97
--- /dev/null
+++ b/gitnexus/test/unit/storage/shared-store.test.ts
@@ -0,0 +1,198 @@
+import { execFileSync } from 'child_process';
+import fs from 'fs/promises';
+import os from 'os';
+import path from 'path';
+import { afterEach, beforeEach, describe, expect, it } from 'vitest';
+import {
+  commitGraphDir,
+  isSharedStoreDisabled,
+  resolveSharedStore,
+  resolveSharedStoreKey,
+  SHARED_STORE_ENV,
+  sharedStoreLayout,
+  type SharedStoreLayout,
+} from '../../../src/storage/shared-store.js';
+import {
+  STORAGE_PATH_ENV,
+  STORAGE_ROOT_ENV,
+  storageSlotName,
+} from '../../../src/storage/storage-resolver.js';
+
+const temporaryPaths: string[] = [];
+const savedHome = process.env.GITNEXUS_HOME;
+let home: string;
+
+const makeTempDir = async (prefix: string): Promise<string> => {
+  const dir = await fs.realpath(await fs.mkdtemp(path.join(os.tmpdir(), prefix)));
+  temporaryPaths.push(dir);
+  return dir;
+};
+
+const git = (cwd: string, ...args: string[]): void => {
+  execFileSync('git', args, { cwd, stdio: 'ignore' });
+};
+
+/** A committed repo; `worktrees` names linked worktrees created beside it. */
+const makeRepo = async (worktrees: string[] = []): Promise<{ main: string; wts: string[] }> => {
+  const parent = await makeTempDir('gn-shared-store-');
+  const main = path.join(parent, 'main');
+  await fs.mkdir(main);
+  git(main, 'init', '-q', '-b', 'main');
+  git(
+    main,
+    '-c',
+    'user.email=t@t',
+    '-c',
+    'user.name=t',
+    'commit',
+    '-q',
+    '--allow-empty',
+    '-m',
+    'init',
+  );
+  const wts = worktrees.map((name) => {
+    const wt = path.join(parent, name);
+    git(main, 'worktree', 'add', '-q', '-b', name, wt);
+    return wt;
+  });
+  return { main, wts };
+};
+
+// Only these keys are read by isSharedStoreDisabled/resolveSharedStoreKey.
+const cleanEnv = (): NodeJS.ProcessEnv => ({});
+
+const layoutOf = (checkoutPath: string): SharedStoreLayout => {
+  const layout = resolveSharedStore(checkoutPath, cleanEnv());
+  expect(layout).not.toBeNull();
+  return layout as SharedStoreLayout;
+};
+
+beforeEach(async () => {
+  home = await makeTempDir('gn-shared-home-');
+  process.env.GITNEXUS_HOME = home;
+});
+
+afterEach(async () => {
+  if (savedHome === undefined) delete process.env.GITNEXUS_HOME;
+  else process.env.GITNEXUS_HOME = savedHome;
+  await Promise.all(
+    temporaryPaths.splice(0).map((p) => fs.rm(p, { recursive: true, force: true })),
+  );
+});
+
+describe('resolveSharedStoreKey', () => {
+  it('gives the main checkout and every linked worktree the same key', async () => {
+    const { main, wts } = await makeRepo(['wt-a', 'wt-b']);
+    const keys = [main, ...wts].map((p) => resolveSharedStoreKey(p, cleanEnv()));
+    expect(keys[0]).toMatch(/^main-[0-9a-f]{12}$/);
+    expect(new Set(keys).size).toBe(1);
+  });
+
+  it('keeps a repository without linked worktrees on local storage', async () => {
+    const { main } = await makeRepo();
+    expect(resolveSharedStoreKey(main, cleanEnv())).toBeNull();
+  });
+
+  it('gives two unrelated repos with the same basename different keys', async () => {
+    const a = await makeRepo(['wt']);
+    const b = await makeRepo(['wt']);
+    const keyA = resolveSharedStoreKey(a.main, cleanEnv());
+    const keyB = resolveSharedStoreKey(b.main, cleanEnv());
+    expect(keyA).not.toBeNull();
+    expect(keyA).not.toBe(keyB);
+  });
+
+  it('does not share a subdirectory of a checkout', async () => {
+    const { main } = await makeRepo(['wt']);
+    const sub = path.join(main, 'pkg');
+    await fs.mkdir(sub);
+    expect(resolveSharedStoreKey(sub, cleanEnv())).toBeNull();
+  });
+
+  it('does not share a non-git folder', async () => {
+    const dir = await makeTempDir('gn-shared-nogit-');
+    expect(resolveSharedStoreKey(dir, cleanEnv())).toBeNull();
+  });
+
+  it('does not treat a gitdir file without commondir (submodule shape) as a worktree', async () => {
+    const dir = await makeTempDir('gn-shared-submodule-');
+    const modules = path.join(dir, 'modules', 'sub');
+    await fs.mkdir(modules, { recursive: true });
+    await fs.writeFile(path.join(dir, '.git'), `gitdir: ${modules}\n`);
+    expect(resolveSharedStoreKey(dir, cleanEnv())).toBeNull();
+  });
+
+  it.each([
+    [{ [SHARED_STORE_ENV]: 'off' }],
+    [{ [SHARED_STORE_ENV]: 'FALSE' }],
+    [{ [SHARED_STORE_ENV]: '0' }],
+    [{ [STORAGE_PATH_ENV]: '/tmp/explicit-index' }],
+    [{ [STORAGE_ROOT_ENV]: '/tmp/index-root' }],
+  ])('returns null for every checkout when disabled by %o', async (env) => {
+    const { main, wts } = await makeRepo(['wt']);
+    expect(isSharedStoreDisabled(env)).toBe(true);
+    expect(resolveSharedStoreKey(main, env)).toBeNull();
+    expect(resolveSharedStoreKey(wts[0], env)).toBeNull();
+  });
+
+  it('treats an unrecognized switch value as enabled', () => {
+    expect(isSharedStoreDisabled({ [SHARED_STORE_ENV]: 'on' })).toBe(false);
+  });
+});
+
+describe('sharedStoreLayout', () => {
+  it('places every area inside the store under GITNEXUS_HOME', async () => {
+    const { main, wts } = await makeRepo(['wt']);
+    const layout = layoutOf(wts[0]);
+    const root = path.join(home, 'stores', layout.key);
+    expect(layout).toEqual({
+      key: resolveSharedStoreKey(main, cleanEnv()),
+      root,
+      cachesDir: path.join(root, 'caches'),
+      commitsDir: path.join(root, 'commits'),
+      checkoutsDir: path.join(root, 'checkouts'),
+      checkoutSlot: path.join(root, 'checkouts', storageSlotName(wts[0])),
+    });
+  });
+
+  it('gives each checkout its own slot', async () => {
+    const { main, wts } = await makeRepo(['wt']);
+    const a = layoutOf(main);
+    const b = layoutOf(wts[0]);
+    expect(a.root).toBe(b.root);
+    expect(a.checkoutSlot).not.toBe(b.checkoutSlot);
+  });
+
+  it('maps a symlinked spelling of a worktree to the same slot', async () => {
+    const { wts } = await makeRepo(['wt']);
+    const link = path.join(await makeTempDir('gn-shared-link-'), 'alias');
+    await fs.symlink(wts[0], link);
+    expect(layoutOf(link).checkoutSlot).toBe(layoutOf(wts[0]).checkoutSlot);
+  });
+
+  it.each(['..', '../escape', 'a/../../b'])(
+    'rejects a key that escapes the stores dir: %s',
+    (key) => {
+      expect(() => sharedStoreLayout(key, '/tmp/x')).toThrow(/escapes the stores directory/);
+    },
+  );
+});
+
+describe('commitGraphDir', () => {
+  const layout = sharedStoreLayout('repo-0123456789ab', '/tmp/checkout');
+
+  it('names one directory per commit and feature key', () => {
+    expect(commitGraphDir(layout, 'abc1234', 'deadbeef')).toBe(
+      path.join(layout.commitsDir, 'abc1234-deadbeef'),
+    );
+  });
+
+  it.each([
+    ['../../x', 'deadbeef'],
+    ['ABC1234', 'deadbeef'],
+    ['abc1234', '../etc'],
+    ['abc1234', 'short'],
+  ])('rejects commit %s / feature key %s', (commit, featureKey) => {
+    expect(() => commitGraphDir(layout, commit, featureKey)).toThrow(/Invalid/);
+  });
+});