diff --git a/gitnexus/src/core/shared-store-analyze.ts b/gitnexus/src/core/shared-store-analyze.ts
index 0cffb4dfc..ed33888ac 100644
--- a/gitnexus/src/core/shared-store-analyze.ts
+++ b/gitnexus/src/core/shared-store-analyze.ts
@@ -491,10 +491,14 @@ const registeredStore = (
 /**
  * Store for a clone with no store of its own: the store a registered sibling
  * clone (same normalized `origin` URL, checkout still present) already uses,
- * or, when siblings exist but none shares yet, a new store keyed on this
- * clone. Graphs are keyed by commit and feature key, so clones only ever share
- * a graph built from the same commit with the same settings. A lone clone
- * keeps its repository-local index.
+ * or, when siblings exist but none shares yet, a new store keyed on the
+ * canonical path that sorts first among this clone and its siblings. Every
+ * sibling computes that same founder key, so clones founding the store
+ * concurrently still land in one store (#3374); the key is only a name, so the
+ * store keeps working if the founder's checkout is later deleted. Graphs are
+ * keyed by commit and feature key, so clones only ever share a graph built
+ * from the same commit with the same settings. A lone clone keeps its
+ * repository-local index.
  */
 const siblingCloneStore = (
   entries: readonly RegistryEntry[],
@@ -515,7 +519,14 @@ const siblingCloneStore = (
     .filter((root): root is string => root !== null)
     .map((root) => path.basename(root))
     .sort();
-  return sharedStoreLayout(keys[0] ?? cloneStoreKey(repoPath), repoPath);
+  if (keys[0]) return sharedStoreLayout(keys[0], repoPath);
+  // Order the way the registry compares paths: case-folded on Windows, whose
+  // slot names hash the folded form too.
+  const fold = (p: string): string => (registryPathEquals('A', 'a') ? p.toLowerCase() : p);
+  const founder = [self, ...siblings.map((e) => canonicalizePath(e.path))].reduce((a, b) =>
+    fold(b) < fold(a) ? b : a,
+  );
+  return sharedStoreLayout(cloneStoreKey(founder), repoPath);
 };
 
 /**
diff --git a/gitnexus/test/integration/shared-store-clone-optin.test.ts b/gitnexus/test/integration/shared-store-clone-optin.test.ts
index e175d418b..150510b61 100644
--- a/gitnexus/test/integration/shared-store-clone-optin.test.ts
+++ b/gitnexus/test/integration/shared-store-clone-optin.test.ts
@@ -3,8 +3,19 @@ import { existsSync } from 'fs';
 import fs from 'fs/promises';
 import path from 'path';
 import { afterAll, afterEach, beforeAll, beforeEach, describe, expect, it } from 'vitest';
-import { getStoragePaths, listRegisteredRepos } from '../../src/storage/repo-manager.js';
-import { resolveSharedStore, type SharedStoreLayout } from '../../src/storage/shared-store.js';
+import { resolveOptedInStore } from '../../src/core/shared-store-analyze.js';
+import { getRemoteUrl } from '../../src/storage/git.js';
+import {
+  getStoragePaths,
+  listRegisteredRepos,
+  registerRepo,
+} from '../../src/storage/repo-manager.js';
+import {
+  cloneStoreKey,
+  resolveSharedStore,
+  sharedStoreLayout,
+  type SharedStoreLayout,
+} from '../../src/storage/shared-store.js';
 import { createTempDir } from '../helpers/test-db.js';
 
 // These suites exercise sharing; an inherited opt-out would silently disable it.
@@ -225,3 +236,77 @@ describe('shared store clone sharing (#3352)', () => {
     );
   }, 240_000);
 });
+
+/**
+ * #3374 S7 — registered sibling clones that found a store at the same time
+ * must pick the same key, or each keeps its own store forever. Resolves the
+ * store directly (no analyze): only the registry and the clones' remotes matter.
+ */
+describe('shared store founder key for concurrent sibling clones (#3374)', () => {
+  let tmpHome: Awaited<ReturnType<typeof createTempDir>>;
+  let tmpRepo: Awaited<ReturnType<typeof createTempDir>>;
+  let savedHome: string | undefined;
+  let root: string;
+
+  /** Clone `source` as `name` and register it at local storage, or at `storeKey`'s slot. */
+  const cloneAndRegister = async (name: string, storeKey?: string): Promise<string> => {
+    const clone = path.join(root, name);
+    git(root, 'clone', '-q', path.join(root, 'source'), clone);
+    git(clone, 'remote', 'set-url', 'origin', REMOTE);
+    const storagePath = storeKey ? sharedStoreLayout(storeKey, clone).checkoutSlot : undefined;
+    await registerRepo(
+      clone,
+      {
+        repoPath: clone,
+        storagePath,
+        lastCommit: git(clone, 'rev-parse', 'HEAD'),
+        indexedAt: new Date(0).toISOString(),
+        remoteUrl: getRemoteUrl(clone),
+      },
+      storagePath ? { storagePath } : undefined,
+    );
+    return clone;
+  };
+
+  beforeEach(async () => {
+    tmpHome = await createTempDir('gitnexus-test-founder-home-');
+    tmpRepo = await createTempDir('gitnexus-test-founder-repo-');
+    savedHome = process.env.GITNEXUS_HOME;
+    process.env.GITNEXUS_HOME = tmpHome.dbPath;
+    root = await fs.realpath(tmpRepo.dbPath);
+    const source = path.join(root, 'source');
+    await fs.mkdir(source);
+    git(source, 'init', '-q', '-b', 'main');
+    await fs.writeFile(path.join(source, 'a.ts'), 'export const a = 1;\n');
+    git(source, 'add', '-A');
+    git(source, '-c', 'user.name=t', '-c', 'user.email=t@t', 'commit', '-q', '-m', 'init');
+  });
+
+  afterEach(async () => {
+    if (savedHome === undefined) delete process.env.GITNEXUS_HOME;
+    else process.env.GITNEXUS_HOME = savedHome;
+    await tmpRepo.cleanup();
+    await tmpHome.cleanup();
+  });
+
+  it('two registered local clones resolve the same new store, keyed on the first path', async () => {
+    // Registered in reverse path order so registry order cannot pick the key.
+    const second = await cloneAndRegister('zeta');
+    const first = await cloneAndRegister('alpha');
+
+    const fromFirst = await resolveOptedInStore(first, undefined);
+    const fromSecond = await resolveOptedInStore(second, undefined);
+
+    expect(fromFirst?.key).toBe(cloneStoreKey(first));
+    expect(fromSecond?.key).toBe(cloneStoreKey(first));
+  });
+
+  it('a later clone joins the existing store rather than founding one', async () => {
+    const existing = cloneStoreKey(path.join(root, 'zz-founder'));
+    await cloneAndRegister('member', existing);
+    await cloneAndRegister('other');
+
+    const joiner = await cloneAndRegister('aaa-joiner');
+    expect((await resolveOptedInStore(joiner, undefined))?.key).toBe(existing);
+  });
+});