diff --git a/.changeset/tenancy-aware-endpoint-bind.md b/.changeset/tenancy-aware-endpoint-bind.md
new file mode 100644
index 000000000..da2dbec02
--- /dev/null
+++ b/.changeset/tenancy-aware-endpoint-bind.md
@@ -0,0 +1,21 @@
+---
+"@objectstack/plugin-auth": patch
+---
+
+fix(auth): create-user membership bind is tenancy-mode-aware; export the ADR-0093 host API
+
+Multi-org runtime verification (real `@objectstack/organizations` linked into a
+live stack) caught a gap in the #2884 endpoint bind: it resolved its target org
+via `resolveDefaultOrgId` (slug='default' first), so in a multi-org deployment —
+where the bootstrap default org coexists with real tenant orgs — `/admin/create-user`
+would have bound the new user into the default org, violating ADR-0093 D3
+("the framework never guesses in multi mode"). The bind now consults the
+`tenancy` service (`getTenancy` on the endpoint deps): single mode → default org,
+multi mode → no bind. Verified live: multi-org create-user and sign-up both leave
+the new user member-less (invites / host hooks own membership there); single-org
+behavior unchanged.
+
+Also exports `reconcile-membership` and `tenancy-service` from the package index
+as the public host API, and adds dogfood integration tests driving the REAL
+better-auth pipeline: sign-up membership via the reconciler hook alone, backfill
+bind + idempotency, invite-only refusal, and the yield-to-host-membership rule.
diff --git a/packages/dogfood/package.json b/packages/dogfood/package.json
index 6cb4fe587..e97da66f0 100644
--- a/packages/dogfood/package.json
+++ b/packages/dogfood/package.json
@@ -12,6 +12,7 @@
     "@objectstack/example-crm": "workspace:*",
     "@objectstack/example-showcase": "workspace:*",
     "@objectstack/objectql": "workspace:*",
+    "@objectstack/plugin-auth": "workspace:*",
     "@objectstack/plugin-security": "workspace:*",
     "@objectstack/spec": "workspace:*",
     "@objectstack/verify": "workspace:*"
diff --git a/packages/dogfood/test/membership-reconciler.dogfood.test.ts b/packages/dogfood/test/membership-reconciler.dogfood.test.ts
new file mode 100644
index 000000000..df2c56c29
--- /dev/null
+++ b/packages/dogfood/test/membership-reconciler.dogfood.test.ts
@@ -0,0 +1,154 @@
+// Copyright (c) 2025 ObjectStack. Licensed under the Apache-2.0 license.
+
+/**
+ * ADR-0093 — membership lifecycle, end-to-end against a real stack.
+ *
+ * The original bug (#2882): user-creation paths outside better-auth's org
+ * flows (`/admin/create-user`, sign-up) never wrote a `sys_member` row, so in
+ * single-org mode those users didn't belong to the Default Organization.
+ * ADR-0093 D2 gives the invariant ONE owner — a reconciler composed into
+ * better-auth's `user.create.after` hook.
+ *
+ * Harness note: `bootStack` deliberately disables the default-org bootstrap
+ * (`autoDefaultOrganization: false` — the ADR-0057 "single-tenant, no org row"
+ * posture) and does not enable the better-auth admin plugin. So these tests
+ * mint the single-org Default Organization themselves (system context, exactly
+ * what the bootstrap would do) and drive the invariant through the REAL
+ * better-auth sign-up pipeline — the path with no endpoint-side bind, where a
+ * membership can only come from the `user.create.after` reconciler. The
+ * endpoint-side create-user bind has its own unit coverage
+ * (plugin-auth/src/admin-user-endpoints.test.ts).
+ */
+
+import { describe, it, expect, beforeAll, afterAll } from 'vitest';
+import showcaseStack from '@objectstack/example-showcase';
+import { bootStack, type VerifyStack } from '@objectstack/verify';
+import { backfillMemberships, reconcileMembership } from '@objectstack/plugin-auth';
+
+const SYSTEM_CTX = { isSystem: true };
+
+async function findRows(ql: any, object: string, where: any, limit = 50): Promise<any[]> {
+  const rows = await ql.find(object, { where, limit }, { context: SYSTEM_CTX });
+  return Array.isArray(rows) ? rows : (rows?.records ?? []);
+}
+
+describe('ADR-0093: membership lifecycle (single-org, real stack)', () => {
+  let stack: VerifyStack;
+  let ql: any;
+  let defaultOrgId: string;
+
+  beforeAll(async () => {
+    stack = await bootStack(showcaseStack, {}); // single-org (no OS_MULTI_ORG_ENABLED)
+    await stack.signIn();
+    ql = await stack.kernel.getServiceAsync<any>('objectql');
+    // Mint the Default Organization the single-org bootstrap would create
+    // (the harness disables that bootstrap — see header). System context is
+    // the legitimate writer for better-auth-managed tables (ADR-0092).
+    const org = await ql.insert(
+      'sys_organization',
+      { name: 'Default Organization', slug: 'default' },
+      { context: SYSTEM_CTX },
+    );
+    defaultOrgId = String(org.id);
+  }, 120_000);
+
+  afterAll(async () => { await stack?.stop?.(); });
+
+  it('sign-up produces a membership through the user.create.after reconciler alone (D2, e2e)', async () => {
+    // No endpoint bind exists on the sign-up path — a membership here can ONLY
+    // come from the reconciler hook consulting tenancy.defaultOrgId().
+    const token = await stack.signUp('reconciled.member@example.com', 'SignUp!Pass123', 'Reconciled Member');
+    expect(token).toBeTruthy();
+    const users = await findRows(ql, 'sys_user', { email: 'reconciled.member@example.com' }, 1);
+    expect(users.length).toBe(1);
+    const userId: string = users[0].id;
+
+    // better-auth defers user.create.after past the signup transaction —
+    // poll briefly before asserting.
+    let members: any[] = [];
+    for (let i = 0; i < 40 && members.length === 0; i++) {
+      members = await findRows(ql, 'sys_member', { user_id: userId }, 5);
+      if (members.length === 0) await new Promise((r) => setTimeout(r, 250));
+    }
+    expect(members.length).toBe(1);
+    expect(members[0].organization_id).toBe(defaultOrgId);
+    expect(members[0].role).toBe('member');
+  }, 30_000);
+
+  it('backfill binds a pre-existing member-less user and is idempotent (D6)', async () => {
+    // A user created OUTSIDE the better-auth pipeline (system-context insert —
+    // e.g. rows that predate the reconciler).
+    const orphan = await ql.insert(
+      'sys_user',
+      { name: 'Orphan User', email: 'orphan.user@example.com' },
+      { context: SYSTEM_CTX },
+    );
+    expect(orphan?.id).toBeTruthy();
+    expect((await findRows(ql, 'sys_member', { user_id: orphan.id })).length).toBe(0);
+
+    const resolveTargetOrg = async () => defaultOrgId;
+    const first = await backfillMemberships(ql, { policy: 'auto', resolveTargetOrg });
+    expect(first.bound).toBeGreaterThanOrEqual(1); // orphan (+ any other member-less user, e.g. the dev admin)
+    const members = await findRows(ql, 'sys_member', { user_id: orphan.id });
+    expect(members.length).toBe(1);
+    expect(members[0].organization_id).toBe(defaultOrgId);
+    expect(members[0].role).toBe('member');
+
+    // Idempotent: a second run binds nothing new.
+    const second = await backfillMemberships(ql, { policy: 'auto', resolveTargetOrg });
+    expect(second.bound).toBe(0);
+    expect((await findRows(ql, 'sys_member', { user_id: orphan.id })).length).toBe(1);
+  }, 30_000);
+
+  it('invite-only policy never auto-binds, on the real engine (D1)', async () => {
+    const loner = await ql.insert(
+      'sys_user',
+      { name: 'Invite Only', email: 'invite.only@example.com' },
+      { context: SYSTEM_CTX },
+    );
+
+    const res = await reconcileMembership(ql, loner.id, {
+      policy: 'invite-only',
+      resolveTargetOrg: async () => defaultOrgId,
+    });
+    expect(res.outcome).toBe('policy-skip');
+    expect((await findRows(ql, 'sys_member', { user_id: loner.id })).length).toBe(0);
+
+    // Backfill refuses under invite-only too.
+    const bf = await backfillMemberships(ql, {
+      policy: 'invite-only',
+      resolveTargetOrg: async () => defaultOrgId,
+    });
+    expect(bf.reason).toBe('policy');
+    expect((await findRows(ql, 'sys_member', { user_id: loner.id })).length).toBe(0);
+  }, 30_000);
+
+  it('reconciler yields to an existing membership instead of double-binding (D2 yield rule)', async () => {
+    // Simulate a host hook having bound the user to some OTHER org first —
+    // the reconciler must respect it and never add a second membership.
+    const hosted = await ql.insert(
+      'sys_user',
+      { name: 'Host Bound', email: 'host.bound@example.com' },
+      { context: SYSTEM_CTX },
+    );
+    const otherOrg = await ql.insert(
+      'sys_organization',
+      { name: 'Host Org', slug: 'host-org' },
+      { context: SYSTEM_CTX },
+    );
+    await ql.insert(
+      'sys_member',
+      { organization_id: String(otherOrg.id), user_id: hosted.id, role: 'owner' },
+      { context: SYSTEM_CTX },
+    );
+
+    const res = await reconcileMembership(ql, hosted.id, {
+      policy: 'auto',
+      resolveTargetOrg: async () => defaultOrgId,
+    });
+    expect(res.outcome).toBe('yielded');
+    const members = await findRows(ql, 'sys_member', { user_id: hosted.id });
+    expect(members.length).toBe(1);
+    expect(members[0].organization_id).toBe(String(otherOrg.id)); // host's bind won
+  }, 30_000);
+});
diff --git a/packages/plugins/plugin-auth/src/admin-user-endpoints.test.ts b/packages/plugins/plugin-auth/src/admin-user-endpoints.test.ts
index 549e66868..f836b8095 100644
--- a/packages/plugins/plugin-auth/src/admin-user-endpoints.test.ts
+++ b/packages/plugins/plugin-auth/src/admin-user-endpoints.test.ts
@@ -397,6 +397,41 @@ describe('runAdminCreateUser', () => {
     expect(m.warn).toHaveBeenCalled();
   });
 
+  it('multi-org via tenancy: does NOT bind even when a slug=default org exists (ADR-0093 D3 regression)', async () => {
+    // A multi-org deployment carries the bootstrap default org NEXT TO real
+    // tenant orgs. Without the tenancy service the raw resolver would prefer
+    // slug='default' and mis-bind the new user into it; the tenancy service
+    // reports multi mode (defaultOrgId → null) and the bind must no-op.
+    const m = makeDepsWithOrgs({
+      orgs: [{ id: 'org_default', slug: 'default' }, { id: 'org_tenant_b' }],
+    });
+    m.deps.getTenancy = () => ({ defaultOrgId: async () => null });
+    const res = await runAdminCreateUser(
+      m.deps,
+      makeRequest({ email: 'a@b.co', generatePassword: true }),
+      ACTOR,
+    );
+    expect(res.status).toBe(200);
+    const data = res.body.data as any;
+    expect(data.organizationId).toBeUndefined();
+    expect(data.membershipCreated).toBe(false);
+    expect(m.engineInsert.mock.calls.some((c) => c[0] === 'sys_member')).toBe(false);
+  });
+
+  it('single-org via tenancy: binds to the org the tenancy service resolves', async () => {
+    const m = makeDepsWithOrgs({ orgs: [{ id: 'org_default', slug: 'default' }] });
+    m.deps.getTenancy = () => ({ defaultOrgId: async () => 'org_default' });
+    const res = await runAdminCreateUser(
+      m.deps,
+      makeRequest({ email: 'a@b.co', generatePassword: true }),
+      ACTOR,
+    );
+    expect(res.status).toBe(200);
+    const data = res.body.data as any;
+    expect(data.organizationId).toBe('org_default');
+    expect(data.membershipCreated).toBe(true);
+  });
+
   it('no-ops the bind (no throw) when the engine has no find surface', async () => {
     // Default makeDeps engine exposes only update/insert — the bind must be a
     // clean no-op, leaving exactly the audit insert.
diff --git a/packages/plugins/plugin-auth/src/admin-user-endpoints.ts b/packages/plugins/plugin-auth/src/admin-user-endpoints.ts
index 35b3913cc..419dae77b 100644
--- a/packages/plugins/plugin-auth/src/admin-user-endpoints.ts
+++ b/packages/plugins/plugin-auth/src/admin-user-endpoints.ts
@@ -82,6 +82,17 @@ export interface AdminUserEndpointDeps {
   noteMustChangePasswordIssued(): void;
   /** Is the better-auth phoneNumber plugin wired (#2766 V1.5)? */
   phoneNumberEnabled?(): boolean;
+  /**
+   * ADR-0093 D3 — accessor for the `tenancy` service. When present, the
+   * create-user membership bind resolves its target org through
+   * `tenancy.defaultOrgId()` (single → default org; MULTI → null, never
+   * guess). Without it the bind falls back to `resolveDefaultOrgId`, which
+   * prefers the `slug='default'` org — correct single-org, but in multi-org
+   * (where a bootstrap default org coexists with real tenants) it would
+   * mis-bind new users into the default org. Wire this everywhere tenancy
+   * is registered.
+   */
+  getTenancy?(): { defaultOrgId(): Promise<string | null> } | undefined;
   logger?: { warn(msg: string): void };
 }
 
@@ -271,9 +282,15 @@ async function bindUserToSoleOrganization(
   userId: string,
 ): Promise<{ organizationId: string | null; membershipCreated: boolean }> {
   const engine = deps.getDataEngine();
+  // ADR-0093 D3 — mode-aware target resolution. The tenancy service returns
+  // null in multi mode (the framework never guesses a tenant), which also
+  // keeps this endpoint bind from grabbing the bootstrap `slug='default'` org
+  // in a multi-org deployment. Fallback (no tenancy wired: lean embeddings,
+  // legacy mocks) keeps the single-org resolution.
+  const tenancy = deps.getTenancy?.();
   const result = await reconcileMembership(engine, userId, {
     policy: 'auto',
-    resolveTargetOrg: () => resolveDefaultOrgId(engine),
+    resolveTargetOrg: () => (tenancy ? tenancy.defaultOrgId() : resolveDefaultOrgId(engine)),
     logger: deps.logger
       ? { warn: (msg, meta) => deps.logger?.warn(`${msg} ${meta ? JSON.stringify(meta) : ''}`.trim()) }
       : undefined,
diff --git a/packages/plugins/plugin-auth/src/auth-plugin.ts b/packages/plugins/plugin-auth/src/auth-plugin.ts
index c5f362ec8..ca9fcd57a 100644
--- a/packages/plugins/plugin-auth/src/auth-plugin.ts
+++ b/packages/plugins/plugin-auth/src/auth-plugin.ts
@@ -1379,6 +1379,10 @@ export class AuthPlugin implements Plugin {
         assertPasswordComplexity: (pw: string) => this.authManager!.checkPasswordComplexity(pw),
         noteMustChangePasswordIssued: () => this.authManager!.noteMustChangePasswordIssued(),
         phoneNumberEnabled: () => this.authManager!.isPhoneNumberEnabled(),
+        // ADR-0093 D3 — mode-aware create-user bind: multi-org resolves NO
+        // target org (never grab the bootstrap default org in a multi-tenant
+        // deployment); single-org resolves the default org.
+        getTenancy: () => this.tenancy ?? undefined,
         logger: ctx.logger,
       });
       const gateAdmin = async (c: any): Promise<{ id: string; email?: string } | Response> => {
diff --git a/packages/plugins/plugin-auth/src/index.ts b/packages/plugins/plugin-auth/src/index.ts
index edbdba0a8..2d6fe3612 100644
--- a/packages/plugins/plugin-auth/src/index.ts
+++ b/packages/plugins/plugin-auth/src/index.ts
@@ -22,4 +22,8 @@ export * from './register-sso-provider.js';
 export * from './send-verification-email.js';
 export * from './objectql-adapter.js';
 export * from './auth-schema-config.js';
+// ADR-0093 — membership reconciler + tenancy service (public host API: hosts
+// compose the reconciler into their own hooks; embeddings query tenancy mode).
+export * from './reconcile-membership.js';
+export * from './tenancy-service.js';
 export type { AuthConfig, AuthProviderConfig, AuthPluginConfig } from '@objectstack/spec/system';
diff --git a/pnpm-lock.yaml b/pnpm-lock.yaml
index 938df9e3a..e33b71c55 100644
--- a/pnpm-lock.yaml
+++ b/pnpm-lock.yaml
@@ -742,6 +742,9 @@ importers:
       '@objectstack/objectql':
         specifier: workspace:*
         version: link:../objectql
+      '@objectstack/plugin-auth':
+        specifier: workspace:*
+        version: link:../plugins/plugin-auth
       '@objectstack/plugin-security':
         specifier: workspace:*
         version: link:../plugins/plugin-security