@@ -0,0 +1,10 @@
+---
+'@objectstack/runtime': patch
+---
+
+fix(runtime): the environment-membership check judges the principal the request resolved
+
+Clause-②: no
+Metadata-protocol: none
+
+On a multi-environment host, the dispatcher refuses a caller who is not a member of the environment a scoped route resolves to (`403 PROJECT_MEMBERSHIP_REQUIRED`). That check now judges the principal the request resolved, whichever credential admitted it: a principal admitted by an OAuth access token on `/mcp` or by an API key is judged as that token's user or that key's owner, in the organization its resolution vetted, including when a session cookie for another user is sent beside it. A principal admitted by a session cookie is judged as before. Hosts without environment scoping, and hosts that turn the check off (`api.enforceProjectMembership: false`), are unchanged.
@@ -83,6 +83,7 @@ import { serveStoredMetadataRead, serveStoredMetadataReadsThrough } from './stor
 import { actorUserFromExecutionContext, resolveActorDisplayName } from './security/actor-user.js';
 import { actionInvocationBranch, actionInvocationFailure, actionInvocationTarget, recordActionInvocation, withActionSurface } from './action-invocation-audit.js';
 import type { HttpProtocolContext } from './http-dispatcher.js';
+import { admittingCredentialKind, type AdmittingCredentialKind } from './security/admitting-credential.js';
 import {
     GLOBAL_ACTION_OBJECT_KEY,
     actionHandlerObjectKeys,
@@ -1619,9 +1620,10 @@ function isIdentityRouteTarget(path: string): boolean {
  * envelope carries its `oauthScopes`; such a token is honoured on the MCP
  * endpoint alone), else what the door's admission recorded — `api_key` or
  * `session`. `undefined` when the door recorded nothing (an anonymous caller,
- * whom the door's own floor answers first).
+ * whom the door's own floor answers first). The one reading is
+ * {@link admittingCredentialKind}, which the environment-membership gate takes too.
  */
-export type ApiForwardCredentialKind = 'session' | 'api_key' | 'oauth';
+export type ApiForwardCredentialKind = AdmittingCredentialKind;
 
 /** Which kind of door an api action's `target` is served by, for the forward. */
 export type ApiForwardTargetDoor = 'identity' | 'mcp' | 'platform';
@@ -1710,8 +1712,7 @@ export function apiForwardAgentOf(requestContext: HttpProtocolContext | undefine
 
 /** The kind of credential that admitted this request's caller (see {@link ApiForwardCredentialKind}). */
 export function apiForwardCredentialKind(requestContext: HttpProtocolContext | undefined): ApiForwardCredentialKind | undefined {
-    if (Array.isArray(requestContext?.executionContext?.oauthScopes)) return 'oauth';
-    return requestContext?.admission?.credential;
+    return admittingCredentialKind(requestContext);
 }
 
 /**
 
@@ -0,0 +1,169 @@
+// Copyright (c) 2026 ObjectStack. Licensed under the Apache-2.0 license.
+
+/**
+ * The environment-membership gate judges the principal the request resolved,
+ * whichever credential admitted it.
+ *
+ * A multi-tenant host: a stub resolver places every request in one
+ * environment, on a tenant kernel whose `sys_environment_member` holds a row
+ * only for {@link MEMBER}. The gate's answer is read off the dispatch result
+ * (`403 PROJECT_MEMBERSHIP_REQUIRED`, naming the user it judged) and off the
+ * membership reads it made.
+ *
+ *  - CONTROL: a cookie session for a non-member is refused, as before.
+ *  - A principal an OAuth access token admitted on `/mcp` is judged: refused
+ *    when it is not a member, and admitted when it is.
+ *  - A token for a non-member sent beside a member's cookie is judged as the
+ *    token's principal, the one the request resolved.
+ *  - A principal an API key admitted is judged as the key's owner, on `/mcp`
+ *    and on a scoped data route alike.
+ */
+
+import { describe, it, expect, vi } from 'vitest';
+// Paid at collection: the dispatcher reaches it through a dynamic import.
+import '@objectstack/metadata';
+import { hashApiKey } from '@objectstack/core';
+import { HttpDispatcher } from './http-dispatcher.js';
+import type { HttpProtocolContext, KernelResolver } from './http-dispatcher.js';
+
+const ENV = 'env-tenant';
+const MEMBER = 'user-member';
+const OUTSIDER = 'user-outsider';
+/** JWT-shaped: three dot-separated segments, which is what the token door reads. */
+const TOKEN = 'eyJhbGciOiJSUzI1NiJ9.eyJzdWIiOiJ4In0.c2ln';
+const API_KEY = 'osk_probe0123456789abcdefghijklmnopqrstuvwxyz';
+const MCP_CALL = { jsonrpc: '2.0', id: 1, method: 'tools/list' };
+
+function kernelWith(services: Record<string, unknown>): any {
+    return {
+        getState: () => 'running',
+        getService: (n: string) => services[n] ?? null,
+        getServiceAsync: async (n: string) => services[n] ?? null,
+        context: { getService: (n: string) => services[n] ?? null },
+    };
+}
+
+/**
+ * The tenant host. `cookieUser` is who the cookie session reads as (none when
+ * absent); `tokenUser` is who a verified OAuth access token names.
+ */
+function tenantHost(opts: { cookieUser?: string; tokenUser?: string; keyOwner?: string }) {
+    const find = vi.fn(async (object: string, query: { where?: Record<string, unknown> }) => {
+        const where = query?.where ?? {};
+        if (object === 'sys_environment_member') {
+            return where.environment_id === ENV && where.user_id === MEMBER
+                ? [{ id: 'm1', environment_id: ENV, user_id: MEMBER }]
+                : [];
+        }
+        if (object === 'sys_api_key' && opts.keyOwner && where.key === hashApiKey(API_KEY)) {
+            return [{ id: 'key-1', key: hashApiKey(API_KEY), user_id: opts.keyOwner }];
+        }
+        if (object === 'sys_user' && typeof where.id === 'string') return [{ id: where.id }];
+        return [];
+    });
+    const objectql = {
+        find,
+        getObjects: vi.fn().mockReturnValue({}),
+        registry: {
+            getObject: vi.fn((name: string) => (name === 'sys_environment_member' ? { name } : null)),
+            getRegisteredTypes: vi.fn().mockReturnValue([]),
+        },
+    };
+    const auth = {
+        getApi: async () => ({
+            getSession: async () => (opts.cookieUser
+                ? { user: { id: opts.cookieUser }, session: { userId: opts.cookieUser } }
+                : null),
+        }),
+        verifyMcpAccessToken: vi.fn(async () => (opts.tokenUser
+            ? { userId: opts.tokenUser, clientId: 'client-1', scopes: ['data:read'] }
+            : null)),
+    };
+    const tenant = kernelWith({ objectql, auth });
+    const resolver: KernelResolver = {
+        resolveKernel: async (c: HttpProtocolContext) => {
+            c.environmentId = ENV;
+            return tenant;
+        },
+    };
+    const dispatcher = new HttpDispatcher(kernelWith({}), undefined, {
+        kernelResolver: resolver,
+        enforceProjectMembership: true,
+    });
+    const membershipReads = () => find.mock.calls
+        .filter((call) => call[0] === 'sys_environment_member')
+        .map((call) => (call[1] as { where: { user_id: string } }).where.user_id);
+    return { dispatcher, membershipReads };
+}
+
+const mcp = (headers: Record<string, string>): HttpProtocolContext => ({
+    request: new Request('http://tenant.example/api/v1/mcp', { method: 'POST', headers }),
+});
+
+function membershipRefusal(result: { response?: { status: number; body?: any } }) {
+    const error = result.response?.body?.error;
+    return result.response?.status === 403 && error?.code === 'PROJECT_MEMBERSHIP_REQUIRED'
+        ? { userId: error?.details?.userId as string | undefined }
+        : undefined;
+}
+
+describe('the environment-membership gate judges the resolved principal', () => {
+    it('CONTROL: a cookie session for a non-member is refused', async () => {
+        const { dispatcher, membershipReads } = tenantHost({ cookieUser: OUTSIDER });
+        const result = await dispatcher.dispatch('POST', '/mcp', MCP_CALL, {}, mcp({}));
+        expect(membershipRefusal(result)).toEqual({ userId: OUTSIDER });
+        expect(membershipReads()).toEqual([OUTSIDER]);
+    });
+
+    it('a principal an OAuth access token admitted, not a member: refused, naming the token\'s user', async () => {
+        const { dispatcher, membershipReads } = tenantHost({ tokenUser: OUTSIDER });
+        const result = await dispatcher.dispatch('POST', '/mcp', MCP_CALL, {}, mcp({ authorization: `Bearer ${TOKEN}` }));
+        expect(membershipRefusal(result), JSON.stringify(result.response?.body)).toEqual({ userId: OUTSIDER });
+        expect(membershipReads()).toEqual([OUTSIDER]);
+    });
+
+    it('a token for a non-member beside a member\'s cookie: judged as the token\'s principal, refused', async () => {
+        const { dispatcher, membershipReads } = tenantHost({ cookieUser: MEMBER, tokenUser: OUTSIDER });
+        const result = await dispatcher.dispatch('POST', '/mcp', MCP_CALL, {}, mcp({ authorization: `Bearer ${TOKEN}` }));
+        expect(membershipRefusal(result), JSON.stringify(result.response?.body)).toEqual({ userId: OUTSIDER });
+        expect(membershipReads()).toEqual([OUTSIDER]);
+    });
+
+    it('a principal an API key admitted, not a member: refused, naming the key\'s owner', async () => {
+        const { dispatcher, membershipReads } = tenantHost({ keyOwner: OUTSIDER });
+        const result = await dispatcher.dispatch('POST', '/mcp', MCP_CALL, {}, mcp({ 'x-api-key': API_KEY }));
+        expect(membershipRefusal(result), JSON.stringify(result.response?.body)).toEqual({ userId: OUTSIDER });
+        expect(membershipReads()).toEqual([OUTSIDER]);
+    });
+
+    it('a principal an API key admitted on a scoped non-MCP route, not a member: refused, naming the key\'s owner', async () => {
+        const { dispatcher, membershipReads } = tenantHost({ keyOwner: OUTSIDER });
+        const context: HttpProtocolContext = {
+            request: new Request('http://tenant.example/api/v1/data/task', { headers: { 'x-api-key': API_KEY } }),
+        };
+        const result = await dispatcher.dispatch('GET', '/data/task', undefined, {}, context);
+        expect(membershipRefusal(result), JSON.stringify(result.response?.body)).toEqual({ userId: OUTSIDER });
+        expect(membershipReads()).toEqual([OUTSIDER]);
+    });
+
+    it('CONTROL: a principal an OAuth access token admitted, a member: passes the gate', async () => {
+        const { dispatcher, membershipReads } = tenantHost({ tokenUser: MEMBER });
+        const result = await dispatcher.dispatch('POST', '/mcp', MCP_CALL, {}, mcp({ authorization: `Bearer ${TOKEN}` }));
+        expect(membershipRefusal(result)).toBeUndefined();
+        expect(membershipReads()).toEqual([MEMBER]);
+    });
+
+    it('CONTROL: a principal an API key admitted, a member: passes the gate', async () => {
+        const { dispatcher, membershipReads } = tenantHost({ keyOwner: MEMBER });
+        const result = await dispatcher.dispatch('POST', '/mcp', MCP_CALL, {}, mcp({ 'x-api-key': API_KEY }));
+        expect(membershipRefusal(result)).toBeUndefined();
+        expect(membershipReads()).toEqual([MEMBER]);
+    });
+
+    it('CONTROL: no credential at all is not judged here', async () => {
+        const { dispatcher, membershipReads } = tenantHost({});
+        const result = await dispatcher.dispatch('POST', '/mcp', MCP_CALL, {}, mcp({}));
+        expect(membershipRefusal(result)).toBeUndefined();
+        expect(membershipReads()).toEqual([]);
+    });
+});
@@ -38,6 +38,7 @@ import { DomainHandlerRegistry, type DomainRoute, type DomainHandlerDeps } from
 // only — `actionExecutionDeps` hands the endpoint step the same `deps` object
 // every domain already passes to `callData`, without this file calling it.
 import type { ActionExecutionDeps } from './action-execution.js';
+import { admittingCredentialKind } from './security/admitting-credential.js';
 import { createAnalyticsDomain, handleAnalyticsRequest } from './domains/analytics.js';
 import { isServiceServeable } from './service-serveable.js';
 import { createI18nDomain, handleI18nRequest } from './domains/i18n.js';
@@ -1673,6 +1674,7 @@ export class HttpDispatcher {
         // and single-tenant setups run without auth.
         let userId: string | undefined;
         let activeOrganizationId: string | undefined;
+        let sessionUnread = false;
         try {
             // [#4127 batch 4] This read was `authService?.api?.getSession?.(…)`
             // with NO `getApi()` fallback — the only one of the three `.api`
@@ -1698,6 +1700,22 @@ export class HttpDispatcher {
             userId = sessionData?.user?.id ?? sessionData?.session?.userId;
             activeOrganizationId = sessionData?.session?.activeOrganizationId;
         } catch {
+            sessionUnread = true;
+        }
+
+        // The gate judges the principal this request RESOLVED
+        // (`resolveRequestScope`), whichever credential admitted it. A
+        // principal an API key or an OAuth access token admitted (with or
+        // without a cookie beside it) is the one judged, in the organization
+        // its resolution vetted; which credential that was is the one reading
+        // the api-action forward takes too (`admittingCredentialKind`). A
+        // session principal is judged by the session read above, as before.
+        const resolved = context.executionContext;
+        const admittedBy = admittingCredentialKind(context);
+        if ((admittedBy === 'api_key' || admittedBy === 'oauth') && resolved?.userId) {
+            userId = resolved.userId;
+            activeOrganizationId = resolved.tenantId;
+        } else if (sessionUnread) {
             // Auth resolution failed — do not block the request on RBAC.
             return null;
         }
 
@@ -0,0 +1,25 @@
+// Copyright (c) 2026 ObjectStack. Licensed under the Apache-2.0 license.
+
+/**
+ * Which kind of credential admitted a request's caller: the one reading the
+ * dispatcher's environment-membership gate and the api-action forward both
+ * take, so the two cannot disagree.
+ *
+ * - `oauth` when the principal was resolved from an OAuth access token (the
+ *   envelope carries its `oauthScopes`; such a token is honoured on the MCP
+ *   endpoint alone, and an in-process forward of it carries them too);
+ * - else what the identity step's admission recorded: `api_key` or `session`;
+ * - `undefined` when it recorded nothing (an anonymous caller).
+ */
+export type AdmittingCredentialKind = 'session' | 'api_key' | 'oauth';
+
+/** The two request facts {@link admittingCredentialKind} reads. */
+export interface AdmittingCredentialFacts {
+    executionContext?: { oauthScopes?: unknown };
+    admission?: { credential: 'api_key' | 'session' };
+}
+
+export function admittingCredentialKind(request: AdmittingCredentialFacts | undefined): AdmittingCredentialKind | undefined {
+    if (Array.isArray(request?.executionContext?.oauthScopes)) return 'oauth';
+    return request?.admission?.credential;
+}