diff --git a/packages/siax-access/src/index.ts b/packages/siax-access/src/index.ts
index e2eae2e..739f633 100644
--- a/packages/siax-access/src/index.ts
+++ b/packages/siax-access/src/index.ts
@@ -1,2 +1,2 @@
-export * from "./relationships";
-export * from "./idempotency";
+export * from './relationships';
+export * from './idempotency';
diff --git a/packages/siax-access/src/relationships.ts b/packages/siax-access/src/relationships.ts
new file mode 100644
index 0000000..6a2bcc8
--- /dev/null
+++ b/packages/siax-access/src/relationships.ts
@@ -0,0 +1,235 @@
+/**
+ * @siax/access — universal project/tenant relationship checks for SIAX Cloud.
+ *
+ * This is the reusable extraction of the pattern first proven in DOC0's
+ * `src/clients/project-relationships.ts`. It is deliberately framework-neutral
+ * (no Next.js, Fastify, Prisma or product types) so every SIAX product can
+ * consume the exact same authorization primitive instead of copying it:
+ *
+ *   0pt      — document upload + structuring scope
+ *   DOC0     — document/version/signature scope
+ *   N0D      — job submission scope
+ *   any app  — tenant/project membership
+ *
+ * IMPORTANT: a relationship match is ONE input to authorization, never a
+ * complete decision. The caller must independently bind tenant and actor to a
+ * verified principal (never to request headers), enforce tenant/project
+ * status, capability, data-handling policy and an authoritative tuple
+ * lifecycle. `resolveRelationship` fails closed: only an explicit
+ * `relationships_match` is treated as allowed.
+ */
+
+export type RelationshipResult = 'allowed' | 'denied' | 'unavailable';
+
+/** The outcome of the composite project check. */
+export type ProjectRelationshipResult = 'relationships_match' | 'denied' | 'unavailable';
+
+export interface ProjectRelationshipScope {
+  tenantId: string;
+  projectId: string;
+  actorId: string;
+}
+
+const objectId = /^[A-Za-z0-9][A-Za-z0-9._@+-]{0,127}$/;
+const ulid = /^[0-9A-HJKMNP-TV-Z]{26}$/;
+const maxResponseBytes = 16 * 1024;
+
+export interface OpenFgaRelationshipsOptions {
+  baseUrl: string;
+  storeId: string;
+  modelId: string;
+  serviceToken: string;
+  timeoutMs?: number;
+  fetchImpl?: typeof fetch;
+  /** Explicit test-only opt-in for an isolated loopback OpenFGA process. */
+  testOnlyAllowLoopbackHttp?: boolean;
+}
+
+/**
+ * Read-only OpenFGA relationship reader. Never writes tuples, never caches,
+ * and never retries: an unavailable authorization source must stay visible to
+ * the caller so it can fail closed.
+ */
+export class OpenFgaRelationships {
+  private readonly baseUrl: string;
+  private readonly fetchImpl: typeof fetch;
+  private readonly storeId: string;
+  private readonly modelId: string;
+  private readonly serviceToken: string;
+  private readonly timeoutMs: number;
+
+  constructor(options: OpenFgaRelationshipsOptions) {
+    const url = new URL(options.baseUrl);
+    const loopbackTest =
+      process.env.NODE_ENV === 'test' &&
+      options.testOnlyAllowLoopbackHttp === true &&
+      url.protocol === 'http:' &&
+      url.hostname === '127.0.0.1';
+    if (
+      (!loopbackTest && url.protocol !== 'https:') ||
+      url.username ||
+      url.password ||
+      url.search ||
+      url.hash ||
+      typeof options.storeId !== 'string' ||
+      typeof options.modelId !== 'string' ||
+      !ulid.test(options.storeId) ||
+      !ulid.test(options.modelId) ||
+      typeof options.serviceToken !== 'string' ||
+      !/^[\x21-\x7e]{1,8192}$/.test(options.serviceToken) ||
+      (options.timeoutMs !== undefined &&
+        (!Number.isSafeInteger(options.timeoutMs) || options.timeoutMs < 100 || options.timeoutMs > 30_000))
+    ) {
+      throw new Error('Invalid trusted OpenFGA configuration');
+    }
+    this.baseUrl = url.toString().replace(/\/+$/, '');
+    this.fetchImpl = options.fetchImpl ?? fetch;
+    this.storeId = options.storeId;
+    this.modelId = options.modelId;
+    this.serviceToken = options.serviceToken;
+    this.timeoutMs = options.timeoutMs ?? 5_000;
+  }
+
+  /**
+   * Both checks are necessary: project membership alone may inherit from a
+   * different parent tenant if bad tuples associate a project with two
+   * tenants. Returns `relationships_match` only when both hold.
+   */
+  async checkProject(scope: ProjectRelationshipScope): Promise<ProjectRelationshipResult> {
+    if (!scope || typeof scope !== 'object' || Array.isArray(scope)) {
+      return 'denied';
+    }
+    const { tenantId, projectId, actorId } = scope;
+    if (![tenantId, projectId, actorId].every((value) => typeof value === 'string' && objectId.test(value))) {
+      return 'denied';
+    }
+    const actor = await this.checkTuple(`user:${actorId}`, 'member', `tenant:${tenantId}`);
+    if (actor !== 'allowed') return actor === 'denied' ? 'denied' : 'unavailable';
+    const parent = await this.checkTuple(`tenant:${tenantId}`, 'tenant', `project:${projectId}`);
+    return parent === 'allowed' ? 'relationships_match' : parent;
+  }
+
+  /** One raw relationship check. Public so products can map their own tuples. */
+  async checkTuple(user: string, relation: string, object: string): Promise<RelationshipResult> {
+    if (
+      ![user, relation, object].every(
+        (value) => typeof value === 'string' && value.length > 0 && value.length <= 256,
+      )
+    ) {
+      return 'denied';
+    }
+    try {
+      const response = await this.fetchImpl(`${this.baseUrl}/stores/${this.storeId}/check`, {
+        method: 'POST',
+        headers: {
+          authorization: `Bearer ${this.serviceToken}`,
+          'content-type': 'application/json',
+        },
+        body: JSON.stringify({
+          authorization_model_id: this.modelId,
+          consistency: 'HIGHER_CONSISTENCY',
+          tuple_key: { user, relation, object },
+        }),
+        redirect: 'error',
+        signal: AbortSignal.timeout(this.timeoutMs),
+      });
+      if (!response.ok || !response.body) return 'unavailable';
+      const reader = response.body.getReader();
+      try {
+        const chunks: Uint8Array[] = [];
+        let total = 0;
+        for (;;) {
+          const part = await reader.read();
+          if (part.done) break;
+          total += part.value.byteLength;
+          if (total > maxResponseBytes) {
+            void reader.cancel().catch(() => undefined);
+            return 'unavailable';
+          }
+          chunks.push(part.value);
+        }
+        const result: unknown = JSON.parse(
+          new TextDecoder('utf-8', { fatal: true }).decode(Buffer.concat(chunks)),
+        );
+        if (
+          !result ||
+          typeof result !== 'object' ||
+          Array.isArray(result) ||
+          typeof (result as { allowed?: unknown }).allowed !== 'boolean'
+        ) {
+          return 'unavailable';
+        }
+        return (result as { allowed: boolean }).allowed ? 'allowed' : 'denied';
+      } finally {
+        reader.releaseLock();
+      }
+    } catch {
+      return 'unavailable';
+    }
+  }
+}
+
+/**
+ * Canonical environment variable names used across the fleet. Reads are
+ * explicit (no implicit defaults) so a missing value is visible.
+ */
+export const OPENFGA_ENV = {
+  baseUrl: 'OPENFGA_API_URL',
+  storeId: 'OPENFGA_STORE_ID',
+  modelId: 'OPENFGA_MODEL_ID',
+  serviceToken: 'OPENFGA_AUTH_TOKEN',
+} as const;
+
+export interface ProjectAccessOptions {
+  /** The already-configured relationship reader, or undefined when OpenFGA is not configured. */
+  relationships?: OpenFgaRelationships;
+  /**
+   * Optional extra policy (project status, capability, data-handling). It runs
+   * only after the relationship check matched; a rejection is final.
+   */
+  additionalPolicy?: (scope: ProjectRelationshipScope) => Promise<boolean> | boolean;
+}
+
+/** Fail closed: only an explicit match with no policy objection is allowed. */
+export async function isProjectAccessAllowed(
+  scope: ProjectRelationshipScope,
+  options: ProjectAccessOptions,
+): Promise<boolean> {
+  if (!options.relationships) return false;
+  const result = await options.relationships.checkProject(scope);
+  if (result !== 'relationships_match') return false;
+  if (options.additionalPolicy) {
+    try {
+      return (await options.additionalPolicy(scope)) === true;
+    } catch {
+      return false;
+    }
+  }
+  return true;
+}
+
+/**
+ * Build a relationship reader from a `NodeJS.ProcessEnv`-shaped object.
+ * Returns `undefined` (fail closed) when any value is missing or invalid.
+ */
+export function openFgaFromEnv(
+  env: NodeJS.ProcessEnv = process.env,
+  fetchImpl?: typeof fetch,
+): OpenFgaRelationships | undefined {
+  const baseUrl = env[OPENFGA_ENV.baseUrl];
+  const storeId = env[OPENFGA_ENV.storeId];
+  const modelId = env[OPENFGA_ENV.modelId];
+  const serviceToken = env[OPENFGA_ENV.serviceToken];
+  if (!baseUrl || !storeId || !modelId || !serviceToken) return undefined;
+  try {
+    return new OpenFgaRelationships({
+      baseUrl,
+      storeId,
+      modelId,
+      serviceToken,
+      ...(fetchImpl ? { fetchImpl } : {}),
+    });
+  } catch {
+    return undefined;
+  }
+}