diff --git a/eslint.config.mjs b/eslint.config.mjs
index e7e631a..eb57f77 100644
--- a/eslint.config.mjs
+++ b/eslint.config.mjs
@@ -30,6 +30,7 @@ export default tseslint.config(
             'apps/gateway/vitest.config.ts',
             'packages/storage/vitest.config.ts',
             'packages/mosaic/__tests__/*.ts',
+            'tools/federation-harness/*.ts',
           ],
         },
       },
diff --git a/tools/federation-harness/README.md b/tools/federation-harness/README.md
new file mode 100644
index 0000000..d403169
--- /dev/null
+++ b/tools/federation-harness/README.md
@@ -0,0 +1,254 @@
+# Federation Test Harness
+
+Local two-gateway federation test infrastructure for Mosaic Stack M3+.
+
+This harness boots two real gateway instances (`gateway-a`, `gateway-b`) on a
+shared Docker bridge network, each backed by its own Postgres (pgvector) +
+Valkey, sharing a single Step-CA. It is the test bed for all M3+ federation
+E2E tests.
+
+## Prerequisites
+
+- Docker with Compose v2 (`docker compose version` ≥ 2.20)
+- pnpm (for running via repo scripts)
+- `infra/step-ca/dev-password` must exist (copy from `infra/step-ca/dev-password.example`)
+
+## Network Topology
+
+```
+Host machine
+├── localhost:14001  →  gateway-a   (Server A — home / requesting)
+├── localhost:14002  →  gateway-b   (Server B — work / serving)
+├── localhost:15432  →  postgres-a
+├── localhost:15433  →  postgres-b
+├── localhost:16379  →  valkey-a
+├── localhost:16380  →  valkey-b
+└── localhost:19000  →  step-ca     (shared CA)
+
+Docker network: fed-test-net (bridge)
+  gateway-a ←──── mTLS ────→ gateway-b
+             ↘             ↗
+               step-ca
+```
+
+Ports are chosen to avoid collision with the base dev stack (5433, 6380, 14242, 9000).
+
+## Starting the Harness
+
+```bash
+# From repo root
+docker compose -f tools/federation-harness/docker-compose.two-gateways.yml up -d
+
+# Wait for all services to be healthy (~60-90s on first boot due to NestJS cold start)
+docker compose -f tools/federation-harness/docker-compose.two-gateways.yml ps
+```
+
+## Seeding Test Data
+
+The seed script provisions three grant scope variants (A, B, C) and walks the
+full enrollment flow so Server A ends up with active peers pointing at Server B.
+
+```bash
+# Assumes stack is already running
+pnpm tsx tools/federation-harness/seed.ts
+
+# Or boot + seed in one step
+pnpm tsx tools/federation-harness/seed.ts --boot
+```
+
+### Scope Variants
+
+| Variant | Resources          | Filters                            | Excluded    | Purpose                         |
+| ------- | ------------------ | ---------------------------------- | ----------- | ------------------------------- |
+| A       | tasks, notes       | include_personal: true             | (none)      | Personal data federation        |
+| B       | tasks              | include_teams: ['T1'], no personal | (none)      | Team-scoped, no personal        |
+| C       | tasks, credentials | include_personal: true             | credentials | Sanity: excluded wins over list |
+
+## Using from Vitest
+
+```ts
+import {
+  bootHarness,
+  tearDownHarness,
+  serverA,
+  serverB,
+  seed,
+} from '../../tools/federation-harness/harness.js';
+import type { HarnessHandle } from '../../tools/federation-harness/harness.js';
+
+let handle: HarnessHandle;
+
+beforeAll(async () => {
+  handle = await bootHarness();
+}, 180_000); // allow 3 min for Docker pull + NestJS cold start
+
+afterAll(async () => {
+  await tearDownHarness(handle);
+});
+
+test('variant A: list tasks returns personal tasks', async () => {
+  // NOTE: Only 'all' is supported for now — per-variant narrowing is M3-11.
+  const seedResult = await seed(handle, 'all');
+  const a = serverA(handle);
+
+  const res = await fetch(`${a.baseUrl}/api/federation/tasks`, {
+    headers: { 'x-federation-grant': seedResult.grants.variantA.id },
+  });
+  expect(res.status).toBe(200);
+});
+```
+
+> **Note:** `seed()` bootstraps a fresh admin user on each gateway via
+> `POST /api/bootstrap/setup`. Both gateways must have zero users (pristine DB).
+> If either gateway already has users, `seed()` throws with a clear error.
+> Reset state with `docker compose down -v`.
+
+The `bootHarness()` function is **idempotent**: if both gateways are already
+healthy, it reuses the running stack and returns `ownedStack: false`. Tests
+should not call `tearDownHarness` when `ownedStack` is false unless they
+explicitly want to shut down a shared stack.
+
+## Vitest Config (pnpm test:federation)
+
+Add to `vitest.config.ts` at repo root (or a dedicated config):
+
+```ts
+// vitest.federation.config.ts
+import { defineConfig } from 'vitest/config';
+
+export default defineConfig({
+  test: {
+    include: ['**/*.federation.test.ts'],
+    testTimeout: 60_000,
+    hookTimeout: 180_000,
+    reporters: ['verbose'],
+  },
+});
+```
+
+Then add to root `package.json`:
+
+```json
+"test:federation": "vitest run --config vitest.federation.config.ts"
+```
+
+## Nuking State
+
+```bash
+# Remove containers AND volumes (ephemeral state — CA keys, DBs, everything)
+docker compose -f tools/federation-harness/docker-compose.two-gateways.yml down -v
+```
+
+On next `up`, Step-CA re-initialises from scratch and generates new CA keys.
+
+## Step-CA Root Certificate
+
+The CA root lives in the `fed-harness-step-ca` Docker volume at
+`/home/step/certs/root_ca.crt`. To extract it to the host:
+
+```bash
+docker run --rm \
+  -v fed-harness-step-ca:/home/step \
+  alpine cat /home/step/certs/root_ca.crt > /tmp/fed-harness-root-ca.crt
+```
+
+## Troubleshooting
+
+### Port conflicts
+
+Default host ports: 14001, 14002, 15432, 15433, 16379, 16380, 19000.
+Override via environment variables before `docker compose up`:
+
+```bash
+GATEWAY_A_HOST_PORT=14101 GATEWAY_B_HOST_PORT=14102 \
+  docker compose -f tools/federation-harness/docker-compose.two-gateways.yml up -d
+```
+
+### Image pull failures
+
+The gateway image is digest-pinned to:
+
+```
+git.mosaicstack.dev/mosaicstack/stack/gateway@sha256:1069117740e00ccfeba357cae38c43f3729fe5ae702740ce474f6512414d7c02
+```
+
+(sha-9f1a081, post-#491 IMG-FIX)
+
+If the registry is unreachable, Docker will use the locally cached image if
+present. If no local image exists, the compose up will fail with a pull error.
+In that case:
+
+1. Ensure you can reach `git.mosaicstack.dev` (VPN, DNS, etc.).
+2. Log in: `docker login git.mosaicstack.dev`
+3. Pull manually: `docker pull git.mosaicstack.dev/mosaicstack/stack/gateway@sha256:1069117740e00ccfeba357cae38c43f3729fe5ae702740ce474f6512414d7c02`
+
+### NestJS cold start
+
+Gateway containers take 40–60 seconds to become healthy on first boot (Node.js
+module resolution + NestJS DI bootstrap). The `start_period: 60s` in the
+compose healthcheck covers this. `bootHarness()` polls for up to 3 minutes.
+
+### Step-CA startup
+
+Step-CA initialises on first boot (generates CA keys). This takes ~5-10s.
+The `start_period: 30s` in the healthcheck covers it. Both gateways wait for
+Step-CA to be healthy before starting (`depends_on: step-ca: condition: service_healthy`).
+
+### dev-password missing
+
+The Step-CA container requires `infra/step-ca/dev-password` to be mounted.
+Copy the example and set a local password:
+
+```bash
+cp infra/step-ca/dev-password.example infra/step-ca/dev-password
+# Edit the file to set your preferred dev CA password
+```
+
+The file is `.gitignore`d — do not commit it.
+
+## Image Digest Note
+
+The gateway image is pinned to `sha256:1069117740e00ccfeba357cae38c43f3729fe5ae702740ce474f6512414d7c02`
+(sha-9f1a081). This is the digest promoted by PR #491 (IMG-FIX). The `latest`
+tag is forbidden per Mosaic image policy. When a new gateway build is promoted,
+update the digest in `docker-compose.two-gateways.yml` and in this file.
+
+## Known Limitations
+
+### BETTER_AUTH_URL enrollment URL bug (upstream production code — not yet fixed)
+
+`apps/gateway/src/federation/federation.controller.ts:145` constructs the
+enrollment URL using `process.env['BETTER_AUTH_URL'] ?? 'http://localhost:14242'`.
+This is an upstream bug: `BETTER_AUTH_URL` is the Better Auth origin (typically
+the web app), not the gateway's own base URL. In non-harness deployments this
+produces an enrollment URL pointing to the wrong host or port.
+
+**How the harness handles this:**
+
+1. **In-cluster calls (container-to-container):** The compose file sets
+   `BETTER_AUTH_URL: 'http://gateway-b:3000'` so the enrollment URL returned by
+   the gateway uses the Docker internal hostname. This lets other containers in the
+   `fed-test-net` network resolve and reach Server B's enrollment endpoint.
+
+2. **Host-side URL rewrite (seed script):** The `seed.ts` script runs on the host
+   machine where `gateway-b` is not a resolvable hostname. Before calling
+   `fetch(enrollmentUrl, ...)`, the seed script rewrites the URL: it extracts only
+   the token path segment from `enrollmentUrl` and reassembles the URL using the
+   host-accessible `serverBUrl` (default: `http://localhost:14002`). This lets the
+   seed script redeem enrollment tokens from the host without being affected by the
+   in-cluster hostname in the returned URL.
+
+**TODO:** Fix `federation.controller.ts` to derive the enrollment URL from its own
+listening address (e.g. `GATEWAY_BASE_URL` env var or a dedicated
+`FEDERATION_ENROLLMENT_BASE_URL` env var) rather than reusing `BETTER_AUTH_URL`.
+Tracked as a follow-up to PR #505 — do not bundle with harness changes.
+
+## Permanent Infrastructure
+
+This harness is designed to outlive M3 and be reused by M4+ milestone tests.
+It is not a throwaway scaffold — treat it as production test infrastructure:
+
+- Keep it idempotent.
+- Do not hardcode test assumptions in the harness layer (put them in tests).
+- Update the seed script when new scope variants are needed.
+- The README and harness should be kept in sync as the federation API evolves.
diff --git a/tools/federation-harness/docker-compose.two-gateways.yml b/tools/federation-harness/docker-compose.two-gateways.yml
new file mode 100644
index 0000000..2a4f980
--- /dev/null
+++ b/tools/federation-harness/docker-compose.two-gateways.yml
@@ -0,0 +1,247 @@
+# tools/federation-harness/docker-compose.two-gateways.yml
+#
+# Two-gateway federation test harness — local-only, no Portainer/Swarm needed.
+#
+# USAGE (manual):
+#   docker compose -f tools/federation-harness/docker-compose.two-gateways.yml up -d
+#   docker compose -f tools/federation-harness/docker-compose.two-gateways.yml down -v
+#
+# USAGE (from harness.ts):
+#   const handle = await bootHarness();
+#   ...
+#   await tearDownHarness(handle);
+#
+# TOPOLOGY:
+#   gateway-a  — "home" instance (Server A, the requesting side)
+#     └── postgres-a  (pgvector/pg17, port 15432)
+#     └── valkey-a    (port 16379)
+#   gateway-b  — "work" instance (Server B, the serving side)
+#     └── postgres-b  (pgvector/pg17, port 15433)
+#     └── valkey-b    (port 16380)
+#   step-ca    — shared CA for both gateways (port 19000)
+#
+# All services share the `fed-test-net` bridge network.
+# Host port ranges (15432-15433, 16379-16380, 14001-14002, 19000) are chosen
+# to avoid collision with the base dev stack (5433, 6380, 14242, 9000).
+#
+# IMAGE:
+#   Pinned to the immutable digest sha256:1069117740e00ccfeba357cae38c43f3729fe5ae702740ce474f6512414d7c02
+#   (sha-9f1a081, post-#491 IMG-FIX, smoke-tested locally).
+#   Update this digest only after a new CI build is promoted to the registry.
+#
+# STEP-CA:
+#   Single shared Step-CA instance. Both gateways connect to it.
+#   CA volume is ephemeral per `docker compose down -v`; regenerated on next up.
+#   The harness seed script provisions the CA roots cross-trust after first boot.
+
+services:
+  # ─── Shared Certificate Authority ────────────────────────────────────────────
+  step-ca:
+    image: smallstep/step-ca:0.27.4
+    container_name: fed-harness-step-ca
+    restart: unless-stopped
+    ports:
+      - '${STEP_CA_HOST_PORT:-19000}:9000'
+    volumes:
+      - step_ca_data:/home/step
+      - ../../infra/step-ca/init.sh:/usr/local/bin/mosaic-step-ca-init.sh:ro
+      - ../../infra/step-ca/templates:/etc/step-ca-templates:ro
+      - ../../infra/step-ca/dev-password:/run/secrets/ca_password:ro
+    entrypoint: ['/bin/sh', '/usr/local/bin/mosaic-step-ca-init.sh']
+    networks:
+      - fed-test-net
+    healthcheck:
+      test:
+        [
+          'CMD',
+          'step',
+          'ca',
+          'health',
+          '--ca-url',
+          'https://localhost:9000',
+          '--root',
+          '/home/step/certs/root_ca.crt',
+        ]
+      interval: 10s
+      timeout: 5s
+      retries: 5
+      start_period: 30s
+
+  # ─── Server A — Home / Requesting Gateway ────────────────────────────────────
+  postgres-a:
+    image: pgvector/pgvector:pg17
+    container_name: fed-harness-postgres-a
+    restart: unless-stopped
+    ports:
+      - '${PG_A_HOST_PORT:-15432}:5432'
+    environment:
+      POSTGRES_USER: mosaic
+      POSTGRES_PASSWORD: mosaic
+      POSTGRES_DB: mosaic
+    volumes:
+      - pg_a_data:/var/lib/postgresql/data
+      - ../../infra/pg-init:/docker-entrypoint-initdb.d:ro
+    networks:
+      - fed-test-net
+    healthcheck:
+      test: ['CMD-SHELL', 'pg_isready -U mosaic']
+      interval: 5s
+      timeout: 3s
+      retries: 5
+
+  valkey-a:
+    image: valkey/valkey:8-alpine
+    container_name: fed-harness-valkey-a
+    restart: unless-stopped
+    ports:
+      - '${VALKEY_A_HOST_PORT:-16379}:6379'
+    volumes:
+      - valkey_a_data:/data
+    networks:
+      - fed-test-net
+    healthcheck:
+      test: ['CMD', 'valkey-cli', 'ping']
+      interval: 5s
+      timeout: 3s
+      retries: 5
+
+  gateway-a:
+    image: git.mosaicstack.dev/mosaicstack/stack/gateway@sha256:1069117740e00ccfeba357cae38c43f3729fe5ae702740ce474f6512414d7c02
+    # Tag for human reference: sha-9f1a081 (post-#491 IMG-FIX; smoke-tested locally)
+    container_name: fed-harness-gateway-a
+    restart: unless-stopped
+    ports:
+      - '${GATEWAY_A_HOST_PORT:-14001}:3000'
+    environment:
+      MOSAIC_TIER: federated
+      DATABASE_URL: postgres://mosaic:mosaic@postgres-a:5432/mosaic
+      VALKEY_URL: redis://valkey-a:6379
+      GATEWAY_PORT: '3000'
+      GATEWAY_CORS_ORIGIN: 'http://localhost:14001'
+      BETTER_AUTH_SECRET: harness-secret-server-a-do-not-use-in-prod
+      BETTER_AUTH_URL: 'http://gateway-a:3000'
+      STEP_CA_URL: 'https://step-ca:9000'
+      FEDERATION_PEER_HOSTNAME: gateway-a
+      # Bootstrap password for POST /api/bootstrap/setup — used by seed.ts to create
+      # the first admin user. Only valid on a pristine (zero-user) database.
+      # Not the same as ADMIN_API_KEY — there is no static API key in the gateway.
+      ADMIN_BOOTSTRAP_PASSWORD: harness-admin-password-a
+    depends_on:
+      postgres-a:
+        condition: service_healthy
+      valkey-a:
+        condition: service_healthy
+      step-ca:
+        condition: service_healthy
+    networks:
+      - fed-test-net
+    healthcheck:
+      test:
+        [
+          'CMD',
+          'node',
+          '-e',
+          "require('http').get('http://127.0.0.1:3000/api/health', r => process.exit(r.statusCode === 200 ? 0 : 1)).on('error', () => process.exit(1))",
+        ]
+      interval: 10s
+      timeout: 5s
+      retries: 5
+      start_period: 60s
+
+  # ─── Server B — Work / Serving Gateway ──────────────────────────────────────
+  postgres-b:
+    image: pgvector/pgvector:pg17
+    container_name: fed-harness-postgres-b
+    restart: unless-stopped
+    ports:
+      - '${PG_B_HOST_PORT:-15433}:5432'
+    environment:
+      POSTGRES_USER: mosaic
+      POSTGRES_PASSWORD: mosaic
+      POSTGRES_DB: mosaic
+    volumes:
+      - pg_b_data:/var/lib/postgresql/data
+      - ../../infra/pg-init:/docker-entrypoint-initdb.d:ro
+    networks:
+      - fed-test-net
+    healthcheck:
+      test: ['CMD-SHELL', 'pg_isready -U mosaic']
+      interval: 5s
+      timeout: 3s
+      retries: 5
+
+  valkey-b:
+    image: valkey/valkey:8-alpine
+    container_name: fed-harness-valkey-b
+    restart: unless-stopped
+    ports:
+      - '${VALKEY_B_HOST_PORT:-16380}:6379'
+    volumes:
+      - valkey_b_data:/data
+    networks:
+      - fed-test-net
+    healthcheck:
+      test: ['CMD', 'valkey-cli', 'ping']
+      interval: 5s
+      timeout: 3s
+      retries: 5
+
+  gateway-b:
+    image: git.mosaicstack.dev/mosaicstack/stack/gateway@sha256:1069117740e00ccfeba357cae38c43f3729fe5ae702740ce474f6512414d7c02
+    # Tag for human reference: sha-9f1a081 (post-#491 IMG-FIX; smoke-tested locally)
+    container_name: fed-harness-gateway-b
+    restart: unless-stopped
+    ports:
+      - '${GATEWAY_B_HOST_PORT:-14002}:3000'
+    environment:
+      MOSAIC_TIER: federated
+      DATABASE_URL: postgres://mosaic:mosaic@postgres-b:5432/mosaic
+      VALKEY_URL: redis://valkey-b:6379
+      GATEWAY_PORT: '3000'
+      GATEWAY_CORS_ORIGIN: 'http://localhost:14002'
+      BETTER_AUTH_SECRET: harness-secret-server-b-do-not-use-in-prod
+      BETTER_AUTH_URL: 'http://gateway-b:3000'
+      STEP_CA_URL: 'https://step-ca:9000'
+      FEDERATION_PEER_HOSTNAME: gateway-b
+      # Bootstrap password for POST /api/bootstrap/setup — used by seed.ts to create
+      # the first admin user. Only valid on a pristine (zero-user) database.
+      # Not the same as ADMIN_API_KEY — there is no static API key in the gateway.
+      ADMIN_BOOTSTRAP_PASSWORD: harness-admin-password-b
+    depends_on:
+      postgres-b:
+        condition: service_healthy
+      valkey-b:
+        condition: service_healthy
+      step-ca:
+        condition: service_healthy
+    networks:
+      - fed-test-net
+    healthcheck:
+      test:
+        [
+          'CMD',
+          'node',
+          '-e',
+          "require('http').get('http://127.0.0.1:3000/api/health', r => process.exit(r.statusCode === 200 ? 0 : 1)).on('error', () => process.exit(1))",
+        ]
+      interval: 10s
+      timeout: 5s
+      retries: 5
+      start_period: 60s
+
+networks:
+  fed-test-net:
+    name: fed-test-net
+    driver: bridge
+
+volumes:
+  step_ca_data:
+    name: fed-harness-step-ca
+  pg_a_data:
+    name: fed-harness-pg-a
+  valkey_a_data:
+    name: fed-harness-valkey-a
+  pg_b_data:
+    name: fed-harness-pg-b
+  valkey_b_data:
+    name: fed-harness-valkey-b
diff --git a/tools/federation-harness/harness.ts b/tools/federation-harness/harness.ts
new file mode 100644
index 0000000..368870a
--- /dev/null
+++ b/tools/federation-harness/harness.ts
@@ -0,0 +1,290 @@
+/**
+ * tools/federation-harness/harness.ts
+ *
+ * Vitest-consumable helpers for the two-gateway federation harness.
+ *
+ * USAGE (in a vitest test file):
+ *
+ *   import { bootHarness, tearDownHarness, serverA, serverB, seed } from
+ *     '../../tools/federation-harness/harness.js';
+ *
+ *   let handle: HarnessHandle;
+ *
+ *   beforeAll(async () => {
+ *     handle = await bootHarness();
+ *   }, 180_000);
+ *
+ *   afterAll(async () => {
+ *     await tearDownHarness(handle);
+ *   });
+ *
+ *   test('variant A — list tasks', async () => {
+ *     const seedResult = await seed(handle, 'all');
+ *     const a = serverA(handle);
+ *     const res = await fetch(`${a.baseUrl}/api/federation/list/tasks`, {
+ *       headers: { Authorization: `Bearer ${seedResult.adminTokenA}` },
+ *     });
+ *     expect(res.status).toBe(200);
+ *   });
+ *
+ * NOTE: The `seed()` helper currently only supports scenario='all'. Passing any
+ * other value throws immediately. Per-variant narrowing is deferred to M3-11.
+ *
+ * ESM / NodeNext: all imports use .js extensions.
+ */
+
+import { execSync, execFileSync } from 'node:child_process';
+import { resolve, dirname } from 'node:path';
+import { fileURLToPath } from 'node:url';
+import { runSeed, type SeedResult } from './seed.js';
+
+// ─── Types ───────────────────────────────────────────────────────────────────
+
+export interface GatewayAccessor {
+  /** Base URL reachable from the host machine, e.g. http://localhost:14001 */
+  baseUrl: string;
+  /** Bootstrap password used for POST /api/bootstrap/setup on a pristine gateway */
+  bootstrapPassword: string;
+  /** Internal Docker network hostname (for container-to-container calls) */
+  internalHostname: string;
+}
+
+export interface HarnessHandle {
+  /** Server A accessor */
+  a: GatewayAccessor;
+  /** Server B accessor */
+  b: GatewayAccessor;
+  /** Absolute path to the docker-compose file */
+  composeFile: string;
+  /** Whether this instance booted the stack (vs. reusing an existing one) */
+  ownedStack: boolean;
+  /** Optional seed result if seed() was called */
+  seedResult?: SeedResult;
+}
+
+/**
+ * Scenario to seed. Currently only 'all' is implemented; per-variant narrowing
+ * is tracked as M3-11. Passing any other value throws immediately with a clear
+ * error rather than silently over-seeding.
+ */
+export type SeedScenario = 'variantA' | 'variantB' | 'variantC' | 'all';
+
+// ─── Constants ────────────────────────────────────────────────────────────────
+
+const __dirname = dirname(fileURLToPath(import.meta.url));
+const COMPOSE_FILE = resolve(__dirname, 'docker-compose.two-gateways.yml');
+
+const GATEWAY_A_URL = process.env['GATEWAY_A_URL'] ?? 'http://localhost:14001';
+const GATEWAY_B_URL = process.env['GATEWAY_B_URL'] ?? 'http://localhost:14002';
+const ADMIN_BOOTSTRAP_PASSWORD_A =
+  process.env['ADMIN_BOOTSTRAP_PASSWORD_A'] ?? 'harness-admin-password-a';
+const ADMIN_BOOTSTRAP_PASSWORD_B =
+  process.env['ADMIN_BOOTSTRAP_PASSWORD_B'] ?? 'harness-admin-password-b';
+
+const READINESS_TIMEOUT_MS = 180_000;
+const READINESS_POLL_MS = 3_000;
+
+// ─── Internal helpers ─────────────────────────────────────────────────────────
+
+async function isGatewayHealthy(baseUrl: string): Promise<boolean> {
+  try {
+    const res = await fetch(`${baseUrl}/api/health`, { signal: AbortSignal.timeout(5_000) });
+    return res.ok;
+  } catch {
+    return false;
+  }
+}
+
+/**
+ * Poll both gateways in parallel until both are healthy or the shared deadline
+ * expires. Polling in parallel (rather than sequentially) avoids the bug where
+ * a slow gateway-a consumes all of the readiness budget before gateway-b is
+ * checked.
+ */
+async function waitForStack(handle: HarnessHandle): Promise<void> {
+  const gateways: Array<{ label: string; url: string }> = [
+    { label: 'gateway-a', url: handle.a.baseUrl },
+    { label: 'gateway-b', url: handle.b.baseUrl },
+  ];
+
+  await Promise.all(
+    gateways.map(async (gw) => {
+      // Each gateway gets its own independent deadline.
+      const deadline = Date.now() + READINESS_TIMEOUT_MS;
+      process.stdout.write(`[harness] Waiting for ${gw.label}...`);
+
+      while (Date.now() < deadline) {
+        if (await isGatewayHealthy(gw.url)) {
+          process.stdout.write(` ready\n`);
+          return;
+        }
+        if (Date.now() + READINESS_POLL_MS > deadline) {
+          throw new Error(
+            `[harness] ${gw.label} did not become healthy within ${READINESS_TIMEOUT_MS.toString()}ms`,
+          );
+        }
+        await new Promise((r) => setTimeout(r, READINESS_POLL_MS));
+        process.stdout.write('.');
+      }
+
+      throw new Error(
+        `[harness] ${gw.label} did not become healthy within ${READINESS_TIMEOUT_MS.toString()}ms`,
+      );
+    }),
+  );
+}
+
+function isStackRunning(): boolean {
+  try {
+    const output = execFileSync(
+      'docker',
+      ['compose', '-f', COMPOSE_FILE, 'ps', '--format', 'json'],
+      { encoding: 'utf8', stdio: ['pipe', 'pipe', 'pipe'] },
+    );
+
+    if (!output.trim()) return false;
+
+    // Parse JSON lines — each running service emits a JSON object per line
+    const lines = output.trim().split('\n').filter(Boolean);
+    const runningServices = lines.filter((line) => {
+      try {
+        const obj = JSON.parse(line) as { State?: string };
+        return obj.State === 'running';
+      } catch {
+        return false;
+      }
+    });
+
+    // Expect at least gateway-a and gateway-b running
+    return runningServices.length >= 2;
+  } catch {
+    return false;
+  }
+}
+
+// ─── Public API ───────────────────────────────────────────────────────────────
+
+/**
+ * Boot the harness stack.
+ *
+ * Idempotent: if the stack is already running and both gateways are healthy,
+ * this function reuses the existing stack and returns a handle with
+ * `ownedStack: false`. Callers that set `ownedStack: false` should NOT call
+ * `tearDownHarness` unless they explicitly want to tear down a pre-existing stack.
+ *
+ * If the stack is not running, it starts it with `docker compose up -d` and
+ * waits for both gateways to pass their /api/health probe.
+ */
+export async function bootHarness(): Promise<HarnessHandle> {
+  const handle: HarnessHandle = {
+    a: {
+      baseUrl: GATEWAY_A_URL,
+      bootstrapPassword: ADMIN_BOOTSTRAP_PASSWORD_A,
+      internalHostname: 'gateway-a',
+    },
+    b: {
+      baseUrl: GATEWAY_B_URL,
+      bootstrapPassword: ADMIN_BOOTSTRAP_PASSWORD_B,
+      internalHostname: 'gateway-b',
+    },
+    composeFile: COMPOSE_FILE,
+    ownedStack: false,
+  };
+
+  // Check if both gateways are already healthy
+  const [aHealthy, bHealthy] = await Promise.all([
+    isGatewayHealthy(handle.a.baseUrl),
+    isGatewayHealthy(handle.b.baseUrl),
+  ]);
+
+  if (aHealthy && bHealthy) {
+    console.log('[harness] Stack already running — reusing existing stack.');
+    handle.ownedStack = false;
+    return handle;
+  }
+
+  console.log('[harness] Starting federation harness stack...');
+  execSync(`docker compose -f "${COMPOSE_FILE}" up -d`, { stdio: 'inherit' });
+  handle.ownedStack = true;
+
+  await waitForStack(handle);
+  console.log('[harness] Stack is ready.');
+
+  return handle;
+}
+
+/**
+ * Tear down the harness stack.
+ *
+ * Runs `docker compose down -v` to remove containers AND volumes (ephemeral state).
+ * Only tears down if `handle.ownedStack` is true unless `force` is set.
+ */
+export async function tearDownHarness(
+  handle: HarnessHandle,
+  opts?: { force?: boolean },
+): Promise<void> {
+  if (!handle.ownedStack && !opts?.force) {
+    console.log(
+      '[harness] Stack not owned by this handle — skipping teardown (pass force: true to override).',
+    );
+    return;
+  }
+
+  console.log('[harness] Tearing down federation harness stack...');
+  execSync(`docker compose -f "${handle.composeFile}" down -v`, { stdio: 'inherit' });
+  console.log('[harness] Stack torn down.');
+}
+
+/**
+ * Return the Server A accessor from a harness handle.
+ * Convenience wrapper for test readability.
+ */
+export function serverA(handle: HarnessHandle): GatewayAccessor {
+  return handle.a;
+}
+
+/**
+ * Return the Server B accessor from a harness handle.
+ * Convenience wrapper for test readability.
+ */
+export function serverB(handle: HarnessHandle): GatewayAccessor {
+  return handle.b;
+}
+
+/**
+ * Seed the harness with test data for one or more scenarios.
+ *
+ * @param handle   The harness handle returned by bootHarness().
+ * @param scenario Which scope variants to provision. Currently only 'all' is
+ *                 supported — passing any other value throws immediately with a
+ *                 clear error. Per-variant narrowing is tracked as M3-11.
+ *
+ * Returns a SeedResult with grant IDs, peer IDs, and admin tokens for each
+ * gateway, which test assertions can reference.
+ *
+ * IMPORTANT: The harness assumes a pristine database on both gateways. The seed
+ * bootstraps an admin user on each gateway via POST /api/bootstrap/setup. If
+ * either gateway already has users, seed() throws with a clear error message.
+ * Run 'docker compose down -v' to reset state.
+ */
+export async function seed(
+  handle: HarnessHandle,
+  scenario: SeedScenario = 'all',
+): Promise<SeedResult> {
+  if (scenario !== 'all') {
+    throw new Error(
+      `seed: scenario narrowing not yet implemented; pass "all" for now. ` +
+        `Got: "${scenario}". Per-variant narrowing is tracked as M3-11.`,
+    );
+  }
+
+  const result = await runSeed({
+    serverAUrl: handle.a.baseUrl,
+    serverBUrl: handle.b.baseUrl,
+    adminBootstrapPasswordA: handle.a.bootstrapPassword,
+    adminBootstrapPasswordB: handle.b.bootstrapPassword,
+  });
+
+  handle.seedResult = result;
+  return result;
+}
diff --git a/tools/federation-harness/seed.ts b/tools/federation-harness/seed.ts
new file mode 100644
index 0000000..6287f49
--- /dev/null
+++ b/tools/federation-harness/seed.ts
@@ -0,0 +1,603 @@
+#!/usr/bin/env tsx
+/**
+ * tools/federation-harness/seed.ts
+ *
+ * Provisions test data for the two-gateway federation harness.
+ * Run via: tsx tools/federation-harness/seed.ts
+ *
+ * What this script does:
+ *  1. (Optional) Boots the compose stack if --boot flag is passed.
+ *  2. Waits for both gateways to be healthy.
+ *  3. Bootstraps an admin user + token on each gateway via POST /api/bootstrap/setup.
+ *  4. Creates three grants on Server B matching the M3 acceptance test scenarios:
+ *     - Scope variant A: tasks + notes, include_personal: true
+ *     - Scope variant B: tasks only, include_teams: ['T1'], exclude T2
+ *     - Scope variant C: tasks + credentials in resources, credentials excluded (sanity)
+ *  5. For each grant, walks the full enrollment flow:
+ *       a. Server B creates a peer keypair (represents the requesting side).
+ *       b. Server B creates the grant referencing that peer.
+ *       c. Server B issues an enrollment token.
+ *       d. Server A creates its own peer keypair (represents its view of B).
+ *       e. Server A redeems the enrollment token at Server B's enrollment endpoint,
+ *          submitting A's CSR → receives signed cert back.
+ *       f. Server A stores the cert on its peer record → peer becomes active.
+ *  6. Inserts representative test tasks/notes/credentials on Server B.
+ *
+ * IMPORTANT: This script uses the real admin REST API — no direct DB writes.
+ * It exercises the full enrollment flow as M3 acceptance tests will.
+ *
+ * ESM / NodeNext: all imports use .js extensions.
+ */
+
+import { execSync } from 'node:child_process';
+import { resolve, dirname } from 'node:path';
+import { fileURLToPath } from 'node:url';
+
+// ─── Constants ───────────────────────────────────────────────────────────────
+
+const __dirname = dirname(fileURLToPath(import.meta.url));
+const COMPOSE_FILE = resolve(__dirname, 'docker-compose.two-gateways.yml');
+
+/** Base URLs as seen from the host machine (mapped host ports). */
+const SERVER_A_URL = process.env['GATEWAY_A_URL'] ?? 'http://localhost:14001';
+const SERVER_B_URL = process.env['GATEWAY_B_URL'] ?? 'http://localhost:14002';
+
+/**
+ * Bootstrap passwords used when calling POST /api/bootstrap/setup on each
+ * gateway. Each gateway starts with zero users and requires a one-time setup
+ * call before any admin-guarded endpoints can be used.
+ */
+const ADMIN_BOOTSTRAP_PASSWORD_A =
+  process.env['ADMIN_BOOTSTRAP_PASSWORD_A'] ?? 'harness-admin-password-a';
+const ADMIN_BOOTSTRAP_PASSWORD_B =
+  process.env['ADMIN_BOOTSTRAP_PASSWORD_B'] ?? 'harness-admin-password-b';
+
+const READINESS_TIMEOUT_MS = 120_000;
+const READINESS_POLL_MS = 3_000;
+
+// ─── Scope variant definitions (for M3 acceptance tests) ─────────────────────
+
+/** Scope variant A — tasks + notes, personal data included. */
+export const SCOPE_VARIANT_A = {
+  resources: ['tasks', 'notes'],
+  filters: {
+    tasks: { include_personal: true },
+    notes: { include_personal: true },
+  },
+  excluded_resources: [] as string[],
+  max_rows_per_query: 500,
+};
+
+/** Scope variant B — tasks only, team T1 only, no personal. */
+export const SCOPE_VARIANT_B = {
+  resources: ['tasks'],
+  filters: {
+    tasks: { include_teams: ['T1'], include_personal: false },
+  },
+  excluded_resources: [] as string[],
+  max_rows_per_query: 500,
+};
+
+/**
+ * Scope variant C — tasks + credentials in resources list, but credentials
+ * explicitly in excluded_resources. Sanity test: credentials must still be
+ * inaccessible even though they appear in resources.
+ */
+export const SCOPE_VARIANT_C = {
+  resources: ['tasks', 'credentials'],
+  filters: {
+    tasks: { include_personal: true },
+  },
+  excluded_resources: ['credentials'],
+  max_rows_per_query: 500,
+};
+
+// ─── Inline types (no import from packages/types — M3-01 branch not yet merged) ─
+
+interface AdminFetchOptions {
+  method?: string;
+  body?: unknown;
+  adminToken: string;
+}
+
+interface PeerRecord {
+  peerId: string;
+  csrPem: string;
+}
+
+interface GrantRecord {
+  id: string;
+  status: string;
+  scope: unknown;
+}
+
+interface EnrollmentTokenResult {
+  token: string;
+  expiresAt: string;
+  enrollmentUrl: string;
+}
+
+interface EnrollmentRedeemResult {
+  certPem: string;
+  certChainPem: string;
+}
+
+interface BootstrapResult {
+  adminUserId: string;
+  adminToken: string;
+}
+
+export interface SeedResult {
+  serverAUrl: string;
+  serverBUrl: string;
+  adminTokenA: string;
+  adminTokenB: string;
+  adminUserIdA: string;
+  adminUserIdB: string;
+  grants: {
+    variantA: GrantRecord;
+    variantB: GrantRecord;
+    variantC: GrantRecord;
+  };
+  peers: {
+    variantA: PeerRecord & { grantId: string };
+    variantB: PeerRecord & { grantId: string };
+    variantC: PeerRecord & { grantId: string };
+  };
+}
+
+// ─── HTTP helpers ─────────────────────────────────────────────────────────────
+
+/**
+ * Authenticated admin fetch. Sends `Authorization: Bearer <adminToken>` which
+ * is the only path supported by AdminGuard (DB-backed sha256 token lookup).
+ * No `x-admin-key` header path exists in the gateway.
+ */
+async function adminFetch<T>(baseUrl: string, path: string, opts: AdminFetchOptions): Promise<T> {
+  const url = `${baseUrl}${path}`;
+  const res = await fetch(url, {
+    method: opts.method ?? 'GET',
+    headers: {
+      'Content-Type': 'application/json',
+      Authorization: `Bearer ${opts.adminToken}`,
+    },
+    body: opts.body !== undefined ? JSON.stringify(opts.body) : undefined,
+  });
+
+  if (!res.ok) {
+    const text = await res.text().catch(() => '(no body)');
+    throw new Error(`${opts.method ?? 'GET'} ${url} → ${res.status}: ${text}`);
+  }
+
+  return res.json() as Promise<T>;
+}
+
+// ─── Admin bootstrap ──────────────────────────────────────────────────────────
+
+/**
+ * Bootstrap an admin user on a pristine gateway.
+ *
+ * Steps:
+ *  1. GET /api/bootstrap/status — confirms needsSetup === true.
+ *  2. POST /api/bootstrap/setup with { name, email, password } — returns
+ *     { user, token: { plaintext } }.
+ *
+ * The harness assumes a fresh DB. If needsSetup is false the harness fails
+ * fast with a clear error rather than proceeding with an unknown token.
+ */
+async function bootstrapAdmin(
+  baseUrl: string,
+  label: string,
+  password: string,
+): Promise<BootstrapResult> {
+  console.log(`[seed] Bootstrapping admin on ${label} (${baseUrl})...`);
+
+  // 1. Check status
+  const statusRes = await fetch(`${baseUrl}/api/bootstrap/status`);
+  if (!statusRes.ok) {
+    throw new Error(`[seed] GET ${baseUrl}/api/bootstrap/status → ${statusRes.status.toString()}`);
+  }
+  const status = (await statusRes.json()) as { needsSetup: boolean };
+
+  if (!status.needsSetup) {
+    throw new Error(
+      `[seed] ${label} at ${baseUrl} already has users (needsSetup=false). ` +
+        `The harness requires a pristine database. Run 'docker compose down -v' to reset.`,
+    );
+  }
+
+  // 2. Bootstrap
+  const setupRes = await fetch(`${baseUrl}/api/bootstrap/setup`, {
+    method: 'POST',
+    headers: { 'Content-Type': 'application/json' },
+    body: JSON.stringify({
+      name: `Harness Admin (${label})`,
+      email: `harness-admin-${label.toLowerCase().replace(/\s+/g, '-')}@example.invalid`,
+      password,
+    }),
+  });
+
+  if (!setupRes.ok) {
+    const body = await setupRes.text().catch(() => '(no body)');
+    throw new Error(
+      `[seed] POST ${baseUrl}/api/bootstrap/setup → ${setupRes.status.toString()}: ${body}`,
+    );
+  }
+
+  const result = (await setupRes.json()) as {
+    user: { id: string };
+    token: { plaintext: string };
+  };
+
+  console.log(`[seed]   ${label} admin user: ${result.user.id}`);
+  console.log(`[seed]   ${label} admin token: ${result.token.plaintext.slice(0, 8)}...`);
+
+  return {
+    adminUserId: result.user.id,
+    adminToken: result.token.plaintext,
+  };
+}
+
+// ─── Readiness probe ──────────────────────────────────────────────────────────
+
+async function waitForGateway(baseUrl: string, label: string): Promise<void> {
+  const deadline = Date.now() + READINESS_TIMEOUT_MS;
+  let lastError: string = '';
+
+  while (Date.now() < deadline) {
+    try {
+      const res = await fetch(`${baseUrl}/api/health`, { signal: AbortSignal.timeout(5_000) });
+      if (res.ok) {
+        console.log(`[seed] ${label} is ready (${baseUrl})`);
+        return;
+      }
+      lastError = `HTTP ${res.status.toString()}`;
+    } catch (err) {
+      lastError = err instanceof Error ? err.message : String(err);
+    }
+    await new Promise((r) => setTimeout(r, READINESS_POLL_MS));
+  }
+
+  throw new Error(
+    `[seed] ${label} did not become ready within ${READINESS_TIMEOUT_MS.toString()}ms — last error: ${lastError}`,
+  );
+}
+
+// ─── Enrollment flow ──────────────────────────────────────────────────────────
+
+/**
+ * Walk the full enrollment flow for one grant.
+ *
+ * The correct two-sided flow (matching the data model's FK semantics):
+ *
+ *  1. On Server B: POST /api/admin/federation/peers/keypair
+ *       → peerId_B (Server B's peer record representing the requesting side)
+ *  2. On Server B: POST /api/admin/federation/grants with peerId: peerId_B
+ *       → grant (FK to Server B's own federation_peers table — no violation)
+ *  3. On Server B: POST /api/admin/federation/grants/:id/tokens
+ *       → enrollmentUrl pointing back to Server B
+ *  4. On Server A: POST /api/admin/federation/peers/keypair
+ *       → peerId_A + csrPem_A (Server A's local record of Server B)
+ *  5. Server A → Server B: POST enrollmentUrl with { csrPem: csrPem_A }
+ *       → certPem signed by Server B's CA
+ *  6. On Server A: PATCH /api/admin/federation/peers/:peerId_A/cert with certPem
+ *       → Server A's peer record transitions to active
+ *
+ * Returns the activated grant (from Server B) and Server A's peer record.
+ */
+async function enrollGrant(opts: {
+  label: string;
+  subjectUserId: string;
+  scope: unknown;
+  adminTokenA: string;
+  adminTokenB: string;
+  serverAUrl: string;
+  serverBUrl: string;
+}): Promise<{ grant: GrantRecord; peer: PeerRecord & { grantId: string } }> {
+  const { label, subjectUserId, scope, adminTokenA, adminTokenB, serverAUrl, serverBUrl } = opts;
+  console.log(`\n[seed] Enrolling grant for scope variant ${label}...`);
+
+  // 1. Create peer keypair on Server B (represents the requesting peer from B's perspective)
+  const peerB = await adminFetch<PeerRecord>(serverBUrl, '/api/admin/federation/peers/keypair', {
+    method: 'POST',
+    adminToken: adminTokenB,
+    body: {
+      commonName: `harness-peer-${label.toLowerCase()}-from-b`,
+      displayName: `Harness Peer ${label} (Server A as seen from B)`,
+      endpointUrl: serverAUrl,
+    },
+  });
+  console.log(`[seed]   Created peer on B: ${peerB.peerId}`);
+
+  // 2. Create grant on Server B referencing B's own peer record
+  const grant = await adminFetch<GrantRecord>(serverBUrl, '/api/admin/federation/grants', {
+    method: 'POST',
+    adminToken: adminTokenB,
+    body: {
+      peerId: peerB.peerId,
+      subjectUserId,
+      scope,
+    },
+  });
+  console.log(`[seed]   Created grant on B: ${grant.id} (status: ${grant.status})`);
+
+  // 3. Generate enrollment token on Server B
+  const tokenResult = await adminFetch<EnrollmentTokenResult>(
+    serverBUrl,
+    `/api/admin/federation/grants/${grant.id}/tokens`,
+    { method: 'POST', adminToken: adminTokenB, body: { ttlSeconds: 900 } },
+  );
+  console.log(`[seed]   Enrollment token: ${tokenResult.token.slice(0, 8)}...`);
+  console.log(`[seed]   Enrollment URL: ${tokenResult.enrollmentUrl}`);
+
+  // 4. Create peer keypair on Server A (Server A's local record of Server B)
+  const peerA = await adminFetch<PeerRecord>(serverAUrl, '/api/admin/federation/peers/keypair', {
+    method: 'POST',
+    adminToken: adminTokenA,
+    body: {
+      commonName: `harness-peer-${label.toLowerCase()}-from-a`,
+      displayName: `Harness Peer ${label} (Server B as seen from A)`,
+      endpointUrl: serverBUrl,
+    },
+  });
+  console.log(`[seed]   Created peer on A: ${peerA.peerId}`);
+
+  // 5. Redeem token at Server B's enrollment endpoint with A's CSR.
+  //    The enrollment endpoint is not admin-guarded — the one-time token IS the credential.
+  //
+  //    The enrollmentUrl returned by the gateway is built using BETTER_AUTH_URL which
+  //    resolves to the in-cluster Docker hostname (gateway-b:3000). That URL is only
+  //    reachable from other containers, not from the host machine running this script.
+  //    We rewrite the host portion to use the host-accessible serverBUrl so the
+  //    seed script can reach the endpoint from the host.
+  const parsedEnrollment = new URL(tokenResult.enrollmentUrl);
+  const tokenSegment = parsedEnrollment.pathname.split('/').pop()!;
+  const redeemUrl = `${serverBUrl}/api/federation/enrollment/${tokenSegment}`;
+  console.log(`[seed]   Rewritten redeem URL (host-accessible): ${redeemUrl}`);
+  const redeemRes = await fetch(redeemUrl, {
+    method: 'POST',
+    headers: { 'Content-Type': 'application/json' },
+    body: JSON.stringify({ csrPem: peerA.csrPem }),
+  });
+
+  if (!redeemRes.ok) {
+    const body = await redeemRes.text().catch(() => '(no body)');
+    throw new Error(`Enrollment redemption failed: ${redeemRes.status.toString()} — ${body}`);
+  }
+
+  const redeemResult = (await redeemRes.json()) as EnrollmentRedeemResult;
+  console.log(`[seed]   Cert issued (${redeemResult.certPem.length.toString()} bytes)`);
+
+  // 6. Store cert on Server A's peer record → transitions to active
+  await adminFetch<unknown>(serverAUrl, `/api/admin/federation/peers/${peerA.peerId}/cert`, {
+    method: 'PATCH',
+    adminToken: adminTokenA,
+    body: { certPem: redeemResult.certPem },
+  });
+  console.log(`[seed]   Cert stored on A — peer ${peerA.peerId} is now active`);
+
+  // Verify grant flipped to active on B
+  const activeGrant = await adminFetch<GrantRecord>(
+    serverBUrl,
+    `/api/admin/federation/grants/${grant.id}`,
+    { adminToken: adminTokenB },
+  );
+  console.log(`[seed]   Grant status on B: ${activeGrant.status}`);
+
+  return { grant: activeGrant, peer: { ...peerA, grantId: grant.id } };
+}
+
+// ─── Test data insertion ──────────────────────────────────────────────────────
+
+/**
+ * Insert representative test data on Server B via its admin APIs.
+ *
+ * NOTE: The gateway's task/note/credential APIs require an authenticated user
+ * session. For the harness, we seed via admin-level endpoints if available,
+ * or document the gap here for M3-11 to fill in with proper user session seeding.
+ *
+ * ASSUMPTION: Server B exposes POST /api/admin/tasks (or similar) for test data.
+ * If that endpoint does not yet exist, this function logs a warning and skips
+ * without failing — M3-11 will add the session-based seeding path.
+ */
+async function seedTestData(
+  subjectUserId: string,
+  scopeLabel: string,
+  serverBUrl: string,
+  adminTokenB: string,
+): Promise<void> {
+  console.log(`\n[seed] Seeding test data on Server B for ${scopeLabel}...`);
+
+  const testTasks = [
+    {
+      title: `${scopeLabel} Task 1`,
+      description: 'Federation harness test task',
+      userId: subjectUserId,
+    },
+    {
+      title: `${scopeLabel} Task 2`,
+      description: 'Team-scoped test task',
+      userId: subjectUserId,
+      teamId: 'T1',
+    },
+  ];
+
+  const testNotes = [
+    {
+      title: `${scopeLabel} Note 1`,
+      content: 'Personal note for federation test',
+      userId: subjectUserId,
+    },
+  ];
+
+  // Attempt to insert — tolerate 404 (endpoint not yet implemented)
+  for (const task of testTasks) {
+    try {
+      await adminFetch<unknown>(serverBUrl, '/api/admin/tasks', {
+        method: 'POST',
+        adminToken: adminTokenB,
+        body: task,
+      });
+      console.log(`[seed]   Inserted task: "${task.title}"`);
+    } catch (err) {
+      const msg = err instanceof Error ? err.message : String(err);
+      if (msg.includes('404') || msg.includes('Cannot POST')) {
+        console.warn(
+          `[seed]   WARN: /api/admin/tasks not found — skipping task insertion (expected until M3-11)`,
+        );
+        break;
+      }
+      throw err;
+    }
+  }
+
+  for (const note of testNotes) {
+    try {
+      await adminFetch<unknown>(serverBUrl, '/api/admin/notes', {
+        method: 'POST',
+        adminToken: adminTokenB,
+        body: note,
+      });
+      console.log(`[seed]   Inserted note: "${note.title}"`);
+    } catch (err) {
+      const msg = err instanceof Error ? err.message : String(err);
+      if (msg.includes('404') || msg.includes('Cannot POST')) {
+        console.warn(
+          `[seed]   WARN: /api/admin/notes not found — skipping note insertion (expected until M3-11)`,
+        );
+        break;
+      }
+      throw err;
+    }
+  }
+
+  console.log(`[seed] Test data seeding for ${scopeLabel} complete.`);
+}
+
+// ─── Main entrypoint ──────────────────────────────────────────────────────────
+
+export async function runSeed(opts?: {
+  serverAUrl?: string;
+  serverBUrl?: string;
+  adminBootstrapPasswordA?: string;
+  adminBootstrapPasswordB?: string;
+  subjectUserIds?: { variantA: string; variantB: string; variantC: string };
+}): Promise<SeedResult> {
+  const aUrl = opts?.serverAUrl ?? SERVER_A_URL;
+  const bUrl = opts?.serverBUrl ?? SERVER_B_URL;
+  const passwordA = opts?.adminBootstrapPasswordA ?? ADMIN_BOOTSTRAP_PASSWORD_A;
+  const passwordB = opts?.adminBootstrapPasswordB ?? ADMIN_BOOTSTRAP_PASSWORD_B;
+
+  // Use provided or default subject user IDs.
+  // In a real run these would be real user UUIDs from Server B's DB.
+  // For the harness, the admin bootstrap user on Server B is used as the subject.
+  // These are overridden after bootstrap if opts.subjectUserIds is not provided.
+  const subjectIds = opts?.subjectUserIds;
+
+  console.log('[seed] Waiting for gateways to be ready...');
+  await Promise.all([waitForGateway(aUrl, 'Server A'), waitForGateway(bUrl, 'Server B')]);
+
+  // Bootstrap admin users on both gateways (requires pristine DBs).
+  console.log('\n[seed] Bootstrapping admin accounts...');
+  const [bootstrapA, bootstrapB] = await Promise.all([
+    bootstrapAdmin(aUrl, 'Server A', passwordA),
+    bootstrapAdmin(bUrl, 'Server B', passwordB),
+  ]);
+
+  // Default subject user IDs to the admin user on Server B (guaranteed to exist).
+  const resolvedSubjectIds = subjectIds ?? {
+    variantA: bootstrapB.adminUserId,
+    variantB: bootstrapB.adminUserId,
+    variantC: bootstrapB.adminUserId,
+  };
+
+  // Enroll all three scope variants sequentially to avoid race conditions on
+  // the step-ca signing queue. Parallel enrollment would work too but
+  // sequential is easier to debug when something goes wrong.
+  console.log('\n[seed] Enrolling scope variants...');
+  const resultA = await enrollGrant({
+    label: 'A',
+    subjectUserId: resolvedSubjectIds.variantA,
+    scope: SCOPE_VARIANT_A,
+    adminTokenA: bootstrapA.adminToken,
+    adminTokenB: bootstrapB.adminToken,
+    serverAUrl: aUrl,
+    serverBUrl: bUrl,
+  });
+  const resultB = await enrollGrant({
+    label: 'B',
+    subjectUserId: resolvedSubjectIds.variantB,
+    scope: SCOPE_VARIANT_B,
+    adminTokenA: bootstrapA.adminToken,
+    adminTokenB: bootstrapB.adminToken,
+    serverAUrl: aUrl,
+    serverBUrl: bUrl,
+  });
+  const resultC = await enrollGrant({
+    label: 'C',
+    subjectUserId: resolvedSubjectIds.variantC,
+    scope: SCOPE_VARIANT_C,
+    adminTokenA: bootstrapA.adminToken,
+    adminTokenB: bootstrapB.adminToken,
+    serverAUrl: aUrl,
+    serverBUrl: bUrl,
+  });
+
+  // Seed test data on Server B for each scope variant
+  await Promise.all([
+    seedTestData(resolvedSubjectIds.variantA, 'A', bUrl, bootstrapB.adminToken),
+    seedTestData(resolvedSubjectIds.variantB, 'B', bUrl, bootstrapB.adminToken),
+    seedTestData(resolvedSubjectIds.variantC, 'C', bUrl, bootstrapB.adminToken),
+  ]);
+
+  const result: SeedResult = {
+    serverAUrl: aUrl,
+    serverBUrl: bUrl,
+    adminTokenA: bootstrapA.adminToken,
+    adminTokenB: bootstrapB.adminToken,
+    adminUserIdA: bootstrapA.adminUserId,
+    adminUserIdB: bootstrapB.adminUserId,
+    grants: {
+      variantA: resultA.grant,
+      variantB: resultB.grant,
+      variantC: resultC.grant,
+    },
+    peers: {
+      variantA: resultA.peer,
+      variantB: resultB.peer,
+      variantC: resultC.peer,
+    },
+  };
+
+  console.log('\n[seed] Seed complete.');
+  console.log('[seed] Summary:');
+  console.log(`  Variant A grant: ${result.grants.variantA.id} (${result.grants.variantA.status})`);
+  console.log(`  Variant B grant: ${result.grants.variantB.id} (${result.grants.variantB.status})`);
+  console.log(`  Variant C grant: ${result.grants.variantC.id} (${result.grants.variantC.status})`);
+
+  return result;
+}
+
+// ─── CLI entry ────────────────────────────────────────────────────────────────
+
+const isCli =
+  process.argv[1] != null &&
+  fileURLToPath(import.meta.url).endsWith(process.argv[1]!.split('/').pop()!);
+
+if (isCli) {
+  const shouldBoot = process.argv.includes('--boot');
+
+  if (shouldBoot) {
+    console.log('[seed] --boot flag detected — starting compose stack...');
+    execSync(`docker compose -f "${COMPOSE_FILE}" up -d`, { stdio: 'inherit' });
+  }
+
+  runSeed()
+    .then(() => {
+      process.exit(0);
+    })
+    .catch((err) => {
+      console.error('[seed] Fatal:', err);
+      process.exit(1);
+    });
+}