diff --git a/README.md b/README.md
index dfc62b1..f952bbc 100644
--- a/README.md
+++ b/README.md
@@ -6,8 +6,9 @@ surfaces.
 
 ## Current status
 
-Phase 3 is complete, and Phase 4A (Core identity foundation), Phase 4B (identity HTTP transport),
-and Phase 4C (Core authorization) have landed. The repository includes
+Phase 3 is complete; Phase 4A (Core identity foundation), Phase 4B (identity HTTP transport), and
+Phase 4C (Core authorization) have landed; and Phase 5A (module authorization integration) is in
+progress. The repository includes
 public module contracts, independently versioned capabilities, static module manifests,
 deterministic automatic discovery, compiled server loading, a staged capability registry,
 dependency-ordered lifecycle with rollback, module-owned Drizzle table contracts, deterministic
@@ -46,11 +47,22 @@ immutable permission registry; an explicit, single-use, concurrency-safe platfor
 command (`pnpm authorization:bootstrap-owner`, never HTTP-exposed, never inferred from signup
 order); and, in `apps/api`, reusable Hono authorization enforcement plus a narrowly scoped
 `/api/v1/grants` HTTP API for managing *explicit* grants only, requiring an exact trusted `Origin`
-on every mutation. Roles/groups/teams/tenants, an owner-management HTTP endpoint, service
-accounts/API tokens, and module-permission registration remain later work. See
-[Authorization](docs/development/authorization.md) and
+on every mutation. Roles/groups/teams/tenants, an owner-management HTTP endpoint, and service
+accounts/API tokens remain later work. See [Authorization](docs/development/authorization.md) and
 [ADR 0013](docs/architecture/decisions/0013-core-authorization-and-owner-bootstrap.md).
 
+Phase 5A (in progress) wires module-declared permissions into that same authorization system: a
+version-3 module manifest declares permissions as complete `{id, baseline, description}`
+declarations (never a bare string), and `apps/api` discovers every installed module and composes
+one immutable platform permission registry — Core's own declarations plus every module's — before
+constructing its database pool, Better Auth instance, or HTTP server. A module's declared
+permission is then enforceable and grantable through the exact same `requirePermission` middleware
+and `/api/v1/grants` HTTP API Core's own permissions already use, with `core.*` reserved and never
+claimable by a module. `pnpm doctor` and `pnpm modules:validate` both validate permission
+composition. Module HTTP route registration and relationship-derived grant creation remain later
+work. See [Authorization](docs/development/authorization.md#module-permission-composition) and
+[Module development](docs/module-development/README.md).
+
 ## Quick start
 
 Prerequisites: Node.js 24 LTS, pnpm 11.12, and Docker with Compose.
@@ -89,6 +101,8 @@ Modules
     0 modules installed
   ✓ Dependency graph valid
     Dependency order is empty.
+  ✓ Permission registry composed
+    1 permission registered
 
 Data definitions
   ✓ Schemas validated
diff --git a/apps/api/src/app.ts b/apps/api/src/app.ts
index 8acd5fa..fce51be 100644
--- a/apps/api/src/app.ts
+++ b/apps/api/src/app.ts
@@ -1,8 +1,8 @@
 import { isAuthenticatedPrincipal } from "@ekklesia/contracts";
-import {
-  type AuthorizationEvaluator,
-  type AuthorizationStore,
-  CORE_PERMISSION_REGISTRY,
+import type {
+  AuthorizationEvaluator,
+  AuthorizationStore,
+  PermissionRegistry,
 } from "@ekklesia/core/authorization";
 import { type EkklesiaIdentityRuntime, IdentityRuntimeError } from "@ekklesia/core/identity";
 import { Hono } from "hono";
@@ -12,6 +12,10 @@ import { createGrantsRoutes } from "./grants-routes.js";
 
 export interface CreateApiAppAuthorizationOptions {
   readonly evaluator: AuthorizationEvaluator;
+  /** The complete platform permission registry (Core's own declarations plus every installed
+   * module's) — always the caller-composed registry `startApi` produces via
+   * `composeStartupPermissionRegistry`, never a Core-only fallback constructed here. */
+  readonly registry: PermissionRegistry;
   readonly store: AuthorizationStore;
 }
 
@@ -80,7 +84,7 @@ export function createApiApp(options: CreateApiAppOptions): Hono {
     "/",
     createGrantsRoutes({
       evaluator: options.authorization.evaluator,
-      registry: CORE_PERMISSION_REGISTRY,
+      registry: options.authorization.registry,
       runtime,
       store: options.authorization.store,
       trustedOrigins: allowedOrigins,
diff --git a/apps/api/src/platform-permissions.ts b/apps/api/src/platform-permissions.ts
new file mode 100644
index 0000000..155dd70
--- /dev/null
+++ b/apps/api/src/platform-permissions.ts
@@ -0,0 +1,104 @@
+import { readFile } from "node:fs/promises";
+import path from "node:path";
+import { fileURLToPath } from "node:url";
+import type { PermissionRegistry } from "@ekklesia/core/authorization";
+import {
+  composePlatformPermissionRegistry,
+  discoverModules,
+  type DiscoveredModule,
+} from "@ekklesia/core/modules";
+
+export type PlatformPermissionConfigurationErrorCode = "PLATFORM_PERMISSION_CONFIGURATION_INVALID";
+
+/** Never carries the underlying package.json path, filesystem error, module directory, manifest
+ * content, or any other machine detail — a broken repository layout is a startup-time
+ * configuration problem, not something to describe in detail to whatever eventually logs it. */
+export class PlatformPermissionConfigurationError extends Error {
+  readonly code: PlatformPermissionConfigurationErrorCode;
+
+  constructor() {
+    super("Module discovery configuration could not be resolved.");
+    this.name = "PlatformPermissionConfigurationError";
+    this.code = "PLATFORM_PERMISSION_CONFIGURATION_INVALID";
+    this.stack = `${this.name}: ${this.message}`;
+    Object.freeze(this);
+  }
+}
+
+/**
+ * Resolves the repository root from this compiled/source file's own location, never
+ * `process.cwd()` — package-scoped test runs execute with `apps/api` as the working directory,
+ * which would silently point discovery at the wrong `modules/` directory. `apps/api/dist/*.js`
+ * (compiled) and `apps/api/src/*.ts` (source, under a test runner transforming TypeScript
+ * on the fly) sit at the identical depth from the repository root, so the same fixed number of
+ * `..` segments resolves correctly in both.
+ */
+function resolveRepositoryRoot(): string {
+  return path.resolve(path.dirname(fileURLToPath(import.meta.url)), "..", "..", "..");
+}
+
+interface PackageVersionFile {
+  readonly version?: unknown;
+}
+
+async function readPackageVersion(packagePath: string): Promise<string> {
+  let packageData: PackageVersionFile;
+  try {
+    packageData = JSON.parse(await readFile(packagePath, "utf8")) as PackageVersionFile;
+  } catch {
+    throw new PlatformPermissionConfigurationError();
+  }
+  if (typeof packageData.version !== "string" || packageData.version.trim().length === 0) {
+    throw new PlatformPermissionConfigurationError();
+  }
+  return packageData.version;
+}
+
+export interface ComposeStartupPermissionRegistryOptions {
+  /** Test-only escape hatch: overrides the directory module discovery scans for installed
+   * modules. Never set by the executable entry point — production always discovers the real
+   * repository's `modules/` directory. Platform and Module SDK versions are always read from the
+   * real repository's own `package.json` files regardless of this override. */
+  readonly modulesDirectoryOverride?: string;
+  /** Test-only escape hatch: supplies an already-discovered module list directly, skipping the
+   * filesystem discovery step entirely, while still running the real `composePlatformPermissionRegistry`
+   * on it. Exists because a genuine exact permission-id collision between two modules — or between
+   * a module and Core — is unreachable through real discovery: module ids are unique (discovery
+   * itself rejects a duplicate) and every permission id is namespaced by its own module's id, so
+   * no two really-discovered modules can ever contribute the same permission id. Proving
+   * composition's own duplicate-rejection behavior, and that startup has created no pool before it
+   * runs, requires supplying deliberately conflicting declarations directly. Never set by the
+   * executable entry point; mutually exclusive with `modulesDirectoryOverride` in practice, since
+   * only one discovery-bypass is ever needed for a given scenario.
+   */
+  readonly discoveredModulesOverride?: readonly DiscoveredModule[];
+}
+
+/**
+ * Discovers every installed module's manifest and composes the one immutable platform permission
+ * registry (Core's own declarations plus every module's) — the production source of truth
+ * `apps/api` enforces every request against. Only manifests are read; no module server entrypoint
+ * is imported or executed, and no module lifecycle hook runs. Read the platform and Module SDK
+ * versions only when this function actually executes, never at import time.
+ */
+export async function composeStartupPermissionRegistry(
+  options: ComposeStartupPermissionRegistryOptions = {},
+): Promise<PermissionRegistry> {
+  if (options.discoveredModulesOverride) {
+    return composePlatformPermissionRegistry(options.discoveredModulesOverride);
+  }
+
+  const repositoryRoot = resolveRepositoryRoot();
+  const [platformVersion, moduleSdkVersion] = await Promise.all([
+    readPackageVersion(path.join(repositoryRoot, "package.json")),
+    readPackageVersion(path.join(repositoryRoot, "packages/module-sdk/package.json")),
+  ]);
+
+  const modules = await discoverModules({
+    moduleSdkVersion,
+    modulesDirectory: options.modulesDirectoryOverride ?? path.join(repositoryRoot, "modules"),
+    platformVersion,
+  });
+
+  return composePlatformPermissionRegistry(modules);
+}
diff --git a/apps/api/src/server.ts b/apps/api/src/server.ts
index c4b2760..754df1b 100644
--- a/apps/api/src/server.ts
+++ b/apps/api/src/server.ts
@@ -5,11 +5,13 @@ import {
   createAuthorizationStore,
 } from "@ekklesia/core/authorization";
 import { createIdentityRuntime, type IdentityRateLimitOverride } from "@ekklesia/core/identity";
+import type { DiscoveredModule } from "@ekklesia/core/modules";
 import { serve, type ServerType } from "@hono/node-server";
 import type { Hono } from "hono";
 import { createApiApp } from "./app.js";
 import { type ApiRuntimeConfig, createApiRuntimeConfig } from "./config.js";
 import { type ApiDatabaseHandle, createApiDatabase } from "./database.js";
+import { composeStartupPermissionRegistry } from "./platform-permissions.js";
 
 const connectionTimeoutMs = 10_000;
 const operationTimeoutMs = 10_000;
@@ -24,6 +26,13 @@ export interface StartApiOptions {
   /** Test-only escape hatch: never set by the executable entry point. See
    * `IdentityRateLimitOverride` for why production must leave this unset. */
   readonly rateLimitOverride?: IdentityRateLimitOverride;
+  /** Test-only escape hatch: overrides the directory permission-composition module discovery
+   * scans. Never set by the executable entry point. See `composeStartupPermissionRegistry`. */
+  readonly modulesDirectoryOverride?: string;
+  /** Test-only escape hatch: supplies an already-discovered module list directly to permission
+   * composition, skipping filesystem discovery entirely. Never set by the executable entry
+   * point. See `composeStartupPermissionRegistry`. */
+  readonly discoveredModulesOverride?: readonly DiscoveredModule[];
 }
 
 async function safeCloseDatabase(handle: ApiDatabaseHandle): Promise<void> {
@@ -54,18 +63,33 @@ function bindServer(app: Hono, host: string, port: number): Promise<ServerType>
 }
 
 /**
- * Explicit runtime composition: constructs the one shared PostgreSQL pool (identity and
- * authorization each get a typed Drizzle view over it — see `./database.js`), the Core identity
- * runtime, the Core authorization store and evaluator, and the Hono application, then starts the
- * Node HTTP server. Returns a handle with an idempotent `shutdown()` so both the executable entry
- * point and tests can stop the server and close the shared pool deterministically. Never applies
- * migrations or generates schema — the database must already be migrated through
- * `pnpm migrations:apply`.
+ * Explicit runtime composition, in a fixed order: discover every installed module's manifest and
+ * compose the one immutable platform permission registry from Core's own declarations plus every
+ * module's (`./platform-permissions.js`); only then construct the one shared PostgreSQL pool
+ * (identity and authorization each get a typed Drizzle view over it — see `./database.js`), the
+ * Core identity runtime, the Core authorization store and evaluator, and the Hono application;
+ * finally start the Node HTTP server. Permission composition happens before any pool, Better
+ * Auth instance, or HTTP server exists, so a discovery or composition failure — an invalid
+ * manifest, a dependency-graph problem, a module claiming the reserved `core.*` permission
+ * namespace, or a duplicate permission declaration — leaves nothing to close: it propagates
+ * directly, and startup never falls back to a Core-only registry. Returns a handle with an
+ * idempotent `shutdown()` so both the executable entry point and tests can stop the server and
+ * close the shared pool deterministically. Never applies migrations or generates schema — the
+ * database must already be migrated through `pnpm migrations:apply`.
  */
 export async function startApi(
   config: ApiRuntimeConfig,
   options: StartApiOptions = {},
 ): Promise<RunningApi> {
+  const permissionRegistry = await composeStartupPermissionRegistry({
+    ...(options.modulesDirectoryOverride === undefined
+      ? {}
+      : { modulesDirectoryOverride: options.modulesDirectoryOverride }),
+    ...(options.discoveredModulesOverride === undefined
+      ? {}
+      : { discoveredModulesOverride: options.discoveredModulesOverride }),
+  });
+
   const databaseHandle = createApiDatabase(config.databaseUrl, {
     connectionTimeoutMs,
     operationTimeoutMs,
@@ -91,7 +115,7 @@ export async function startApi(
   });
 
   const app = createApiApp({
-    authorization: { evaluator, store: authorizationStore },
+    authorization: { evaluator, registry: permissionRegistry, store: authorizationStore },
     runtime,
     trustedOrigins: config.identity.trustedOrigins,
   });
diff --git a/apps/api/tests/api-http.integration.ts b/apps/api/tests/api-http.integration.ts
index c997385..812d963 100644
--- a/apps/api/tests/api-http.integration.ts
+++ b/apps/api/tests/api-http.integration.ts
@@ -8,12 +8,13 @@ import {
   createAuthorizationDrizzleDatabase,
   createAuthorizationStore,
 } from "@ekklesia/core/authorization";
+import { diagnoseEkklesia } from "@ekklesia/core/diagnostics";
 import {
   applyModuleMigrations,
   createPostgresMigrationDatabase,
   discoverAllMigrations,
 } from "@ekklesia/core/migrations";
-import { mkdtemp, rm } from "node:fs/promises";
+import { mkdir, mkdtemp, rm, writeFile } from "node:fs/promises";
 import { tmpdir } from "node:os";
 import { Client, type QueryResultRow } from "pg";
 import { afterAll, describe, expect, it, vi } from "vitest";
@@ -198,10 +199,60 @@ async function applyCoreMigrations(url: string): Promise<void> {
   }
 }
 
+const trackedModulesDirectories: string[] = [];
+
+async function createFixtureModulesDirectory(): Promise<string> {
+  const directory = await mkdtemp(path.join(tmpdir(), "ekklesia-api-installed-modules-"));
+  trackedModulesDirectories.push(directory);
+  return directory;
+}
+
+/** Writes a minimal, structurally valid module fixture straight to disk — real discovery and
+ * composition run against it, nothing hand-built. The server entrypoint throws immediately if
+ * ever imported/executed, so a scenario that completes without that throw firing is direct
+ * evidence permission composition and Doctor never ran module lifecycle code. */
+async function createFixtureModule(
+  modulesDirectory: string,
+  id: string,
+  manifestOverrides: Record<string, unknown> = {},
+): Promise<void> {
+  const moduleDirectory = path.join(modulesDirectory, id);
+  const manifest = {
+    manifestVersion: 3,
+    id,
+    name: `${id} module`,
+    version: "0.1.0",
+    compatibility: { platform: "^0.1.0", moduleSdk: "^0.1.0" },
+    permissions: [],
+    entrypoints: { server: "./dist/server/index.js" },
+    ...manifestOverrides,
+  };
+  await mkdir(path.join(moduleDirectory, "dist/server"), { recursive: true });
+  await Promise.all([
+    writeFile(
+      path.join(moduleDirectory, "package.json"),
+      JSON.stringify({
+        ekklesia: { layer: "module" },
+        name: `@test/module-${id}`,
+        type: "module",
+        version: "0.1.0",
+      }),
+    ),
+    writeFile(path.join(moduleDirectory, "ekklesia.module.json"), JSON.stringify(manifest)),
+    writeFile(
+      path.join(moduleDirectory, "dist/server/index.js"),
+      'throw new Error("permission composition and Doctor must never execute a module server entrypoint");',
+    ),
+  ]);
+}
+
 afterAll(async () => {
   for (const databaseName of [...trackedDatabases]) {
     await dropDatabase(databaseName).catch(() => undefined);
   }
+  for (const directory of trackedModulesDirectories.splice(0)) {
+    await rm(directory, { recursive: true, force: true }).catch(() => undefined);
+  }
 });
 
 const identityBaseUrl = "https://identity.example.test";
@@ -216,7 +267,10 @@ function apiOrigin(running: RunningApi): string {
   return `http://127.0.0.1:${address.port}`;
 }
 
-async function startMigratedApi(url: string): Promise<RunningApi> {
+async function startMigratedApi(
+  url: string,
+  modulesDirectoryOverride?: string,
+): Promise<RunningApi> {
   const config = createApiRuntimeConfig(
     {
       databaseUrl: url,
@@ -230,10 +284,15 @@ async function startMigratedApi(url: string): Promise<RunningApi> {
     },
     { environment: "production" },
   );
-  // A narrow, explicit test-only override: production composition (server.ts's `main`) never
-  // sets this. Better Auth's built-in sign-in/sign-up rate limit (3 requests / 10s) would
-  // otherwise reject this suite's many sequential requests.
-  return startApi(config, { rateLimitOverride: { max: 1000, window: 1 } });
+  // Narrow, explicit test-only overrides: production composition (server.ts's `main`) never sets
+  // either. Better Auth's built-in sign-in/sign-up rate limit (3 requests / 10s) would otherwise
+  // reject this suite's many sequential requests; `modulesDirectoryOverride` lets a scenario point
+  // permission composition at a real, disposable module fixture instead of this repository's own
+  // (empty) `modules/` directory.
+  return startApi(config, {
+    rateLimitOverride: { max: 1000, window: 1 },
+    ...(modulesDirectoryOverride === undefined ? {} : { modulesDirectoryOverride }),
+  });
 }
 
 function setCookieValue(response: Response): string | undefined {
@@ -1147,3 +1206,198 @@ describe.sequential("live API — authorization infrastructure failure is never
     await running.shutdown().catch(() => undefined);
   });
 });
+
+describe.sequential("live API — installed module permission end-to-end", () => {
+  let database: { name: string; url: string };
+  let modulesDirectory: string;
+  let running: RunningApi;
+  let origin: string;
+  let ownerUserId: string;
+  let ownerCookie: string;
+  let createdGrantId: string;
+
+  const ownerEmail = "authz-module-owner@example.test";
+  const password = "correct horse battery staple";
+  const modulePermission = "calendar.events.read";
+  const absentPermission = "scheduler.jobs.read";
+
+  it("recognizes a real installed module and passes permission composition in Doctor, without executing its server entrypoint", async () => {
+    modulesDirectory = await createFixtureModulesDirectory();
+    await createFixtureModule(modulesDirectory, "calendar", {
+      permissions: [
+        {
+          baseline: "restricted",
+          description: "Read calendar events.",
+          id: modulePermission,
+        },
+      ],
+    });
+
+    const report = await diagnoseEkklesia({
+      moduleDiscovery: {
+        moduleSdkVersion: "0.1.0",
+        modulesDirectory,
+        platformVersion: "0.1.0",
+      },
+      runtimeRange: ">=0.0.0",
+      runtimeVersion: process.versions.node,
+    });
+
+    // Only the module-discovery and permission-composition checks are asserted here — this
+    // fixture's temporary directory has no `core/drizzle` sibling for migrations discovery to
+    // find, which is unrelated to what this scenario proves and would otherwise make the check
+    // fail for a reason that has nothing to do with permission composition.
+    const moduleCheck = report.checks.find((entry) => entry.id === "modules.discovery");
+    const permissionsCheck = report.checks.find((entry) => entry.id === "modules.permissions");
+    expect(moduleCheck).toMatchObject({ status: "pass" });
+    expect(moduleCheck?.details).toEqual(["calendar 0.1.0"]);
+    expect(permissionsCheck).toMatchObject({
+      details: ["2 permissions registered"],
+      status: "pass",
+    });
+  });
+
+  it("starts a freshly migrated API that discovers the fixture module and signs up its eventual owner", async () => {
+    database = await createDisposableDatabase("authz_module");
+    await applyCoreMigrations(database.url);
+    running = await startMigratedApi(database.url, modulesDirectory);
+    origin = apiOrigin(running);
+
+    const ownerSignUp = await signUp(origin, ownerEmail, password);
+    ownerUserId = ownerSignUp.body.user.id;
+    const ownerSignIn = await signIn(origin, ownerEmail, password);
+    expect(ownerSignIn.cookiePair).toBeDefined();
+    ownerCookie = ownerSignIn.cookiePair as string;
+
+    // Existing identity HTTP behavior — sign-up, sign-in, principal resolution — is unaffected by
+    // module permission composition running before it.
+    const { body: principal } = await fetchPrincipal(origin, { cookie: ownerCookie });
+    expect(principal).toEqual({ kind: "authenticated", userId: ownerUserId });
+  });
+
+  it("lets an authorized platform owner create an explicit grant for the installed module's permission, with an atomic audit entry using its declared id", async () => {
+    const { close, db } = createAuthorizationDrizzleDatabase(database.url);
+    try {
+      await bootstrapPlatformOwner(db, { userId: parseEkklesiaUserId(ownerUserId) });
+    } finally {
+      await close();
+    }
+
+    const response = await fetch(`${origin}/api/v1/grants`, {
+      body: JSON.stringify({ permission: modulePermission, subjectUserId: ownerUserId }),
+      headers: {
+        "content-type": "application/json",
+        cookie: ownerCookie,
+        origin: dashboardOrigin,
+      },
+      method: "POST",
+    });
+    const body = (await response.json()) as {
+      grant: { id: string; permission: string; sourceKind: string };
+    };
+
+    expect(response.status).toBe(201);
+    expect(body.grant.permission).toBe(modulePermission);
+    expect(body.grant.sourceKind).toBe("explicit");
+    createdGrantId = body.grant.id;
+
+    const admin = await openAdministrativeClient(database.url);
+    try {
+      const rows = await safeQuery<{ action: string; permission_id: string }>(
+        admin,
+        `select action, permission_id from "ekklesia_core_authorization_audit" where grant_id = $1`,
+        [createdGrantId],
+      );
+      expect(rows).toEqual([{ action: "grant.create", permission_id: modulePermission }]);
+    } finally {
+      await safeEnd(admin);
+    }
+  });
+
+  it("rejects a well-formed permission absent from the composed registry, writing no grant or audit row", async () => {
+    const admin = await openAdministrativeClient(database.url);
+    try {
+      const before = await safeQuery<{ count: string }>(
+        admin,
+        `select count(*)::text as count from "ekklesia_core_grant" where permission_id = $1`,
+        [absentPermission],
+      );
+      expect(before[0]?.count).toBe("0");
+
+      const response = await fetch(`${origin}/api/v1/grants`, {
+        body: JSON.stringify({ permission: absentPermission, subjectUserId: ownerUserId }),
+        headers: {
+          "content-type": "application/json",
+          cookie: ownerCookie,
+          origin: dashboardOrigin,
+        },
+        method: "POST",
+      });
+      expect(response.status).toBe(403);
+      expect(await response.json()).toEqual({ code: "AUTHORIZATION_DENIED" });
+
+      const after = await safeQuery<{ count: string }>(
+        admin,
+        `select count(*)::text as count from "ekklesia_core_grant" where permission_id = $1`,
+        [absentPermission],
+      );
+      const audit = await safeQuery<{ count: string }>(
+        admin,
+        `select count(*)::text as count from "ekklesia_core_authorization_audit" where permission_id = $1`,
+        [absentPermission],
+      );
+      expect(after[0]?.count).toBe("0");
+      expect(audit[0]?.count).toBe("0");
+    } finally {
+      await safeEnd(admin);
+    }
+  });
+
+  it("shuts down cleanly, then restarting with the module absent rejects new grants for its former permission while the historical grant remains listable", async () => {
+    await running.shutdown();
+    const remainingConnections = await activeConnectionCount(database.name);
+    expect(remainingConnections).toBe(0);
+
+    const emptyModulesDirectory = await createFixtureModulesDirectory();
+    running = await startMigratedApi(database.url, emptyModulesDirectory);
+    origin = apiOrigin(running);
+
+    // The owner's existing session survives the restart (sessions live in the database, not the
+    // process), so the same cookie proves ordinary identity behavior is unaffected here too.
+    const listResponse = await fetch(`${origin}/api/v1/grants?userId=${ownerUserId}`, {
+      headers: { cookie: ownerCookie },
+    });
+    const listBody = (await listResponse.json()) as {
+      grants: Array<{ id: string; permission: string }>;
+    };
+    expect(listResponse.status).toBe(200);
+    expect(listBody.grants.some((grant) => grant.id === createdGrantId)).toBe(true);
+
+    const response = await fetch(`${origin}/api/v1/grants`, {
+      body: JSON.stringify({ permission: modulePermission, subjectUserId: ownerUserId }),
+      headers: {
+        "content-type": "application/json",
+        cookie: ownerCookie,
+        origin: dashboardOrigin,
+      },
+      method: "POST",
+    });
+    expect(response.status).toBe(403);
+    expect(await response.json()).toEqual({ code: "AUTHORIZATION_DENIED" });
+
+    const admin = await openAdministrativeClient(database.url);
+    try {
+      const rows = await safeQuery<{ id: string }>(
+        admin,
+        `select id from "ekklesia_core_grant" where id = $1 and revoked_at is null`,
+        [createdGrantId],
+      );
+      expect(rows).toHaveLength(1);
+    } finally {
+      await safeEnd(admin);
+    }
+
+    await running.shutdown();
+    await dropDatabase(database.name);
+  });
+});
diff --git a/apps/api/tests/app.test.ts b/apps/api/tests/app.test.ts
index d2d590c..a7d9e27 100644
--- a/apps/api/tests/app.test.ts
+++ b/apps/api/tests/app.test.ts
@@ -1,5 +1,10 @@
 import { ANONYMOUS_PRINCIPAL, createAuthenticatedPrincipal } from "@ekklesia/contracts";
-import type { AuthorizationEvaluator, AuthorizationStore } from "@ekklesia/core/authorization";
+import {
+  type AuthorizationEvaluator,
+  type AuthorizationStore,
+  CORE_PERMISSION_REGISTRY,
+  type PermissionRegistry,
+} from "@ekklesia/core/authorization";
 import type { EkklesiaIdentityRuntime } from "@ekklesia/core/identity";
 import { IdentityRuntimeError } from "@ekklesia/core/identity";
 import { describe, expect, it, vi } from "vitest";
@@ -17,9 +22,13 @@ function fakeRuntime(overrides: Partial<EkklesiaIdentityRuntime> = {}): Ekklesia
 }
 
 /** A fully stubbed authorization dependency, denying everything by default — tests that need a
- * specific decision or store behavior override the relevant method explicitly. */
+ * specific decision or store behavior override the relevant method explicitly. The registry
+ * defaults to Core's own (containing `core.authorization.grants.manage`, exactly what the
+ * mounted grants routes require) rather than an app-composed one — these tests exercise
+ * `createApiApp`'s wiring, not permission composition itself. */
 function fakeAuthorization(overrides: {
   readonly evaluator?: Partial<AuthorizationEvaluator>;
+  readonly registry?: PermissionRegistry;
   readonly store?: Partial<AuthorizationStore>;
 }): CreateApiAppAuthorizationOptions {
   const evaluator: AuthorizationEvaluator = {
@@ -36,7 +45,7 @@ function fakeAuthorization(overrides: {
     revokePlatformOwner: vi.fn(),
     ...overrides.store,
   };
-  return { evaluator, store };
+  return { evaluator, registry: overrides.registry ?? CORE_PERMISSION_REGISTRY, store };
 }
 
 describe("Ekklesia API Hono application", () => {
@@ -253,6 +262,32 @@ describe("Ekklesia API Hono application", () => {
       expect(evaluatorAuthorize).toHaveBeenCalledTimes(1);
     });
 
+    it("uses exactly the injected registry, never an internal Core-only fallback", async () => {
+      // An evaluator that would allow anything, paired with a registry that does not register
+      // core.authorization.grants.manage at all — if createApiApp silently fell back to some
+      // internal registry containing it, this request would reach the evaluator and succeed
+      // (empty grant list, 200). It must instead fail closed on the registry lookup itself.
+      const evaluatorAuthorize = vi.fn().mockResolvedValue({ allowed: true });
+      const emptyRegistry: PermissionRegistry = {
+        declarations: [],
+        get: () => undefined,
+        has: () => false,
+      };
+      const app = createApiApp({
+        authorization: fakeAuthorization({
+          evaluator: { authorize: evaluatorAuthorize },
+          registry: emptyRegistry,
+        }),
+        runtime: fakeRuntime(),
+        trustedOrigins: [trustedOrigin],
+      });
+
+      const response = await app.request("/api/v1/grants?userId=user_123");
+
+      expect(response.status).toBe(403);
+      expect(evaluatorAuthorize).not.toHaveBeenCalled();
+    });
+
     it("applies the same CORS policy to /api/v1/grants as the rest of /api/v1/*", async () => {
       const app = createApiApp({
         authorization: fakeAuthorization({}),
diff --git a/apps/api/tests/authorization-middleware.test.ts b/apps/api/tests/authorization-middleware.test.ts
index 9c42136..bb4aee2 100644
--- a/apps/api/tests/authorization-middleware.test.ts
+++ b/apps/api/tests/authorization-middleware.test.ts
@@ -151,6 +151,153 @@ describe("requirePermission", () => {
   });
 });
 
+describe("requirePermission with module-owned declarations", () => {
+  // requirePermission has no concept of "Core" versus "module" — it only ever consults whatever
+  // registry it is given. These declarations are shaped exactly like what
+  // composePlatformPermissionRegistry would contribute from a real installed module (an id
+  // namespaced under a module, not `core.*`), proving the shared enforcement mechanism works
+  // identically for module-owned permissions without any module-specific code path.
+  const modulePublicPermission = parsePermissionId("calendar.events.list");
+  const moduleAuthenticatedPermission = parsePermissionId("calendar.events.subscribe");
+  const moduleRestrictedPermission = parsePermissionId("calendar.events.read");
+  const moduleRegistry = createPermissionRegistry([
+    {
+      baseline: "public",
+      description: "List public calendar events.",
+      id: modulePublicPermission,
+    },
+    {
+      baseline: "authenticated",
+      description: "Subscribe to calendar event updates.",
+      id: moduleAuthenticatedPermission,
+    },
+    {
+      baseline: "restricted",
+      description: "Read calendar events.",
+      id: moduleRestrictedPermission,
+    },
+  ]);
+
+  function buildModuleApp(
+    permissionId: typeof modulePublicPermission,
+    options: Partial<AuthorizationMiddlewareOptions> = {},
+  ) {
+    const runtime = options.runtime ?? fakeRuntime();
+    const evaluator = options.evaluator ?? fakeEvaluator();
+    const middlewareOptions: AuthorizationMiddlewareOptions = {
+      evaluator,
+      registry: moduleRegistry,
+      runtime,
+    };
+    const app = new Hono();
+    app.get("/module-protected", requirePermission(middlewareOptions, permissionId), (c) =>
+      c.json({ ok: true }),
+    );
+    return { app, evaluator, runtime };
+  }
+
+  it("resolves a module public permission's trusted baseline and passes it to the evaluator unchanged", async () => {
+    const evaluator = fakeEvaluator();
+    const { app } = buildModuleApp(modulePublicPermission, { evaluator });
+
+    const response = await app.request("/module-protected");
+
+    expect(response.status).toBe(200);
+    expect(evaluator.authorize).toHaveBeenCalledWith(
+      expect.objectContaining({ baseline: "public", permission: modulePublicPermission }),
+    );
+  });
+
+  it("resolves a module authenticated permission's trusted baseline and passes it to the evaluator unchanged", async () => {
+    const evaluator = fakeEvaluator();
+    const { app } = buildModuleApp(moduleAuthenticatedPermission, { evaluator });
+
+    await app.request("/module-protected");
+
+    expect(evaluator.authorize).toHaveBeenCalledWith(
+      expect.objectContaining({
+        baseline: "authenticated",
+        permission: moduleAuthenticatedPermission,
+      }),
+    );
+  });
+
+  it("resolves a module restricted permission's trusted baseline and passes it to the evaluator unchanged", async () => {
+    const evaluator = fakeEvaluator();
+    const { app } = buildModuleApp(moduleRestrictedPermission, { evaluator });
+
+    await app.request("/module-protected");
+
+    expect(evaluator.authorize).toHaveBeenCalledWith(
+      expect.objectContaining({
+        baseline: "restricted",
+        permission: moduleRestrictedPermission,
+      }),
+    );
+  });
+
+  it("allows an anonymous principal for a module permission the evaluator authorizes", async () => {
+    const runtime = fakeRuntime({
+      resolvePrincipal: vi.fn().mockResolvedValue(ANONYMOUS_PRINCIPAL),
+    });
+    const { app } = buildModuleApp(modulePublicPermission, { runtime });
+
+    const response = await app.request("/module-protected");
+
+    expect(response.status).toBe(200);
+  });
+
+  it("allows an authenticated principal for a module permission the evaluator authorizes", async () => {
+    const runtime = fakeRuntime({
+      resolvePrincipal: vi.fn().mockResolvedValue(createAuthenticatedPrincipal("user_1")),
+    });
+    const { app } = buildModuleApp(moduleRestrictedPermission, { runtime });
+
+    const response = await app.request("/module-protected");
+
+    expect(response.status).toBe(200);
+  });
+
+  it("denies a module permission the evaluator does not authorize", async () => {
+    const evaluator = fakeEvaluator({ authorize: vi.fn().mockResolvedValue({ allowed: false }) });
+    const { app } = buildModuleApp(moduleRestrictedPermission, { evaluator });
+
+    const response = await app.request("/module-protected");
+
+    expect(response.status).toBe(403);
+    expect(await response.json()).toEqual({ code: "AUTHORIZATION_DENIED" });
+  });
+
+  it("denies a module permission absent from the registry before ever resolving the principal or consulting the evaluator", async () => {
+    const runtime = fakeRuntime();
+    const evaluator = fakeEvaluator();
+    const absentModulePermission = parsePermissionId("scheduler.jobs.read");
+    const { app } = buildModuleApp(absentModulePermission, { evaluator, runtime });
+
+    const response = await app.request("/module-protected");
+
+    expect(response.status).toBe(403);
+    expect(await response.json()).toEqual({ code: "AUTHORIZATION_DENIED" });
+    expect(runtime.resolvePrincipal).not.toHaveBeenCalled();
+    expect(evaluator.authorize).not.toHaveBeenCalled();
+  });
+
+  it("never lets a client-supplied baseline influence the decision for a module permission", async () => {
+    const evaluator = fakeEvaluator();
+    const { app } = buildModuleApp(moduleRestrictedPermission, { evaluator });
+
+    // A hostile client cannot make a restricted module permission behave as public — there is no
+    // request field the middleware reads for a baseline override.
+    await app.request("/module-protected", {
+      headers: { "x-baseline": "public", "x-permission-baseline": "public" },
+    });
+
+    expect(evaluator.authorize).toHaveBeenCalledWith(
+      expect.objectContaining({ baseline: "restricted" }),
+    );
+  });
+});
+
 describe("getAuthorizedPrincipal", () => {
   it("throws when called on a route requirePermission never protected", async () => {
     const { app } = buildTestApp();
diff --git a/apps/api/tests/lifecycle.test.ts b/apps/api/tests/lifecycle.test.ts
index 6e5a3dd..c58cdae 100644
--- a/apps/api/tests/lifecycle.test.ts
+++ b/apps/api/tests/lifecycle.test.ts
@@ -1,4 +1,7 @@
-import { beforeEach, describe, expect, it, vi } from "vitest";
+import { mkdir, mkdtemp, rm, writeFile } from "node:fs/promises";
+import { tmpdir } from "node:os";
+import path from "node:path";
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
 
 const poolConstructor = vi.hoisted(() => vi.fn());
 const betterAuthMock = vi.hoisted(() => vi.fn());
@@ -11,6 +14,13 @@ vi.mock("better-auth", () => ({ betterAuth: betterAuthMock }));
 vi.mock("better-auth/adapters/drizzle", () => ({ drizzleAdapter: drizzleAdapterMock }));
 vi.mock("@hono/node-server", () => ({ serve: serveMock }));
 
+import { parseModuleManifest } from "@ekklesia/contracts";
+import {
+  type DiscoveredModule,
+  ModuleDiscoveryError,
+  ModulePermissionCompositionError,
+} from "@ekklesia/core/modules";
+import { PermissionRegistryError } from "@ekklesia/core/authorization";
 import { createApiRuntimeConfig } from "../src/config.js";
 import { installShutdownSignalHandlers, startApi } from "../src/server.js";
 
@@ -30,6 +40,84 @@ function validConfig() {
   );
 }
 
+const temporaryDirectories: string[] = [];
+
+async function createTemporaryModulesDirectory(): Promise<string> {
+  const directory = await mkdtemp(path.join(tmpdir(), "ekklesia-api-modules-"));
+  temporaryDirectories.push(directory);
+  return directory;
+}
+
+/** Writes a minimal, structurally valid module fixture (package.json + ekklesia.module.json +
+ * server entrypoint) directly to disk — real discovery, not a hand-built double. The server
+ * entrypoint throws immediately if ever imported, so a test that reaches a successful assertion
+ * without that throw firing is direct evidence permission composition never executed it. */
+async function createFixtureModule(
+  modulesDirectory: string,
+  id: string,
+  manifestOverrides: Record<string, unknown> = {},
+): Promise<void> {
+  const moduleDirectory = path.join(modulesDirectory, id);
+  const manifest = {
+    manifestVersion: 3,
+    id,
+    name: `${id} module`,
+    version: "0.1.0",
+    compatibility: { platform: "^0.1.0", moduleSdk: "^0.1.0" },
+    permissions: [],
+    entrypoints: { server: "./dist/server/index.js" },
+    ...manifestOverrides,
+  };
+  await mkdir(path.join(moduleDirectory, "dist/server"), { recursive: true });
+  await Promise.all([
+    writeFile(
+      path.join(moduleDirectory, "package.json"),
+      JSON.stringify({
+        ekklesia: { layer: "module" },
+        name: `@test/module-${id}`,
+        type: "module",
+        version: "0.1.0",
+      }),
+    ),
+    writeFile(path.join(moduleDirectory, "ekklesia.module.json"), JSON.stringify(manifest)),
+    writeFile(
+      path.join(moduleDirectory, "dist/server/index.js"),
+      'throw new Error("permission composition must never execute a module server entrypoint");',
+    ),
+  ]);
+}
+
+/** A minimal, in-memory `DiscoveredModule` double built from a real, validated manifest — used
+ * only to reach `composePlatformPermissionRegistry`'s own duplicate-detection failure, which is
+ * unreachable through real filesystem discovery (module ids are unique and every permission id is
+ * namespaced by its own module's id, so no two genuinely discovered modules can ever collide). */
+function fakeDiscoveredModule(
+  id: string,
+  permissions: ReadonlyArray<{
+    id: string;
+    baseline: "public" | "authenticated" | "restricted";
+    description: string;
+  }>,
+): DiscoveredModule {
+  const manifest = parseModuleManifest({
+    capabilities: [],
+    compatibility: { moduleSdk: "^0.1.0", platform: "^0.1.0" },
+    entrypoints: { server: "./dist/does-not-exist/index.js" },
+    id,
+    manifestVersion: 3,
+    name: `${id} module`,
+    permissions,
+    version: "0.1.0",
+  });
+  return {
+    directory: `/modules/${id}`,
+    entrypoints: { server: `/modules/${id}/dist/does-not-exist/index.js` },
+    manifest,
+    manifestPath: `/modules/${id}/ekklesia.module.json`,
+    packageName: `@test/${id}`,
+  };
+}
+
 /** A minimal fake `ServerType`: enough of Node's `http.Server` shape for `bindServer`/`shutdown`
  * to drive without a real socket. `close` and `once` are spies so tests can assert call counts;
  * `triggerError` simulates the server's own `'error'` event (e.g. a bind failure). */
@@ -285,3 +373,148 @@ describe("Ekklesia API shutdown lifecycle", () => {
     restore();
   });
 });
+
+describe("Ekklesia API module permission composition at startup", () => {
+  afterEach(async () => {
+    await Promise.all(
+      temporaryDirectories.splice(0).map((directory) => rm(directory, { recursive: true })),
+    );
+  });
+
+  beforeEach(() => {
+    poolConstructor.mockReset();
+    betterAuthMock.mockReset();
+    drizzleAdapterMock.mockClear();
+    serveMock.mockReset();
+  });
+
+  it("discovers and composes permissions before constructing the database pool, Better Auth, or the HTTP server", async () => {
+    const modulesDirectory = await createTemporaryModulesDirectory();
+    // An incompatible platform version fails discovery itself (MODULE_PLATFORM_INCOMPATIBLE),
+    // before permission composition — and, since composition runs before any pool exists, before
+    // any pool, Better Auth instance, or HTTP server could ever be constructed.
+    await createFixtureModule(modulesDirectory, "incompatible", {
+      compatibility: { moduleSdk: "^0.1.0", platform: ">=99.0.0" },
+    });
+
+    await expect(
+      startApi(validConfig(), { modulesDirectoryOverride: modulesDirectory }),
+    ).rejects.toBeInstanceOf(ModuleDiscoveryError);
+
+    expect(poolConstructor).not.toHaveBeenCalled();
+    expect(betterAuthMock).not.toHaveBeenCalled();
+    expect(serveMock).not.toHaveBeenCalled();
+  });
+
+  it("fails startup when a module claims the reserved core.* permission namespace, creating no pool, Better Auth instance, or server", async () => {
+    const modulesDirectory = await createTemporaryModulesDirectory();
+    // Only a module whose own id is literally "core" can produce a "core."-prefixed permission
+    // that still passes the manifest's own module-namespace check.
+    await createFixtureModule(modulesDirectory, "core", {
+      permissions: [
+        { baseline: "restricted", description: "Not really Core's.", id: "core.rogue.permission" },
+      ],
+    });
+
+    let captured: unknown;
+    try {
+      await startApi(validConfig(), { modulesDirectoryOverride: modulesDirectory });
+    } catch (error) {
+      captured = error;
+    }
+
+    expect(captured).toBeInstanceOf(ModulePermissionCompositionError);
+    expect(captured).toMatchObject({ code: "MODULE_PERMISSION_RESERVED_NAMESPACE" });
+    expect(poolConstructor).not.toHaveBeenCalled();
+    expect(betterAuthMock).not.toHaveBeenCalled();
+    expect(serveMock).not.toHaveBeenCalled();
+  });
+
+  it("fails startup on a duplicate permission declaration, creating no pool, Better Auth instance, or server", async () => {
+    const first = fakeDiscoveredModule("calendar", [
+      { baseline: "restricted", description: "Read events.", id: "calendar.events.read" },
+    ]);
+    const second = fakeDiscoveredModule("calendar", [
+      { baseline: "public", description: "Also read events.", id: "calendar.events.read" },
+    ]);
+
+    let captured: unknown;
+    try {
+      await startApi(validConfig(), { discoveredModulesOverride: [first, second] });
+    } catch (error) {
+      captured = error;
+    }
+
+    expect(captured).toBeInstanceOf(PermissionRegistryError);
+    expect(captured).toMatchObject({ code: "PERMISSION_REGISTRY_DUPLICATE_DECLARATION" });
+    expect(poolConstructor).not.toHaveBeenCalled();
+    expect(betterAuthMock).not.toHaveBeenCalled();
+    expect(serveMock).not.toHaveBeenCalled();
+  });
+
+  it("keeps a reserved-namespace failure and a generic duplicate-declaration failure distinguishable", async () => {
+    const modulesDirectory = await createTemporaryModulesDirectory();
+    await createFixtureModule(modulesDirectory, "core", {
+      permissions: [
+        {
+          baseline: "restricted",
+          description: "Attempting to shadow Core's own permission.",
+          id: "core.authorization.grants.manage",
+        },
+      ],
+    });
+
+    let reservedNamespaceError: unknown;
+    try {
+      await startApi(validConfig(), { modulesDirectoryOverride: modulesDirectory });
+    } catch (error) {
+      reservedNamespaceError = error;
+    }
+
+    const duplicateModule = fakeDiscoveredModule("calendar", [
+      { baseline: "restricted", description: "Read events.", id: "calendar.events.read" },
+    ]);
+    const duplicateModuleAgain = fakeDiscoveredModule("calendar", [
+      { baseline: "restricted", description: "Read events, again.", id: "calendar.events.read" },
+    ]);
+    let duplicateError: unknown;
+    try {
+      await startApi(validConfig(), {
+        discoveredModulesOverride: [duplicateModule, duplicateModuleAgain],
+      });
+    } catch (error) {
+      duplicateError = error;
+    }
+
+    // Even though the first scenario's module attempted to exactly collide with an existing
+    // Core-owned permission id, it is reported as a reserved-namespace violation, never as a
+    // generic duplicate — the two failure families never overlap.
+    expect(reservedNamespaceError).toBeInstanceOf(ModulePermissionCompositionError);
+    expect(reservedNamespaceError).not.toBeInstanceOf(PermissionRegistryError);
+    expect(duplicateError).toBeInstanceOf(PermissionRegistryError);
+    expect(duplicateError).not.toBeInstanceOf(ModulePermissionCompositionError);
+  });
+
+  it("still constructs exactly one pool and succeeds when zero modules are installed", async () => {
+    const modulesDirectory = await createTemporaryModulesDirectory();
+    const poolEnd = vi.fn().mockResolvedValue(undefined);
+    poolConstructor.mockImplementation(function fakePool() {
+      return { end: poolEnd, on: vi.fn() };
+    });
+    betterAuthMock.mockReturnValue({ api: { getSession: vi.fn() }, handler: vi.fn() });
+    mockSuccessfulServe();
+
+    const running = await startApi(validConfig(), { modulesDirectoryOverride: modulesDirectory });
+
+    expect(poolConstructor).toHaveBeenCalledTimes(1);
+    await running.shutdown();
+    expect(poolEnd).toHaveBeenCalledTimes(1);
+  });
+
+  it("does not perform module discovery merely by importing the platform-permissions module", async () => {
+    const module = await import("../src/platform-permissions.js");
+
+    expect(typeof module.composeStartupPermissionRegistry).toBe("function");
+    expect(poolConstructor).not.toHaveBeenCalled();
+  });
+});
diff --git a/core/src/cli/validate-modules.ts b/core/src/cli/validate-modules.ts
index 5e7b7ba..71b3f70 100644
--- a/core/src/cli/validate-modules.ts
+++ b/core/src/cli/validate-modules.ts
@@ -1,8 +1,11 @@
 import { readFile } from "node:fs/promises";
 import path from "node:path";
+import { PermissionRegistryError } from "../authorization/index.js";
 import {
+  composePlatformPermissionRegistry,
   discoverModuleSchemas,
   ModuleDiscoveryError,
+  ModulePermissionCompositionError,
   ModuleSchemaDiscoveryError,
 } from "../modules/index.js";
 
@@ -31,6 +34,12 @@ async function main(): Promise<void> {
   });
   const modules = schemaAggregate.discoveredModules;
 
+  // Reuses the modules discoverModuleSchemas already discovered — never rediscovers modules
+  // solely for permission composition. A manifest that passes structural validation but claims
+  // the reserved core.* namespace, or duplicates another declaration, fails validation here
+  // before this module is considered installable.
+  composePlatformPermissionRegistry(modules);
+
   if (modules.length === 0) {
     console.log("Module validation passed (0 modules installed).");
     return;
@@ -57,6 +66,10 @@ main().catch((error: unknown) => {
       const issuePath = issue.path ? ` (${issue.path})` : "";
       console.error(`[${issue.code}] ${issue.moduleId}${issuePath}: ${issue.message}`);
     }
+  } else if (error instanceof ModulePermissionCompositionError) {
+    console.error(`[${error.code}] ${error.message}`);
+  } else if (error instanceof PermissionRegistryError) {
+    console.error(`[${error.code}] ${error.message}`);
   } else {
     console.error(error instanceof Error ? error.message : String(error));
   }
diff --git a/core/src/diagnostics/doctor-report.ts b/core/src/diagnostics/doctor-report.ts
index e9a5267..bd4003c 100644
--- a/core/src/diagnostics/doctor-report.ts
+++ b/core/src/diagnostics/doctor-report.ts
@@ -11,7 +11,7 @@ function section(title: string, checkIds: readonly DoctorCheckId[]): DoctorSecti
 
 const sections: readonly DoctorSection[] = Object.freeze([
   section("Platform", ["platform.runtime"]),
-  section("Modules", ["modules.discovery", "modules.dependencies"]),
+  section("Modules", ["modules.discovery", "modules.dependencies", "modules.permissions"]),
   section("Data definitions", ["schemas.discovery", "migrations.discovery"]),
   section("PostgreSQL", ["postgresql.connection", "postgresql.plan"]),
   section("Identity", ["identity.configuration"]),
diff --git a/core/src/diagnostics/doctor.ts b/core/src/diagnostics/doctor.ts
index 53498ac..88934a5 100644
--- a/core/src/diagnostics/doctor.ts
+++ b/core/src/diagnostics/doctor.ts
@@ -1,6 +1,12 @@
 import path from "node:path";
-import { isVersionCompatible, moduleIdSchema, semanticVersionSchema } from "@ekklesia/contracts";
+import {
+  isVersionCompatible,
+  moduleIdSchema,
+  permissionIdSchema,
+  semanticVersionSchema,
+} from "@ekklesia/contracts";
 import { getTableName } from "drizzle-orm";
+import { type PermissionRegistry, PermissionRegistryError } from "../authorization/index.js";
 import {
   type CreateIdentityConfigOptions,
   createIdentityConfig,
@@ -18,9 +24,11 @@ import {
   planModuleMigrations,
 } from "../migrations/index.js";
 import {
+  composePlatformPermissionRegistry,
   type DiscoveredModule,
   discoverModuleSchemas,
   discoverModules,
+  ModulePermissionCompositionError,
   type ModuleDiscoveryOptions,
   type ModuleMigrationAggregate,
   type ModuleSchemaAggregate,
@@ -30,6 +38,7 @@ export type DoctorCheckId =
   | "platform.runtime"
   | "modules.discovery"
   | "modules.dependencies"
+  | "modules.permissions"
   | "schemas.discovery"
   | "migrations.discovery"
   | "postgresql.connection"
@@ -68,6 +77,9 @@ export interface DoctorOptions {
 }
 
 export interface DoctorDependencies {
+  /** Synchronous, matching the real implementation: pure in-memory composition over an
+   * already-discovered module list, never re-discovering modules itself. */
+  readonly composePermissions: (modules: readonly DiscoveredModule[]) => PermissionRegistry;
   readonly createMigrationDatabase: (
     connectionString: string,
     options: PostgresMigrationDatabaseOptions,
@@ -101,6 +113,7 @@ async function discoverAllMigrationsForRuntime(
 }
 
 const productionDependencies: DoctorDependencies = Object.freeze({
+  composePermissions: composePlatformPermissionRegistry,
   createMigrationDatabase: createPostgresMigrationDatabase,
   discoverMigrations: discoverAllMigrationsForRuntime,
   discoverModules,
@@ -163,6 +176,57 @@ function safeMigrationReference(identity: string, checksum: string): string {
   return `${identity} · ${checksum.slice(0, 12)}`;
 }
 
+function safeModuleId(value: string): string {
+  if (!moduleIdSchema.safeParse(value).success) {
+    throw new TypeError("Unsafe module diagnostic value.");
+  }
+  return value;
+}
+
+function safePermissionId(value: string): string {
+  if (!permissionIdSchema.safeParse(value).success) {
+    throw new TypeError("Unsafe permission diagnostic value.");
+  }
+  return value;
+}
+
+/** Distinguishes the two composition-error families rather than describing a reserved `core.*`
+ * declaration as a generic duplicate. Never includes a module directory, manifest path, caught
+ * cause, or stack — only the module id and permission id the structured errors already carry,
+ * each re-validated before being interpolated (defense in depth against a future drift bug,
+ * matching `safeModuleDetails`/`safeMigrationReference` elsewhere in this file); if that
+ * re-validation itself fails, this falls back to a fixed generic detail rather than throwing. */
+function permissionCompositionFailure(error: unknown): DoctorCheck {
+  if (error instanceof ModulePermissionCompositionError) {
+    try {
+      const moduleId = safeModuleId(error.moduleId);
+      const permissionId = safePermissionId(error.permissionId);
+      return freezeCheck("modules.permissions", "failure", "Permission composition failed", [
+        `Module "${moduleId}" declared permission "${permissionId}" in the reserved core.* namespace.`,
+      ]);
+    } catch {
+      return freezeCheck("modules.permissions", "failure", "Permission composition failed", [
+        "A module declared a permission in the reserved core.* namespace.",
+      ]);
+    }
+  }
+  if (error instanceof PermissionRegistryError) {
+    try {
+      const permissionId = safePermissionId(error.permissionId);
+      return freezeCheck("modules.permissions", "failure", "Permission composition failed", [
+        `Permission "${permissionId}" is declared more than once.`,
+      ]);
+    } catch {
+      return freezeCheck("modules.permissions", "failure", "Permission composition failed", [
+        "A permission is declared more than once.",
+      ]);
+    }
+  }
+  return freezeCheck("modules.permissions", "failure", "Permission composition failed", [
+    "The installed permission declarations could not be composed safely.",
+  ]);
+}
+
 function moduleDiscoveryFailure(error: unknown): DoctorCheck {
   const details =
     error instanceof Error && error.name === "ModuleDiscoveryError"
@@ -386,6 +450,33 @@ export async function diagnoseEkklesia(
     ]);
   }
 
+  // Reuses the already-discovered module list above — never re-discovers modules solely for
+  // permission composition. Independent of the schema/migration checks below: a permission
+  // composition failure here (a reserved-namespace or duplicate declaration) does not skip those,
+  // since neither depends on the permission registry, only on discovery having succeeded.
+  let permissionsCheck: DoctorCheck;
+  if (!discoveredModules) {
+    permissionsCheck = freezeCheck(
+      "modules.permissions",
+      "skipped",
+      "Permission composition skipped",
+      ["Complete module discovery is required."],
+    );
+  } else {
+    try {
+      const registry = dependencies.composePermissions(discoveredModules);
+      const count = registry.declarations.length;
+      permissionsCheck = freezeCheck(
+        "modules.permissions",
+        "pass",
+        "Permission registry composed",
+        [`${count} ${plural(count, "permission")} registered`],
+      );
+    } catch (error) {
+      permissionsCheck = permissionCompositionFailure(error);
+    }
+  }
+
   let schemaCheck: DoctorCheck;
   let migrationCheck: DoctorCheck;
   let migrations: ModuleMigrationAggregate | undefined;
@@ -466,6 +557,7 @@ export async function diagnoseEkklesia(
     runtimeCheck,
     moduleCheck,
     dependencyCheck,
+    permissionsCheck,
     schemaCheck,
     migrationCheck,
     postgresConnectionCheck,
diff --git a/core/tests/doctor-report.test.ts b/core/tests/doctor-report.test.ts
index ac7349d..7a11f49 100644
--- a/core/tests/doctor-report.test.ts
+++ b/core/tests/doctor-report.test.ts
@@ -6,6 +6,7 @@ import {
 } from "../src/diagnostics/index.js";
 import type { MigrationDatabase } from "../src/migrations/index.js";
 import type { ModuleMigrationAggregate, ModuleSchemaAggregate } from "../src/modules/index.js";
+import { composePlatformPermissionRegistry } from "../src/modules/permissions.js";
 
 const originalNoColor = process.env.NO_COLOR;
 
@@ -36,6 +37,7 @@ function zeroMigrationAggregate(): ModuleMigrationAggregate {
 
 function noConfigurationDependencies(): DoctorDependencies {
   return {
+    composePermissions: composePlatformPermissionRegistry,
     createMigrationDatabase: () => {
       throw new Error("database must not be constructed");
     },
@@ -75,6 +77,8 @@ Modules
     0 modules installed
   ✓ Dependency graph valid
     Dependency order is empty.
+  ✓ Permission registry composed
+    1 permission registered
 
 Data definitions
   ✓ Schemas validated
diff --git a/core/tests/doctor.test.ts b/core/tests/doctor.test.ts
index 2c8f505..1a76ac5 100644
--- a/core/tests/doctor.test.ts
+++ b/core/tests/doctor.test.ts
@@ -1,5 +1,6 @@
-import type { ModuleManifest } from "@ekklesia/contracts";
+import { parsePermissionId, type ModuleManifest } from "@ekklesia/contracts";
 import { describe, expect, it, vi } from "vitest";
+import { createPermissionRegistry, PermissionRegistryError } from "../src/authorization/index.js";
 import {
   type DoctorCheck,
   type DoctorDependencies,
@@ -21,6 +22,7 @@ import type {
   ModuleMigrationAggregate,
   ModuleSchemaAggregate,
 } from "../src/modules/index.js";
+import { ModulePermissionCompositionError } from "../src/modules/permissions.js";
 
 const checksumA = "a".repeat(64);
 const checksumB = "b".repeat(64);
@@ -125,6 +127,11 @@ function plan(entries: MigrationPlanResult["entries"] = []): MigrationPlanResult
 
 function dependencies(overrides: Partial<DoctorDependencies> = {}): DoctorDependencies {
   return {
+    composePermissions: vi.fn((modules: readonly DiscoveredModule[]) =>
+      createPermissionRegistry(
+        modules.flatMap((discovered) => [...discovered.manifest.permissions]),
+      ),
+    ),
     createMigrationDatabase: vi.fn(() => emptyDatabase()),
     discoverMigrations: vi.fn(async () => migrationAggregate()),
     discoverModules: vi.fn(async () => Object.freeze([])),
@@ -196,6 +203,7 @@ describe("Ekklesia Doctor diagnostics", () => {
       "platform.runtime",
       "modules.discovery",
       "modules.dependencies",
+      "modules.permissions",
       "schemas.discovery",
       "migrations.discovery",
       "postgresql.connection",
@@ -205,6 +213,10 @@ describe("Ekklesia Doctor diagnostics", () => {
     ]);
     expect(check(report, "modules.discovery").details).toEqual(["people 0.1.0", "calendar 0.2.0"]);
     expect(check(report, "modules.dependencies").details).toEqual(["Order: people → calendar"]);
+    expect(check(report, "modules.permissions")).toMatchObject({
+      details: ["0 permissions registered"],
+      status: "pass",
+    });
     expect(check(report, "migrations.discovery").details).toEqual([
       "1 migration · checksums valid",
       "people:0000_initial · aaaaaaaaaaaa",
@@ -369,6 +381,7 @@ describe("Ekklesia Doctor diagnostics", () => {
     expect(check(report, "modules.discovery").status).toBe("failure");
     for (const id of [
       "modules.dependencies",
+      "modules.permissions",
       "schemas.discovery",
       "migrations.discovery",
       "postgresql.connection",
@@ -385,6 +398,114 @@ describe("Ekklesia Doctor diagnostics", () => {
     expect(JSON.stringify(report)).not.toContain("password");
   });
 
+  it("passes permission composition with a safe declaration count when modules are installed", async () => {
+    const calendar = discoveredModule("calendar");
+    const report = await diagnoseEkklesia(
+      options(),
+      dependencies({
+        composePermissions: vi.fn(() =>
+          createPermissionRegistry([
+            {
+              baseline: "restricted",
+              description: "Read calendar events.",
+              id: parsePermissionId("calendar.events.read"),
+            },
+          ]),
+        ),
+        discoverModules: vi.fn(async () => Object.freeze([calendar])),
+      }),
+    );
+
+    expect(check(report, "modules.permissions")).toMatchObject({
+      details: ["1 permission registered"],
+      status: "pass",
+    });
+  });
+
+  it("reuses the already-discovered module list rather than rediscovering modules for permission composition", async () => {
+    const calendar = discoveredModule("calendar");
+    const composePermissions = vi.fn(() => createPermissionRegistry([]));
+    const discoverModules = vi.fn(async () => Object.freeze([calendar]));
+    await diagnoseEkklesia(options(), dependencies({ composePermissions, discoverModules }));
+
+    expect(discoverModules).toHaveBeenCalledOnce();
+    expect(composePermissions).toHaveBeenCalledOnce();
+    expect(composePermissions).toHaveBeenCalledWith([calendar]);
+  });
+
+  it("fails permission composition when a module claims the reserved core.* namespace, distinctly from a generic duplicate", async () => {
+    const report = await diagnoseEkklesia(
+      options(),
+      dependencies({
+        composePermissions: vi.fn(() => {
+          throw new ModulePermissionCompositionError("core", "core.rogue.permission");
+        }),
+      }),
+    );
+
+    expect(check(report, "modules.permissions")).toMatchObject({
+      details: [
+        'Module "core" declared permission "core.rogue.permission" in the reserved core.* namespace.',
+      ],
+      status: "failure",
+    });
+    expect(report.result).toBe("not-ready");
+  });
+
+  it("fails permission composition on a generic duplicate declaration, distinctly from a reserved-namespace violation", async () => {
+    const report = await diagnoseEkklesia(
+      options(),
+      dependencies({
+        composePermissions: vi.fn(() => {
+          throw new PermissionRegistryError("calendar.events.read");
+        }),
+      }),
+    );
+
+    expect(check(report, "modules.permissions")).toMatchObject({
+      details: ['Permission "calendar.events.read" is declared more than once.'],
+      status: "failure",
+    });
+    expect(report.result).toBe("not-ready");
+  });
+
+  it("sanitizes an unexpected permission-composition exception to a fixed generic detail", async () => {
+    const hostile = "postgresql://admin:password@host.invalid/db /home/operator/.env";
+    const report = await diagnoseEkklesia(
+      options(),
+      dependencies({
+        composePermissions: vi.fn(() => {
+          throw new Error(hostile);
+        }),
+      }),
+    );
+
+    expect(check(report, "modules.permissions")).toMatchObject({
+      details: ["The installed permission declarations could not be composed safely."],
+      status: "failure",
+    });
+    expect(JSON.stringify(report)).not.toContain(hostile);
+  });
+
+  it("continues independent schema, migration, and PostgreSQL checks after permission composition fails", async () => {
+    const planMigrations = vi.fn(async () => plan());
+    const report = await diagnoseEkklesia(
+      options("postgresql://not-rendered.invalid/ekklesia"),
+      dependencies({
+        composePermissions: vi.fn(() => {
+          throw new PermissionRegistryError("calendar.events.read");
+        }),
+        planMigrations,
+      }),
+    );
+
+    expect(check(report, "modules.permissions").status).toBe("failure");
+    expect(check(report, "schemas.discovery").status).toBe("pass");
+    expect(check(report, "migrations.discovery").status).toBe("pass");
+    expect(check(report, "postgresql.connection").status).toBe("pass");
+    expect(planMigrations).toHaveBeenCalledOnce();
+  });
+
   it("continues independent migration and PostgreSQL checks after schema failure", async () => {
     const planMigrations = vi.fn(async () => plan());
     const report = await diagnoseEkklesia(
diff --git a/docs/development/api.md b/docs/development/api.md
index 699d324..36b4237 100644
--- a/docs/development/api.md
+++ b/docs/development/api.md
@@ -206,6 +206,18 @@ trusted-proxy IP policy is configured, Better Auth may fall back to a single sha
 rather than a genuine per-client bucket. This is not yet the final, distributed, production-grade
 abuse-control design.
 
+## Module permission discovery at startup
+
+Before any database pool, Better Auth instance, or HTTP server exists, `startApi()` discovers
+every installed module's manifest and composes the one immutable platform permission registry
+(Core's own declarations plus every module's) that the rest of startup — and every subsequent
+request — enforces against. A discovery or composition failure (an invalid manifest, a module
+claiming the reserved `core.*` permission namespace, a duplicate permission declaration, ...) fails
+startup outright, before there is anything to close; startup never falls back to a Core-only
+registry. See [Authorization](authorization.md#api-startup-discovery-composition-and-enforcement)
+for the full flow, and [Module development](module-development/README.md) for the manifest shape
+that produces these declarations.
+
 ## Database connection limits
 
 `apps/api/src/server.ts` configures the one shared PostgreSQL pool (`createApiDatabase`,
diff --git a/docs/development/authorization.md b/docs/development/authorization.md
index 8a1f3bd..9a66ce5 100644
--- a/docs/development/authorization.md
+++ b/docs/development/authorization.md
@@ -96,9 +96,73 @@ Composition itself performs no filesystem, environment, database, network, or mo
 operation — it is pure, synchronous, in-memory composition over data a caller already discovered
 and validated (`core/src/authorization` and `core/src/modules` remain independent of each other in
 the other direction: authorization never imports module discovery or runtime, only the module
-integration layer depends on authorization). This pass stops at producing the combined registry;
-wiring it into `apps/api` in place of `CORE_PERMISSION_REGISTRY`, and any module-owned HTTP
-enforcement built on it, is later work.
+integration layer depends on authorization).
+
+## API startup: discovery, composition, and enforcement
+
+`apps/api` builds its production permission registry from the complete installed-module inventory
+every time it starts, in a fixed order, before anything else:
+
+1. Resolve module-discovery configuration (the repository's `modules/` directory, and the
+   platform/Module SDK versions from their own `package.json` files).
+2. Discover installed modules (`discoverModules`).
+3. Compose the platform permission registry (`composePlatformPermissionRegistry`).
+4. Construct the shared PostgreSQL pool.
+5. Construct Identity and Authorization.
+6. Create the Hono application, injecting the composed registry.
+7. Bind the HTTP server.
+
+`apps/api/src/platform-permissions.ts`'s `composeStartupPermissionRegistry` performs steps 1–3.
+Discovery and composition happen strictly before any pool, Better Auth instance, or HTTP server
+exists, so a failure there — an invalid manifest, a broken dependency graph, a module claiming the
+reserved `core.*` namespace, or a duplicate permission declaration — leaves nothing to close: it
+propagates directly out of `startApi()`, and startup never falls back to `CORE_PERMISSION_REGISTRY`
+or any Core-only registry. The repository root is resolved from this compiled/source file's own
+location (two directories up from `apps/api/dist/*.js` or `apps/api/src/*.ts`, which sit at
+identical depth), never `process.cwd()` — a package-scoped test run has `apps/api` as its working
+directory, which would otherwise point discovery at the wrong `modules/` directory.
+
+The resulting registry is a **static, immutable snapshot** taken once at startup. There is no hot
+installation, removal, or dynamic mutation of installed modules or their permissions while the
+process is running — an installed-module change (add, remove, or edit a manifest) takes effect
+only on the next API restart. `createApiApp` requires this registry as part of its injected
+authorization options; `requirePermission` and the explicit-grant routes
+(`GET/POST /api/v1/grants`, `POST /api/v1/grants/:grantId/revoke`) consult exactly the same
+registry Core's own `core.authorization.grants.manage` permission is registered in, so a
+module-owned permission and Core's own are enforced, and grantable, through the identical
+mechanism — there is no module-specific enforcement path. Removing a module from the next
+startup's discovered inventory makes its permission unknown to new authorization checks and new
+grants (a well-formed but unregistered permission denies exactly as before); it never deletes,
+revokes, or otherwise modifies a historical grant or audit row already recorded for that
+permission — a previously granted, still-unrevoked explicit grant for a now-uninstalled module's
+permission remains visible through `GET /api/v1/grants` and simply stops being something anyone
+can newly grant, revoke, or have authorized against.
+
+Module HTTP route registration (a module owning its own mounted endpoints) and relationship-derived
+grant creation both remain deferred, unrelated slices — this work only makes a module's *declared*
+permission usable through Core's existing, generic explicit-grant HTTP API and enforcement
+middleware.
+
+## Doctor and modules:validate
+
+`pnpm doctor`'s `modules.permissions` check composes the platform permission registry from the
+same already-discovered module list `modules.discovery`/`modules.dependencies` already produced —
+it never re-discovers modules itself. A successful composition reports a pass with a safe
+declaration count (for example `1 permission registered` with zero installed modules, since Core's
+own declaration is always present); zero installed modules still composes successfully. If module
+discovery itself failed, the check is skipped, matching the existing schema/migration checks'
+pattern; if discovery succeeded but composition failed, schema and migration checks still run
+normally — they are independent diagnostics that do not depend on the permission registry. A
+reserved-namespace violation (`ModulePermissionCompositionError`,
+`MODULE_PERMISSION_RESERVED_NAMESPACE`) and a generic duplicate declaration
+(`PermissionRegistryError`, `PERMISSION_REGISTRY_DUPLICATE_DECLARATION`) are reported as distinct
+failure details — a reserved `core.*` claim is never described as an ordinary duplicate, even
+though (see above) a module attempting to exactly collide with a Core-owned id always surfaces as
+the reserved-namespace failure specifically. Composition requires no database and never imports or
+executes a module's server entrypoint. `pnpm modules:validate` composes the same registry from its
+own already-discovered modules for the identical reason: a manifest that is otherwise structurally
+valid but claims the `core.*` namespace, or duplicates a permission id, now fails validation before
+that module is considered installable.
 
 ### Two layers, not one: shape versus registration
 
@@ -265,7 +329,7 @@ cannot smuggle a server-derived field through a request that otherwise looks wel
 | Field | Required | Notes |
 | --- | --- | --- |
 | `subjectUserId` | Yes | Must parse as a valid `EkklesiaUserId`. |
-| `permission` | Yes | Must parse as a valid `PermissionId` **and** be registered in `CORE_PERMISSION_REGISTRY` — see [Two layers, not one](#two-layers-not-one-shape-versus-registration). A malformed value is a `400`; a well-formed but unregistered one is a `403`. |
+| `permission` | Yes | Must parse as a valid `PermissionId` **and** be registered in the running API's composed platform registry (Core's own declarations plus every installed module's — see [API startup](#api-startup-discovery-composition-and-enforcement)). A malformed value is a `400`; a well-formed but unregistered one is a `403`. |
 | `scope` | No | Must parse as a valid, non-empty `AuthorizationScope` if present. |
 | `expiresAt` | No | An ISO date string if present. |
 
@@ -357,13 +421,27 @@ there are or whether the failure happened during startup.
   its audit row, nonexistent-user rollback, second-bootstrap rejection, a revoked historical owner
   still blocking reuse, and the concurrency scenario described above.
 - `apps/api/tests/authorization-middleware.test.ts` — allow/deny/unregistered/infrastructure-
-  failure paths for `requirePermission`, and `requireTrustedOrigin`'s allow/reject behavior.
+  failure paths for `requirePermission`, and `requireTrustedOrigin`'s allow/reject behavior;
+  a dedicated block proves `requirePermission` enforces module-owned `public`/`authenticated`/
+  `restricted` declarations identically to Core's own, that the evaluator receives the registry's
+  exact id/baseline, and that an absent module permission denies before principal resolution or
+  evaluator access — the shared mechanism, not a module-specific code path.
+- `core/tests/doctor.test.ts` / `core/tests/doctor-report.test.ts` — the `modules.permissions`
+  check: passing count (including zero installed modules), reuse of the already-discovered module
+  list (no rediscovery), distinct reserved-namespace vs. generic-duplicate failure detail, an
+  unexpected-exception fallback to a fixed generic detail, schema/migration checks continuing
+  independently after a composition failure, and the exact documented terminal report shape.
 - `apps/api/tests/grants-routes.test.ts` — all three routes' allow/deny paths, disallowed-field
   and malformed-field rejection, a well-formed but unregistered target permission rejected with
   `403` and `store.createGrant` never called, exact server-derived-argument assertions,
   sourceKind-filtered revoke returning 404 for a non-explicit grant, origin enforcement, and
   store-error mapping.
-- `apps/api/tests/lifecycle.test.ts` — shared-pool-closes-exactly-once coverage described above.
+- `apps/api/tests/lifecycle.test.ts` — shared-pool-closes-exactly-once coverage described above,
+  plus module-permission composition at startup: discovery/composition run before any pool,
+  Better Auth instance, or HTTP server is constructed; a discovery failure, a reserved-namespace
+  failure, and a generic duplicate-declaration failure each create zero pools/Better Auth
+  instances/servers and remain distinguishable from one another; zero installed modules still
+  succeeds; and import safety for `platform-permissions.ts`.
 - `core/tests/authorization-evaluator.test.ts` — includes a malformed permission id denying under
   every baseline (`public`, `authenticated`, `restricted`), never just `restricted`, and never
   consulting the grant or platform-owner ports for `public`/`authenticated`.
@@ -373,11 +451,21 @@ there are or whether the failure happened during startup.
   account then managing grants as a non-owner, a well-formed but unregistered target permission
   rejected with `403` (with no grant or audit row written) even for a caller with full
   grant-management authority, a create/list/revoke cycle with audit-trail verification (using
-  `core.authorization.grants.manage` itself as the granted permission, since module permission
-  registration does not exist yet), disallowed-field rejection, missing/untrusted-origin rejection,
-  a real relationship-derived grant proven invisible to LIST and unreachable by REVOKE, and an
-  infrastructure-failure scenario (database dropped out from under a live session) asserting a
-  `503` rather than a false `403` or `200`.
+  `core.authorization.grants.manage` itself as the granted permission), disallowed-field
+  rejection, missing/untrusted-origin rejection, a real relationship-derived grant proven
+  invisible to LIST and unreachable by REVOKE, and an infrastructure-failure scenario (database
+  dropped out from under a live session) asserting a `503` rather than a false `403` or `200`.
+- `apps/api/tests/api-http.integration.ts`, "installed module permission end-to-end" (live
+  PostgreSQL) — a real temporary module fixture (structurally valid manifest, a server entrypoint
+  that throws if ever imported) proves the complete chain against a real API process and real
+  Doctor run: Doctor recognizes the installed module and passes permission composition without
+  executing its entrypoint; API startup discovers the fixture and makes its declared permission
+  enforceable; a bootstrapped platform owner creates a real explicit grant for the module's
+  permission with an atomic audit row recorded under its exact declared id; a well-formed
+  permission absent from the composed registry is rejected with no grant or audit write; and,
+  after a clean shutdown and restart with the module absent, the same permission can no longer be
+  newly granted while the earlier, still-unrevoked grant remains listed — with ordinary identity
+  HTTP behavior (sign-up, sign-in, principal resolution) unaffected throughout.
 
 Run the focused suites:
 
@@ -389,7 +477,8 @@ pnpm test:migrations:integration   # requires Docker
 
 ## Next phase
 
-Phase 4C, pass 3: deferred from this slice — module discovery/Module SDK permission
-registration, roles/groups/teams/tenants, an owner-management HTTP endpoint, service
-accounts/API tokens for non-browser automation, and relationship-derived grant creation itself
-(a roster or similar module operation, not the generic grants API).
+Phase 5A, pass 3: deferred from this slice — module HTTP route-registration contracts (a module
+mounting its own endpoints), a module runtime-context authorization service, relationship-derived
+grant creation itself (a roster or similar module operation, not the generic grants API),
+roles/groups/teams/tenants, an owner-management HTTP endpoint, and service accounts/API tokens for
+non-browser automation.
diff --git a/docs/development/repository-workflow.md b/docs/development/repository-workflow.md
index 1d1e266..b1a56a8 100644
--- a/docs/development/repository-workflow.md
+++ b/docs/development/repository-workflow.md
@@ -101,6 +101,8 @@ Modules
     0 modules installed
   ✓ Dependency graph valid
     Dependency order is empty.
+  ✓ Permission registry composed
+    1 permission registered
 
 Data definitions
   ✓ Schemas validated
diff --git a/docs/module-development/README.md b/docs/module-development/README.md
index 53947a7..b11449f 100644
--- a/docs/module-development/README.md
+++ b/docs/module-development/README.md
@@ -121,6 +121,17 @@ every discovered module's declarations, alongside its own, into the one platform
 registry). A duplicate permission id within one manifest, or a permission id outside the module's
 own namespace, fails validation the same way a duplicate or misnamed capability id does.
 
+`apps/api` composes this registry once, at startup, from every module installed at that time (see
+[Authorization](../development/authorization.md#api-startup-discovery-composition-and-enforcement));
+installing, removing, or editing a module's manifest takes effect only on the next API restart, not
+while it is running. Once installed and discovered, a module's declared permission is enforceable
+and grantable through exactly the same mechanism as any Core-owned permission — the same
+`requirePermission` middleware and the same `/api/v1/grants` explicit-grant HTTP API a platform
+owner (or anyone they delegate `core.authorization.grants.manage` to) already uses. Mounting a
+module's own HTTP routes, and creating relationship-derived grants automatically from a module's
+own business logic, are both still later work; this only makes a declared permission *usable*
+through Core's existing, generic surface.
+
 Server and optional schema entrypoints must be compiled ESM `.js` or `.mjs` files. The optional
 migration entrypoint is a directory. All entrypoints begin with `./`, remain inside the package,
 and exist. Production never imports source TypeScript directly. Modules with no tables omit