diff --git a/packages/authz/src/catalog.ts b/packages/authz/src/catalog.ts
new file mode 100644
index 00000000..0ffb77b5
--- /dev/null
+++ b/packages/authz/src/catalog.ts
@@ -0,0 +1,119 @@
+import type { AttributeMeta, AuthzCatalog, AuthzModule, PermissionMeta } from "./types.ts";
+import type { DecisionPolicy } from "./advanced.ts";
+
+export interface CatalogSource {
+  /** File or package that declared this module, used in conflict messages. */
+  source: string;
+  module: AuthzModule;
+}
+
+/** Structural equality for declaration metadata. Key order is irrelevant. */
+function deepEqual(a: unknown, b: unknown): boolean {
+  if (Object.is(a, b)) return true;
+  if (typeof a !== "object" || typeof b !== "object" || a === null || b === null) return false;
+  if (Array.isArray(a) !== Array.isArray(b)) return false;
+  const left = a as Record<string, unknown>;
+  const right = b as Record<string, unknown>;
+  const keys = new Set([...Object.keys(left), ...Object.keys(right)]);
+  for (const key of keys) if (!deepEqual(left[key], right[key])) return false;
+  return true;
+}
+
+/** A frozen Map that throws on mutation, so the catalog cannot drift after boot. */
+function frozenMap<V>(entries: Iterable<[string, V]>): ReadonlyMap<string, V> {
+  const map = new Map(entries);
+  const reject = () => {
+    throw new Error("WRN-AUTHZ-FROZEN: the authorization catalog is frozen after boot.");
+  };
+  map.set = reject as never;
+  map.delete = reject as never;
+  map.clear = reject as never;
+  return map;
+}
+
+export function emptyCatalog(): AuthzCatalog {
+  return {
+    permissions: frozenMap<PermissionMeta>([]),
+    roles: frozenMap<readonly string[]>([]),
+    policies: frozenMap<DecisionPolicy<never, never>>([]),
+    attributes: frozenMap<AttributeMeta>([]),
+    bindings: frozenMap<readonly string[]>([]),
+  };
+}
+
+export function mergeCatalogs(sources: CatalogSource[]): AuthzCatalog {
+  const permissions = new Map<string, PermissionMeta>();
+  const roles = new Map<string, readonly string[]>();
+  const policies = new Map<string, DecisionPolicy<never, never>>();
+  const attributes = new Map<string, AttributeMeta>();
+  const bindings = new Map<string, Set<string>>();
+  const origin = new Map<string, string>();
+
+  const claim = (
+    kind: string,
+    key: string,
+    source: string,
+    existingValue: unknown,
+    value: unknown,
+  ) => {
+    const previous = origin.get(`${kind}:${key}`);
+    if (previous === undefined) {
+      origin.set(`${kind}:${key}`, source);
+      return;
+    }
+    if (!deepEqual(existingValue, value)) {
+      throw new Error(
+        `WRN-AUTHZ-CONFLICT: ${kind} '${key}' is declared differently in ${previous} and ${source}.`,
+      );
+    }
+  };
+
+  for (const { source, module } of sources) {
+    for (const [id, meta] of Object.entries(module.permissions ?? {})) {
+      claim("permission", id, source, permissions.get(id), meta);
+      permissions.set(id, meta);
+    }
+    for (const [name, grants] of Object.entries(module.roles ?? {})) {
+      claim("role", name, source, roles.get(name), grants);
+      roles.set(name, grants);
+    }
+    for (const [name, policy] of Object.entries(module.policies ?? {})) {
+      // Two closures are never deep-equal, so identity is the only sane test.
+      const existing = policies.get(name);
+      if (existing && existing !== policy) {
+        throw new Error(
+          `WRN-AUTHZ-CONFLICT: policy '${name}' is declared differently in ${origin.get(`policy:${name}`)} and ${source}.`,
+        );
+      }
+      origin.set(`policy:${name}`, source);
+      policies.set(name, policy);
+    }
+    for (const [name, meta] of Object.entries(module.attributes ?? {})) {
+      claim("attribute", name, source, attributes.get(name), meta);
+      attributes.set(name, meta);
+    }
+    for (const [permission, names] of Object.entries(module.bindings ?? {})) {
+      const set = bindings.get(permission) ?? new Set<string>();
+      for (const name of names) set.add(name);
+      bindings.set(permission, set);
+    }
+  }
+
+  for (const [permission, names] of bindings) {
+    for (const name of names) {
+      if (!policies.has(name)) {
+        throw new Error(
+          `WRN-AUTHZ-CONFLICT: binding for '${permission}' names policy '${name}', which no module declares.`,
+        );
+      }
+    }
+  }
+
+  return {
+    permissions: frozenMap(permissions),
+    roles: frozenMap(roles),
+    policies: frozenMap(policies),
+    attributes: frozenMap(attributes),
+    bindings: frozenMap([...bindings].map(([k, v]) => [k, [...v]] as [string, readonly string[]])),
+  };
+}
diff --git a/packages/authz/test/catalog.test.ts b/packages/authz/test/catalog.test.ts
new file mode 100644
index 00000000..cc9a00db
--- /dev/null
+++ b/packages/authz/test/catalog.test.ts
@@ -0,0 +1,77 @@
+import { describe, expect, test } from "bun:test";
+import { defineAuthz } from "../src/registry.ts";
+import { emptyCatalog, mergeCatalogs } from "../src/catalog.ts";
+
+describe("mergeCatalogs", () => {
+  test("merges disjoint modules", () => {
+    const catalog = mergeCatalogs([
+      { source: "a.ts", module: defineAuthz({ permissions: { "post:read": {} } }) },
+      { source: "b.ts", module: defineAuthz({ permissions: { "user:read": {} } }) },
+    ]);
+    expect([...catalog.permissions.keys()].sort()).toEqual(["post:read", "user:read"]);
+  });
+
+  test("re-declaring a permission with deep-equal metadata is a no-op", () => {
+    const meta = { title: "View posts", risk: "low" as const };
+    const catalog = mergeCatalogs([
+      { source: "a.ts", module: defineAuthz({ permissions: { "post:read": meta } }) },
+      { source: "b.ts", module: defineAuthz({ permissions: { "post:read": { ...meta } } }) },
+    ]);
+    expect(catalog.permissions.size).toBe(1);
+  });
+
+  test("conflicting metadata is a boot error naming both files", () => {
+    expect(() =>
+      mergeCatalogs([
+        { source: "a.ts", module: defineAuthz({ permissions: { "post:read": { risk: "low" } } }) },
+        { source: "b.ts", module: defineAuthz({ permissions: { "post:read": { risk: "high" } } }) },
+      ]),
+    ).toThrow(/a\.ts.*b\.ts|b\.ts.*a\.ts/s);
+  });
+
+  test("conflicting role definitions are a boot error", () => {
+    expect(() =>
+      mergeCatalogs([
+        { source: "a.ts", module: defineAuthz({ roles: { editor: ["post:read"] } }) },
+        { source: "b.ts", module: defineAuthz({ roles: { editor: ["post:write"] } }) },
+      ]),
+    ).toThrow(/editor/);
+  });
+
+  test("bindings for the same permission union across modules", () => {
+    const p1 = defineAuthz({
+      permissions: { "post:write": {} },
+      policies: { ownsPost: async () => ({ allowed: true }) },
+      bindings: { "post:write": ["ownsPost"] },
+    });
+    const p2 = defineAuthz({
+      policies: { notLocked: async () => ({ allowed: true }) },
+      bindings: { "post:write": ["notLocked"] },
+    });
+    const catalog = mergeCatalogs([
+      { source: "a.ts", module: p1 },
+      { source: "b.ts", module: p2 },
+    ]);
+    expect([...catalog.bindings.get("post:write")!].sort()).toEqual(["notLocked", "ownsPost"]);
+  });
+
+  test("a binding referencing a policy no module declares is a boot error", () => {
+    expect(() =>
+      mergeCatalogs([
+        {
+          source: "a.ts",
+          module: { permissions: { "post:write": {} }, bindings: { "post:write": ["ghost"] } },
+        },
+      ]),
+    ).toThrow(/ghost/);
+  });
+
+  test("the merged catalog is frozen", () => {
+    const catalog = mergeCatalogs([]);
+    expect(() => (catalog.permissions as Map<string, never>).set("x:y", {} as never)).toThrow();
+  });
+
+  test("emptyCatalog has no entries", () => {
+    expect(emptyCatalog().permissions.size).toBe(0);
+  });
+});