diff --git a/lib/utils/transport-error.util.ts b/lib/utils/transport-error.util.ts
index f594f33..0d33896 100644
--- a/lib/utils/transport-error.util.ts
+++ b/lib/utils/transport-error.util.ts
@@ -23,13 +23,13 @@ function load(pkg: '@nestjs/websockets' | '@nestjs/microservices', name: string)
  * The one place a denial becomes a response. `@Can()` (in the guard) and
  * `authorize()` (an `AuthorizationError` leaving a handler) both come here.
  * Every transport gets the body of Nest's own `UnauthorizedException` or
- * `ForbiddenException`, like any other 401 or 403:
+ * `ForbiddenException`, with the denial reason as `errorCode`:
  *
  * | Context | Error | Client sees |
  * | --- | --- | --- |
- * | `http`, `graphql` | `UnauthorizedException` / `ForbiddenException` | `{"message":"Forbidden","statusCode":403}`, or the `UNAUTHENTICATED` / `FORBIDDEN` code |
- * | `ws` | `WsException` | `{ status: 'error', message: 'Forbidden', statusCode: 403 }` |
- * | `rpc` | `RpcException` | `{ message: 'Forbidden', statusCode: 403 }` |
+ * | `http`, `graphql` | `UnauthorizedException` / `ForbiddenException` | `{"message":"Forbidden","statusCode":403,"errorCode":"forbidden"}`; GraphQL keeps its `UNAUTHENTICATED` / `FORBIDDEN` code |
+ * | `ws` | `WsException` | `{ status: 'error', message: 'Forbidden', statusCode: 403, errorCode: 'forbidden' }` |
+ * | `rpc` | `RpcException` | `{ message: 'Forbidden', statusCode: 403, errorCode: 'forbidden' }` |
  *
  * The ws and rpc filters report `HttpException`s as "Internal server error",
  * hence their own classes, loaded only when those packages are installed.
@@ -38,8 +38,8 @@ function load(pkg: '@nestjs/websockets' | '@nestjs/microservices', name: string)
 export async function toTransportError(context: ExecutionContext, denial: AuthorizationError): Promise<Error> {
   const exception =
     denial.reason === 'unauthenticated'
-      ? new UnauthorizedException(undefined, { cause: denial })
-      : new ForbiddenException(undefined, { cause: denial });
+      ? new UnauthorizedException(undefined, { cause: denial, errorCode: denial.reason })
+      : new ForbiddenException(undefined, { cause: denial, errorCode: denial.reason });
   const body = exception.getResponse() as object;
 
   switch (context.getType<string>()) {
diff --git a/tests/authorization.e2e.spec.ts b/tests/authorization.e2e.spec.ts
index 70b3aee..fcc8e41 100644
--- a/tests/authorization.e2e.spec.ts
+++ b/tests/authorization.e2e.spec.ts
@@ -4,8 +4,8 @@ import { adapters, createApp } from './support/adapters.js';
 import { AuthorizationEvents, type AuthorizationDeniedEvent } from '../lib/index.js';
 import { AppModule, MisconfiguredAppModule, users } from './fixtures.js';
 
-const UNAUTHORIZED = { message: 'Unauthorized', statusCode: 401 };
-const FORBIDDEN = { message: 'Forbidden', statusCode: 403 };
+const UNAUTHORIZED = { message: 'Unauthorized', statusCode: 401, errorCode: 'unauthenticated' };
+const FORBIDDEN = { message: 'Forbidden', statusCode: 403, errorCode: 'forbidden' };
 
 describe.each(adapters.map((a) => a.name))('authorization e2e (%s)', (adapter) => {
   let app: INestApplication;
diff --git a/tests/guard.spec.ts b/tests/guard.spec.ts
index b80e85c..4d1b8bd 100644
--- a/tests/guard.spec.ts
+++ b/tests/guard.spec.ts
@@ -171,7 +171,11 @@ describe('AuthorizationGuard', () => {
       .canActivate(httpContext(SteppingController, SteppingController.prototype.guarded, users.alice))
       .catch((e: unknown) => e)) as HttpException;
 
-    expect(error.getResponse()).toEqual({ message: 'Forbidden', statusCode: 403 });
+    expect(error.getResponse()).toEqual({
+      message: 'Forbidden',
+      statusCode: 403,
+      errorCode: 'forbidden'
+    });
     expect(error.cause).toBeInstanceOf(AuthorizationError);
     expect(error.cause).toMatchObject({ policy: 'SteppingPolicy', ability: 'second', reason: 'forbidden' });
   });
@@ -435,7 +439,12 @@ describe('toTransportError', () => {
     const error = await toTransportError(contextOf('ws'), guest);
 
     expect(error).toBeInstanceOf(WsException);
-    expect((error as WsException).getError()).toEqual({ status: 'error', message: 'Unauthorized', statusCode: 401 });
+    expect((error as WsException).getError()).toEqual({
+      status: 'error',
+      message: 'Unauthorized',
+      statusCode: 401,
+      errorCode: 'unauthenticated',
+    });
     expect(error.cause).toBe(guest);
   });
 
@@ -450,7 +459,11 @@ describe('toTransportError', () => {
     const error = await toTransportError(contextOf('kafka-custom'), forbidden);
 
     expect(error).toBeInstanceOf(ForbiddenException);
-    expect((error as HttpException).getResponse()).toEqual({ message: 'Forbidden', statusCode: 403 });
+    expect((error as HttpException).getResponse()).toEqual({
+      message: 'Forbidden',
+      statusCode: 403,
+      errorCode: 'forbidden'
+    });
   });
 });
 
@@ -493,7 +506,12 @@ describe('AuthorizationErrorInterceptor', () => {
     ).catch((e: unknown) => e);
 
     expect(mapped).toBeInstanceOf(WsException);
-    expect((mapped as WsException).getError()).toEqual({ status: 'error', message: 'Forbidden', statusCode: 403 });
+    expect((mapped as WsException).getError()).toEqual({
+      status: 'error',
+      message: 'Forbidden',
+      statusCode: 403,
+      errorCode: 'forbidden',
+    });
   });
 });
 
diff --git a/tests/http.e2e.spec.ts b/tests/http.e2e.spec.ts
index 37bbc5e..a9b7da0 100644
--- a/tests/http.e2e.spec.ts
+++ b/tests/http.e2e.spec.ts
@@ -140,7 +140,11 @@ describe.each(adapters.map((a) => a.name))('HTTP denials and failures (%s)', (ad
   });
 
   it('answers 401 when a @Can() ability dereferences a guest, and still checks signed-in users', async () => {
-    await request(app.getHttpServer()).get('/reports/view').expect(401, { message: 'Unauthorized', statusCode: 401 });
+    await request(app.getHttpServer()).get('/reports/view').expect(401, {
+      message: 'Unauthorized',
+      statusCode: 401,
+      errorCode: 'unauthenticated',
+    });
     await request(app.getHttpServer()).get('/reports/view').set('x-user', 'alice').expect(403);
     await request(app.getHttpServer()).get('/reports/view').set('x-user', 'eve').expect(200, { views: 1 });
   });
@@ -170,11 +174,15 @@ describe.each(adapters.map((a) => a.name))('HTTP denials and failures (%s)', (ad
   });
 
   it('denies a route that declares no check, and names it at startup', async () => {
-    await request(app.getHttpServer()).get('/reports/forgotten').expect(401, { message: 'Unauthorized', statusCode: 401 });
+    await request(app.getHttpServer()).get('/reports/forgotten').expect(401, {
+      message: 'Unauthorized',
+      statusCode: 401,
+      errorCode: 'unauthenticated',
+    });
     await request(app.getHttpServer())
       .get('/reports/forgotten')
       .set('x-user', 'root')
-      .expect(403, { message: 'Forbidden', statusCode: 403 });
+      .expect(403, { message: 'Forbidden', statusCode: 403, errorCode: 'forbidden' });
 
     expect(logged.map(({ cause }) => cause)).toEqual([
       expect.objectContaining({ reason: 'unauthenticated', policy: null, ability: null }),
diff --git a/tests/resilience.integration.spec.ts b/tests/resilience.integration.spec.ts
index 75ce88b..b2d1c70 100644
--- a/tests/resilience.integration.spec.ts
+++ b/tests/resilience.integration.spec.ts
@@ -84,8 +84,16 @@ describe.each(adapters.map((a) => a.name))('with @nestjs/resilience (%s)', (adap
     });
 
     it('answers a denial once, with its own status: no retry, no fallback', async () => {
-      await http().get('/reports/backup').query({ user: 'alice' }).expect(403, { message: 'Forbidden', statusCode: 403 });
-      await http().get('/reports/backup').expect(401, { message: 'Unauthorized', statusCode: 401 });
+      await http().get('/reports/backup').query({ user: 'alice' }).expect(403, {
+        message: 'Forbidden',
+        statusCode: 403,
+        errorCode: 'forbidden',
+      });
+      await http().get('/reports/backup').expect(401, {
+        message: 'Unauthorized',
+        statusCode: 401,
+        errorCode: 'unauthenticated',
+      });
 
       expect(app.get(ReportsService).calls).toBe(2);
       expect(events).toEqual([]);
diff --git a/tests/transports.e2e.spec.ts b/tests/transports.e2e.spec.ts
index a071109..9303335 100644
--- a/tests/transports.e2e.spec.ts
+++ b/tests/transports.e2e.spec.ts
@@ -119,12 +119,12 @@ describe('WebSocket gateway (platform-ws)', () => {
 
     expect(await ask('eve', 'create')).toEqual({
       event: 'exception',
-      data: { status: 'error', message: 'Forbidden', statusCode: 403 },
+      data: { status: 'error', message: 'Forbidden', statusCode: 403, errorCode: 'forbidden' },
     });
 
     expect(await ask(undefined, 'create')).toEqual({
       event: 'exception',
-      data: { status: 'error', message: 'Unauthorized', statusCode: 401 },
+      data: { status: 'error', message: 'Unauthorized', statusCode: 401, errorCode: 'unauthenticated' },
     });
   });
 
@@ -132,17 +132,19 @@ describe('WebSocket gateway (platform-ws)', () => {
     expect(await ask('bob', 'draft')).toEqual({ event: 'draft', data: 'Bob drafts' });
     expect(await ask('alice', 'draft')).toEqual({
       event: 'exception',
-      data: { status: 'error', message: 'Forbidden', statusCode: 403 },
+      data: { status: 'error', message: 'Forbidden', statusCode: 403, errorCode: 'forbidden' },
     });
   });
 
   it("carries the body of Nest's own ForbiddenException, after the ws filter's status", async () => {
     expect(await send('eve', 'create')).toBe(
-      '{"event":"exception","data":{"status":"error","message":"Forbidden","statusCode":403}}',
+      '{"event":"exception","data":{"status":"error","message":"Forbidden","statusCode":403,"errorCode":"forbidden"}}',
     );
     const { data } = (await ask('eve', 'create')) as { data: Record<string, unknown> };
     const { status: _, ...body } = data;
-    expect(JSON.stringify(body)).toBe(JSON.stringify(new ForbiddenException().getResponse()));
+    expect(JSON.stringify(body)).toBe(
+      JSON.stringify(new ForbiddenException(undefined, { errorCode: 'forbidden' }).getResponse()),
+    );
   });
 });
 
@@ -246,7 +248,10 @@ describe('WebSocket gateway behind per-message authentication (platform-ws)', ()
     const next = () => (replies.length ? Promise.resolve(replies.shift()) : new Promise((resolve) => waiting.push(resolve)));
     return { send, next, ask: (event: string, data?: object) => (send(event, data), next()) };
   };
-  const unauthenticated = { event: 'exception', data: { status: 'error', message: 'Unauthorized', statusCode: 401 } };
+  const unauthenticated = {
+    event: 'exception',
+    data: { status: 'error', message: 'Unauthorized', statusCode: 401, errorCode: 'unauthenticated' },
+  };
 
   beforeAll(async () => {
     app = await createApp('express', RoomsAppModule, { setup: (a) => void a.useWebSocketAdapter(new WsAdapter(a)) });
@@ -349,19 +354,28 @@ describe('GraphQL (Apollo, express)', () => {
 
     expect((await gql('{ create }', 'eve').expect(200)).body.errors[0]).toMatchObject({
       message: 'Forbidden',
-      extensions: { code: 'FORBIDDEN' },
+      extensions: {
+        code: 'FORBIDDEN',
+        originalError: { message: 'Forbidden', statusCode: 403, errorCode: 'forbidden' },
+      },
     });
 
     expect((await gql('{ create }').expect(200)).body.errors[0]).toMatchObject({
       message: 'Unauthorized',
-      extensions: { code: 'UNAUTHENTICATED' },
+      extensions: {
+        code: 'UNAUTHENTICATED',
+        originalError: { message: 'Unauthorized', statusCode: 401, errorCode: 'unauthenticated' },
+      },
     });
   });
 
   it('authorize() denials in a service carry the same code', async () => {
     expect((await gql('{ draft }').expect(200)).body.errors[0]).toMatchObject({
       message: 'Unauthorized',
-      extensions: { code: 'UNAUTHENTICATED' },
+      extensions: {
+        code: 'UNAUTHENTICATED',
+        originalError: { message: 'Unauthorized', statusCode: 401, errorCode: 'unauthenticated' },
+      },
     });
   });
 });
@@ -634,18 +648,18 @@ describe('microservice handlers (rpc)', () => {
     expect(await guard.canActivate(rpcContext(users.alice))).toBe(true);
     const error = await guard.canActivate(rpcContext(users.eve)).catch((e) => e);
     expect(error).toBeInstanceOf(RpcException);
-    expect(error.getError()).toEqual({ message: 'Forbidden', statusCode: 403 });
+    expect(error.getError()).toEqual({ message: 'Forbidden', statusCode: 403, errorCode: 'forbidden' });
     expect(error.cause).toMatchObject({ reason: 'forbidden', policy: 'PostPolicy', ability: 'create' });
   });
 
   it("carries the bodies of Nest's own UnauthorizedException and ForbiddenException, as over HTTP", async () => {
     const forbidden = (await guard.canActivate(rpcContext(users.eve)).catch((e) => e)) as RpcException;
-    expect(JSON.stringify(forbidden.getError())).toBe('{"message":"Forbidden","statusCode":403}');
-    expect(forbidden.getError()).toEqual(new ForbiddenException().getResponse());
+    expect(JSON.stringify(forbidden.getError())).toBe('{"message":"Forbidden","statusCode":403,"errorCode":"forbidden"}');
+    expect(forbidden.getError()).toEqual(new ForbiddenException(undefined, { errorCode: 'forbidden' }).getResponse());
 
     const guest = (await guard.canActivate(rpcContext(null)).catch((e) => e)) as RpcException;
-    expect(JSON.stringify(guest.getError())).toBe('{"message":"Unauthorized","statusCode":401}');
-    expect(guest.getError()).toEqual(new UnauthorizedException().getResponse());
+    expect(JSON.stringify(guest.getError())).toBe('{"message":"Unauthorized","statusCode":401,"errorCode":"unauthenticated"}');
+    expect(guest.getError()).toEqual(new UnauthorizedException(undefined, { errorCode: 'unauthenticated' }).getResponse());
   });
 
   it('an AuthorizationError leaving the handler becomes the same RpcException', async () => {
@@ -655,7 +669,7 @@ describe('microservice handlers (rpc)', () => {
     });
     const error = await lastValueFrom(result).catch((e: RpcException) => e);
     expect(error).toBeInstanceOf(RpcException);
-    expect((error as RpcException).getError()).toEqual({ message: 'Unauthorized', statusCode: 401 });
+    expect((error as RpcException).getError()).toEqual({ message: 'Unauthorized', statusCode: 401, errorCode: 'unauthenticated' });
   });
 
   it('leaves other errors alone', async () => {
@@ -739,19 +753,21 @@ describe('microservice over TCP (a real transport)', () => {
   it('@Can() reads the user the authentication guard left on the transport context', async () => {
     expect(await send('refunds.approve', { orderId: 3, token: 'finance-token' })).toEqual({ reply: { approved: 3 } });
     expect(await send('refunds.approve', { orderId: 3 })).toEqual({
-      error: { message: 'Unauthorized', statusCode: 401 },
+      error: { message: 'Unauthorized', statusCode: 401, errorCode: 'unauthenticated' },
     });
   });
 
   it('never takes the user from the message payload, which the producer writes', async () => {
     const admin = { id: 1, roles: ['admin', 'finance'] };
     expect(await send('refunds.approve', { orderId: 3, user: admin })).toEqual({
-      error: { message: 'Unauthorized', statusCode: 401 },
+      error: { message: 'Unauthorized', statusCode: 401, errorCode: 'unauthenticated' },
     });
   });
 
   it('authorize() denials in a service reach the client in the same shape', async () => {
-    expect(await send('drafts.show', {})).toEqual({ error: { message: 'Unauthorized', statusCode: 401 } });
+    expect(await send('drafts.show', {})).toEqual({
+      error: { message: 'Unauthorized', statusCode: 401, errorCode: 'unauthenticated' },
+    });
   });
 });
 
@@ -802,7 +818,9 @@ describe('hybrid app (HTTP plus a connected microservice)', () => {
   // Nest gives a connected microservice the app's global enhancers (APP_GUARD
   // included) only with inheritAppConfig: true. The README says so.
   it('enforces @Can() on message handlers with inheritAppConfig: true, and only then', async () => {
-    expect(await guestCall(true)).toEqual({ error: { message: 'Unauthorized', statusCode: 401 } });
+    expect(await guestCall(true)).toEqual({
+      error: { message: 'Unauthorized', statusCode: 401, errorCode: 'unauthenticated' },
+    });
     expect(await guestCall(false)).toEqual({ reply: 'approved' });
   });
 });
diff --git a/tests/transports.integration.spec.ts b/tests/transports.integration.spec.ts
index 86de390..56bab47 100644
--- a/tests/transports.integration.spec.ts
+++ b/tests/transports.integration.spec.ts
@@ -129,7 +129,9 @@ describe('hybrid app: authorize() in message handlers', () => {
 
   it('maps a service denial to an RpcException with inheritAppConfig: true', async () => {
     running = await hybrid(HybridAppModule, true);
-    expect(await send(running.client, 'refunds.show')).toEqual({ error: { message: 'Unauthorized', statusCode: 401 } });
+    expect(await send(running.client, 'refunds.show')).toEqual({
+      error: { message: 'Unauthorized', statusCode: 401, errorCode: 'unauthenticated' },
+    });
   });
 
   it('leaves it to the rpc filter without inheritAppConfig, which reports "Internal server error"', async () => {
@@ -163,8 +165,12 @@ describe('hybrid app: a getUser that trusts a gateway-verified user in RPC paylo
 
   it('reads the user from the message payload over RPC', async () => {
     expect(await send(running.client, 'refunds.approve', { user: finance })).toEqual({ reply: 'approved' });
-    expect(await send(running.client, 'refunds.approve', { user: users.alice })).toEqual({ error: { message: 'Forbidden', statusCode: 403 } });
-    expect(await send(running.client, 'refunds.approve')).toEqual({ error: { message: 'Unauthorized', statusCode: 401 } });
+    expect(await send(running.client, 'refunds.approve', { user: users.alice })).toEqual({
+      error: { message: 'Forbidden', statusCode: 403, errorCode: 'forbidden' },
+    });
+    expect(await send(running.client, 'refunds.approve')).toEqual({
+      error: { message: 'Unauthorized', statusCode: 401, errorCode: 'unauthenticated' },
+    });
   });
 
   it('delegates HTTP to defaultGetUser, which reads request.user and never a payload', async () => {