swagger path gets doubled when path set to 'api-docs'

[milo-v]

Is there an existing issue for this?

• I have searched the existing issues

Current behavior

I was following this guide to setup swagger ui: https://docs.nestjs.com/openapi/introduction

However, when I set the path in SwaggerModule.setup to 'api-docs', I see the browser looks for assets in (for example) http://localhost:8000/api-docs/api-docs/swagger-ui.css. It is incorrect path and returns 404 not found.

It does not happen when I set any other value, for example, 'docs' gives me http://localhost:8000/docs/swagger-ui.css, which is the correct behavior.

Minimum reproduction code

https://docs.nestjs.com/openapi/introduction

Steps to reproduce

No response

Expected behavior

'api-docs' should not be duplicated

NestJS version

11.0.1

Packages versions

[System Information]
OS Version : Linux6.14.0-33-generic
NodeJS Version : v24.9.0
(node:17880) [DEP0190] DeprecationWarning: Passing args to a child process with shell option true can lead to security vulnerabilities, as the arguments are not escaped, only concatenated.
(Use node --trace-deprecation ... to show where the warning was created)
PNPM Version : 10.18.0

[Nest CLI]
Nest CLI Version : 11.0.10

[Nest Platform Information]
platform-express version : 11.1.8
schematics version : 11.0.9
swagger version : 11.2.1
testing version : 11.1.8
common version : 11.1.8
config version : 4.0.2
core version : 11.1.8
cli version : 11.0.10

Node.js version

24.9.0

In which operating systems have you tested?

• macOS
• Windows
• Linux

Other

No response

[kamilmysliwiec]

Please provide a minimum reproduction repository (Git repository/StackBlitz/CodeSandbox project).

[mag123c]

import { NestFactory } from '@nestjs/core';
import { AppModule } from './app.module';
import { DocumentBuilder, SwaggerModule } from '@nestjs/swagger';

async function bootstrap() {
  const app = await NestFactory.create(AppModule);

  const config = new DocumentBuilder()
    .setTitle('Cats example')
    .setDescription('The cats API description')
    .setVersion('1.0')
    .addTag('cats')
    .build();
  const documentFactory = () => SwaggerModule.createDocument(app, config);
  SwaggerModule.setup('/api-docs', app, documentFactory); // 'api-docs' also no problem 

  await app.listen(process.env.PORT ?? 3000);
}
bootstrap();

In the same environment, I set up the swagger by looking at the nest official document, but I could not reproduce this phenomenon. Can you provide the minimum reproduction code?

[rodrigo-itao]

I noticed that this bug only occurs when I add a / at the end of the URL, for example http://localhost:3001/docs/.
If I remove the trailing /, the Swagger docs load correctly (http://localhost:3001/docs).

[mag123c]

The root cause is a bug in SwaggerModule's path calculation logic, not the server framework.

When setup('docs/') is called with a trailing slash:

1. SwaggerModule bug

const urlLastSubdirectory = finalPath.split('/').slice(-1).pop() || '';
// '/docs/'.split('/') → ['', 'docs', ''] → pop() → '' (empty string!)

2. Relative path generation (line 147)

const baseUrlForSwaggerUI = normalizeRelPath(`./${urlLastSubdirectory}/`);
// = normalizeRelPath('.//') → './'

3. Browser behavior

• /docs → base is / → ./file.css resolves to /file.css
• /docs/ → base is /docs/ → ./file.css resolves to /docs/file.css

The server framework (NestJS/Express) works correctly - it serves HTML for both /docs and /docs/.
The problem is that SwaggerModule uses relative paths in the HTML, which browsers interpret differently based on trailing slash presence.

I think if this is something that needs to be resolved, I think @nestjs/swagger should handle it.

[kamilmysliwiec]

Would you like to create a PR for this issue?