mount() produces incorrect URL path when Elysia instance has prefix option

[notcome]

What version of Elysia is running?

elysia@1.4.21

What platform is your computer?

Darwin 25.1.0 arm64 arm

What environment are you using?

Bun 1.3.6

Are you using dynamic mode?

No, default aot: true. Issue also reproduces with aot: false.

What steps can reproduce the bug?

import { describe, expect, it } from 'bun:test'
import { Elysia } from 'elysia'

describe('Mount with prefix', () => {
  it('mount without prefix - strips mount path', async () => {
    const app = new Elysia()
      .mount('/sdk/problems-domain', (request) => {
        return Response.json({ path: new URL(request.url).pathname })
      })

    const response = await app
      .handle(new Request('http://localhost/sdk/problems-domain/problems'))
      .then((x) => x.json())

    expect(response.path).toBe('/problems') // ✅ PASSES
  })

  it('mount with prefix - should strip both prefix and mount path', async () => {
    const sdkApp = new Elysia({ prefix: '/sdk' })
      .mount('/problems-domain', (request) => {
        return Response.json({ path: new URL(request.url).pathname })
      })

    const app = new Elysia().use(sdkApp)

    const response = await app
      .handle(new Request('http://localhost/sdk/problems-domain/problems'))
      .then((x) => x.json())

    expect(response.path).toBe('/problems') // ❌ FAILS
  })
})

What is the expected behavior?

When using mount() on an Elysia instance that has a prefix option, the mounted handler should receive the URL with the full prefix + mount path stripped.

For request to /sdk/problems-domain/problems:

• Expected: handler receives path /problems

What do you see instead?

The handler receives /main/problems - a mysterious /main/ prefix appears from nowhere.

without prefix: { path: "/problems" }    ✅
with prefix:    { path: "/main/problems" } ❌

Additional information

• The issue only occurs when combining prefix option with mount()
• Using mount() without prefix works correctly
• The /main/ string doesn't appear anywhere in my code

Have you tried removing the node_modules and bun.lockb?

Yes, issue persists with fresh install.

[notcome]

Before you ban me from using Claude Code to post this issue, I did supervise the whole process. It is just that filling out the whole questionnaire for bug report is a bit error prone so I decided to automate this.