From a5558502479f00bdbd6f7314a99d5cce09624542 Mon Sep 17 00:00:00 2001
From: saltyaom <saltyaom@gmail.com>
Date: Wed, 14 Jan 2026 10:56:47 +0700
Subject: [PATCH] :wrench: fix: #1671 mount produce incorrect url

---
 CHANGELOG.md               |  1 +
 example/a.ts               | 17 ++++++++++++++++-
 src/adapter/bun/compose.ts |  2 +-
 src/index.ts               |  6 +++++-
 src/utils.ts               | 10 +++++++---
 test/core/mount.test.ts    | 33 +++++++++++++++++++++++++++++++++
 6 files changed, 63 insertions(+), 6 deletions(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index d680bb14..113c3466 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,6 @@
 # 1.4.22
 Bug fix:
+- [#1671](https://github.com/elysiajs/elysia/issues/1671) mount() produces incorrect URL path when Elysia instance has prefix option
 - ValueError with summary missing types
 - Elysia not using Bun.routes
 
diff --git a/example/a.ts b/example/a.ts
index c19a2462..bac7371a 100644
--- a/example/a.ts
+++ b/example/a.ts
@@ -1,3 +1,18 @@
 import { Elysia } from '../src'
 
-const app = new Elysia().get('/', () => 'a').listen(3000)
+const sdkApp = new Elysia({ prefix: '/sdk' }).mount(
+	'/problems-domain',
+	(request) => {
+		console.log(request.url)
+
+		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.text())
+
+console.log(response)
diff --git a/src/adapter/bun/compose.ts b/src/adapter/bun/compose.ts
index b0b05915..1f30e8cf 100644
--- a/src/adapter/bun/compose.ts
+++ b/src/adapter/bun/compose.ts
@@ -33,7 +33,7 @@ const createContext = (
 	const getQi =
 		`const u=request.url,` +
 		`s=u.indexOf('/',${standardHostname ? 11 : 7}),` +
-		`qi=u.indexOf('?', s + 1)\n`
+		`qi=u.indexOf('?',s+1)\n`
 
 	const needsQuery =
 		inference.query ||
diff --git a/src/index.ts b/src/index.ts
index de83bb87..c9c011be 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -5613,8 +5613,12 @@ export default class Elysia<
 							throw new Error('Invalid handler')
 						})()
 
-		const length = path.length - (path.endsWith('*') ? 1 : 0)
+		const fullPath =
+			typeof path === 'string' && this.config.prefix
+				? this.config.prefix + path
+				: path
 
+		const length = fullPath.length - (path.endsWith('*') ? 1 : 0)
 		const handler: Handler = ({ request, path }) =>
 			handle(
 				new Request(
diff --git a/src/utils.ts b/src/utils.ts
index 761c167b..18fffdea 100644
--- a/src/utils.ts
+++ b/src/utils.ts
@@ -28,9 +28,13 @@ import { ElysiaFile } from './universal/file'
 export const hasHeaderShorthand = 'toJSON' in new Headers()
 
 export const replaceUrlPath = (url: string, pathname: string) => {
-	const urlObject = new URL(url)
-	urlObject.pathname = pathname
-	return urlObject.toString()
+	const pathStartIndex = url.indexOf('/', 11)
+	const queryIndex = url.indexOf('?', pathStartIndex)
+
+	if (queryIndex === -1)
+		return `${url.slice(0, pathStartIndex)}${pathname.charCodeAt(0) === 47 ? '' : '/'}${pathname}`
+
+	return `${url.slice(0, pathStartIndex)}${pathname.charCodeAt(0) === 47 ? '' : '/'}${pathname}${url.slice(queryIndex)}`
 }
 
 export const isClass = (v: Object) =>
diff --git a/test/core/mount.test.ts b/test/core/mount.test.ts
index 0f44df44..ff17b5b1 100644
--- a/test/core/mount.test.ts
+++ b/test/core/mount.test.ts
@@ -137,4 +137,37 @@ describe('Mount', () => {
 			'x-test': 'test'
 		})
 	})
+
+	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() as Promise<{ path: string }>)
+
+		expect(response.path).toBe('/problems')
+	})
+
+	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() as Promise<{ path: string }>)
+
+		expect(response.path).toBe('/problems')
+	})
 })