`set.headers` appends to Response headers instead of merging, causing duplicate values

[truehazker]

What version of Elysia is running?

elysia@1.4.17

What platform is your computer?

Darwin 25.1.0 arm64 arm

What environment are you using

1.3.2

Are you using dynamic mode?

no

What steps can reproduce the bug?

When a handler returns a Response with custom headers, and set.headers contains the same header key, values are appended instead of merged:

import { Elysia } from 'elysia'

const app = new Elysia()
    .onRequest(({ set }) => {
        set.headers['Content-Type'] = 'application/json'
    })
    .get('/', () => {
        return new Response('data', {
            headers: { 'Content-Type': 'text/plain' }
        })
    })

const res = await app.handle(new Request('http://localhost/'))
console.log(res.headers.get('Content-Type'))
// Output: "text/plain, application/json"

What is the expected behavior?

Headers should be merged without duplication. Either:

• Response headers take precedence → text/plain
• Or set.headers takes precedence → application/json

Not both appended together.

What do you see instead?

Content-Type: text/plain, application/json

This is invalid for most headers. Content-Type should have a single value.

Additional information

The issue is in src/adapter/utils.ts in createResponseHandler(). It uses response.headers.append() which adds values instead of replacing:

for (const key in set.headers)
    response.headers.append(key, set.headers[key]) // append creates duplicates

Should use proper merge logic that handles conflicts.

Have you try removing the node_modules and bun.lockb and try again yet?

yes

Fix implementation

#1591