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`Next Start` Built-In Compression Never Applies To Route Handlers (Including Metadata Routes) — Sitemap.Xml Served Uncompressed

In Next.js 16.2.10, when using `next start`, page renders are correctly compressed by the built-in `compression` middleware, but all Route Handler responses (including static/prerendered metadata routes and dynamic route handlers) are sent without any `Content-Encoding`. This occurs even when the response body is well above the 1KB compression threshold and the content type is compressible (e.g., `application/xml`). The issue violates the official documentation's promise that Next.js compresses "rendered content and static files" by default, leading to wasted bandwidth and degraded performance for sitemaps, JSON APIs, and other large Route Handler responses in self-hosted environments.

highConfidence 85%Next.js Affected V16.2.10

Origin Analysis

In `server/lib/router-server.js`, the `requestHandlerImpl` function invokes the `compression` middleware for every request by calling `compress(req, res, () => {})`. This middleware works by modifying the `res` object's methods (e.g., `writeHead`, `write`, `end`) to inject compression. However, Route Handler responses are written through a different path that bypasses these modified methods—likely because Route Handlers operate on a different response object (or directly on the original `res` without the middleware's patches). The observed header casing difference (page responses use `Content-Type` while Route Handler responses use lowercase `content-type`) confirms that Route Handler responses are not going through the `compression` middleware's wrappers, leaving them uncompressed.

1. Create a Next.js 16.2.10 project with a page route (`app/page.js`), a metadata route (`app/sitemap.js`, body >1KB), and a force-dynamic route handler (`app/d.xml/route.js`, body >1KB). 2. Run `npm run build && npm run start`. 3. Execute `curl -s -H 'Accept-Encoding: gzip, br' -D - -o /dev/null http://127.0.0.1:3000/` and observe `Content-Encoding: gzip` in the response headers. 4. Execute the same command for `/sitemap.xml` and `/d.xml`. The response headers will lack `Content-Encoding` and all headers will be lowercase, indicating the compression middleware was bypassed.

Fixing Code Block

0x7F 45 4C 46 02 01 01 000x90 12 AB 04 C1 7E 33 08
0xA4 1C 90 3E 77 0B D2 580x11 F6 6A 2C 80 19 DE 44
0x00 00 00 00 03 00 3E 000x01 00 B7 4D 55 90 0C 61
0x6C 69 62 63 2E 73 6F 2E0x36 00 8E 17 42 AA 05 F3
0xDE C0 DE 01 8B 45 FC 890xC7 5D 20 6F 91 03 E4 78
0x13 37 00 4F 9A 2D E1 700x08 B4 5C 22 71 D0 4A 16
0x48 8D 3D 22 01 00 00 E80x91 FF 0A 63 2E 57 B8 04
0x5A 91 C3 0E 44 70 1B 880x02 6F 39 C4 15 80 ED 27
0xFF 15 40 20 60 00 48 890xC3 90 7A 01 4C 28 B1 56
0x0F 1F 44 00 00 55 48 890xE5 C3 18 6D 92 0B 33 A7

Edge Case Audit

Using a custom server may disable certain Next.js optimizations and features that rely on `next start` (e.g., Turbopack integration, automatic port handling, graceful shutdown). It adds an external dependency (`compression`) and requires manual process management in production. If the official fix is released, remove this workaround and revert to `next start`. Rollback: delete `server.js`, remove the `compression` package, and restore the original `next start` command.