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Next.Js TestProxy Breaks Fetch To HTTP/2 Servers Due To ALPN Mismatch

When experimental.testProxy is enabled, the global socket interceptor from @mswjs/interceptors patches net.Socket.prototype.connect for all outbound sockets, including fetch/undici. In passthrough mode (no active test context), it opens a real TLS socket copying the original ALPN list. If the server negotiates HTTP/2, the interceptor still writes HTTP/1.1 plaintext and parses the response with an HTTP/1.1 parser, causing fetch failures.

highConfidence 85%Next.js Affected V16.4.0-Canary.1

Origin Analysis

The socket-interceptor in @mswjs/interceptors (vendored by Next.js for testProxy) assumes HTTP/1.1 framing throughout. When reconstructing a passthrough connection, it copies TLS options including ALPNProtocols. On TLS servers that support h2, the connection negotiates HTTP/2, but the interceptor sends an HTTP/1.1 request and attempts to parse the HTTP/2 response with an HTTP/1.1 parser, leading to SocketError or HTTPParserError.

git clone https://github.com/melkir/testproxy-h2-repro cd testproxy-h2-repro npm install npm test The test proxy is enabled in next.config.ts. The test performs a simple fetch to an HTTPS server that supports HTTP/2. The request fails with TypeError: fetch failed or HTTPParserError.

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

Forcing HTTP/1.1 globally may break connections to servers that only support HTTP/2 (e.g., some modern APIs). The patch affects all TLS sockets in the process, including non-fetch libraries, potentially altering their behavior. It should only be enabled in test environments and must be removed once the official fix is available. Concurrency is not impacted, but TLS session reuse and ALPN caching could be affected. Rollback: remove the patch and set experimental.testProxy to false.