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Socket Rewrites Connection Reuse Can Lead To A Race Condition And Intermittent 500S

Next.js 16.3.x uses httpxy for rewrites with a shared keep-alive agent, which can reuse an idle socket at the same moment the upstream closes it, causing a race condition that intermittently returns 500 on the second socket connection.

highConfidence 72%Next.js Affected V16.3.0-Canary.98 Affected V16.3.6

Origin Analysis

httpxy by default uses a shared keep-alive agent for proxy requests; when an upstream service closes an idle connection, the agent may still resurrect the socket for a new request, leading to an ECONNRESET/IPE and a 500 response.

1. Clone https://github.com/aleksi-abr/nextjs-external-rewrite-socket-reuse-repro; 2. Run `pnpm install`; 3. Run `pnpm upstream` to start mock upstream; 4. Run `pnpm dev` to start Next; 5. Run `pnpm probe` to make two socket connections; 6. Observe first connection returns 200, second returns 500.

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

Disabling keep-alive increases the per-request latency and connection establishment overhead, especially for high-throughput proxying. In multi-process/cluster deployments, ensure agent instances are created per worker to avoid cross-thread sharing issues; in serverless environments, module-level agents may keep connections from being properly released after function termination. If this patch is applied to a future Next.js version where the internal structure changes, it may conflict; rollback by reverting the agent changes and rebuilding. A more flexible solution would expose a configuration option (e.g., `experimental.proxyClientKeepAlive`) to allow users to choose the trade-off.