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CacheComponents: @Vercel/Otel'S Http Instrumentation Tracer Acquired During Register() Bypasses AfterRegistration Span Patch Causing Math.Random() Prerender Error

Next.js cacheComponents span-creation exemption is applied after instrumentation.register() returns, but @vercel/otel FetchInstrumentation captures a concrete Tracer during register(); subsequent outgoing HTTP requests in prerender create spans through that unpatched tracer and trip the sync-IO guard, producing a Math.random() prerender error.

highConfidence 89%Next.js Affected V16.3.6 Affected V16.3.8 Affected V16.4.0-Canary.54

Origin Analysis

The `extendTracerProviderForCacheComponents` patch is only applied in `afterRegistration()`, after `instrumentation.register()` has already run. `@vercel/otel`'s `FetchInstrumentation` calls `tracerProvider.getTracer('@vercel/otel/fetch', …)` during `register()` and closes over the returned concrete `Tracer`. Because the global provider's delegate is set by then, the returned tracer is the real SDK tracer, not a ProxyTracer, and never passes through the patched methods. Later span creation for outgoing HTTP requests uses that concrete tracer, bypassing the sync-IO exemption and hitting `Math.random()` inside `RandomIdGenerator.generateSpanId`.

1. Use a Next.js app with cacheComponents and instrumentation.ts that calls registerOTel({ serviceName: 'repro' }) from @vercel/otel. 2. Create a page with a 'use cache' function that awaits searchParams then makes an outbound https.request (e.g., via stripe). 3. Run next dev and curl the page. 4. Observe error 'Next.js encountered the unstable value Math.random() while prerendering.'

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

Double-patching may occur if the existing `afterRegistration` call remains; ensure `extendTracerProviderForCacheComponents` is idempotent (e.g., guard with a symbol on the provider) or remove the afterRegistration call. Multiple calls to `setGlobalTracerProvider` (e.g., by different instrumentations) will each be patched; if a provider is replaced after a tracer was already captured, that tracer remains unpatched. Also, some OTel API implementations may not allow overriding `setGlobalTracerProvider` after the global provider has been set; this wrapper should be installed before any registration. Test on Node 18/20/24 and with common instrumentations. Rollback: remove the wrapper and restore original `setGlobalTracerProvider` before deploying.