Minimal Cloudflare Worker demonstrating that RpcSerialization.msgPack silently fails when decoding messages with 3+ same-structure objects.

new Packr() with no options enables msgpackr's record/structure path (undefined != false is true). When the Unpackr encounters 3+ objects with the same key structure in a single decode, it JIT-compiles a fast reader via new Function() — which CF Workers blocks during request handling.

The error is silently swallowed by RpcSerialization.msgPack's catch { return [] }.

• Cloudflare account (miniflare does not enforce new Function() restriction)
• allow_eval_during_startup compat flag (default for compatibility_date >= 2025-06-01)

npm install
npx wrangler deploy
curl https://<your-worker>.workers.dev

{
  "bug": {
    "raw_error": "EvalError: Code generation from strings disallowed for this context",
    "rpcSerialization_result": [],
    "rpcSerialization_length": 0,
    "silently_swallowed": true
  },
  "fix": {
    "useRecords_false_result": [{ "_tag": "Chunk", ... }],
    "useRecords_false_length": 1
  }
}

• bug.raw_error — the actual EvalError from new Function() being blocked
• bug.silently_swallowed — true because catch { return [] } hides the error
• fix — { useRecords: false } prevents the JIT path entirely

PR: Effect-TS/effect#6161

// Before (silently fails on CF Workers with nested objects)
RpcSerialization.layerMsgPack

// After
RpcSerialization.layerMsgPack({ useRecords: false })

• CF Workers new Function() restriction
• msgpackr useRecords docs
• CF allow_eval_during_startup compat flag
• livestore investigation