fix(rpc): make RpcSerialization.msgPack options configurable

[bohdanbirdie]

Summary

• Adds makeMsgPack(options) factory for creating MessagePack serialization with custom msgpackr options
• Existing msgPack and layerMsgPack exports are unchanged (no breaking change)
• Fixes silent error swallowing in msgPack decode — non-incomplete errors are now rethrown instead of returning []

Problem

On Cloudflare Workers with allow_eval_during_startup (default for compatibility_date >= 2025-06-01), RpcSerialization.msgPack silently fails when decoding messages containing 3+ objects with the same key structure.

Root cause: new Packr() with no options sets this.structures = [] because undefined != false evaluates to true. This enables msgpackr's record/structure path. When the Unpackr's readObject.count exceeds the JIT threshold (2), msgpackr calls new Function() to compile a fast reader — which CF Workers blocks during request handling with EvalError: Code generation from strings disallowed for this context.

The existing catch { return [] } in RpcSerialization.msgPack silently swallows this error, returning an empty response. The caller never receives data.

Reproduction

https://github.com/bohdanbirdie/repro-effect-rpc-msgpack-cf-workers

• main branch — "before" state: reproduces the bug using inline msgpackr code (same as RpcSerialization.msgPack)
• fixed branch — "after" state: uses bun patch on @effect/rpc to apply this PR's changes, worker uses RpcSerialization.makeMsgPack({ useRecords: false })

Deploy either branch to CF Workers with npx wrangler deploy and curl the endpoint.

Fix

Use makeMsgPack to create a serialization with custom options:

import { Layer } from "effect"
import { RpcSerialization } from "@effect/rpc"

Layer.succeed(
  RpcSerialization,
  RpcSerialization.makeMsgPack({ useRecords: false })
)

This skips this.structures = [] in the Packr constructor, preventing the record/JIT path entirely.

Test plan

• New unit tests for msgPack and makeMsgPack with custom options
• Existing 101 e2e RpcServer tests pass (http, ws, tcp, worker × ndjson, msgpack, jsonrpc)
• CF Workers deployment verified with { useRecords: false }

References

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

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[bohdanbirdie]

I'm not sure if the solution is good enough, but I think that the underlying issue is confirmed. I'm rather new to effect so PR changes might need some guidance, but I'm happy to carry on with it.

Reproduction repository should be easy to use and try.

Tim, let me know what you think (didn't want to ping and create noise).