CapsuleRSC: Safe Server/Client Boundary Enforcement for React Server Components

Source: Dev.to

Overview

CapsuleRSC is a minimal framework that enforces safe server/client boundaries for React Server Components (RSC) with strong, mechanical guarantees.

It does not rely on conventions or developer discipline. Instead, CapsuleRSC enforces correctness through three explicit layers of defense:

• Type-level – TypeScript types prevent non‑serializable data from crossing boundaries at compile time
• Build-time – An ESLint plugin detects boundary violations during development
• Runtime – assertSerializable provides a final line of defense at execution time

Why Boundaries Matter

React Server Components introduce a new constraint: data must cross from server to client in a safe, serializable form.

Without strict boundaries, subtle but serious problems appear:

• Functions passed to client components cause runtime failures
• Date objects silently become strings
• Class instances lose their methods
• Circular references crash serialization

CapsuleRSC prevents these issues by enforcing boundaries at every layer, not by assuming developers will remember the rules.

Core Principle

Do not assume boundaries are respected — enforce them mechanically.

CapsuleRSC exists to validate this principle through implementation, not theory.

How CapsuleRSC Differs from tRPC

CapsuleRSC and tRPC solve different problems at different layers.

• tRPC focuses on type‑safe API communication between server and client. It assumes the execution boundary already exists (HTTP/RPC).
• CapsuleRSC focuses on mechanically enforcing the server/client execution boundary itself, which is especially fragile in React Server Components.

In short:

• tRPC is about type‑safe communication.
• CapsuleRSC is about mechanically enforced execution boundaries.

They are not competitors and can be used together.

Demo

A live demo of CapsuleRSC is available as part of the author’s portfolio.

• Server/client boundary enforcement
• Serialization guarantees
• Capability‑based side effects

Demo URL:
https://yuu1ch13-portfollio-tau-sepia.vercel.app/capsule-rsc

Repository

https://github.com/yuuichieguchi/capsule-rsc