Yjs in Next.js 16, tRPC, and shadcn/ui

Yjs 13.6.31 in an App Router codebase — the decisions collaborative editing forces and how to settle them, written so an agent applies them while writing or reviewing code. Each rule names the wrong default it corrects; there is no rule for things the model already gets right.

Most of these failures are silent. Yjs converges, the editor keeps working, and the data loss surfaces later as "my change reverted" — so the rules below lead with the evidence of the failure, most of it measured directly against Yjs 13.6.31 rather than asserted.

When to Apply

• Building collaborative editing, presence, or offline sync into a Next.js 16 App Router app
• Choosing shared types for a document model, or reviewing one that loses edits under concurrency
• Deciding where the sync backend runs, or wiring tRPC procedures alongside a Yjs provider
• Persisting Y.Doc state to a database, or compacting stored updates
• Binding a Yjs document to React — provider lifecycle, subscriptions, Strict Mode, SSR
• Wiring Tiptap or shadcn/ui form controls to a live document
• Debugging a document that syncs but duplicates, blanks, or reverts content

This skill does not cover general Next.js App Router patterns (use the nextjs skill), general tRPC usage (trpc), or shadcn/ui component conventions (shadcn) — only where those intersect with a CRDT.

Version Pin

Rules target Yjs 13.6.31 with y-websocket 3.0.0, y-protocols 1.0.7, and y-indexeddb 9.0.12. Yjs is mid-migration to v14 under the @y/* scope, and several packages have already moved latest or main onto that prerelease track — @y/websocket-server@0.1.5 depends on yjs@^14.0.0-7. See host-pin-the-yjs-13-track before installing anything.

Rule Categories

Quick Reference

1. Sync Topology & Dependencies

• host-route-handlers-cannot-upgrade — App Router has no socket-upgrade API; the sync server needs a life independent of a request
• host-vercel-lacks-instance-affinity — Vercel serves WebSockets but does not pin a room to an instance; in-memory registries split one document in two
• host-websocket-server-is-not-production — the reference server is a starting point, and YPERSISTENCE no longer exists
• host-pin-the-yjs-13-track — installing latest pulls a Yjs 14 prerelease into a v13 app
• host-trpc-is-not-the-sync-channel — SSE is one-way and binary input is POST-only; use tRPC for authorization and snapshots
• host-authorize-rooms-not-updates — a CRDT update cannot be schema-validated or permission-checked, so the room is the trust boundary

2. Binary Transport & Persistence

• wire-rehydrate-to-uint8array — applyUpdate accepts a number[] and silently applies nothing
• wire-base64-not-superjson — superjson round-trips typed arrays at 3.69x payload; base64 costs 1.33x
• wire-send-updates-not-state — 24 bytes of delta versus 5041 bytes of full state for one keystroke
• wire-compaction-needs-a-doc-roundtrip — mergeUpdates re-encodes but never reclaims deleted content

3. Modeling State in Shared Types

• model-nested-types-not-plain-objects — a plain object in a Y.Map is one conflict unit; concurrent edits to different fields lose one
• model-yarray-has-no-move — delete-then-insert duplicates the row when two people drag it at once
• model-ytext-only-for-prose — two people retyping a Y.Text title produce both values spliced together
• model-seed-the-document-once — an emptiness check passes on every client, so the template lands twice

4. Doc Lifecycle & React Binding

• react-stable-doc-and-provider — own creation and teardown in one effect, because Strict Mode runs setup, cleanup, setup
• react-subscribe-via-usesyncexternalstore — no official binding exists, and toJSON() returns a new reference every call
• react-keep-yjs-client-only — providers need browser globals, and 'use client' still renders on the server

5. Undo, Snapshots & Offline Load

• hist-scope-undo-by-origin — the default undo manager tracks remote edits and will revert a colleague's work
• hist-snapshots-require-gc-disabled — decided at construction; restoring a snapshot otherwise throws
• hist-wait-for-indexeddb-sync — an unloaded document is indistinguishable from an empty one

6. Awareness & Cursors

• pres-awareness-is-ephemeral — presence in the document is permanent, versioned, and never cleaned up
• pres-relative-positions-for-cursors — an index stops meaning the same place as soon as anyone types above it

7. Editor & Form Integration

• ui-do-not-drive-inputs-from-the-doc — assigning value is specified to throw the caret to the end of the field
• ui-tiptap-collaboration-wiring — Tiptap v3 uses its own fork, and content re-seeds on every load

How to Use

Read a reference file when its decision comes up. Each rule names the wrong default it corrects, then shows the canonical way, with an incorrect/correct contrast where the wrong way is a real trap.

When debugging rather than writing, start from the symptom:

Reference Files

Related Skills

• nextjs — App Router caching, Server Components, and routing outside the CRDT layer
• trpc — tRPC v11 router, link, and subscription rules in full
• shadcn — component composition and form patterns this skill builds on
• zod — the validators used in the tRPC procedure examples here