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name: yjs-nextjs description: Corrects the wrong defaults a capable model has when building Yjs collaborative editing into a Next.js 16 App Router app with tRPC v11 and shadcn/ui — pinned to Yjs 13.6.31. Covers the decisions whose failure mode is silence rather than an error — plain objects in a Y.Map discarding concurrent edits, updates that vanish because a JSON boundary turned a Uint8Array into a number array, undo that reverts a colleague's paragraph, initial content seeded once per client, and carets thrown to the end of a shadcn Input on every remote keystroke. Also covers where the sync loop can actually run, since route handlers cannot upgrade, Vercel Functions have no instance affinity, and tRPC subscriptions are one-way — plus which packages have already moved to the Yjs 14 prerelease track. Use when writing, reviewing, or debugging collaborative editing, presence, offline sync, or CRDT persistence in this stack.

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

#	Category	Prefix	Covers
1	Sync Topology & Dependencies	host-	Where the sync loop runs, which backend to build on, where the trust boundary sits, which versions are on the v13 track
2	Binary Transport & Persistence	wire-	Moving Uint8Array updates across JSON boundaries and into storage
3	Modeling State in Shared Types	model-	Which shared type per field — decides whether concurrent edits merge or destroy each other
4	Doc Lifecycle & React Binding	react-	One document across renders, Strict Mode, subscriptions, the server boundary
5	Undo, Snapshots & Offline Load	hist-	Origin-scoped history, garbage collection, load ordering
6	Awareness & Cursors	pres-	Ephemeral presence and positions that survive concurrent edits
7	Editor & Form Integration	ui-	Tiptap and shadcn/ui form controls against a live document

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:

Symptom	Read first
Document loads blank	wire-rehydrate-to-uint8array, hist-wait-for-indexeddb-sync
Content appears twice	model-seed-the-document-once, ui-tiptap-collaboration-wiring
Someone's edit silently reverted	model-nested-types-not-plain-objects, hist-scope-undo-by-origin
List items duplicate when dragged	model-yarray-has-no-move
Two editors never see each other	host-vercel-lacks-instance-affinity
A user edited a field they should not reach	host-authorize-rooms-not-updates
A collaborator's avatar lingers after they leave	pres-awareness-is-ephemeral
Caret jumps to the end while typing	ui-do-not-drive-inputs-from-the-doc
Infinite render loop or snapshot error	react-subscribe-via-usesyncexternalstore
Works in production, broken in dev	react-stable-doc-and-provider

Reference Files

File	Description
references/_sections.md	Category definitions and ordering
assets/templates/_template.md	Template for new rules
AGENTS.md	Auto-built table of contents across all rules
metadata.json	Version and source references

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