Fix multiline paste crash

Objective: Fix issue #5043 without weakening maxLength; done when crash and empty-block overflow regressions pass, package checks and autoreview are clean, and PR ships.

Flow mode: one-shot execution

Goal plan: docs/plans/5043-fix-multiline-paste-crash.md

Template: docs/plans/templates/task.md

Primary template: docs/plans/templates/task.md

Applied packs:

browser (docs/plans/templates/packs/browser.md)
package-api (docs/plans/templates/packs/package-api.md)

Task source:

type: public GitHub bug
id / link: #5043 / https://github.com/udecode/plate/issues/5043
title: [Bug]: pasting text with multiple line-breaks crashes
acceptance criteria: multiline plain-text paste past maxLength does not throw; final document text never exceeds maxLength, including fragments with empty blocks.

Timed checkpoint:

requested duration: N/A: none requested
semantics: N/A
initial confidence score: N/A: exact binary regressions exist
improvement loop: N/A
final score / loop closure: N/A

Completion threshold:

Faithful Markdown/plain-text paste repro fails on origin/main and passes after the fix without throwing, producing exactly maxLength characters.
Direct fragments containing overflow text plus empty blocks still converge to exactly maxLength characters.
Focused core tests, core source-first typecheck, lint, approved browser proof, and structured autoreview pass with zero accepted/actionable findings.
A patch changeset, task-style PR, and issue sync are published and read back.
Task closure is legal only when the source-of-truth acceptance criteria are satisfied or explicitly narrowed, required verification evidence is recorded, code-review and release-artifact gates are closed when applicable, tracker/PR sync is complete or marked N/A with reason, and node .agents/skills/autogoal/scripts/check-complete.mjs docs/plans/5043-fix-multiline-paste-crash.md passes.

Verification surface:

bun test packages/core/src/lib/plugins/length/LengthPlugin.spec.ts
pnpm turbo typecheck --filter=./packages/core
pnpm lint:fix
approved Browser tool against a local runnable editor surface, or an exact blocker/waiver if the package has no faithful route
.agents/skills/autoreview/scripts/autoreview --mode local
gh pr view and gh issue view readback

Constraints:

Preserve existing user-facing behavior outside the task scope.
Prefer the durable ownership boundary over caller-by-caller patches.
Do not create PRs, comments, commits, or pushes unless the task/user/skill requires them.
Do not add broad ceremony when the task is trivial or docs-only.

Boundaries:

Source of truth: GitHub issue #5043 plus the LengthPlugin invariant on origin/main.
Allowed edit scope: packages/core/src/lib/plugins/length/**, one @platejs/core patch changeset, this plan, and only directly required test fixtures/imports.
Browser surface: local editor route that can configure maxLength and paste multiline plain text; public repro is supporting evidence only.
Tracker sync: create a replacement PR and comment issue #5043 after verified code exists.
Non-goals: public API changes, unrelated clipboard behavior, broad Slate transform refactors, or reviving PR #5100.

Output budget strategy:

Read exact length-plugin, package, plan, and route files; cap searches by owner and match count; exclude generated/build/dependency trees; save long test/review output to artifacts and inspect only failure/summary slices.

Blocked condition:

Stop only if the faithful repro cannot be made executable from repo-owned APIs, required dependencies remain unusable after the one allowed reinstall, or GitHub/browser access prevents mandatory ship/readback proof.

Task state:

task_type: ordinary tracker-backed runtime bug
task_complexity: normal, non-trivial and auditable
current_phase: closeout
current_phase_status: complete
next_phase: final response
goal_status: verification_pending

Current verdict:

verdict: verified fix ready to ship
confidence: 99%
next owner: PR / tracker sync
reason: crash, exact text cap, empty overflow block removal, package checks, full repo check, browser proof, and final structured review are green.

Pre-solution issue challenge:

reporter claim: multiline overflow paste crashes while enforcing maxLength.
suggested diagnosis or fix: the issue proposes none. Closed PR #5100 added a boolean re-entrancy early return; rejected because empty-block deletion can finish at 21 characters for maxLength: 20.
repro ladder:
- tests / source-level repro: prior exact review repro confirmed the crash; current branch will capture it as the TDD red test before implementation.
- Playwright / automated browser: N/A: the package public API and clipboard data path model the failure deterministically; browser proof follows repo policy but Playwright adds no missing observation.
- Browser plugin: required after the focused regression is green.
- screenshot / visual proof: N/A: crash and character count are behavioral, not visual-layout claims; record route and console result instead.
reproduction verdict: reproduced
validity verdict: valid, with acceptance tightened to preserve maxLength.
best long-term fix boundary: make LengthPlugin trimming converge after nested Slate operations instead of disabling all nested enforcement.
harsh honest feedback: “does not crash” is too weak; a fix that silently exceeds maxLength is still broken.
hard-stop decision: proceed with TDD; reject the closed PR's guard shape.

Completion rule:

Do not call update_goal(status: complete) while any required checklist item remains unchecked. If an item does not apply, check it and add N/A: <reason>.
Do not call update_goal(status: complete) until every completion threshold above is satisfied, final handoff evidence is recorded, and node .agents/skills/autogoal/scripts/check-complete.mjs docs/plans/5043-fix-multiline-paste-crash.md passes.
Do not create hook state for this goal. This file plus the active goal are the durable state.

Start Gates:

Gate	Applies	Evidence
Timed checkpoint parsed	no	N/A: no duration requested
Skill analysis before edits	yes	`task`, `autogoal`, `autoreview`, `tdd`, and `changeset` read; Browser skill deferred until proof
Active goal checked or created	yes	goal created with this plan path
Source of truth read before edits	yes	`gh issue view 5043 --comments` equivalent JSON; zero comments
Tracker comments and attachments read	yes	zero comments; linked reproduction commit noted; no video
Video transcript evidence required	no	N/A: no video or screen recording
Pre-solution issue challenge required	yes	valid; criterion tightened to preserve exact length
Reproduction verdict before implementation	yes	reproduced in prior exact review; fresh TDD red required before code
Repro escalation ladder selected	yes	public API test first; Browser after green; visual proof waived
Suggested fix reviewed against durable boundary	yes	PR #5100 guard rejected after exact 21/20 counterexample
`docs/solutions` checked for non-trivial existing-code work	yes	focused search found clipboard boundary guidance but no LengthPlugin solution
TDD decision before behavior change or bug fix	yes	one faithful crash test red first; invariant companion coverage retained
Branch decision for code-changing task	yes	dedicated `codex/fix-5043-multiline-paste` from fresh `origin/main`
Release artifact decision	yes	one `@platejs/core` patch changeset
Browser tool decision for browser surface	yes	approved in-app Browser required after green package proof
PR expectation decision	yes	task workflow requires replacement PR after `check`
Tracker sync expectation decision	yes	comment issue #5043 after PR creation
Output budget strategy recorded	yes	exact scoped reads/searches with output caps
Browser pack selected	yes	materialized `browser` pack
Browser route / app surface identified	yes	local editor surface; exact route selected during narrow app search
Browser tool decision recorded	yes	use approved in-app Browser; no standalone browser automation
Console/network caveat policy recorded	yes	record paste outcome and console; unrelated network noise called out
Package/API pack selected	yes	materialized `package-api` pack for published runtime behavior
Public surface or package boundary identified	yes	`@platejs/core` `LengthPlugin` runtime enforcement
Release artifact path selected	yes	`.changeset/*.md` for `@platejs/core`: patch
`changeset` skill loaded when `.changeset` is required	yes	loaded; one package, imperative user-impact prose, no minor
Barrel/export impact decision recorded	yes	N/A: no exports or file layout change planned

Work Checklist:

Completion Gates:

Gate	Applies	Required action	Evidence
Named verification threshold	yes	Run named proof	Focused 9/9, core typecheck 5/5, browser exact, autoreview clean, `pnpm check` exit 0
Pre-solution issue challenge verdict	yes	Record verdict	Valid issue; “no crash” tightened to exact cap and no retained overflow; PR #5100 guard rejected
Repro escalation ladder	yes	Record each applicable level	Source test red; Playwright N/A; approved Browser red/green; visual screenshot N/A for nonvisual claim
Bug reproduced before fix	yes	Record failing repro	Faithful test threw exact `node.text` TypeError; 7 pass, 1 fail
Targeted behavior verification	yes	Run focused proof	Final focused suite 9 pass, 0 fail
TypeScript or typed config changed	yes	Run typecheck	Core 5/5 and full package typecheck 54/54
Package exports or file layout changed	no	N/A	No exports or public file layout changed; `pnpm brl` not required
Package manifests, lockfile, or install graph changed	no	N/A	No manifest or lockfile changes
Agent rules or skills changed	no	N/A	No agent/tooling files changed
Workspace authority proof	yes	Verify in owner	Commands ran in repo/core; Browser ran local www route against rebuilt core dist; GitHub readback via `gh`
Browser surface changed	yes	Capture proof	Local full EditorKit exact paste passed
Browser final proof	yes	Record exact caveat/artifact	One DOM child, exact 20-character text, zero fresh console errors; screenshot waived because behavior is nonvisual
CI-controlled template output changed	no	N/A	No `templates/**` changes; temporary playground config restored
Package behavior or public API changed	yes	Add changeset	`.changeset/core-fix-multiline-max-length.md` patches `@platejs/core`
User-visible registry output changed	no	N/A	No final registry diff; registry changelog not applicable
Docs or content changed	no	N/A	Only internal goal ledger changed; no public docs/content/API examples
High-risk mini gate	yes	Record risk/boundary/proof	Failure modes: cross-block merge crash, 21/20 leak, retained blank blocks; owner is LengthPlugin; tests + full EditorKit Browser prove boundary
Agent-native review for agent/tooling changes	no	N/A	No agent/tooling changes
Local install corruption suspected	no	N/A	No install-corruption signal; artifact-facing stale dist fixed by core build
Autoreview for non-trivial implementation changes	yes	Run until clean	First P2 accepted/fixed; final local autoreview clean with zero findings
PR create or update	yes	Run check and create	`pnpm check` exit 0 before PR; https://github.com/udecode/plate/pull/5101
Task-style PR body verified	yes	Read back body	`gh pr view 5101 --json body`: auto-release, issue/confidence, exact table, and four required sections present; no self-link
PR proof image hosting	no	N/A	No image used; exact DOM/console proof is textual
Tracker sync-back	yes	Comment issue	https://github.com/udecode/plate/issues/5043#issuecomment-5348836498
Final handoff contract	yes	Fill fields	Completed below with PR, issue, confidence, flow, browser, outcome, caveat, design, and verification
Final lint	yes	Run lint	`pnpm lint:fix`: 3,286 files; final formatting applied; `pnpm check` lint has 0 errors
Output budget discipline	yes	Audit output	Commands capped; one buffered dev-server flood recorded and subsequent server shutdown capped
Timed checkpoint	no	N/A	No duration requested
Goal plan complete	yes	Run checker	`check-complete.mjs` exit 0 after all plan phases closed
Browser interaction proof	yes	Exercise paste	Approved in-app Browser on local `/blocks/playground`; exact multiline paste passed
Browser console/network check	yes	Record state	Zero fresh console errors; no relevant network failures
Browser final proof artifact	yes	Record route/DOM caveat	Local route, one DOM child, exact 20 characters; screenshot N/A for nonvisual behavior
Public API / package boundary proof	yes	Audit impact	`@platejs/core` runtime behavior only; no export, type, manifest, or public shape change
Release artifact classification	yes	Classify	Published `@platejs/core` runtime bug fix
Published package changeset	yes	Add patch changeset	One-package patch changeset; no forbidden `minor`
Registry changelog	no	N/A	Not registry-only and no registry diff
No release artifact	no	N/A	Release artifact is required and present
Package typecheck/build/test	yes	Run owner checks	Core typecheck 5/5; core build passed; focused 9/9; full check exit 0
Barrel/export generation	no	N/A	No exports or exported file layout changed

Phase / pass table:

Phase	Status	Evidence	Next
Intake and source read	complete	issue, skills, owner files, focused solutions search, branch and red repro recorded	implementation
Implementation	complete	leaf-scoped convergent trim; exact crash/invariant/structure regressions green	verification
Verification	complete	focused 9/9; core typecheck 5/5; browser one block/20 chars/no errors; autoreview clean; `pnpm check` exit 0	PR / tracker sync
PR / tracker sync	complete	PR #5101 open/mergeable with verified body; issue comment posted	closeout
Closeout	complete	PR/issue/body synced; plan checker exit 0; final CI watch follows the last push	final response

Findings:

Issue #5043 has no comments or video evidence.
LengthPlugin enforces the limit inside its apply override and recursive deletes re-enter that override.
Existing tests cover single-block text/fragment truncation only.
Focused docs/solutions search found no existing LengthPlugin fix.

Decisions and tradeoffs:

Preserve the package API and exact hard limit -> fix convergence inside the plugin owner -> avoid caller patches and the broken unconditional early return.
Use one faithful paste crash test plus one empty-block invariant test -> catch both reported failure and the rejected patch's regression without test bloat.

Implementation notes:

Added per-editor isTrimming and shouldTrimAgain closure state.
Nested trim operations apply normally but request one outer recheck; trimming deletes only within the current text leaf. At an empty leaf boundary it moves to the previous leaf without merging or removing structural nodes.
The outer loop runs only while a nested delete/selection operation occurred and stops once measured text is within the limit or no progress is possible.
Added faithful Markdown insertData crash coverage and the exact empty-block fragment counterexample that rejected PR #5100.
Added one @platejs/core patch changeset.

Review fixes:

Scope baseline frozen before autoreview: issue #5043; branch codex/fix-5043-multiline-paste; owner LengthPlugin; intended behavior is crash-free exact enforcement; final code scope is plugin + focused spec; release/ledger scope is one changeset + this plan.
Autoreview P2 “Truncation leaves overflow blocks behind” -> accepted -> structural regression changed from length-only to exact one-block output; trim removes an empty top-level overflow block atomically and never merges across blocks. Focused red showed three retained empty paragraphs; green is exact one paragraph.
Final rerun: .agents/skills/autoreview/scripts/autoreview --mode local --stream-engine-output -> clean, zero findings, patch correct at 0.86 confidence.

Error attempts:

Error / failed attempt	Count	Next different move	Resolution
Browser route loaded stale `@platejs/core/dist` and reproduced the old crash	2	Build the artifact-facing core package, reload, and timestamp-filter fresh console logs	`pnpm --filter @platejs/core build`; exact paste then passed with zero fresh errors
Dev-server shutdown returned oversized buffered watcher output	1	Stop broad output immediately; use capped exact commands only	No repo change; recorded here and resumed with scoped reads
Final core typecheck found nullable `editor.api.start([])` target	1	Guard the unresolved point and rerun the same package check	Guard added; 5/5 typecheck tasks pass

Verification evidence:

bun test packages/core/src/lib/plugins/length/LengthPlugin.spec.ts in repo root on origin/main implementation -> 7 pass, faithful multiline paste test fails with TypeError: undefined is not an object (evaluating 'node.text').
Same focused command after implementation -> 9 pass, 0 fail, including faithful crash and 21/20 empty-block invariant coverage.
Browser skill, http://localhost:3001/blocks/playground, rebuilt core dist, temporary empty editor with maxLength: 20: pasted 123456789012345678901\n\ntrailing text; DOM contained exactly 12345678901234567890 plus two empty blocks and fresh console errors were []. Temporary playground configuration was restored; targeted diff exit 0.
pnpm turbo typecheck --filter=./packages/core after the final null guard -> 5/5 tasks pass.
pnpm lint:fix after final implementation -> 3,286 files checked, no fixes.
Final focused bun test packages/core/src/lib/plugins/length/LengthPlugin.spec.ts -> 9 pass, 0 fail, 10 expectations.
Post-review Browser rerun on rebuilt core at http://localhost:3000/blocks/playground: exact paste produced one DOM child, innerText/textContent of exactly 12345678901234567890, and zero fresh console errors; temporary config restored with targeted diff exit 0.
Post-review fix checks: core typecheck 5/5; lint checked 3,286 and formatted one source file; focused suite 9/9.
pnpm check in /Users/zbeyens/git/plate -> exit 0; lint 0 errors (one pre-existing hook warning), package build 54/54, package typecheck 54/54, fast 3,459/3,459, slow 352/352 plus isolated slow rows, and slowest suite completed with zero failures.

Final handoff contract:

PR line: https://github.com/udecode/plate/pull/5101
Issue / tracker line: #5043 synced at https://github.com/udecode/plate/issues/5043#issuecomment-5348836498
Confidence line: 99%
Flow table:
- Reproduced: faithful test exact TypeError; local old artifact full EditorKit crash
- Verified: focused 9/9, full check exit 0; Browser one block / 20 chars / zero errors
Browser check: approved in-app Browser on local /blocks/playground with rebuilt core
Outcome: multiline overflow paste is crash-free, exactly capped, and drops overflow paragraphs
Caveat: nested schemas retain required empty structural blocks; plain-text top-level overflow is fully discarded
Design:
- Chosen boundary: LengthPlugin leaf-scoped trim plus atomic top-level empty-block removal
- Why not quick patch: unconditional re-entrancy return leaks 21/20 and misses convergence
- Why not broader change: no Slate transform or public API redesign is needed
Verified: focused/package/full/browser/autoreview gates green
PR body verified: exact gh pr view 5101 --json body readback matches task format

Task-style PR body contract:

Preserve any existing  block. If a changeset is part of the diff and repo policy expects auto release, include that block.
Use the accepted kitcn PR #270 visual format. The body starts with an emoji issue/tracker/fix line, for example 🐛 Fixes #123 or 🐛 Fixes ➖ N/A, then an emoji confidence line like 🟢 95-100% confidence.
Use this exact table header: | Phase | 🧪 Tests | 🌐 Browser |.
Use Reproduced and Verified rows. Mark passing proof with 🟢, repro or failing proof with 🔴, and non-applicable cells with ➖ N/A.
Use bold emoji section headings: **✅ Outcome**, **⚠️ Caveat**, **🏗️ Design**, and **🧪 Verified**.
Never include a line that links to the current PR itself. The current PR URL belongs in the final response, not in its own description.
Do not replace this with a generic Summary / Verification PR body, an adaptive prose body from a git helper skill, plain ## Outcome sections, or an unrelated generated badge footer unless the caller or repo template explicitly asks for it.
Proof is gh pr view --json body output or a concise source-backed summary of that output.

Final handoff / sync:

PR: https://github.com/udecode/plate/pull/5101
Issue / tracker: #5043 comment https://github.com/udecode/plate/issues/5043#issuecomment-5348836498
Browser proof: one DOM child, exact 20-character text, zero fresh console errors
Caveats: CI final readback remains before goal completion

Timeline:

2026-08-19T22:13:13.161Z Task goal plan created.
2026-08-19T22:41:21Z Changeset policy rejected single-quoted YAML key; switched to the repo-required double-quoted form and revalidated locally.
2026-08-20T00:13:28+02:00 Issue, skills, active-goal state, branch, exact owner files, and prior fix counterexample reviewed; goal created.
2026-08-20T00:16:00+02:00 TDD red reproduced exact issue via createPlateEditor + MarkdownPlugin + plain-text insertData: 7 pass, 1 fail.
2026-08-20T00:19:00+02:00 Implemented progress-triggered trim recheck; focused suite green at 9 pass, 0 fail; patch changeset added.
2026-08-20T00:25:00+02:00 Browser initially exposed stale dist; rebuilt @platejs/core, reran exact local paste, observed 20 text characters and no fresh console errors, then restored the temporary route configuration.
2026-08-20T00:28:00+02:00 Final typecheck caught and resolved a nullable start-point guard; core typecheck 5/5, lint 3,286 clean, focused tests 9/9.
2026-08-20T00:35:00+02:00 Accepted autoreview P2, reproduced retained blank blocks, added exact structure assertion, removed empty top-level overflow blocks atomically, and re-proved one block / 20 chars / zero browser errors.
2026-08-20T00:40:00+02:00 Final autoreview clean; full pnpm check exit 0.

Reboot status:

Question	Answer
Where am I?	All implementation and verification gates green; shipping PR
Where am I going?	Commit/push, PR body readback, issue sync, plan closeout
What is the goal?	Fix multiline overflow paste without weakening `maxLength`
What have I learned?	Safe truncation needs leaf-scoped text deletion plus atomic removal of empty overflow blocks
What have I done?	Fixed crash/invariant/blank-block regression and proved tests, package checks, and browser behavior

Open risks:

None known. Existing maxLength counts JavaScript string length while Slate locates deletion targets by character; this patch preserves that contract.