Answer architecture round 1: every container joining several walks sums their projections before it joins, the portable reduction refuses a list whose spelling is refused, and heldTextLength moves beside textLength

src/adf/document.ts

@@ -86,6 +86,17 @@ export function textLength(node: AdfNode): number {
   return node.text?.length ?? 0
 }
 
+// A line spells every character of its nodes' text at least once, so this much text cannot fit.
+export function heldTextLength(nodes: readonly AdfNode[]): number {
+  let length = 0
+  const pending = [...nodes]
+  for (let node = pending.pop(); node !== undefined; node = pending.pop()) {
+    length += textLength(node)
+    for (const child of nodeContent(node)) pending.push(child)
+  }
+  return length
+}
+
 export function mergeAdjacentText(nodes: readonly AdfNode[], joins: (previous: AdfNode, node: AdfNode) => boolean): AdfNode[] {
   const merged: AdfNode[] = []
   for (const node of nodes) {

src/markdown/emit/adf-to-markdown.test.ts

@@ -69,6 +69,21 @@ test('refuses a directive whose opener and body together would pass V8\'s longes
   assert.equal(markdown(adfToLosslessMarkdown(document(expand))), tooLong)
 })
 
+test('refuses a list whose items together would pass V8\'s longest string, before joining them', () => {
+  const item: AdfNode = { content: [{ attrs: {}, content: [{ text: 'a'.repeat(2 ** 28), type: 'text' }], type: 'codeBlock' }], type: 'listItem' }
+  const result = adfToLosslessMarkdown(document({ content: [item, item], type: 'bulletList' }))
+  assert.equal(markdown(result), tooLong)
+  assert.deepEqual(path(result), ['content', 0, 'content', 1])
+})
+
+test('refuses a pipe table whose rows their containers would prefix past V8\'s longest string', () => {
+  const row: AdfNode = { content: [{ content: [{ type: 'paragraph' }], type: 'tableCell' }], type: 'tableRow' }
+  const header: AdfNode = { content: [{ content: [{ type: 'paragraph' }], type: 'tableHeader' }], type: 'tableRow' }
+  let quote: AdfNode = { content: [header, ...Array.from({ length: 1_100_000 }, () => row)], type: 'table' }
+  for (let level = 0; level < 245; level += 1) quote = { content: [quote], type: 'blockquote' }
+  assert.equal(markdown(adfToLosslessMarkdown(document(quote))), tooLong)
+})
+
 test('refuses text past V8\'s longest string before spelling it', () => {
   const long = 'a'.repeat(2 ** 28)
   const result = adfToLosslessMarkdown(document(paragraph({ text: long, type: 'text' }, { text: long, type: 'text' }, { text: long, type: 'text' })))

src/markdown/emit/adf-to-markdown.ts

@@ -24,7 +24,8 @@ type Delimiters = readonly [string, string]
 type EmittedBlock = SpelledBlock & { headroom: number }
 type KeptSpelling = { block: EmittedBlock | undefined; depth: number }
 type PlacedBlock = SpelledBlock & { node: AdfNode }
-type PlacedBlocks = { blocks: readonly PlacedBlock[]; headroom: number }
+// projected: the blocks' length once joined and their containers prefix their lines; it never undercounts.
+type PlacedBlocks = { blocks: readonly PlacedBlock[]; headroom: number; projected: number }
 // CommonMark starts a new list where the marker changes, so every second list in a run of one kind takes `alternateText`.
 type SpelledBlock = { spelling: 'commonmark' | 'directive'; text: string } | { alternateText: string; spelling: 'list'; text: string }
 // Keyed by reference: only a caller building one object per position (the parse, the portable reduction) passes one; a consumer's document may share a node.
@@ -38,6 +39,7 @@ const bulletWidth = 2
 export const largestListMarker = 999999999
 // Bare because tryList admits no item carrying attributes, marks or text.
 const listItemOpener = spellDirectiveOpener('listItem', undefined, '')
+const listItemCloser = spellDirectiveCloser('listItem')
 const orderedDelimiters: Delimiters = ['.', ')']
 
 export function adfToLosslessMarkdown(document: AdfDocument): Result<string> {
@@ -59,17 +61,21 @@ function walkBlocks(nodes: readonly AdfNode[], path: ConvertErrorPath, depth: nu
   let headroom = largestNesting - depth
   if (headroom < 0) return tooDeep(path)
   const blocks: PlacedBlock[] = []
-  // Each block's length once its containers prefix its lines, and the two characters parting it from the next.
   let projected = 0
   for (const [index, node] of nodes.entries()) {
     const block = emitBlock(node, [...path, 'content', index], depth, writing)
     if (!block.ok) return block
-    projected += prefixedLength(block.value.text, writing.indent) + 2
+    projected += (index === 0 ? 0 : separatorLength(writing)) + prefixedLength(block.value.text, writing.indent) + 1
     if (projected > longestOutput) return tooLong([...path, 'content', index])
     headroom = Math.min(headroom, block.value.headroom)
     blocks.push({ ...block.value, node })
   }
-  return success({ blocks, headroom })
+  return success({ blocks, headroom, projected })
+}
+
+// A block counts the newline ending it; a blank line between two is prefixed as their lines are.
+function separatorLength(writing: Writing): number {
+  return 1 + writing.indent
 }
 
 function prefixedLength(text: string, indent: number): number {
@@ -155,7 +161,10 @@ function spellReadableBlock(node: AdfNode, path: ConvertErrorPath, depth: number
   if (node.type === 'mediaSingle') return readableText(tryImage(node))
   if (node.type === 'paragraph') return tryParagraph(node, path, writing.flavour)
   if (node.type === 'rule') return readableText(tryRule(node))
-  if (node.type === 'table') return readableText(tryPipeTable(node, path, writing.flavour))
+  if (node.type === 'table') {
+    const table = tryPipeTable(node, path, writing)
+    return table === undefined || !table.ok ? table : success(commonMarkText(table.value))
+  }
   return undefined
 }
 
@@ -171,8 +180,10 @@ function spellPortableBlock(node: AdfNode, path: ConvertErrorPath, depth: number
 }
 
 function quotedUnder(head: string, node: AdfNode, path: ConvertErrorPath, depth: number, writing: Writing): Result<EmittedBlock> {
-  const inner = walkBlocks(nodeContent(node), path, depth + 1, indented(writing, quoteWidth))
+  const quotedWriting = indented(writing, quoteWidth)
+  const inner = walkBlocks(nodeContent(node), path, depth + 1, quotedWriting)
   if (!inner.ok) return inner
+  if (prefixedLength(head, quotedWriting.indent) + 1 + separatorLength(quotedWriting) + inner.value.projected > longestOutput) return tooLong(path)
   const body = joinBlocks(inner.value.blocks, 'document')
   return success(commonMarkText(quoted(body === '' ? head : `${head}\n\n${body}`), inner.value.headroom))
 }
@@ -188,6 +199,8 @@ function quoted(text: string): string {
 
 function tryTaskList(node: AdfNode, path: ConvertErrorPath, depth: number, writing: Writing): Result<EmittedBlock> {
   const items: PlacedBlock[][] = []
+  // Each walk alone stayed within the bound, so the join is checked before it builds.
+  let projected = 0
   let headroom = largestNesting - depth - 1
   // A child other than a task nests in the task before it.
   for (const [index, child] of nodeContent(node).entries()) {
@@ -195,6 +208,8 @@ function tryTaskList(node: AdfNode, path: ConvertErrorPath, depth: number, writi
     const itemWriting = indented(writing, bulletWidth)
     const walk = task ? taskBlocks(child, [...path, 'content', index], depth + 1, itemWriting) : placedBlock(child, [...path, 'content', index], depth + 1, itemWriting)
     if (!walk.ok) return walk
+    projected += (index === 0 ? 0 : separatorLength(writing)) + walk.value.projected
+    if (projected > longestOutput) return tooLong([...path, 'content', index])
     headroom = Math.min(headroom, walk.value.headroom)
     const previous = items.at(-1)
     if (task || previous === undefined) items.push([...walk.value.blocks])
@@ -212,7 +227,7 @@ function tryTaskList(node: AdfNode, path: ConvertErrorPath, depth: number, writi
 
 function placedBlock(node: AdfNode, path: ConvertErrorPath, depth: number, writing: Writing): Result<PlacedBlocks> {
   const block = emitBlock(node, path, depth, writing)
-  return block.ok ? success({ blocks: [{ ...block.value, node }], headroom: block.value.headroom }) : block
+  return block.ok ? success({ blocks: [{ ...block.value, node }], headroom: block.value.headroom, projected: prefixedLength(block.value.text, writing.indent) + 1 }) : block
 }
 
 // The marker leads the first paragraph, or stands as one where the blocks open with another.
@@ -223,13 +238,15 @@ function taskBlocks(task: AdfNode, path: ConvertErrorPath, depth: number, writin
     const content = nodeContent(task)
     const line = content.length === 0 ? success('') : emitInlineLine(content, 'paragraph', path, writing.flavour)
     if (!line.ok) return line
-    return success({ blocks: [{ ...markerBlock, text: line.value === '' ? marker : `${marker} ${line.value}` }], headroom: Number.POSITIVE_INFINITY })
+    const text = line.value === '' ? marker : `${marker} ${line.value}`
+    return success({ blocks: [{ ...markerBlock, text }], headroom: Number.POSITIVE_INFINITY, projected: prefixedLength(text, writing.indent) + 1 })
   }
   const walk = walkBlocks(nodeContent(task), path, depth, writing)
   if (!walk.ok) return walk
   const [first, ...rest] = walk.value.blocks
   const blocks = first?.node.type === 'paragraph' ? [{ ...first, text: `${marker} ${first.text}` }, ...rest] : [markerBlock, ...walk.value.blocks]
-  return success({ blocks, headroom: walk.value.headroom })
+  // The marker either leads the first line or stands as a block of its own.
+  return success({ blocks, headroom: walk.value.headroom, projected: walk.value.projected + marker.length + separatorLength(writing) + writing.indent + 1 })
 }
 
 function readableText(text: string | undefined): Result<EmittedBlock> | undefined {
@@ -268,9 +285,8 @@ function emitDirectiveBody(node: AdfNode, model: BlockNodeModel, opener: string,
   }
   const walk = walkBody()
   if (!walk.ok) return walk
-  const body = joinBlocks(walk.value.blocks, 'directive')
-  if (opener.length + body.length > longestOutput) return tooLong(path)
-  return success(directivePair(node, opener, body, walk.value.headroom))
+  if (opener.length + walk.value.projected > longestOutput) return tooLong(path)
+  return success(directivePair(node, opener, joinBlocks(walk.value.blocks, 'directive'), walk.value.headroom))
 }
 
 function tryBlockquote(node: AdfNode, path: ConvertErrorPath, depth: number, writing: Writing): Result<EmittedBlock> | undefined {
@@ -334,6 +350,8 @@ function tryList(node: AdfNode, path: ConvertErrorPath, depth: number, writing:
   if (start === undefined || items.length === 0) return undefined
   if (items.some((item) => item.type !== 'listItem' || !holdsOnlyAttributes(item, []))) return undefined
   const walked: WalkedItem[] = []
+  // Each walk alone stayed within the bound, so the join is checked before it builds; the directive form adds an opener and a closer line to each item.
+  let projected = 0
   let headroom = Number.POSITIVE_INFINITY
   const itemWriting = indented(writing, ordered ? `${start + items.length - 1}. `.length : bulletWidth)
   for (const [offset, item] of items.entries()) {
@@ -341,6 +359,8 @@ function tryList(node: AdfNode, path: ConvertErrorPath, depth: number, writing:
     if (!walk.ok) return walk
     headroom = Math.min(headroom, walk.value.headroom)
     walked.push({ node: item, walk: walk.value })
+    projected += (offset === 0 ? 0 : separatorLength(writing)) + walk.value.projected + directiveItemLength(writing)
+    if (projected > longestOutput) return tooLong([...path, 'content', offset])
   }
   const inners = walked.map((item) => {
     const inner = joinBlocks(item.walk.blocks, 'list-item')
@@ -351,7 +371,7 @@ function tryList(node: AdfNode, path: ConvertErrorPath, depth: number, writing:
   if (list !== undefined) return success(list)
   // The directive form spends a level the walk did not count.
   if (headroom < 1) return tooDeep(path)
-  return emitDirectiveBlock(node, ordered ? blockNodes.orderedList : blockNodes.bulletList, path, depth, () => success({ blocks: directiveItems(walked), headroom: headroom - 1 }))
+  return emitDirectiveBlock(node, ordered ? blockNodes.orderedList : blockNodes.bulletList, path, depth, () => success({ blocks: directiveItems(walked), headroom: headroom - 1, projected }))
 }
 
 function spelledList(inners: readonly string[], number: (offset: number) => string, [usual, alternate]: Delimiters, headroom: number): EmittedBlock | undefined {
@@ -370,6 +390,10 @@ function listLines(inners: readonly string[], number: (offset: number) => string
   return lines.join('\n')
 }
 
+function directiveItemLength(writing: Writing): number {
+  return listItemOpener.length + listItemCloser.length + 2 * (1 + writing.indent)
+}
+
 function directiveItems(items: readonly WalkedItem[]): PlacedBlock[] {
   return items.map((item) => ({ ...directivePair(item.node, listItemOpener, joinBlocks(item.walk.blocks, 'directive')), node: item.node }))
 }

src/markdown/emit/inline-line.ts

@@ -10,7 +10,7 @@ import { escapeUnbalanced, spellDestination, spellLinkTarget } from '../commonma
 import { failure, success, type ConvertErrorPath, type Result } from '../../result.ts'
 import { flavourClaims, highlightDelimiter } from '../portable/conventions.ts'
 import { holdsNullCharacter, trimTrailingSpace } from '../commonmark/grammar.ts'
-import { identicalMark, isSpellableText, nodeAttrs, nodeContent, nodeMarks, textLength } from '../../adf/document.ts'
+import { heldTextLength, identicalMark, isSpellableText, nodeAttrs, nodeContent, nodeMarks } from '../../adf/document.ts'
 import { inlineNodeModel } from '../../adf/inline-nodes.ts'
 import { joinsWhenRead, textBreakSpelling } from '../adjacent-text.ts'
 import { largestNesting } from '../../nesting.ts'
@@ -101,17 +101,6 @@ function emitLine(nodes: readonly AdfNode[], container: LineContainer, path: Con
   }
 }
 
-// The line spells every character of its text at least once, so this much text cannot fit.
-export function heldTextLength(nodes: readonly AdfNode[]): number {
-  let length = 0
-  const pending = [...nodes]
-  for (let node = pending.pop(); node !== undefined; node = pending.pop()) {
-    length += textLength(node)
-    for (const child of nodeContent(node)) pending.push(child)
-  }
-  return length
-}
-
 function takeFallback(fallbacks: LineFallbacks, fallback: NodeRange | 'opening-link', path: ConvertErrorPath): Result<null> {
   if (fallback === 'opening-link') {
     if (fallbacks.openingLinkAsDirective) return failure('unsupported-node-shape', 'an opening link spelled as a directive still reads as a link definition, so the line has no spelling left', path)

src/markdown/emit/pipe-table.ts

@@ -1,14 +1,18 @@
 import type { AdfNode } from '../../adf/document.ts'
-import type { ConvertErrorPath } from '../../result.ts'
 import type { WrittenFlavour } from '../portable/conventions.ts'
 import { holdsOnlyAttributes, nodeContent } from '../../adf/document.ts'
+import { longestOutput, tooLong } from '../../output-length.ts'
 import { spellPipeDelimiter, spellPipeRow } from '../pipe-table-syntax.ts'
+import { success, type ConvertErrorPath, type Result } from '../../result.ts'
 import { tryPipeCell } from './inline-line.ts'
 
-export function tryPipeTable(node: AdfNode, path: ConvertErrorPath, flavour: WrittenFlavour): string | undefined {
+// indent: the characters the containers around the table prefix each of its lines with.
+export function tryPipeTable(node: AdfNode, path: ConvertErrorPath, { flavour, indent }: { flavour: WrittenFlavour; indent: number }): Result<string> | undefined {
   const rows = pipeRows(node)
   if (rows === undefined) return undefined
   const lines: string[] = []
+  // A row spells ` | ` beside each cell, and a newline and the indent after it.
+  let projected = 0
   for (const [rowIndex, row] of rows.entries()) {
     const cells: string[] = []
     for (const [cellIndex, paragraph] of row.entries()) {
@@ -16,11 +20,14 @@ export function tryPipeTable(node: AdfNode, path: ConvertErrorPath, flavour: Wri
       const line = content.length === 0 ? '' : tryPipeCell(content, [...path, 'content', rowIndex, 'content', cellIndex, 'content', 0], flavour)
       if (line === undefined) return undefined
       cells.push(line)
+      projected += line.length + 3
     }
+    projected += 2 + indent + (rowIndex === 0 ? spellPipeDelimiter(cells.length).length + 1 + indent : 0)
+    if (projected > longestOutput) return tooLong([...path, 'content', rowIndex])
     lines.push(spellPipeRow(cells))
     if (rowIndex === 0) lines.push(spellPipeDelimiter(cells.length))
   }
-  return lines.join('\n')
+  return success(lines.join('\n'))
 }
 
 function pipeRows(node: AdfNode): AdfNode[][] | undefined {

src/markdown/portable/adf-to-portable-markdown.test.ts

@@ -76,6 +76,12 @@ test('refuses a paragraph whose text would pass V8\'s longest string, joining no
   assert.equal(portableDocument(document(merged)), 'unsupported-node-shape at /content/0')
 })
 
+test('refuses an expand whose title and body together would pass V8\'s longest string', () => {
+  let quote: AdfNode = node('expand', { title: 'a'.repeat(1_000_000) }, { attrs: {}, content: [text('a\n'.repeat(1_108_000))], type: 'codeBlock' })
+  for (let level = 0; level < 240; level += 1) quote = { content: [quote], type: 'blockquote' }
+  assert.equal(portableDocument(document(quote)), `unsupported-node-shape at /${Array.from({ length: 241 }, () => 'content/0').join('/')}`)
+})
+
 test('refuses nesting past 500 levels wherever the reduction walks', () => {
   const lowest = (bottom: AdfNode): string => {
     let deep = bottom

src/markdown/portable/adf-to-portable-markdown.ts

@@ -117,14 +117,16 @@ function concatenated(results: readonly Result<AdfNode[]>[]): Result<AdfNode[]>
   return success(blocks)
 }
 
-// A list still taking the directive form gives way to its items' blocks.
+// A list still taking the directive form gives way to its items' blocks; one whose spelling is refused, too deep or too long, refuses the reduction (the maintainer, 2026-10-12).
 function portableSequence(blocks: readonly AdfNode[], reduction: Reduction): Result<AdfNode[]> {
   const pending = blocks.filter((block) => block.type !== 'paragraph' || nodeContent(block).length > 0).reverse()
   const sequence: AdfNode[] = []
   for (let next = pending.pop(); next !== undefined; next = pending.pop()) {
     const block = numberedPastMarkers(next)
     const listed = block.type === 'bulletList' || block.type === 'orderedList'
-    if (!listed || (block === next && commonMarkSpelling(block, reduction.path, reduction.depth, { flavour: 'portable', indent: 0, memo: reduction.memo })?.ok === true)) {
+    const spelled = listed && block === next ? commonMarkSpelling(block, reduction.path, reduction.depth, { flavour: 'portable', indent: 0, memo: reduction.memo }) : undefined
+    if (spelled !== undefined && !spelled.ok) return spelled
+    if (!listed || spelled !== undefined) {
       sequence.push(block)
       continue
     }

src/markdown/portable/inline-reduction.ts

@@ -6,8 +6,8 @@ import { failure, success, type ConvertErrorPath, type Result } from '../../resu
 import { joinsWhenEditorNormal, sameMarkWhenEditorNormal } from './editor-normal.ts'
 import { largestNesting } from '../../nesting.ts'
 import { longestOutput, tooLong } from '../../output-length.ts'
-import { mergeAdjacentText, nodeAttrs, nodeContent, nodeMarks } from '../../adf/document.ts'
-import { heldTextLength, portableLineFallback, type PortableLineFallback } from '../emit/inline-line.ts'
+import { heldTextLength, mergeAdjacentText, nodeAttrs, nodeContent, nodeMarks } from '../../adf/document.ts'
+import { portableLineFallback, type PortableLineFallback } from '../emit/inline-line.ts'
 import { spellDestination, spellLinkTarget } from '../commonmark/link-syntax.ts'
 
 const highlight = 'backgroundColor'