diff --git a/docs/decisions.md b/docs/decisions.md
index eb9c17f..4b111e1 100644
--- a/docs/decisions.md
+++ b/docs/decisions.md
@@ -669,8 +669,12 @@ Every writer, and the reader's CommonMark ask, refuses markdown longer than 2^29
 converts alike everywhere. Each builder that joins pieces — blocks, quoted and list lines, directive
 pairs and openers, fences, pipe rows, the carry's JSON — joins them through `joinedWithin`, which
 stops once they pass the bound, so none builds a string no engine was asked for. A walk of blocks
-also forecasts each block's length with the indent its containers will prefix every line with, so a
-deep subtree refuses before any container copies it.
+also forecasts each block's length with the indent its containers surely prefix every line that is
+not blank with, so a subtree deep in quotes or task lists refuses before any container copies it.
+The forecast never overcounts, so it refuses only markdown past the bound: a plain list's marker
+indent is left out, since the list may give way to the directive form, which prefixes nothing. A
+deep list refuses once a level's text passes the bound, after the levels below it re-indented theirs
+(`todo.md` item 109).
 
 A line is held to its text's length alone; what its spelling adds — escapes, link targets,
 attributes — passes V8's longest string only by throwing, and `withinEngineLimits` refuses that
diff --git a/src/canonical-json.test.ts b/src/canonical-json.test.ts
index c480ca2..a06af18 100644
--- a/src/canonical-json.test.ts
+++ b/src/canonical-json.test.ts
@@ -2,7 +2,7 @@ import assert from 'node:assert/strict'
 import test from 'node:test'
 
 import type { JsonValue } from './json-value.ts'
-import { serializeCanonicalJson, serializeCanonicalJsonWithin, spellsCanonicalJson } from './canonical-json.ts'
+import { serializeCanonicalJson, spellsCanonicalJson } from './canonical-json.ts'
 
 test('sorts object keys recursively', () => {
   const value = { b: 1, a: { d: 2, c: 3 } }
@@ -62,8 +62,3 @@ test('holds a text to the canonical spelling, whole', () => {
   assert.ok(!spellsCanonicalJson(value, 'two-space', '{"a":[1,2]}'))
   assert.ok(!spellsCanonicalJson(1e21, 'compact', '1e21'))
 })
-
-test('spells canonical JSON up to a length, and nothing past it', () => {
-  assert.equal(serializeCanonicalJsonWithin({ a: [1, 2] }, 'compact', 11), '{"a":[1,2]}')
-  assert.equal(serializeCanonicalJsonWithin({ a: [1, 2] }, 'compact', 10), undefined)
-})
diff --git a/src/canonical-json.ts b/src/canonical-json.ts
index 985754b..f6909f9 100644
--- a/src/canonical-json.ts
+++ b/src/canonical-json.ts
@@ -7,51 +7,32 @@ type Member = { label: string; value: JsonValue }
 type Pending = string | { depth: number; value: JsonValue }
 
 export function serializeCanonicalJson(value: JsonValue, spelling: JsonSpelling): string {
-  const text: string[] = []
-  writeCanonicalJson(value, spelling, (piece) => {
-    text.push(piece)
-    return true
-  })
-  return text.join('')
-}
-
-// `undefined` where the spelling runs past `longest` characters, where it stops building.
-export function serializeCanonicalJsonWithin(value: JsonValue, spelling: JsonSpelling, longest: number): string | undefined {
-  const text: string[] = []
-  let length = 0
-  const whole = writeCanonicalJson(value, spelling, (piece) => {
-    length += piece.length
-    text.push(piece)
-    return length <= longest
-  })
-  return whole ? text.join('') : undefined
+  return [...canonicalJsonPieces(value, spelling)].join('')
 }
 
 // Stops at the first piece that does not match `text`: re-spelled with two-space indentation, a short compact text can pass the engine's longest string.
 export function spellsCanonicalJson(value: JsonValue, spelling: JsonSpelling, text: string): boolean {
   let offset = 0
-  const whole = writeCanonicalJson(value, spelling, (piece) => {
+  for (const piece of canonicalJsonPieces(value, spelling)) {
     if (!text.startsWith(piece, offset)) return false
     offset += piece.length
-    return true
-  })
-  return whole && offset === text.length
+  }
+  return offset === text.length
 }
 
-function writeCanonicalJson(value: JsonValue, spelling: JsonSpelling, write: (piece: string) => boolean): boolean {
+export function* canonicalJsonPieces(value: JsonValue, spelling: JsonSpelling): Generator<string> {
   const indent = spelling === 'compact' ? '' : '  '
   const pending: Pending[] = [{ depth: 0, value }]
   for (let next = pending.pop(); next !== undefined; next = pending.pop()) {
     if (typeof next === 'string') {
-      if (!write(next)) return false
+      yield next
       continue
     }
     const { depth, value: held } = next
     if (Array.isArray(held)) schedule(pending, '[', held.map((item) => ({ label: '', value: item })), ']', indent, depth)
     else if (held !== null && typeof held === 'object') schedule(pending, '{', objectMembers(held, indent), '}', indent, depth)
-    else if (!write(Object.is(held, -0) ? '-0' : JSON.stringify(held))) return false
+    else yield Object.is(held, -0) ? '-0' : JSON.stringify(held)
   }
-  return true
 }
 
 function objectMembers(value: { [key: string]: JsonValue }, indent: string): Member[] {
diff --git a/src/conformance/markdown-property.test.ts b/src/conformance/markdown-property.test.ts
index 126d4f6..7d17ea7 100644
--- a/src/conformance/markdown-property.test.ts
+++ b/src/conformance/markdown-property.test.ts
@@ -25,7 +25,7 @@ import {
   spellStringAttribute,
   spellVocabulary,
 } from '../markdown/directive-syntax.ts'
-import { fencedCodeBlock } from '../markdown/commonmark/backtick-runs.ts'
+import { fencePieces } from '../markdown/commonmark/backtick-runs.ts'
 import { inlineNodes } from '../adf/inline-nodes.ts'
 import { markAttributes } from '../adf/mark-attributes.ts'
 import { markSpelling } from '../markdown/mark-spellings.ts'
@@ -174,8 +174,8 @@ const carriedNode = fc.oneof(
 const spellCarry = (json: string) => spellInlineLeafDirective(carryName, `{json=${spellStringAttribute(json)}}`)
 const inlineCarry = carriedNode.map((node) => spellCarry(serializeCanonicalJson(node, 'compact')))
 const hostileInlineCarry = fc.oneof(carriedNode, jsonValue).map((node) => spellCarry(JSON.stringify(node, null, 1)))
-const blockCarry = carriedNode.map((node) => fencedCodeBlock(carryName, serializeCanonicalJson(node, 'two-space')))
-const hostileBlockCarry = fc.oneof(carriedNode, jsonValue).map((node) => fencedCodeBlock(carryName, JSON.stringify(node)))
+const blockCarry = carriedNode.map((node) => fencePieces(carryName, serializeCanonicalJson(node, 'two-space')).join(''))
+const hostileBlockCarry = fc.oneof(carriedNode, jsonValue).map((node) => fencePieces(carryName, JSON.stringify(node)).join(''))
 
 function prefixLines(body: string, first: string, rest: (index: number) => string): string {
   return body
diff --git a/src/markdown/commonmark/backtick-runs.ts b/src/markdown/commonmark/backtick-runs.ts
index 20efa79..8cdce83 100644
--- a/src/markdown/commonmark/backtick-runs.ts
+++ b/src/markdown/commonmark/backtick-runs.ts
@@ -15,10 +15,6 @@ export function closingBacktickRun(text: string, from: number, opener: number):
   return undefined
 }
 
-export function fencedCodeBlock(info: string, body: string): string {
-  return fencePieces(info, body).join('')
-}
-
 export function fencePieces(info: string, body: string): string[] {
   const fence = '`'.repeat(Math.max(3, longestBacktickRun(body) + 1))
   return body === '' ? [fence, info, '\n', fence] : [fence, info, '\n', body, '\n', fence]
diff --git a/src/markdown/emit/adf-to-markdown.test.ts b/src/markdown/emit/adf-to-markdown.test.ts
index bda2fd4..d3c3152 100644
--- a/src/markdown/emit/adf-to-markdown.test.ts
+++ b/src/markdown/emit/adf-to-markdown.test.ts
@@ -54,7 +54,7 @@ test('refuses a carry whose JSON would run past V8\'s longest string, on every e
   assert.equal(markdown(result), tooLong)
 })
 
-test('refuses lines their containers would prefix past V8\'s longest string, before any container copies them', () => {
+test('refuses lines their quotes would prefix past V8\'s longest string', () => {
   let quote: AdfNode = { attrs: {}, content: [{ text: 'a\n'.repeat(1_100_000), type: 'text' }], type: 'codeBlock' }
   for (let level = 0; level < 245; level += 1) quote = { content: [quote], type: 'blockquote' }
   const result = adfToLosslessMarkdown(document(quote))
@@ -67,7 +67,7 @@ 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', () => {
+test('refuses a list whose items together would pass V8\'s longest string', () => {
   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)
@@ -92,7 +92,7 @@ test('refuses a directive opener whose attributes together would pass V8\'s long
   assert.equal(markdown(adfToLosslessMarkdown(document(panel))), tooLong)
 })
 
-test('refuses text past V8\'s longest string before spelling it', () => {
+test('refuses text past V8\'s longest string', () => {
   const long = 'a'.repeat(2 ** 28)
   const result = adfToLosslessMarkdown(document(paragraph({ text: long, type: 'text' }, { text: long, type: 'text' }, { text: long, type: 'text' })))
   assert.equal(markdown(result), tooLong)
diff --git a/src/markdown/emit/adf-to-markdown.ts b/src/markdown/emit/adf-to-markdown.ts
index b6bc511..2e30e36 100644
--- a/src/markdown/emit/adf-to-markdown.ts
+++ b/src/markdown/emit/adf-to-markdown.ts
@@ -5,12 +5,12 @@ import { adfDocumentFault, holdsOnlyAttributes, isUnmarkedSpellableText, nodeAtt
 import { alertMarker, flavourClaims, foldedAlertMarker, leadingMarker, readAlertMarker, readTaskMarker, taskMarker } from '../portable/conventions.ts'
 import { blockDirectiveForm, documentSpelling } from '../block-directive.ts'
 import { blockNodeModel, blockNodes, isTaskItem } from '../../adf/block-nodes.ts'
+import { boundedThen, joinedWithin, longestMarkdown, separatedBy, tooLong, withinEngineLimits } from '../length-bound.ts'
 import { carriedBlock } from '../opaque-carry.ts'
 import { emitInlineLine } from './inline-line.ts'
 import { failure, faulted, success, type ConvertErrorPath, type Result } from '../../result.ts'
 import { fencePieces } from '../commonmark/backtick-runs.ts'
 import { holdsNullCharacter, isBlankLine, isThematicBreak, markerInterruptsParagraph } from '../commonmark/grammar.ts'
-import { boundedThen, joinedWithin, longestMarkdown, separatedBy, tooLong, withinEngineLimits } from '../length-bound.ts'
 import { languageSlot, type LanguageSlot } from '../code-language.ts'
 import { largestNesting } from '../../nesting.ts'
 import { spellBlockDirectiveOpener } from './block-directive-spelling.ts'
@@ -27,10 +27,12 @@ type PlacedBlock = SpelledBlock & { node: AdfNode }
 type PlacedBlocks = { blocks: readonly PlacedBlock[]; headroom: 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 }
+// joined: each item's text; start: the first item's number.
+type SpelledItems = { joined: readonly string[]; start: number; walked: readonly WalkedItem[] }
 // 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.
 export type SpellingMemo = Map<AdfNode, KeptSpelling>
 type WalkedItem = { node: AdfNode; walk: PlacedBlocks }
-// indent: the characters the containers around a block prefix each of its lines with.
+// indent: the characters the containers around a block surely prefix each of its lines with.
 export type Writing = { flavour: WrittenFlavour; indent: number; memo: SpellingMemo | undefined }
 
 const bulletDelimiters: Delimiters = ['-', '*']
@@ -60,7 +62,7 @@ function walkBlocks(nodes: readonly AdfNode[], path: ConvertErrorPath, depth: nu
   let headroom = largestNesting - depth
   if (headroom < 0) return tooDeep(path)
   const blocks: PlacedBlock[] = []
-  // A forecast, so a deep subtree refuses where its containers would prefix its lines past the bound, before any of them copies those lines.
+  // A forecast that never overcounts, so a deep subtree refuses where its containers would prefix its lines past the bound, before any of them copies those lines.
   let forecast = 0
   for (const [index, node] of nodes.entries()) {
     const block = emitBlock(node, [...path, 'content', index], depth, writing)
@@ -73,10 +75,16 @@ function walkBlocks(nodes: readonly AdfNode[], path: ConvertErrorPath, depth: nu
   return success({ blocks, headroom })
 }
 
+// A blank line takes no indent in a list item and one `>` in a quote, so only the others are charged it.
 function prefixedLength(text: string, indent: number): number {
-  let lines = 1
-  for (let at = text.indexOf('\n'); at !== -1; at = text.indexOf('\n', at + 1)) lines += 1
-  return text.length + lines * indent
+  let length = text.length
+  for (let start = 0; start <= text.length; ) {
+    const end = text.indexOf('\n', start)
+    const stop = end === -1 ? text.length : end
+    if (stop > start) length += indent
+    start = stop + 1
+  }
+  return length
 }
 
 function indented(writing: Writing, width: number): Writing {
@@ -184,7 +192,8 @@ function quotedUnder(head: string, node: AdfNode, path: ConvertErrorPath, depth:
 }
 
 function quotedBlock(text: string | undefined, headroom: number): Result<EmittedBlock> {
-  return boundedThen(text, (whole) => boundedThen(joinedWithin(quotedPieces(whole)), (quotedText) => success(commonMarkText(quotedText, headroom))))
+  const quoted = (whole: string): Result<EmittedBlock> => boundedThen(joinedWithin(quotedPieces(whole)), (quotedText) => success(commonMarkText(quotedText, headroom)))
+  return boundedThen(text, quoted)
 }
 
 
@@ -214,16 +223,19 @@ function tryTaskList(node: AdfNode, path: ConvertErrorPath, depth: number, writi
     if (task || previous === undefined) items.push([...walk.value.blocks])
     else for (const block of walk.value.blocks) previous.push(block)
   }
+  const inners = joinedItems(items.map((blocks) => blockPieces(blocks, 'list-item')))
   // The portable reduction leaves no task a list item cannot hold: a directive here would break the flavour.
-  return boundedThen(joinedItems(items.map((blocks) => blockPieces(blocks, 'list-item'))), (inners) => spelledList(inners, () => '', bulletDelimiters, headroom) ?? failure('unsupported-node-shape', 'a task holds blocks no list item spells', path))
+  const unspellable = failure<EmittedBlock>('unsupported-node-shape', 'a task holds blocks no list item spells', path)
+  return boundedThen(inners, (spelled) => spelledList(spelled, () => '', bulletDelimiters, headroom) ?? unspellable)
 }
 
-// Each item's text, or `undefined` once all of them together pass the bound; each alone stayed within the forecast.
+// Each item's text, or `undefined` where one, or all of them together, would pass the bound: every item is joined before the list is.
 function joinedItems(items: readonly Iterable<string>[]): string[] | undefined {
   const inners: string[] = []
   let length = 0
   for (const pieces of items) {
-    const inner = [...pieces].join('')
+    const inner = joinedWithin(pieces)
+    if (inner === undefined) return undefined
     length += inner.length
     if (length > longestMarkdown) return undefined
     inners.push(inner)
@@ -361,30 +373,34 @@ function tryList(node: AdfNode, path: ConvertErrorPath, depth: number, writing:
   if (items.some((item) => item.type !== 'listItem' || !holdsOnlyAttributes(item, []))) return undefined
   const walked: WalkedItem[] = []
   let headroom = Number.POSITIVE_INFINITY
-  const itemWriting = indented(writing, ordered ? `${start + items.length - 1}. `.length : bulletWidth)
   for (const [offset, item] of items.entries()) {
-    const walk = walkBlocks(nodeContent(item), [...path, 'content', offset], depth + 1, itemWriting)
+    // Unindented: the list may give way to the directive form, which prefixes nothing, and the forecast must not overcount.
+    const walk = walkBlocks(nodeContent(item), [...path, 'content', offset], depth + 1, writing)
     if (!walk.ok) return walk
     headroom = Math.min(headroom, walk.value.headroom)
     walked.push({ node: item, walk: walk.value })
   }
-  return boundedThen(joinedItems(walked.map((item) => blockPieces(item.walk.blocks, 'list-item'))), (joined) => {
-    // GitHub reads a task marker opening any item's first paragraph as a checkbox, whatever its siblings hold.
-    const inners = joined.map((inner) => (flavourClaims[writing.flavour].taskMarkers && leadingMarker(inner, readTaskMarker) !== undefined ? `\\${inner}` : inner))
-    const list = ordered ? spelledList(inners, (offset) => `${start + offset}`, orderedDelimiters, headroom) : spelledList(inners, () => '', bulletDelimiters, headroom)
-    if (list !== undefined) return list
-    // The directive form spends a level the walk did not count.
-    if (headroom < 1) return tooDeep(path)
-    return emitDirectiveBlock(node, ordered ? blockNodes.orderedList : blockNodes.bulletList, path, depth, () => directiveItems(walked, headroom - 1))
-  })
+  return boundedThen(joinedItems(walked.map((item) => blockPieces(item.walk.blocks, 'list-item'))), (joined) => spelledOrDirectiveList(node, { joined, start, walked }, path, depth, headroom, writing))
+}
+
+// The list spelling where every item holds lines a list item spells, else the directive form.
+function spelledOrDirectiveList(node: AdfNode, { joined, start, walked }: SpelledItems, path: ConvertErrorPath, depth: number, headroom: number, writing: Writing): Result<EmittedBlock> {
+  const ordered = node.type === 'orderedList'
+  // GitHub reads a task marker opening any item's first paragraph as a checkbox, whatever its siblings hold.
+  const inners = joined.map((inner) => (flavourClaims[writing.flavour].taskMarkers && leadingMarker(inner, readTaskMarker) !== undefined ? `\\${inner}` : inner))
+  const list = ordered ? spelledList(inners, (offset) => `${start + offset}`, orderedDelimiters, headroom) : spelledList(inners, () => '', bulletDelimiters, headroom)
+  if (list !== undefined) return 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, () => directiveItems(walked, headroom - 1))
 }
 
 function spelledList(inners: readonly string[], number: (offset: number) => string, [usual, alternate]: Delimiters, headroom: number): Result<EmittedBlock> | undefined {
   const pieces = listLines(inners, number, usual)
   const alternatePieces = listLines(inners, number, alternate)
   if (pieces === undefined || alternatePieces === undefined) return undefined
-  // The alternate delimiter is as wide as the usual one, so its text is as long.
-  return boundedThen(joinedWithin(pieces), (text) => success({ alternateText: [...alternatePieces].join(''), headroom, spelling: 'list', text }))
+  const spelled = (text: string, alternateText: string): Result<EmittedBlock> => success({ alternateText, headroom, spelling: 'list', text })
+  return boundedThen(joinedWithin(pieces), (text) => boundedThen(joinedWithin(alternatePieces), (alternateText) => spelled(text, alternateText)))
 }
 
 // `undefined` where an item holds a line no list item spells.
diff --git a/src/markdown/emit/block-directive-spelling.ts b/src/markdown/emit/block-directive-spelling.ts
index 7942a82..e80e895 100644
--- a/src/markdown/emit/block-directive-spelling.ts
+++ b/src/markdown/emit/block-directive-spelling.ts
@@ -1,10 +1,10 @@
 import type { AdfNode } from '../../adf/document.ts'
 import type { BlockNodeModel } from '../../adf/block-nodes.ts'
 import { attributeNestingMessage, nodeAttrs, nodeMarks } from '../../adf/document.ts'
+import { attributePieces, directiveOpenerPieces, isBareToken, spellJsonAttribute, spellVocabulary } from '../directive-syntax.ts'
 import { blockArgument, markValues, marksAttribute } from '../block-directive.ts'
 import { failure, success, type ConvertErrorPath, type Result } from '../../result.ts'
-import { joinedWithin, tooLong } from '../length-bound.ts'
-import { attributePieces, directiveOpenerPieces, isBareToken, spellJsonAttribute, spellVocabulary } from '../directive-syntax.ts'
+import { boundedThen, joinedWithin } from '../length-bound.ts'
 import { overNested } from '../../json-value.ts'
 import { spellEmptyKeys } from '../empty-keys.ts'
 import { vocabularyPairs } from '../../adf/attribute-vocabulary.ts'
@@ -24,7 +24,7 @@ export function spellBlockDirectiveOpener(node: AdfNode, model: BlockNodeModel,
     spelledPairs.push([marksAttribute, spellJsonAttribute(values)])
   }
   const opener = joinedWithin(directiveOpenerPieces(node.type, slot.argument, attributePieces(spelledPairs)))
-  return opener === undefined ? tooLong() : success(opener)
+  return boundedThen(opener, success)
 }
 
 // `undefined` where the argument slot holds a value no bare token spells.
diff --git a/src/markdown/emit/pipe-table.ts b/src/markdown/emit/pipe-table.ts
index 9e2fc47..0a1be57 100644
--- a/src/markdown/emit/pipe-table.ts
+++ b/src/markdown/emit/pipe-table.ts
@@ -1,8 +1,8 @@
 import type { AdfNode } from '../../adf/document.ts'
 import type { WrittenFlavour } from '../portable/conventions.ts'
+import { boundedThen, joinedWithin, longestMarkdown, separatedBy, tooLong } from '../length-bound.ts'
 import { delimiterCells, pipeRowPieces } from '../pipe-table-syntax.ts'
 import { holdsOnlyAttributes, nodeContent } from '../../adf/document.ts'
-import { boundedThen, joinedWithin, longestMarkdown, separatedBy, tooLong } from '../length-bound.ts'
 import { success, type ConvertErrorPath, type Result } from '../../result.ts'
 import { tryPipeCell } from './inline-line.ts'
 
@@ -10,7 +10,7 @@ export function tryPipeTable(node: AdfNode, path: ConvertErrorPath, flavour: Wri
   const rows = pipeRows(node)
   if (rows === undefined) return undefined
   const spelled: string[][] = []
-  // The cells alone must fit before the rows around them are built.
+  // Every cell is spelled before the rows join, so the cells together are held to the bound as they come.
   let length = 0
   for (const [rowIndex, row] of rows.entries()) {
     const cells: string[] = []
diff --git a/src/markdown/opaque-carry.ts b/src/markdown/opaque-carry.ts
index 9ea679e..7a69f36 100644
--- a/src/markdown/opaque-carry.ts
+++ b/src/markdown/opaque-carry.ts
@@ -2,16 +2,16 @@ import type { AdfNode } from '../adf/document.ts'
 import type { DirectiveSpan, Read } from './directive-syntax.ts'
 import type { JsonSpelling } from '../canonical-json.ts'
 import type { JsonValue } from '../json-value.ts'
+import { boundedThen, joinedWithin } from './length-bound.ts'
 import { directiveEscape, malformedDirective, readSoleStringAttribute, spellAttributes, spellDirectiveOpener, spellInlineLeafDirective, spellStringAttribute, unsupportedNodeShape } from './directive-syntax.ts'
 import { failure, success, type ConvertErrorPath, type Result } from '../result.ts'
 import { fencePieces } from './commonmark/backtick-runs.ts'
 import { infoStringCarries } from './commonmark/grammar.ts'
 import { isAdfNode } from '../adf/document.ts'
 import { isJsonValue, nestingDepth, overNested } from '../json-value.ts'
-import { boundedThen, joinedWithin, longestMarkdown } from './length-bound.ts'
 import { largestNesting } from '../nesting.ts'
 import { parseJsonText } from '../json-text.ts'
-import { serializeCanonicalJsonWithin, spellsCanonicalJson } from '../canonical-json.ts'
+import { canonicalJsonPieces, spellsCanonicalJson } from '../canonical-json.ts'
 
 export const carryFencePrefix = 'adf:'
 
@@ -60,7 +60,7 @@ function carriedJson(node: AdfNode, spelled: object, spelling: JsonSpelling, pat
   if (!isJsonValue(spelled) || headroom < 0) {
     return failure('unsupported-nesting-depth', `a carried node's JSON nests deeper than the ${levels} levels its position leaves`, path)
   }
-  const json = serializeCanonicalJsonWithin(spelled, spelling, longestMarkdown)
+  const json = joinedWithin(canonicalJsonPieces(spelled, spelling))
   return boundedThen(json, (spelled) => success({ headroom, json: spelled }))
 }
 
diff --git a/src/markdown/parse/markdown-to-adf.test.ts b/src/markdown/parse/markdown-to-adf.test.ts
index 2816373..616b77b 100644
--- a/src/markdown/parse/markdown-to-adf.test.ts
+++ b/src/markdown/parse/markdown-to-adf.test.ts
@@ -457,7 +457,7 @@ test('refuses compact JSON in the carry fence without building its two-space spe
 })
 
 test('refuses a directive whose CommonMark spelling would pass V8\'s longest string', () => {
-  const markdown = `!adf:blockquote\n${'- '.repeat(490)}a\\\n${'b\\\n'.repeat(560_000)}c\n!adf:/blockquote\n`
+  const markdown = `!adf:blockquote\n${'>'.repeat(490)} a\\\n${'b\\\n'.repeat(560_000)}c\n!adf:/blockquote\n`
   assert.equal(content(losslessMarkdownToAdf(markdown)), 'unsupported-node-shape: spelled as markdown, this would be longer than 536870888 characters, the longest string V8 holds')
 })
 
diff --git a/src/markdown/parse/markdown-to-adf.ts b/src/markdown/parse/markdown-to-adf.ts
index 0e32cd2..e981388 100644
--- a/src/markdown/parse/markdown-to-adf.ts
+++ b/src/markdown/parse/markdown-to-adf.ts
@@ -51,8 +51,9 @@ function readDocument(markdown: string, claims: Claims): Result<AdfDocument, Par
   )
 }
 
-function readMarkdown(markdown: string, claims: Claims): Result<AdfDocument, ParseError> {
-  if (typeof markdown !== 'string') return positioned(failure('not-a-string', `markdown is a string: found ${typeName(markdown)}`, []), { line: 1, offset: 0 })
+// A failure naming no position stands at the start of the input, which readDocument gives it.
+function readMarkdown(markdown: string, claims: Claims): Result<AdfDocument> {
+  if (typeof markdown !== 'string') return failure('not-a-string', `markdown is a string: found ${typeName(markdown)}`, [])
   const parsed = parseBlocks(markdown, claims)
   const [only] = parsed.blocks
   if (parsed.blocks.length === 1 && only?.kind === 'directive' && only.name === documentName) {
@@ -60,7 +61,7 @@ function readMarkdown(markdown: string, claims: Claims): Result<AdfDocument, Par
     return fault === undefined ? success({ type: 'doc', version: 1 }) : positioned(faulted(fault, []), only.position)
   }
   const reading: Reading = { claims, definitions: parsed.definitions, inExpand: false, memo: new Map() }
-  const content = positioned(readBlocks(parsed.blocks, reading, [], 0), { line: 1, offset: 0 })
+  const content = readBlocks(parsed.blocks, reading, [], 0)
   if (!content.ok) return content
   const document: AdfDocument = { content: content.value, type: 'doc', version: 1 }
   if (claims.taskMarkers) mintTaskIds(document, markdown)
diff --git a/src/markdown/portable/adf-to-portable-markdown.test.ts b/src/markdown/portable/adf-to-portable-markdown.test.ts
index 36cb02b..fb92f11 100644
--- a/src/markdown/portable/adf-to-portable-markdown.test.ts
+++ b/src/markdown/portable/adf-to-portable-markdown.test.ts
@@ -82,6 +82,11 @@ test('refuses an expand whose title and body together would pass V8\'s longest s
   assert.equal(portableDocument(document(quote)), 'unsupported-node-shape at /')
 })
 
+test('refuses a code block whose text nodes together would pass V8\'s longest string, joining none of them', () => {
+  const long = text('a'.repeat(2 ** 28))
+  assert.equal(portableDocument(document({ content: [long, long, long], type: 'codeBlock' })), 'unsupported-node-shape at /')
+})
+
 test('refuses nesting past 500 levels wherever the reduction walks', () => {
   const lowest = (bottom: AdfNode): string => {
     let deep = bottom
diff --git a/src/markdown/portable/adf-to-portable-markdown.ts b/src/markdown/portable/adf-to-portable-markdown.ts
index 796bca8..b07a496 100644
--- a/src/markdown/portable/adf-to-portable-markdown.ts
+++ b/src/markdown/portable/adf-to-portable-markdown.ts
@@ -1,6 +1,7 @@
 import type { AdfDocument, AdfNode } from '../../adf/document.ts'
 import { adfDocumentFault, nodeAttrs, nodeContent, nodeMarks } from '../../adf/document.ts'
 import { blockNodeModel, isTaskItem } from '../../adf/block-nodes.ts'
+import { boundedThen, joinedWithin, withinEngineLimits } from '../length-bound.ts'
 import { centeredImage, externalMedia } from '../external-image.ts'
 import { commonMarkSpelling, largestListMarker, writeMarkdown, type SpellingMemo } from '../emit/adf-to-markdown.ts'
 import { failure, faulted, success, type ConvertErrorPath, type Result } from '../../result.ts'
@@ -12,7 +13,6 @@ import { largestNesting } from '../../nesting.ts'
 import { linkDefinition } from '../mark-spellings.ts'
 import { splitTaskList } from './task-list.ts'
 import { toEditorNormal } from './editor-normal.ts'
-import { withinEngineLimits } from '../length-bound.ts'
 
 // depth: the level the node reduced stands at, counted as the emitter counts it.
 type Reduction = { depth: number; memo: SpellingMemo; path: ConvertErrorPath }
@@ -135,7 +135,7 @@ function portableSequence(blocks: readonly Sourced[], reduction: Reduction): Res
       sequence.push(block)
       continue
     }
-    // Back on `pending`: a replacement may take the directive form itself.
+    // Back on `pending`: a replacement may take the directive form itself. Its offsets are the reduced item's, the caller's while the item's blocks reduce one to one.
     const { path } = next
     const replacement = block === next.block ? nodeContent(block).flatMap((item, index) => nodeContent(item).map((node, offset) => ({ block: node, path: [...path, 'content', index, 'content', offset] }))) : [{ block, path }]
     for (const node of replacement.reverse()) pending.push(node)
@@ -221,10 +221,11 @@ function reduceHeading(node: AdfNode, reduction: Reduction): Result<AdfNode[]> {
 function reduceCodeBlock(node: AdfNode, reduction: Reduction): Result<AdfNode[]> {
   const leaves = inlineLeaves(nodeContent(node), 'paragraph', reduction.path, reduction.depth)
   if (!leaves.ok) return leaves
-  const code = leaves.value.map((leaf) => leaf.text ?? '\n').join('')
   const slot = languageSlot(nodeAttrs(node)['language'], flavourClaims.portable)
-  const block: AdfNode = { content: code === '' ? [] : [text(code)], type: 'codeBlock' }
-  return success([slot.kind === 'fence' ? { ...block, attrs: { language: slot.info } } : block])
+  return boundedThen(joinedWithin(leaves.value.map((leaf) => leaf.text ?? '\n')), (code) => {
+    const block: AdfNode = { content: code === '' ? [] : [text(code)], type: 'codeBlock' }
+    return success([slot.kind === 'fence' ? { ...block, attrs: { language: slot.info } } : block])
+  })
 }
 
 function reduceList(node: AdfNode, reduction: Reduction): Result<AdfNode[]> {