diff --git a/CHANGELOG.md b/CHANGELOG.md
index 605fd51..e9ea8cd 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -83,6 +83,10 @@
 - Fix a cyclic document, in which an object holds itself: `adfToLosslessMarkdown` and
   `adfToPortableMarkdown` refuse it as `not-an-adf-document` and `isAdfDocument` returns `false`,
   where all three hung.
+- Fix a document whose markdown runs past the longest string the JavaScript engine holds, about 537
+  million characters on V8: `adfToLosslessMarkdown` and `adfToPortableMarkdown` refuse it as
+  `unsupported-node-shape`, where they threw. `losslessMarkdownToAdf` refuses compact JSON in an
+  `adf:` fence in time linear in it, where a megabyte of deeply nested JSON threw.
 - Add `adfToPortableMarkdown` and `portableMarkdownToAdf`, a lossy pair converting ADF to and from
   markdown GitHub, GitLab and Obsidian render: alerts, callouts, task lists, `==highlights==` and
   pipe tables. A task item keeps its paragraphs, then its nested task lists; the rest of the item,
diff --git a/README.md b/README.md
index c630ae8..b2f09f7 100644
--- a/README.md
+++ b/README.md
@@ -211,7 +211,7 @@ Each call's name says which markdown it reads or writes:
 | Call | Markdown | Refuses with |
 | --- | --- | --- |
 | `adfToLosslessMarkdown` | writes the lossless flavour: CommonMark plus GFM's pipe tables and `~~`, with `!adf:` directives and `adf:` fences for everything else | `not-an-adf-document`, `unsupported-document-version`, `unsupported-nesting-depth`, `unsupported-node-shape` |
-| `adfToPortableMarkdown` | writes markdown GitHub, GitLab and Obsidian render; drops what they cannot show, and writes no directive | `not-an-adf-document`, `unsupported-document-version`, `unsupported-nesting-depth` |
+| `adfToPortableMarkdown` | writes markdown GitHub, GitLab and Obsidian render; drops what they cannot show, and writes no directive | `not-an-adf-document`, `unsupported-document-version`, `unsupported-nesting-depth`, `unsupported-node-shape` |
 | `commonMarkToAdf` | reads strict CommonMark as its spec says: a directive, a pipe table, `~~`, `==` and an `adf:` fence read as text and code | `not-a-string`, `unmappable-html`, `unsupported-nesting-depth` |
 | `losslessMarkdownToAdf` | reads the lossless flavour, markdown `adfToLosslessMarkdown` wrote, edited or not | every code in [The errors](#the-errors) except `not-an-adf-document` and `unsupported-document-version` |
 | `portableMarkdownToAdf` | reads CommonMark, GFM's pipe tables and `~~`, alerts and task markers, Obsidian's callouts and `==`; a directive and an `adf:` fence read as text and code | `malformed-pipe-table`, `not-a-string`, `unmappable-html`, `unsupported-nesting-depth` |
@@ -442,7 +442,7 @@ The emitter raises these, and so does a parse, which asks the emitter how to spe
 | Code | Fires when | What you can do |
 | --- | --- | --- |
 | `unsupported-nesting-depth` | blocks, marks, an attribute's JSON or a carried node's JSON nest past 500 levels | keep the ADF and pass the document over, or show it read-only; flatten the input where you are the one who wrote it |
-| `unsupported-node-shape` | parsing, in `losslessMarkdownToAdf`: markdown spells a node with an attribute, value, argument or body its type does not take, or without one it needs, wraps a directive mark around nothing or only hard breaks, writes as a directive a node or mark the lossless flavour spells as CommonMark, or puts a reserved directive out of place: `!adf:textBreak{}` parting nothing, `!adf:doc` anywhere but as the whole document. Emitting, in `adfToLosslessMarkdown`: a node not riding the carry that carries one mark type twice, a block or a non-text inline node holding `text`, or a leaf block or a non-text inline node holding `content` | fix what the message names; [`spec/flavour.md`](https://github.com/larvit/adf-codec/blob/main/spec/flavour.md) lists every type's attributes and body |
+| `unsupported-node-shape` | parsing, in `losslessMarkdownToAdf`: markdown spells a node with an attribute, value, argument or body its type does not take, or without one it needs, wraps a directive mark around nothing or only hard breaks, writes as a directive a node or mark the lossless flavour spells as CommonMark, or puts a reserved directive out of place: `!adf:textBreak{}` parting nothing, `!adf:doc` anywhere but as the whole document. Emitting, in `adfToLosslessMarkdown`: a node not riding the carry that carries one mark type twice, a block or a non-text inline node holding `text`, or a leaf block or a non-text inline node holding `content`. Emitting, in both writers: a document whose markdown runs past the longest string the JavaScript engine holds, about 537 million characters on V8 | fix what the message names; [`spec/flavour.md`](https://github.com/larvit/adf-codec/blob/main/spec/flavour.md) lists every type's attributes and body |
 
 ## The guarantees
 
diff --git a/docs/decisions.md b/docs/decisions.md
index b262414..2092611 100644
--- a/docs/decisions.md
+++ b/docs/decisions.md
@@ -461,6 +461,9 @@ handles one cause alike whichever node, attribute or direction raised it.
   knows which direction it called and parting them after `0.1.0` is MAJOR (2026-09-23).
 - A non-finite number takes two codes: `unsupported-node-shape` parsing, `not-an-adf-document`
   emitting — no document holds one, so no round-trip crosses them (2026-09-23).
+- A document whose markdown runs past the longest string the JavaScript engine holds is
+  `unsupported-node-shape` at the document's path, in either writer: the document does not convert,
+  and the bound is the engine's own (2026-10-11).
 - The markdown readers and `htmlToAdf` refuse a value that is not a string as `not-a-string`, at
   position `{ line: 1, offset: 0 }` with an empty path: no existing code reads true of it, and
   `not-an-adf-document` names the emitters' input (2026-10-05).
diff --git a/src/canonical-json.test.ts b/src/canonical-json.test.ts
index 26cdf8f..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 } 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 } }
@@ -52,3 +52,13 @@ test('spells a value nesting far past the levels a recursive walk survives', ()
   assert.equal(serializeCanonicalJson(array, 'compact'), `${'['.repeat(levels)}1${']'.repeat(levels)}`)
   assert.equal(serializeCanonicalJson(object, 'compact'), `${'{"a":'.repeat(levels)}1${'}'.repeat(levels)}`)
 })
+
+test('holds a text to the canonical spelling, whole', () => {
+  const value = { a: [1, 2] }
+  assert.ok(spellsCanonicalJson(value, 'compact', '{"a":[1,2]}'))
+  assert.ok(spellsCanonicalJson(value, 'two-space', serializeCanonicalJson(value, 'two-space')))
+  assert.ok(!spellsCanonicalJson(value, 'compact', '{"a":[1,2]} '))
+  assert.ok(!spellsCanonicalJson(value, 'compact', '{"a":[1,2]'))
+  assert.ok(!spellsCanonicalJson(value, 'two-space', '{"a":[1,2]}'))
+  assert.ok(!spellsCanonicalJson(1e21, 'compact', '1e21'))
+})
diff --git a/src/canonical-json.ts b/src/canonical-json.ts
index a1fd6ec..00e4d7f 100644
--- a/src/canonical-json.ts
+++ b/src/canonical-json.ts
@@ -7,20 +7,37 @@ type Member = { label: string; value: JsonValue }
 type Pending = string | { depth: number; value: JsonValue }
 
 export function serializeCanonicalJson(value: JsonValue, spelling: JsonSpelling): string {
-  const indent = spelling === 'compact' ? '' : '  '
   const text: string[] = []
+  writeCanonicalJson(value, spelling, (piece) => text.push(piece) > 0)
+  return text.join('')
+}
+
+// Stops at the first piece `text` does not hold: two-space indentation can spell a short compact text past the engine's longest string.
+export function spellsCanonicalJson(value: JsonValue, spelling: JsonSpelling, text: string): boolean {
+  let offset = 0
+  const whole = writeCanonicalJson(value, spelling, (piece) => {
+    if (!text.startsWith(piece, offset)) return false
+    offset += piece.length
+    return true
+  })
+  return whole && offset === text.length
+}
+
+// `write` returns false to stop the walk, and the walk then returns false.
+function writeCanonicalJson(value: JsonValue, spelling: JsonSpelling, write: (piece: string) => boolean): boolean {
+  const indent = spelling === 'compact' ? '' : '  '
   const pending: Pending[] = [{ depth: 0, value }]
   for (let next = pending.pop(); next !== undefined; next = pending.pop()) {
     if (typeof next === 'string') {
-      text.push(next)
+      if (!write(next)) return false
       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 text.push(Object.is(held, -0) ? '-0' : JSON.stringify(held))
+    else if (!write(Object.is(held, -0) ? '-0' : JSON.stringify(held))) return false
   }
-  return text.join('')
+  return true
 }
 
 function objectMembers(value: { [key: string]: JsonValue }, indent: string): Member[] {
diff --git a/src/markdown/emit/adf-to-markdown.test.ts b/src/markdown/emit/adf-to-markdown.test.ts
index 05198af..ec283fe 100644
--- a/src/markdown/emit/adf-to-markdown.test.ts
+++ b/src/markdown/emit/adf-to-markdown.test.ts
@@ -44,6 +44,15 @@ test('refuses a value that is not an ADF document, naming the check it failed',
   assert.equal(markdown(adfToLosslessMarkdown({ type: 'doc', version: Number.NaN })), 'not-an-adf-document: an ADF document holds a version number: found NaN')
 })
 
+// About 2.2 billion characters of carry JSON, past V8's, JavaScriptCore's and SpiderMonkey's longest string.
+test('refuses a document whose markdown runs past the engine\'s longest string', () => {
+  let value: JsonValue = new Array<JsonValue>(2_200_000).fill(0)
+  for (let level = 0; level < 495; level += 1) value = [value]
+  const result = adfToLosslessMarkdown(document({ attrs: { value }, type: 'unknownBlock' }))
+  assert.match(markdown(result), /^unsupported-node-shape: the output runs past what this JavaScript engine holds: /)
+  assert.deepEqual(path(result), [])
+})
+
 test('refuses a document version the markdown cannot carry', () => {
   assert.equal(code(adfToLosslessMarkdown({ type: 'doc', version: 2 })), 'unsupported-document-version')
 })
diff --git a/src/markdown/emit/adf-to-markdown.ts b/src/markdown/emit/adf-to-markdown.ts
index b775500..6506743 100644
--- a/src/markdown/emit/adf-to-markdown.ts
+++ b/src/markdown/emit/adf-to-markdown.ts
@@ -7,7 +7,7 @@ import { blockDirectiveForm, documentSpelling } from '../block-directive.ts'
 import { blockNodeModel, blockNodes, isTaskItem } from '../../adf/block-nodes.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 { failure, faulted, success, withinEngineLimits, type ConvertErrorPath, type Result } from '../../result.ts'
 import { fencedCodeBlock } from '../commonmark/backtick-runs.ts'
 import { holdsNullCharacter, isBlankLine, isThematicBreak, markerInterruptsParagraph } from '../commonmark/grammar.ts'
 import { languageSlot, type LanguageSlot } from '../code-language.ts'
@@ -38,7 +38,7 @@ const listItemOpener = spellDirectiveOpener('listItem', undefined, '')
 const orderedDelimiters: Delimiters = ['.', ')']
 
 export function adfToLosslessMarkdown(document: AdfDocument): Result<string> {
-  return writeMarkdown(document, 'lossless')
+  return withinEngineLimits(() => writeMarkdown(document, 'lossless'))
 }
 
 export function writeMarkdown(document: AdfDocument, flavour: WrittenFlavour): Result<string> {
diff --git a/src/markdown/opaque-carry.ts b/src/markdown/opaque-carry.ts
index ef32c59..e7673d5 100644
--- a/src/markdown/opaque-carry.ts
+++ b/src/markdown/opaque-carry.ts
@@ -10,7 +10,7 @@ import { isAdfNode } from '../adf/document.ts'
 import { isJsonValue, nestingDepth, overNested } from '../json-value.ts'
 import { largestNesting } from '../nesting.ts'
 import { parseJsonText } from '../json-text.ts'
-import { serializeCanonicalJson } from '../canonical-json.ts'
+import { serializeCanonicalJson, spellsCanonicalJson } from '../canonical-json.ts'
 
 export const carryFencePrefix = 'adf:'
 
@@ -75,7 +75,7 @@ function readCarriedJson(raw: string, spelling: JsonSpelling, levels: number, ty
   if (overNested(held, levels)) {
     return { fault: { code: 'unsupported-nesting-depth', message: `a carried node's JSON nests deeper than the ${levels} levels its position leaves` } }
   }
-  if (serializeCanonicalJson(value, spelling) !== raw) {
+  if (!spellsCanonicalJson(value, spelling, raw)) {
     const shape = spelling === 'compact' ? 'compact, keys sorted' : 'two-space indent, keys sorted'
     return { fault: unsupportedNodeShape(`the opaque carry spells its node's JSON canonically: ${shape}; ${wayOut}`) }
   }
diff --git a/src/markdown/parse/markdown-to-adf.test.ts b/src/markdown/parse/markdown-to-adf.test.ts
index 22d337b..23090f6 100644
--- a/src/markdown/parse/markdown-to-adf.test.ts
+++ b/src/markdown/parse/markdown-to-adf.test.ts
@@ -448,6 +448,14 @@ test('names the canonical spelling a carried JSON reads alone', () => {
   assert.equal(content(losslessMarkdownToAdf('!adf:carry{json="{\\"type\\":\\"blockCard\\",\\"attrs\\":{}}"}\n')), `${canonically}compact, keys sorted${literal}`)
 })
 
+test('refuses compact JSON in the carry fence without spelling it two-space past the engine\'s longest string', () => {
+  const json = `{"attrs":{"value":${'['.repeat(495)}${new Array(2_200_000).fill(0).join(',')}${']'.repeat(495)}}}`
+  assert.equal(
+    content(losslessMarkdownToAdf(`\`\`\`adf:unknownBlock\n${json}\n\`\`\`\n`)),
+    "unsupported-node-shape: the opaque carry spells its node's JSON canonically: two-space indent, keys sorted; !adf:codeBlock {language=\"adf:unknownBlock\"} around a bare fence keeps it a code block",
+  )
+})
+
 test('names the node JSON an opaque carry restores alone', () => {
   const node = "unsupported-node-shape: the opaque carry holds one ADF node's JSON: this JSON is no ADF node; "
   assert.equal(content(losslessMarkdownToAdf('```adf:x\n[]\n```\n')), `${node}!adf:codeBlock {language="adf:x"} around a bare fence keeps it a code block`)
diff --git a/src/markdown/portable/adf-to-portable-markdown.test.ts b/src/markdown/portable/adf-to-portable-markdown.test.ts
index de56314..e6571ab 100644
--- a/src/markdown/portable/adf-to-portable-markdown.test.ts
+++ b/src/markdown/portable/adf-to-portable-markdown.test.ts
@@ -70,6 +70,12 @@ test('refuses what the document guard refuses, and nothing else', () => {
   assert.match(portableDocument(document(paragraph(deepInline))), /^unsupported-nesting-depth at /)
 })
 
+test('refuses a document whose text merges past the engine\'s longest string', () => {
+  const long = 'a'.repeat(2 ** 28)
+  const merged = paragraph(...Array.from({ length: 9 }, () => text(long)))
+  assert.equal(portableDocument(document(merged)), '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 3c953e5..ee485d2 100644
--- a/src/markdown/portable/adf-to-portable-markdown.ts
+++ b/src/markdown/portable/adf-to-portable-markdown.ts
@@ -3,7 +3,7 @@ import { adfDocumentFault, nodeAttrs, nodeContent, nodeMarks } from '../../adf/d
 import { blockNodeModel, isTaskItem } from '../../adf/block-nodes.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'
+import { failure, faulted, success, withinEngineLimits, type ConvertErrorPath, type Result } from '../../result.ts'
 import { flavourClaims, taskMarker } from './conventions.ts'
 import { inlineLeaves, isBlockNodeType, oneLine, portableLink, reduceInline, writableHref } from './inline-reduction.ts'
 import { inlineNodeModel } from '../../adf/inline-nodes.ts'
@@ -46,8 +46,10 @@ const blockReducers: Readonly<Record<string, BlockReducer>> = {
 }
 
 export function adfToPortableMarkdown(document: AdfDocument): Result<string> {
-  const reduced = reduceToPortable(document)
-  return reduced.ok ? writeMarkdown(reduced.value, 'portable') : reduced
+  return withinEngineLimits(() => {
+    const reduced = reduceToPortable(document)
+    return reduced.ok ? writeMarkdown(reduced.value, 'portable') : reduced
+  })
 }
 
 export function reduceToPortable(document: AdfDocument): Result<AdfDocument> {
diff --git a/src/result.test.ts b/src/result.test.ts
index 8503d13..27ee428 100644
--- a/src/result.test.ts
+++ b/src/result.test.ts
@@ -4,6 +4,8 @@ import { fileURLToPath } from 'node:url'
 import { readFileSync, readdirSync } from 'node:fs'
 import test from 'node:test'
 
+import { success, withinEngineLimits } from './result.ts'
+
 const sourceRoot = dirname(fileURLToPath(import.meta.url))
 const union = /export type ConvertErrorCode =\n((?:\s+\| '[a-z-]+'\n)+)/
 const declared = /'([a-z-]+)'/g
@@ -29,3 +31,23 @@ function calledCodes(): string[] {
 test('every ConvertErrorCode is the code of a production call site, and every call site names a declared one', () => {
   assert.deepEqual(calledCodes(), declaredCodes())
 })
+
+test('refuses where SpiderMonkey runs out of string, and lets any other throw through', () => {
+  const overflow = new Error('allocation size overflow')
+  overflow.name = 'InternalError'
+  assert.deepEqual(
+    withinEngineLimits(() => {
+      throw overflow
+    }),
+    { error: { code: 'unsupported-node-shape', message: 'the output runs past what this JavaScript engine holds: allocation size overflow', path: [] }, ok: false },
+  )
+  assert.throws(() =>
+    withinEngineLimits(() => {
+      throw new TypeError('a bug')
+    }),
+  )
+  assert.deepEqual(
+    withinEngineLimits(() => success('x')),
+    { ok: true, value: 'x' },
+  )
+})
diff --git a/src/result.ts b/src/result.ts
index 90166f7..8d859c2 100644
--- a/src/result.ts
+++ b/src/result.ts
@@ -41,3 +41,13 @@ export function positioned<T>(result: Result<T>, position: SourcePosition): Resu
 export function success<T>(value: T): { ok: true; value: T } {
   return { ok: true, value }
 }
+
+// Any string concatenation throws past the engine's longest string: RangeError in V8 and JavaScriptCore, InternalError in SpiderMonkey.
+export function withinEngineLimits<T>(convert: () => Result<T>): Result<T> {
+  try {
+    return convert()
+  } catch (error) {
+    if (!(error instanceof RangeError || (error instanceof Error && error.name === 'InternalError'))) throw error
+    return failure('unsupported-node-shape', `the output runs past what this JavaScript engine holds: ${error.message}`, [])
+  }
+}
diff --git a/todo.md b/todo.md
index 4518da2..878e03f 100644
--- a/todo.md
+++ b/todo.md
@@ -25,12 +25,10 @@
 
 | ID | Release | Exempt | Item | R | S | A | G | Goals | Score |
 |---|---|---|---|---|---|---|---|---|---|
-| 83 | 0.2.0 | defect | **Refuse a document whose markdown would pass the engine's string limit with a result, never a `RangeError`.** | 3 | 4 | 5 | 8 | 1, 9 | 22.0 |
 | 103 | 0.2.0 |  | **Fold `unspellable-whitespace` into `unsupported-node-shape`, so a line ending and a null character in a content slot take one code.** | 2 | 2 | 5 | 6 | 1 | 19.5 |
 | 84 | 0.2.0 |  | **Read GFM's pipe table without leading pipes, and with alignment colons, in `portableMarkdownToAdf`, dropping the alignment.** | 3 | 4 | 7 | 7 | 4, 7 | 19.4 |
 | 52 | 0.2.0 |  | **Spell `colwidth` as a comma list, `colwidth="340,420"`.** | 3 | 3 | 5 | 6 | 6 | 11.5 |
 | 53 | 0.2.0 |  | **Put a block's `marks` spelling to the writer panel and adopt its pick.** | 4 | 5 | 5 | 6 | 6 | 10.0 |
-| 105 | 0.2.1 |  | **Prove with a test that `adfToPortableMarkdown` never returns `unsupported-node-shape`, or list the code in the README's table of calls.** | 1 | 2 | 3 | 5 | 1 | 14.5 |
 | 89 | 0.2.1 |  | **Build a `date`, `emoji`, `inlineCard`, `mention` or `status` holding no mark Atlassian's schema withholds from it, in every reader.** | 4 | 4 | 5 | 6 | 4 | 13.0 |
 | 99 | 0.2.1 | defect | **Write a linked external image standing inline or in a table cell in the spelling a reader panel picks, in `adfToPortableMarkdown`.** | 3 | 3 | 6 | 7 | 7 | 12.9 |
 | 100 | 0.2.1 | decision | **Put the reading of an image's title as its caption back to a reader panel, now that a writer panel reads a title as hover text.** | 3 | 3 | 4 | 5 | 4, 6 | 10.9 |
@@ -73,14 +71,6 @@
 
 ## Details
 
-### 83. Refuse a document whose markdown would pass the engine's string limit with a result, never a `RangeError`.
-
-The block carry indents its JSON two spaces a level, so one unknown block holding 497 nested arrays
-around 540,000 numbers, about 1.1 MB as JSON, spells about 540 million characters, past V8's 2^29 −
-24. `serializeCanonicalJson`'s `join` throws, and `adfToLosslessMarkdown` throws with it. Any output
-past the limit does the same. Refuse it with a code from the closed list, and add a fixture. Found
-by the README-goals audit, 2026-10-08.
-
 ### 103. Fold `unspellable-whitespace` into `unsupported-node-shape`, so a line ending and a null character in a content slot take one code.
 
 `slotFault` in `directive-syntax.ts` gives a newline in an `emoji`, `mention` or `status` content
@@ -112,12 +102,6 @@ Today `marks="[{\"attrs\":{\"mode\":\"wide\"},\"type\":\"breakout\"}]"`, the mar
 escaped JSON. Breaking where the panel picks another spelling: `MIGRATION.md`'s Spellings table
 gains its row.
 
-### 105. Prove with a test that `adfToPortableMarkdown` never returns `unsupported-node-shape`, or list the code in the README's table of calls.
-
-`spellableLine` (`src/markdown/portable/inline-reduction.ts`) can return `unsupported-node-shape`, and
-no test reaches it; the README lists three codes for the call, and an app switching on them would
-meet a fourth.
-
 ### 89. Build a `date`, `emoji`, `inlineCard`, `mention` or `status` holding no mark Atlassian's schema withholds from it, in every reader.
 
 `*!adf:mention[@a]{id=…}*` and `[!adf:status[Done]{…}](/u)` build a node holding `em` or `link`.