4b - the walk reports its headroom, so a list falling back to the directive form refuses at zero and the list limit stays 500

AGENTS.md

@@ -290,10 +290,13 @@ someone spells it or pins it.
   again at every level, doubling per level (4b).
 - Nothing recurses unbounded: the guards walk iteratively, and blocks, marks and JSON values — an
   attribute's and a carried node's alike — are all held to 500 levels, so a deep document is a
-  `Result` rather than the stack overflow that waits near 2000. A block's level is its count of
-  block ancestors — a list item's children two below the list — in either direction and whichever
-  spelling holds them, so the guards agree at the boundary and no spelling recurses twice per
-  level it counts once (the maintainer, 2026-09-18).
+  `Result` rather than the stack overflow that waits near 2000. A level is one block-list
+  recursion in either direction: a readable list's items sit one below it, its directive
+  spelling's two. So a list giving way after its walk owes the directive form a level the walk
+  did not count, and the walk reports its headroom — the least slack any depth guard below it
+  has — for the fallback to refuse at zero rather than walk again; counting every list twice
+  halved the list limit, counting the directive form once doubled the parser's frames per level
+  (the maintainer, 2026-09-18).
 - A reader takes the text and an index — a sticky regex whose `lastIndex` the caller sets on the
   line before it reads, `indexOf` — never a fresh slice per character, and a per-character walk
   hoists the scan that does not vary with the character. The pipeline persona feeds documents

src/json-value.ts

@@ -19,15 +19,20 @@ export function isJsonValue(value: unknown): value is JsonValue {
   return true
 }
 
-export function overNested(value: JsonValue, levels: number = largestNesting): boolean {
-  const pending: { depth: number; item: JsonValue }[] = [{ depth: 0, item: value }]
+export function nestingDepth(value: unknown): number {
+  const pending: { depth: number; item: unknown }[] = [{ depth: 0, item: value }]
+  let deepest = 0
   while (pending.length > 0) {
     const entry = pending.pop()
     if (entry === undefined) continue
     const { depth, item } = entry
-    if (depth > levels) return true
+    deepest = Math.max(deepest, depth)
     if (Array.isArray(item)) for (const child of item) pending.push({ depth: depth + 1, item: child })
     else if (item !== null && typeof item === 'object') for (const child of Object.values(item)) pending.push({ depth: depth + 1, item: child })
   }
-  return false
+  return deepest
+}
+
+export function overNested(value: unknown, levels: number = largestNesting): boolean {
+  return nestingDepth(value) > levels
 }

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

@@ -514,12 +514,20 @@ test('refuses a document nested deeper than the emitter carries', () => {
     for (let level = 1; level < levels; level += 1) list = { content: [{ content: [...first, list], type: 'listItem' }], type: 'bulletList' }
     return list
   }
-  const lists = largestNesting / 2
-  assert.ok(adfToMarkdown(document(listed(lists, []))).ok)
-  assert.equal(code(adfToMarkdown(document(listed(lists + 1, [])))), 'unsupported-nesting-depth')
-  const deep = document(listed(lists, [{ type: 'rule' }]))
+  assert.ok(adfToMarkdown(document(listed(largestNesting, [paragraph({ text: 'x', type: 'text' })]))).ok)
+  assert.equal(code(adfToMarkdown(document(listed(largestNesting + 1, [paragraph({ text: 'x', type: 'text' })])))), 'unsupported-nesting-depth')
+  const directiveLists = largestNesting / 2
+  const deep = document(listed(directiveLists, [{ type: 'rule' }]))
   assert.deepEqual(markdownToAdf(markdown(adfToMarkdown(deep))), { ok: true, value: deep })
-  assert.equal(code(adfToMarkdown(document(listed(lists + 1, [{ type: 'rule' }])))), 'unsupported-nesting-depth')
+  assert.equal(code(adfToMarkdown(document(listed(directiveLists + 1, [{ type: 'rule' }])))), 'unsupported-nesting-depth')
+  const chain = (levels: number): AdfNode => {
+    let card: AdfNode = { type: 'blockCard' }
+    for (let level = 0; level < levels; level += 1) card = { content: [card], type: 'blockCard' }
+    return card
+  }
+  const carriedInItem = (levels: number): AdfDocument => document(listed(1, [{ type: 'rule' }, chain(levels)]))
+  assert.deepEqual(markdownToAdf(markdown(adfToMarkdown(carriedInItem(248)))), { ok: true, value: carriedInItem(248) })
+  assert.equal(code(adfToMarkdown(carriedInItem(249))), 'unsupported-nesting-depth')
 })
 
 test('emits an empty list item without trailing whitespace', () => {

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

@@ -18,11 +18,13 @@ import { tryPipeTable } from './pipe-table.ts'
 
 type BlockContainer = 'directive' | 'document' | 'list-item'
 type BlockSpelling = 'commonmark' | 'directive' | 'list'
-type EmittedBlock = { spelling: BlockSpelling; text: string }
+type EmittedBlock = { headroom: number; spelling: BlockSpelling; text: string }
 type PlacedBlock = EmittedBlock & { node: AdfNode }
-type WalkedItem = { blocks: readonly PlacedBlock[]; node: AdfNode }
+type Walk = { blocks: readonly PlacedBlock[]; headroom: number }
+type WalkedItem = { node: AdfNode; walk: Walk }
 
 const largestListMarker = 999999999
+// Bare because emitList admits no item carrying attributes, marks or text.
 const listItemOpener = spellDirectiveOpener('listItem', undefined, '')
 
 export function adfToMarkdown(document: AdfDocument): Result<string> {
@@ -35,20 +37,27 @@ export function adfToMarkdown(document: AdfDocument): Result<string> {
 }
 
 function emitBlocks(nodes: readonly AdfNode[], container: BlockContainer, path: ConvertErrorPath, depth: number): Result<string> {
-  const blocks = walkBlocks(nodes, path, depth)
-  if (!blocks.ok) return blocks
-  return success(joinBlocks(blocks.value, container))
+  const walk = walkBlocks(nodes, path, depth)
+  if (!walk.ok) return walk
+  return success(joinBlocks(walk.value.blocks, container))
 }
 
-function walkBlocks(nodes: readonly AdfNode[], path: ConvertErrorPath, depth: number): Result<PlacedBlock[]> {
-  if (depth > largestNesting) return failure('unsupported-nesting-depth', `the document nests deeper than the ${largestNesting} levels the emitter carries`, path)
+// The walk's headroom is the least slack any depth guard below it has, so a spelling that sinks the walked blocks a level can refuse rather than walk again.
+function walkBlocks(nodes: readonly AdfNode[], path: ConvertErrorPath, depth: number): Result<Walk> {
+  let headroom = largestNesting - depth
+  if (headroom < 0) return tooDeep(path)
   const blocks: PlacedBlock[] = []
   for (const [index, node] of nodes.entries()) {
     const block = emitBlock(node, [...path, 'content', index], depth)
     if (!block.ok) return block
+    headroom = Math.min(headroom, block.value.headroom)
     blocks.push({ ...block.value, node })
   }
-  return success(blocks)
+  return success({ blocks, headroom })
+}
+
+function tooDeep(path: ConvertErrorPath): Result<never> {
+  return failure('unsupported-nesting-depth', `the document nests deeper than the ${largestNesting} levels the emitter carries`, path)
 }
 
 function joinBlocks(blocks: readonly PlacedBlock[], container: BlockContainer): string {
@@ -111,20 +120,20 @@ function readableText(text: string | undefined): Result<EmittedBlock> | undefine
   return text === undefined ? undefined : success(commonMarkText(text))
 }
 
-function commonMarkLine(text: Result<string>): Result<EmittedBlock> {
-  if (!text.ok) return text
-  return success(commonMarkText(text.value))
+function commonMarkLine(carried: Result<{ headroom: number; text: string }>): Result<EmittedBlock> {
+  if (!carried.ok) return carried
+  return success({ ...carried.value, spelling: 'commonmark' })
 }
 
-function commonMarkText(text: string): EmittedBlock {
-  return { spelling: 'commonmark', text }
+function commonMarkText(text: string, headroom: number = Number.POSITIVE_INFINITY): EmittedBlock {
+  return { headroom, spelling: 'commonmark', text }
 }
 
-function directivePair(node: AdfNode, opener: string, body: string): EmittedBlock {
-  return { spelling: 'directive', text: `${opener}\n${body === '' ? '' : `${body}\n`}${spellDirectiveCloser(node.type)}` }
+function directivePair(node: AdfNode, opener: string, body: string, headroom: number = Number.POSITIVE_INFINITY): EmittedBlock {
+  return { headroom, spelling: 'directive', text: `${opener}\n${body === '' ? '' : `${body}\n`}${spellDirectiveCloser(node.type)}` }
 }
 
-function emitDirectiveBlock(node: AdfNode, directive: BlockDirective, path: ConvertErrorPath, depth: number, walkBody: () => Result<PlacedBlock[]>): Result<EmittedBlock> {
+function emitDirectiveBlock(node: AdfNode, directive: BlockDirective, path: ConvertErrorPath, depth: number, walkBody: () => Result<Walk>): Result<EmittedBlock> {
   if (node.text !== undefined) return failure('unsupported-node-shape', `a ${node.type} carries no text: this one holds text`, path)
   if (blockDirectiveForm(node.type) === 'leaf' && nodeContent(node).length > 0) return failure('unsupported-node-shape', `a ${node.type} holds no content: this one holds some`, path)
   if (directive.contentModel === 'code') return emitCodeDirective(node, directive, path, depth)
@@ -133,27 +142,27 @@ function emitDirectiveBlock(node: AdfNode, directive: BlockDirective, path: Conv
   return emitDirectiveBody(node, directive, opener, path, walkBody)
 }
 
-function emitDirectiveBody(node: AdfNode, directive: BlockDirective, opener: string, path: ConvertErrorPath, walkBody: () => Result<PlacedBlock[]>): Result<EmittedBlock> {
-  if (blockDirectiveForm(node.type) === 'leaf') return success({ spelling: 'directive', text: opener })
+function emitDirectiveBody(node: AdfNode, directive: BlockDirective, opener: string, path: ConvertErrorPath, walkBody: () => Result<Walk>): Result<EmittedBlock> {
+  if (blockDirectiveForm(node.type) === 'leaf') return success({ headroom: Number.POSITIVE_INFINITY, spelling: 'directive', text: opener })
   if (directive.contentModel === 'inline') {
     const line = emitInlineLine(nodeContent(node), 'paragraph', path)
     if (!line.ok) return line
     return success(directivePair(node, opener, line.value))
   }
-  const blocks = walkBody()
-  if (!blocks.ok) return blocks
-  return success(directivePair(node, opener, joinBlocks(blocks.value, 'directive')))
+  const walk = walkBody()
+  if (!walk.ok) return walk
+  return success(directivePair(node, opener, joinBlocks(walk.value.blocks, 'directive'), walk.value.headroom))
 }
 
 function emitBlockquote(node: AdfNode, path: ConvertErrorPath, depth: number): Result<EmittedBlock> | undefined {
   if (!carriesOnly(node, [])) return undefined
-  const inner = emitBlocks(nodeContent(node), 'document', path, depth + 1)
+  const inner = walkBlocks(nodeContent(node), path, depth + 1)
   if (!inner.ok) return inner
-  const text = inner.value
+  const text = joinBlocks(inner.value.blocks, 'document')
     .split('\n')
     .map((line) => (line === '' ? '>' : `> ${line}`))
     .join('\n')
-  return success(commonMarkText(text))
+  return success(commonMarkText(text, inner.value.headroom))
 }
 
 function emitCodeBlock(node: AdfNode, path: ConvertErrorPath): Result<EmittedBlock> | undefined {
@@ -216,21 +225,24 @@ function emitList(node: AdfNode, path: ConvertErrorPath, depth: number): Result<
   if (items.some((item) => item.type !== 'listItem' || !carriesOnly(item, []))) return undefined
   const walked: WalkedItem[] = []
   for (const [offset, item] of items.entries()) {
-    const blocks = walkBlocks(nodeContent(item), [...path, 'content', offset], depth + 2)
-    if (!blocks.ok) return blocks
-    walked.push({ blocks: blocks.value, node: item })
+    const walk = walkBlocks(nodeContent(item), [...path, 'content', offset], depth + 1)
+    if (!walk.ok) return walk
+    walked.push({ node: item, walk: walk.value })
   }
+  const headroom = Math.min(...walked.map((item) => item.walk.headroom))
   const lines: string[] = []
   for (const [offset, item] of walked.entries()) {
-    const line = listItemLines(item.blocks, ordered ? `${start + offset}. ` : '- ')
-    if (line === undefined) return emitDirectiveBlock(node, ordered ? blockDirectives.orderedList : blockDirectives.bulletList, path, depth, () => success(directiveItems(walked)))
+    const line = listItemLines(item.walk.blocks, ordered ? `${start + offset}. ` : '- ')
+    if (line === undefined) return headroom < 1 ? tooDeep(path) : emitDirectiveBlock(node, ordered ? blockDirectives.orderedList : blockDirectives.bulletList, path, depth, () => success(directiveItems(walked)))
     lines.push(line)
   }
-  return success({ spelling: 'list', text: lines.join('\n') })
+  return success({ headroom, spelling: 'list', text: lines.join('\n') })
 }
 
-function directiveItems(items: readonly WalkedItem[]): PlacedBlock[] {
-  return items.map((item) => ({ ...directivePair(item.node, listItemOpener, joinBlocks(item.blocks, 'directive')), node: item.node }))
+// The directive form sinks each item's blocks a level below where the walk read them.
+function directiveItems(items: readonly WalkedItem[]): Walk {
+  const blocks = items.map((item) => ({ ...directivePair(item.node, listItemOpener, joinBlocks(item.walk.blocks, 'directive'), item.walk.headroom - 1), node: item.node }))
+  return { blocks, headroom: Math.min(...blocks.map((block) => block.headroom)) }
 }
 
 function listStart(node: AdfNode, items: number): number | undefined {

src/markdown/opaque-carry.ts

@@ -3,7 +3,7 @@ import type { DirectiveSpan, Read } from './directive-syntax.ts'
 import type { JsonSpelling } from '../canonical-json.ts'
 import { failure, success, type ConvertErrorPath, type Result } from '../result.ts'
 import { isAdfNode } from '../adf/document.ts'
-import { isJsonValue, overNested } from '../json-value.ts'
+import { isJsonValue, nestingDepth, overNested } from '../json-value.ts'
 import { fencedCodeBlock } from './backtick-runs.ts'
 import { largestNesting } from '../nesting.ts'
 import { malformedDirective, readSoleStringAttribute, spellAttributes, spellInlineLeafDirective, spellStringAttribute, unsupportedNodeShape } from './directive-syntax.ts'
@@ -13,16 +13,16 @@ export const carryName = 'carry'
 
 const jsonAttribute = 'json'
 
-export function carriedBlock(node: AdfNode, path: ConvertErrorPath, depth: number): Result<string> {
+export function carriedBlock(node: AdfNode, path: ConvertErrorPath, depth: number): Result<{ headroom: number; text: string }> {
   const json = carriedJson(node, 'two-space', path, largestNesting - depth)
   if (!json.ok) return json
-  return success(fencedCodeBlock(carryName, json.value))
+  return success({ headroom: json.value.headroom, text: fencedCodeBlock(carryName, json.value.json) })
 }
 
 export function carriedInline(node: AdfNode, path: ConvertErrorPath): Result<string> {
   const json = carriedJson(node, 'compact', path, largestNesting)
   if (!json.ok) return json
-  return success(spellInlineLeafDirective(carryName, spellAttributes([[jsonAttribute, spellStringAttribute(json.value)]])))
+  return success(spellInlineLeafDirective(carryName, spellAttributes([[jsonAttribute, spellStringAttribute(json.value.json)]])))
 }
 
 export function readCarriedBlock(body: string, depth: number): Read<AdfNode> {
@@ -36,11 +36,12 @@ export function readCarriedInline(span: DirectiveSpan): Read<AdfNode> | undefine
   return readCarriedJson(spelled.value, 'compact', largestNesting)
 }
 
-function carriedJson(node: AdfNode, spelling: JsonSpelling, path: ConvertErrorPath, levels: number): Result<string> {
-  if (!isJsonValue(node) || overNested(node, levels)) {
+function carriedJson(node: AdfNode, spelling: JsonSpelling, path: ConvertErrorPath, levels: number): Result<{ headroom: number; json: string }> {
+  const headroom = levels - nestingDepth(node)
+  if (!isJsonValue(node) || headroom < 0) {
     return failure('unsupported-nesting-depth', `a carried node's JSON nests deeper than the ${levels} levels its position leaves`, path)
   }
-  return success(serializeCanonicalJson(node, spelling))
+  return success({ headroom, json: serializeCanonicalJson(node, spelling) })
 }
 
 function readCarriedJson(raw: string, spelling: JsonSpelling, levels: number): Read<AdfNode> {

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

@@ -676,11 +676,11 @@ test('refuses input nested deeper than the parser carries', () => {
   assert.deepEqual(position(markdownToAdf(nest(['expand', ...repeated('panel'), 'expand'], 'Part.\n'))), { line: 501, offset: 5501 })
   assert.equal(code(markdownToAdf(nest(['panel', ...repeated('expand'), 'panel'], 'Part.\n'))), 'unsupported-nesting-depth')
   const listed = (levels: number): string => `${'!adf:bulletList\n!adf:listItem\n---\n'.repeat(levels)}${'!adf:/listItem\n!adf:/bulletList\n'.repeat(levels)}`
-  const lists = largestNesting / 2
-  assert.ok(markdownToAdf(listed(lists)).ok)
-  assert.equal(code(markdownToAdf(listed(lists + 1))), 'unsupported-nesting-depth')
-  assert.ok(markdownToAdf(`${'- '.repeat(lists)}a\n`).ok)
-  assert.equal(code(markdownToAdf(`${'- '.repeat(lists + 1)}a\n`)), 'unsupported-nesting-depth')
+  const directiveLists = largestNesting / 2
+  assert.ok(markdownToAdf(listed(directiveLists)).ok)
+  assert.equal(code(markdownToAdf(listed(directiveLists + 1))), 'unsupported-nesting-depth')
+  assert.ok(markdownToAdf(`${'- '.repeat(largestNesting)}a\n`).ok)
+  assert.equal(code(markdownToAdf(`${'- '.repeat(largestNesting + 1)}a\n`)), 'unsupported-nesting-depth')
 })
 
 test('decodes the backslash escapes CommonMark spells, and keeps the rest literal', () => {

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

@@ -150,7 +150,7 @@ function withContent(node: AdfNode, content: readonly AdfNode[]): AdfNode {
 function listNode(node: AdfNode, items: readonly Block[][], definitions: LinkDefinitions, path: ConvertErrorPath, depth: number): Result<AdfNode> {
   const content: AdfNode[] = []
   for (const [index, blocks] of items.entries()) {
-    const item = containerNode({ type: 'listItem' }, blocks, definitions, [...path, 'content', index], depth + 1)
+    const item = containerNode({ type: 'listItem' }, blocks, definitions, [...path, 'content', index], depth)
     if (!item.ok) return item
     content.push(item.value)
   }

src/nesting.ts

@@ -1,2 +1,2 @@
-// A block's depth is its count of block ancestors, a list item's children two below the list, in either direction and whichever spelling holds them — or the two guards disagree.
+// Levels count per AGENTS.md §11.
 export const largestNesting = 500

todo.md

@@ -64,10 +64,11 @@ proves 12, 13 spells 11's gaps in 12's grammar, and 12 rewrites code 4b and 4c c
       export persona runs in bulk walks the document twice. Both walks are linear, so this is a
       constant factor rather than 4b's class change, and the parting is what gives depth its own
       code (§8) — measure before joining them back.
-      **Settled** (the maintainer, 2026-09-18): the one walk cannot know the spelling it will take,
-      so a list and its item count two levels in every spelling and both directions — the readable
-      list counted one and the directive form two, and letting the directive form count one put
-      twice the frames on the stack at 500 — leaving 250 nested lists the limit.
+      **Settled** (the maintainer, 2026-09-18): the limit stays 500 readable lists. The one walk
+      counts items one below the list, as the readable form does, and reports its headroom; a
+      list falling back to the directive form refuses when that form's extra level no longer
+      fits. Counting every list twice was rejected for halving the limit, counting the directive
+      form once for doubling the parser's frames per level.
       **Measured** (2026-09-18): `adfDocumentFault` walks a 9 MB document in 52 ms against 314 ms
       for the emit, so its two walks stay parted.
 - [ ] **4c — The scanning rule's remaining sites (`0.2.0`).** A trailing-anchored regex re-walks
@@ -90,7 +91,7 @@ proves 12, 13 spells 11's gaps in 12's grammar, and 12 rewrites code 4b and 4c c
       complexity (the maintainer, 2026-09-16). A sixth 4b leaves behind: the parser asks
       `commonMarkSpelling` at every directive-spelled list it reads, and the answer spells the whole
       subtree below, so nested directive lists cost O(depth × subtree) — 250 rule-first levels parse
-      in 1.3 s at 16.5 kB, 1.5 MB of the same shape in 0.6 s — bounded by the depth guard like
+      in 1.3 s at 16.5 kB, 1.5 MB of that shape at 250 levels in 0.6 s — bounded by the depth guard like
       `readNestedDirective` (the maintainer, 2026-09-18).
 - [ ] **4d — What the gate says while it runs (`0.2.1`).** `ci.sh` runs nine legs and announces
       none of them, so five minutes of a Gitea run read as silence and a hang cannot be told from