diff --git a/AGENTS.md b/AGENTS.md
index 80494a0..ec04b15 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -109,9 +109,9 @@ live Atlassian APIs; property-generated ADF trees; the CommonMark spec suite aga
   `Result<T>` — `{ ok: true; value } | { ok: false; error: ConvertError }` — nothing throws.
   `try/catch` only wrapped tightly around a call that genuinely throws, converted to a result on
   the spot.
-- Nothing recurses unbounded: the document guard walks iteratively and block emission refuses past
-  500 nesting levels, so a deep document is a `Result` rather than the stack overflow that waits
-  near 3000.
+- Nothing recurses unbounded: the guards walk iteratively, and blocks, marks and attribute values
+  are all held to 500 levels, so a deep document is a `Result` rather than the stack overflow that
+  waits near 2000.
 - No casts: `as`, `as unknown as`, non-null `!`. A boundary owes a type guard validating the
   fields it claims (`isAdfDocument`); past it everything is typed. Make invalid states
   unrepresentable.
diff --git a/src/adf-to-markdown.test.ts b/src/adf-to-markdown.test.ts
index e6d25ea..04a212d 100644
--- a/src/adf-to-markdown.test.ts
+++ b/src/adf-to-markdown.test.ts
@@ -73,6 +73,8 @@ test('refuses a link destination CommonMark cannot spell', () => {
   assert.equal(code(adfToMarkdown(link('https://example.com/a\\b'))), 'unspellable-link-destination')
   assert.equal(code(adfToMarkdown(link('https://example.com/?a=1&amp;b=2'))), 'unspellable-link-destination')
   assert.equal(code(adfToMarkdown(link('https://example.com/a\nb'))), 'unspellable-link-destination')
+  const entity = 'https://example.com/?a=1&amp;b=2'
+  assert.equal(code(adfToMarkdown(document(paragraph({ marks: [{ attrs: { href: entity }, type: 'link' }], text: entity, type: 'text' })))), 'unspellable-link-destination')
   assert.equal(markdown(adfToMarkdown(link('https://en.example.com/a_(b)'))), '[t](https://en.example.com/a_(b))\n')
 })
 
@@ -100,6 +102,7 @@ test('refuses whitespace CommonMark cannot hold', () => {
   assert.equal(code(adfToMarkdown(document(paragraph({ text: '  lead', type: 'text' })))), 'unspellable-whitespace')
   assert.equal(code(adfToMarkdown(document(paragraph({ text: 'trail ', type: 'text' })))), 'unspellable-whitespace')
   assert.equal(code(adfToMarkdown(document(paragraph({ text: 'a\nb', type: 'text' })))), 'unspellable-whitespace')
+  assert.equal(code(adfToMarkdown(document(paragraph({ text: '\fa', type: 'text' })))), 'unspellable-whitespace')
   assert.equal(code(adfToMarkdown(document(paragraph({ marks: [{ type: 'em' }], text: 'x ', type: 'text' }, { text: 'y', type: 'text' })))), 'unspellable-whitespace')
 })
 
@@ -177,13 +180,34 @@ test('wraps adjacent nodes carrying one mark once, and a differing mark twice',
   assert.equal(emitted(marked('a', link('http://x')), marked('b', link('http://y'))), '[a](http://x)[b](http://y)\n')
 })
 
+test('refuses a mark spelling that cannot open or close where it sits', () => {
+  const marked = (text: string, ...marks: AdfMark[]): AdfNode => ({ marks, text, type: 'text' })
+  const emitted = (...content: AdfNode[]): string => markdown(adfToMarkdown(document(paragraph(...content))))
+  const strong: AdfMark = { type: 'strong' }
+  assert.equal(code(adfToMarkdown(document(paragraph({ text: 'un', type: 'text' }, marked('-real', strong), { text: 'istic', type: 'text' })))), 'unspellable-mark')
+  assert.equal(code(adfToMarkdown(document(paragraph(marked('C++', { type: 'em' }), { text: 'ish', type: 'text' })))), 'unspellable-mark')
+  assert.equal(code(adfToMarkdown(document(paragraph({ text: 'x', type: 'text' }, marked('.a', strong))))), 'unspellable-mark')
+  assert.equal(emitted({ text: 'un ', type: 'text' }, marked('-real', strong), { text: ' istic', type: 'text' }), 'un **-real** istic\n')
+  assert.equal(emitted(marked('a.', strong)), '**a.**\n')
+  assert.equal(emitted({ text: 'x', type: 'text' }, marked('a', strong), { text: 'y', type: 'text' }), 'x**a**y\n')
+})
+
+test('refuses marks and attributes nested deeper than the emitter carries', () => {
+  const marks: AdfMark[] = Array.from({ length: 600 }, (_, index) => ({ type: index % 2 === 0 ? 'em' : 'strong' }))
+  assert.equal(code(adfToMarkdown(document(paragraph({ marks, text: 'x', type: 'text' })))), 'unsupported-node-shape')
+  let attrs: AdfMark['attrs'] = { depth: 'x' }
+  for (let depth = 0; depth < 600; depth += 1) attrs = { depth: attrs }
+  assert.equal(code(adfToMarkdown(document(paragraph({ marks: [{ attrs, type: 'em' }], text: 'x', type: 'text' })))), 'not-an-adf-document')
+})
+
 test('escapes a literal delimiter that would merge with an emitted one', () => {
   const marked = (text: string, ...marks: AdfMark[]): AdfNode => ({ marks, text, type: 'text' })
   const emitted = (...content: AdfNode[]): string => markdown(adfToMarkdown(document(paragraph(...content))))
   assert.equal(emitted(marked('a_', { type: 'em' })), '_a\\__\n')
   assert.equal(emitted(marked('_a', { type: 'em' })), '_\\_a_\n')
   assert.equal(emitted(marked('a*', { type: 'strong' })), '**a\\***\n')
-  assert.equal(emitted({ text: 'x', type: 'text' }, marked('~a', { type: 'strike' })), 'x~~\\~a~~\n')
+  assert.equal(emitted(marked('~a', { type: 'strike' })), '~~\\~a~~\n')
+  assert.equal(code(adfToMarkdown(document(paragraph({ text: 'x', type: 'text' }, marked('~a', { type: 'strike' }))))), 'unspellable-mark')
   assert.equal(emitted({ text: '`', type: 'text' }, marked('x', { type: 'code' })), '\\``x`\n')
   assert.equal(emitted(marked('x', { type: 'code' }), { text: '`', type: 'text' }), '`x`\\`\n')
   assert.equal(emitted({ text: '!', type: 'text' }, marked('x', { attrs: { href: 'https://example.com/' }, type: 'link' })), '\\![x](https://example.com/)\n')
@@ -207,6 +231,7 @@ test('refuses the characters CommonMark rewrites', () => {
   assert.equal(code(adfToMarkdown(document({ content: [{ text: 'a\rb', type: 'text' }], type: 'codeBlock' }))), 'unspellable-whitespace')
   assert.equal(code(adfToMarkdown(document(paragraph({ text: 'a\u0000b', type: 'text' })))), 'unspellable-character')
   assert.equal(code(adfToMarkdown(document({ content: [{ text: 'a\u0000b', type: 'text' }], type: 'codeBlock' }))), 'unspellable-character')
+  assert.equal(code(adfToMarkdown(document({ content: [{ text: '', type: 'text' }], type: 'codeBlock' }))), 'unsupported-node-shape')
 })
 
 test('refuses a text node carrying no text at all', () => {
diff --git a/src/adf-to-markdown.ts b/src/adf-to-markdown.ts
index 99bf1d7..ca49af6 100644
--- a/src/adf-to-markdown.ts
+++ b/src/adf-to-markdown.ts
@@ -4,10 +4,10 @@ import { emitInlineLine } from './markdown-inline.ts'
 import { failure, success, type ConvertErrorPath, type Result } from './result.ts'
 import { holdsNullCharacter, isThematicBreak } from './commonmark-grammar.ts'
 import { isAdfDocument } from './adf-document.ts'
+import { largestNesting } from './nesting.ts'
 import { longestBacktickRun } from './backtick-runs.ts'
 
 const largestListMarker = 999999999
-const largestNesting = 500
 const listTypes = ['bulletList', 'orderedList']
 
 export function adfToMarkdown(document: AdfDocument): Result<string> {
@@ -72,7 +72,7 @@ function emitCodeBlock(node: AdfNode, path: ConvertErrorPath): Result<string> {
   let text = ''
   for (const [index, child] of (node.content ?? []).entries()) {
     const childPath = [...path, 'content', index]
-    if (child.type !== 'text' || typeof child.text !== 'string' || (child.marks ?? []).length > 0 || Object.keys(child.attrs ?? {}).length > 0) {
+    if (child.type !== 'text' || typeof child.text !== 'string' || child.text === '' || (child.marks ?? []).length > 0 || Object.keys(child.attrs ?? {}).length > 0) {
       return failure('unsupported-node-shape', 'a codeBlock holds plain text nodes only', childPath)
     }
     if (/\r/.test(child.text)) return failure('unspellable-whitespace', 'a codeBlock holds no carriage return CommonMark keeps', childPath)
diff --git a/src/json-value.ts b/src/json-value.ts
index 6762f15..88b548a 100644
--- a/src/json-value.ts
+++ b/src/json-value.ts
@@ -1,17 +1,22 @@
+import { largestNesting } from './nesting.ts'
+
 export type JsonValue = JsonValue[] | boolean | null | number | string | { [key: string]: JsonValue }
 
 export function isJsonValue(value: unknown): value is JsonValue {
-  const pending: unknown[] = [value]
+  const pending: { depth: number; item: unknown }[] = [{ depth: 0, item: value }]
   while (pending.length > 0) {
-    const item = pending.pop()
+    const entry = pending.pop()
+    if (entry === undefined) continue
+    const { depth, item } = entry
+    if (depth > largestNesting) return false
     if (item === null || typeof item === 'boolean' || typeof item === 'string') continue
     if (typeof item === 'number') {
       if (!Number.isFinite(item)) return false
       continue
     }
     // A hole is not a JSON value, and Array.prototype methods skip holes — spreading materialises them.
-    if (Array.isArray(item)) pending.push(...item)
-    else if (typeof item === 'object') pending.push(...Object.values(item))
+    if (Array.isArray(item)) for (const child of [...item]) pending.push({ depth: depth + 1, item: child })
+    else if (typeof item === 'object') for (const child of Object.values(item)) pending.push({ depth: depth + 1, item: child })
     else return false
   }
   return true
diff --git a/src/markdown-escaping.ts b/src/markdown-escaping.ts
index 7e13931..034d007 100644
--- a/src/markdown-escaping.ts
+++ b/src/markdown-escaping.ts
@@ -5,9 +5,11 @@ export type InlineSegment = {
   text: string
 }
 
+export type AssembledLine = { line: string; unspellableDelimiter: string | undefined }
+
 export type LineContainer = 'heading' | 'paragraph'
 
-const delimiters = ['`', '*', '_', '~']
+const delimiters = ['*', '_', '`', '~']
 
 const asciiPunctuation = /[!"#$%&'()*+,\-./:;<=>?@[\\\]^_`{|}~]/
 const htmlConstructs = [/^<[!?]/, /^<\/?[A-Za-z][A-Za-z0-9-]*(?:[\s/>]|$)/, /^<[^\s<>@]+@[^\s<>@]+>/]
@@ -15,7 +17,7 @@ const inlineDirective = /^:[a-z][A-Za-z0-9]*[[{]/
 const linkOpener = /\](?=[([:])/
 const unicodePunctuation = /[\p{P}\p{S}]/u
 
-export function assembleInlineLine(segments: readonly InlineSegment[], container: LineContainer): string {
+export function assembleInlineLine(segments: readonly InlineSegment[], container: LineContainer): AssembledLine {
   return escape(resolveEmphasis(segments), container)
 }
 
@@ -40,6 +42,7 @@ function resolveEmphasis(segments: readonly InlineSegment[]): InlineSegment[] {
     if (openerIndex === undefined || opener === undefined) continue
     const openOffset = offsets[openerIndex] ?? 0
     const closeOffset = offsets[index] ?? 0
+    if (opener.text !== '_') continue
     if (!isWordCharacter(charAt(scan, openOffset - 1)) && !isWordCharacter(charAt(scan, closeOffset + 1))) continue
     opener.text = '*'
     segment.text = '*'
@@ -47,11 +50,12 @@ function resolveEmphasis(segments: readonly InlineSegment[]): InlineSegment[] {
   return resolved
 }
 
-function escape(segments: readonly InlineSegment[], container: LineContainer): string {
+function escape(segments: readonly InlineSegment[], container: LineContainer): AssembledLine {
   const scan = segments.map((segment) => segment.text).join('')
   const kinds: InlineSegment['kind'][] = []
   for (const segment of segments) for (let index = 0; index < segment.text.length; index += 1) kinds.push(segment.kind)
   const escaped = new Set<number>()
+  const placements: number[] = []
   let output = ''
   for (let index = 0; index < scan.length; index += 1) {
     const kind = kinds[index]
@@ -60,9 +64,24 @@ function escape(segments: readonly InlineSegment[], container: LineContainer): s
       output += '\\'
       escaped.add(index)
     }
+    placements.push(output.length)
     output += scan.charAt(index)
   }
-  return output
+  return { line: output, unspellableDelimiter: unspellableDelimiter(segments, output, placements) }
+}
+
+function unspellableDelimiter(segments: readonly InlineSegment[], output: string, placements: readonly number[]): string | undefined {
+  let cursor = 0
+  for (const segment of segments) {
+    const start = placements[cursor] ?? 0
+    cursor += segment.text.length
+    if (segment.kind !== 'emphasis-close' && segment.kind !== 'emphasis-open') continue
+    const before = charAt(output, start - 1)
+    const after = output.charAt(start + segment.text.length)
+    const spellable = segment.kind === 'emphasis-open' ? isLeftFlanking(before, after) : isRightFlanking(before, after)
+    if (!spellable) return segment.text
+  }
+  return undefined
 }
 
 function mergesWithSyntax(scan: string, kinds: readonly (InlineSegment['kind'] | undefined)[], index: number): boolean {
diff --git a/src/markdown-inline.ts b/src/markdown-inline.ts
index 9547a9e..d92f68c 100644
--- a/src/markdown-inline.ts
+++ b/src/markdown-inline.ts
@@ -1,5 +1,6 @@
 import type { AdfMark, AdfNode } from './adf-document.ts'
 import { assembleInlineLine, type InlineSegment, type LineContainer } from './markdown-escaping.ts'
+import { largestNesting } from './nesting.ts'
 import { claimsLine, holdsControlCharacter, holdsEntityReference, holdsNullCharacter, isAutolink, isUnicodeWhitespace } from './commonmark-grammar.ts'
 import { failure, success, type ConvertErrorPath, type Result } from './result.ts'
 import { longestBacktickRun } from './backtick-runs.ts'
@@ -19,9 +20,13 @@ const linkAttributes = ['href', 'title']
 export function emitInlineLine(nodes: readonly AdfNode[], container: LineContainer, path: ConvertErrorPath): Result<string> {
   const segments = emitRun(nodes, 0, 0, { atBlockEnd: true, container, inLinkText: false, path })
   if (!segments.ok) return segments
-  const line = assembleInlineLine(segments.value, container)
+  const assembled = assembleInlineLine(segments.value, container)
+  if (assembled.unspellableDelimiter !== undefined) {
+    return failure('unspellable-mark', `the ${assembled.unspellableDelimiter} spelling cannot open or close where it sits`, path)
+  }
+  const line = assembled.line
   for (const [index, single] of line.split('\n').entries()) {
-    if (/^[ \t]|[ \t]$/.test(single)) {
+    if (/^[ \t\v\f]|[ \t\v\f]$/.test(single)) {
       return failure('unspellable-whitespace', 'a line begins or ends with whitespace CommonMark strips', path)
     }
     if (container === 'paragraph' && claimsLine(single, index === 0 ? 'first' : 'later')) {
@@ -32,6 +37,9 @@ export function emitInlineLine(nodes: readonly AdfNode[], container: LineContain
 }
 
 function emitRun(nodes: readonly AdfNode[], depth: number, firstIndex: number, context: InlineContext): Result<InlineSegment[]> {
+  if (depth > largestNesting) {
+    return failure('unsupported-node-shape', `the marks nest deeper than the ${largestNesting} levels the emitter carries`, context.path)
+  }
   const runs = inlineRuns(nodes, depth, firstIndex)
   const segments: InlineSegment[] = []
   for (const [offset, run] of runs.entries()) {
@@ -94,10 +102,7 @@ function emitMarkedRun(nodes: readonly AdfNode[], mark: AdfMark, depth: number,
   if (!inner.ok) return inner
   const text = inner.value.map((segment) => segment.text).join('')
   if (holdsEdgeWhitespace(text)) return failure('unspellable-whitespace', `the ${mark.type} spelling cannot open or close beside whitespace`, path)
-  if (mark.type === 'em') {
-    return success([{ kind: 'emphasis-open', text: spelling }, ...inner.value, { kind: 'emphasis-close', text: spelling }])
-  }
-  return success([{ kind: 'syntax', text: spelling }, ...inner.value, { kind: 'syntax', text: spelling }])
+  return success([{ kind: 'emphasis-open', text: spelling }, ...inner.value, { kind: 'emphasis-close', text: spelling }])
 }
 
 function emitCodeSpan(nodes: readonly AdfNode[], depth: number, path: ConvertErrorPath): Result<InlineSegment[]> {
@@ -115,7 +120,7 @@ function emitCodeSpan(nodes: readonly AdfNode[], depth: number, path: ConvertErr
 }
 
 function holdsEdgeWhitespace(text: string): boolean {
-  return text !== '' && (isUnicodeWhitespace(text.charAt(0)) || isUnicodeWhitespace(text.charAt(text.length - 1)))
+  return isUnicodeWhitespace(text.charAt(0)) || isUnicodeWhitespace(text.charAt(text.length - 1))
 }
 
 function needsPadding(text: string): boolean {
diff --git a/src/nesting.ts b/src/nesting.ts
new file mode 100644
index 0000000..1019551
--- /dev/null
+++ b/src/nesting.ts
@@ -0,0 +1 @@
+export const largestNesting = 500