diff --git a/AGENTS.md b/AGENTS.md
index 614fcc0..b0d3f6f 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -54,6 +54,7 @@ In `docs/decisions.md`:
 - The flavour spec is read as a source
 - The node tables answer to Atlassian's schema
 - Nothing recurses unbounded
+- No value `JSON.parse` or `structuredClone` builds loops a call
 - Nothing spreads an unbounded array
 - A retry loop checks its own termination
 - Readers scan by index
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 3123e48..5e1c5cc 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -31,6 +31,8 @@
   `collection`, `id` and `occurrenceKey` directly where they rode the opaque carry.
 - Fix an image inside another image's description: it flattens into the alt text, where it was
   refused.
+- Fix a cyclic document, in which an object holds itself: `adfToMarkdown` and `adfToPlainMarkdown`
+  refuse it as `not-an-adf-document` and `isAdfDocument` returns `false`, where all three hung.
 
 ## 0.1.0
 
diff --git a/README.md b/README.md
index e0e48f1..2b2c6d2 100644
--- a/README.md
+++ b/README.md
@@ -33,7 +33,7 @@ The most useful ADF conversion library available, judged by these goals, in prio
 5. **No surprises: output reads and edits the way its audience expects.**
 6. **Lossy conversion drops form, never content.**
 7. **Runs in any JavaScript engine, with no runtime dependencies and nothing to configure or connect.**
-8. **Fast, and linear in the document's size.**
+8. **Fast, and linear in the document's size as JSON.**
 9. **Easy to find, and clear at a glance what it does.**
 
 ## Audience
@@ -185,7 +185,7 @@ Emitting — `adfToMarkdown`, and `adfToHtml` at `0.2.0`:
 
 | Code | Fires when | What you can do |
 | --- | --- | --- |
-| `not-an-adf-document` | the value handed in is no ADF document — a missing or wrong `type`, a stray key, a node that is not a node | guard the boundary you receive JSON at with `isAdfDocument`; the message names the branch that refused |
+| `not-an-adf-document` | the value handed in is no ADF document — a missing or wrong `type`, a stray key, a node that is not a node, an object in the document holding itself | guard the boundary you receive JSON at with `isAdfDocument`; the message names the branch that refused |
 | `unsupported-document-version` | the document's `version` is not 1 | keep the ADF and pass the document over, or show it read-only; the version is the site's, not yours to change |
 
 Either direction. The emitter's own refusals are in this group — a parse reaches them by asking it
@@ -238,8 +238,8 @@ Serves Goals 1, 3 and 4.
   input.
 - Past that and `~~`, no GFM: an autolink literal and a `- [ ]` marker stay text, and a checklist
   is the `taskList` directive — `plainMarkdownToAdf` turns the marker into a `taskList`.
-- A document nested deeper than 500 levels is an error result, not a stack overflow, and no input
-  makes a call loop forever.
+- A document nested deeper than 500 levels is an error result, not a stack overflow, and no call
+  loops forever on a string, or on a value `JSON.parse` or `structuredClone` builds.
 - The emitted formats are semver surface
   ([`docs/decisions.md`](https://gitea.larvit.se/larvit/adf-codec/src/branch/main/docs/decisions.md#the-formats-are-api)).
 - **`0.2.0`** — `htmlToAdf(adfToHtml(doc))` deep-equals `doc`; fidelity HTML cannot express rides
diff --git a/docs/decisions.md b/docs/decisions.md
index 1f0c535..c5c18a2 100644
--- a/docs/decisions.md
+++ b/docs/decisions.md
@@ -384,10 +384,10 @@ input reads `message`.
   must invert before it reads as a failure — and where the flavour's claim refuses ordinary prose
   it names the escape that unclaims the form claimed: `\!adf:` for a directive, block line and
   inline alike, `\|` for every pipe row.
-- `not-an-adf-document` carries the document's own path throughout: seven of the guard's eight
-  branches read the document's own shape, and threading a path to the eighth — a malformed node
-  anywhere in the tree — wants the manual stack §Nothing recurses unbounded forces. The message
-  names the violation instead.
+- `not-an-adf-document` carries the document's own path throughout: seven of the guard's nine
+  branches read the document's own shape, and threading a path to the other two, which look anywhere
+  in the tree — a malformed node, or an object holding itself — wants the manual stack §Nothing
+  recurses unbounded forces. The message names the violation instead.
 
 ## Publish on a version bump
 
@@ -563,6 +563,15 @@ format, as its parser does. A list giving way to the directive form refuses at z
 than walking again; counting every list twice halved the list limit, counting the directive form
 once doubled the parser's frames per level.
 
+## No value `JSON.parse` or `structuredClone` builds loops a call
+
+2026-10-04, the maintainer. Goals 1 and 8. Valid while every persona hands in plain data, as
+`JSON.parse` or `structuredClone` builds it.
+
+The guards walk a value by `Object.values` and refuse a cycle in it. An inherited or non-enumerable
+key, a getter, a Proxy or a custom iterator is the caller's to rule out: no persona builds one.
+Goal 8's size counts ADF as JSON, so a shared object costs once per place it is held.
+
 ## Nothing spreads an unbounded array
 
 2026-09-18, the maintainer. Goal 1. Valid while engines cap a call's arguments.
diff --git a/src/adf/document.test.ts b/src/adf/document.test.ts
index 20e3b37..5b66c8f 100644
--- a/src/adf/document.test.ts
+++ b/src/adf/document.test.ts
@@ -82,3 +82,23 @@ test("reads a node's siblings as a walk rather than as one call's arguments", ()
   assert.equal(isAdfDocument(wide), true)
   assert.equal(fault(wide), 'accepted')
 })
+
+test('refuses an object holding itself, and accepts one shared', () => {
+  const holdsItself = 'an ADF document is a tree: an object in it holds itself'
+  const paragraph: { content: unknown[]; type: string } = { content: [], type: 'paragraph' }
+  paragraph.content.push(paragraph)
+  assert.equal(fault({ content: [paragraph], type: 'doc', version: 1 }), holdsItself)
+  const document: { content: unknown[]; type: string; version: number } = { content: [], type: 'doc', version: 1 }
+  document.content.push(document)
+  assert.equal(fault(document), holdsItself)
+  const attrs: Record<string, unknown> = {}
+  attrs['a'] = [attrs]
+  assert.equal(fault({ content: [{ attrs, type: 'paragraph' }], type: 'doc', version: 1 }), holdsItself)
+  assert.equal(fault({ content: [{ marks: [{ attrs, type: 'link' }], text: 'x', type: 'text' }], type: 'doc', version: 1 }), holdsItself)
+  assert.equal(isAdfDocument({ content: [{ attrs, type: 'paragraph' }], type: 'doc', version: 1 }), false)
+  const bold = { type: 'strong' }
+  const text = { marks: [bold], text: 'x', type: 'text' }
+  const shared = { content: [text, text], type: 'paragraph' }
+  const values = { b: 1 }
+  assert.equal(fault({ content: [shared, shared, { attrs: { a: [values, values] }, type: 'paragraph' }], type: 'doc', version: 1 }), 'accepted')
+})
diff --git a/src/adf/document.ts b/src/adf/document.ts
index 772dd34..6f45a23 100644
--- a/src/adf/document.ts
+++ b/src/adf/document.ts
@@ -43,6 +43,7 @@ export function adfDocumentFault(value: unknown): ConvertFault | undefined {
   const held: unknown = value['content']
   if (!Array.isArray(held)) return notADocument(`an ADF document's content is an array: found ${describe(held)}`)
   const content: readonly unknown[] = held
+  if (holdsItself(content)) return notADocument('an ADF document is a tree: an object in it holds itself')
   if (!isNodeArray(content)) return notADocument("an ADF document's content holds ADF nodes: one of them is not")
   return nestingFault(content)
 }
@@ -122,6 +123,23 @@ export function nodeMarks(node: { marks?: AdfMark[] }): readonly AdfMark[] {
   return node.marks ?? []
 }
 
+// Tracks ancestors alone: a tree may share an object, but an object holding itself walks the guards forever.
+function holdsItself(root: object): boolean {
+  const pending = [{ depth: 0, item: root }]
+  const ancestors: object[] = []
+  const onPath = new Set<unknown>()
+  while (pending.length > 0) {
+    const entry = pending.pop()
+    if (entry === undefined) continue
+    while (ancestors.length > entry.depth) onPath.delete(ancestors.pop())
+    if (onPath.has(entry.item)) return true
+    ancestors.push(entry.item)
+    onPath.add(entry.item)
+    for (const child of Object.values(entry.item)) if (typeof child === 'object' && child !== null) pending.push({ depth: entry.depth + 1, item: child })
+  }
+  return false
+}
+
 function isNodeArray(value: readonly unknown[]): value is readonly AdfNode[] {
   const pending: unknown[] = [...value]
   while (pending.length > 0) {
diff --git a/todo.md b/todo.md
index 54b9978..5b1ebc7 100644
--- a/todo.md
+++ b/todo.md
@@ -6,7 +6,7 @@
 
 `Bar = 9`
 
-`Next ID = 66`
+`Next ID = 67`
 
 | Goal | W |
 |---|---|
@@ -24,12 +24,12 @@
 
 | ID | Release | Exempt | Item | R | S | A | G | Goals | Score |
 |---|---|---|---|---|---|---|---|---|---|
-| 63 | 0.2.0 | defect | **Refuse a cyclic input as `not-an-adf-document`.** | 2 | 2 | 6 | 9 | 1 | 27.5 |
 | 7 | 0.2.0 |  | **Ship HTML: `adfToHtml`, `htmlToAdf`, and `markdownToHtml` / `htmlToMarkdown` composed through ADF.** | 6 | 9 | 9 | 9 | 2, 3 | 25.8 |
 | 55 | 0.2.0 | defect | **Spell every document the emitter refuses today for a carriage return, a NUL, a code-span line start, a newline inside an emoji, mention or status, or a text node whose text is empty or missing, or which holds content.** | 5 | 5 | 7 | 9 | 1 | 25.8 |
 | 45 | 0.2.0 |  | **Replace `isAdfDocument` with a reader returning `Result<AdfDocument>`.** | 2 | 3 | 6 | 8 | 1 | 25.2 |
 | 6 | 0.2.0 | decision | **Specify the HTML dialect.** | 2 | 6 | 7 | 8 | 2, 3 | 24.7 |
 | 64 | 0.2.0 | defect | **Read back unchanged on V8 every JSON key the emitter writes, with a fixture whose key holds `\`, `"` or a control character.** | 4 | 4 | 6 | 8 | 1, 7 | 23.0 |
+| 66 | 0.2.0 | defect | **Refuse an attribute value that is not a plain object, array or JSON primitive.** | 3 | 2 | 5 | 8 | 1 | 22.5 |
 | 43 | 0.2.0 | decision | **Give each markdown input its own reader, strict to its own standard.** | 6 | 7 | 8 | 9 | 3, 4 | 22.3 |
 | 65 | 0.2.0 | defect | **Read an image not alone in its paragraph as its alt text linked to its URL, and a titled image alone in its paragraph as the image captioned with its title.** | 5 | 5 | 7 | 7 | 3, 4, 6 | 18.7 |
 | 49 | 0.2.0 |  | **Read a list whose bullet or ordered delimiter changes as two lists, in both readers, and retire `!adf:listBreak`.** | 4 | 5 | 5 | 8 | 3, 4 | 17.2 |
@@ -57,13 +57,6 @@
 
 ## Details
 
-### 63. Refuse a cyclic input as `not-an-adf-document`.
-
-`adfDocumentFault` walks with `isNodeArray` (`adf/document.ts`) and `isJsonValue` (`json-value.ts`),
-worklists that record no visited object, so a node whose `content` holds itself hangs
-`adfToMarkdown`, `adfToPlainMarkdown` and `isAdfDocument`; README §The guarantees promises no input
-loops forever.
-
 ### 7. Ship HTML: `adfToHtml`, `htmlToAdf`, and `markdownToHtml` / `htmlToMarkdown` composed through ADF.
 
 Lands after items 6, 59, 60, 61 and 62. The CommonMark spec suite also runs against
@@ -109,6 +102,13 @@ Deno and Chrome the round-trip would refuse its own output or read back a differ
 tells which. Confirm with a fixture first; then read JSON with our own parser or record the gap with
 an ending item. Found by the README-goals audit, 2026-10-03.
 
+### 66. Refuse an attribute value that is not a plain object, array or JSON primitive.
+
+`isJsonValue` (`json-value.ts`) walks any object by `Object.values`, so a `Date`, `Map`, `Set` or
+typed array in `attrs`, as `structuredClone` keeps it, passes the guard and is written as `{}` or a
+key per element: `ok: true` with content lost. Accept an object whose prototype is
+`Object.prototype` or `null`, and an array. Found by the stability review of item 63, 2026-10-04.
+
 ### 43. Give each markdown input its own reader, strict to its own standard.
 
 Today `markdownToAdf` reads CommonMark and the lossless flavour as one input: text shaped like a