Answer stability round 4: the decision's title backs the loop guarantee alone, item 66 filed

todo.md

@@ -6,7 +6,7 @@
 
 `Bar = 9`
 
-`Next ID = 66`
+`Next ID = 67`
 
 | Goal | W |
 |---|---|
@@ -29,6 +29,7 @@
 | 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 |  | **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 |
@@ -101,6 +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