diff --git a/.changeset/retire-record-details-section-collapsed.md b/.changeset/retire-record-details-section-collapsed.md
new file mode 100644
index 0000000000..7399c135b4
--- /dev/null
+++ b/.changeset/retire-record-details-section-collapsed.md
@@ -0,0 +1,32 @@
+---
+'@object-ui/types': minor
+---
+
+**BREAKING** — `RecordDetailsComponentProps.sections[]` no longer declares `collapsed`.
+
+**FROM** `collapsed` **TO** `defaultCollapsed`.
+
+```ts
+// before
+sections: [{ label: 'Address', fields: ['street', 'city'], collapsed: true }]
+// after
+sections: [{ label: 'Address', fields: ['street', 'city'], defaultCollapsed: true }]
+```
+
+`@objectstack/spec` `RecordDetailsProps.sections[]` refuses `collapsed` by name
+(`unrecognized_keys`), and `defaultCollapsed` is the spelling it declares for that
+state — the spelling `DetailSection` has always read. So the retired key never
+reached the renderer through a spec parse, and nothing in this repository read it:
+this is a type-face narrowing, not a data or runtime change. There is nothing to
+migrate — rename the key at the authoring site.
+
+`collapsible` is unaffected; it stays.
+
+⚠️ The census behind this removal covers this repository only. A TypeScript
+consumer of `@object-ui/types` outside it that wrote `collapsed` is not
+observable from here, and gets a compile error (TS2353) naming the key — which
+is why the FROM/TO is spelled out above.
+
+Retires the last member of the divergence objectui#8583 measured. The six keys
+this type used to omit are item 1 of the same card; its changeset is still
+pending, so both halves ship in this release.
diff --git a/packages/types/src/__tests__/p1-spec-alignment.test.ts b/packages/types/src/__tests__/p1-spec-alignment.test.ts
index e036072e39..f2e3c7b727 100644
--- a/packages/types/src/__tests__/p1-spec-alignment.test.ts
+++ b/packages/types/src/__tests__/p1-spec-alignment.test.ts
@@ -498,7 +498,7 @@ describe('P1.5 Record Components', () => {
       layout: 'stacked',
       sections: [
         { label: 'Basic Info', fields: ['name', 'email', 'phone'], collapsible: true },
-        { label: 'Address', fields: ['street', 'city', 'state'], collapsed: true },
+        { label: 'Address', fields: ['street', 'city', 'state'], defaultCollapsed: true },
       ],
       fields: ['name', 'email'],
       aria: { ariaLabel: 'Account Details' },
diff --git a/packages/types/src/__tests__/record-details-section-members-8583.test.ts b/packages/types/src/__tests__/record-details-section-members-8583.test.ts
index 0d0f147423..243342fcd4 100644
--- a/packages/types/src/__tests__/record-details-section-members-8583.test.ts
+++ b/packages/types/src/__tests__/record-details-section-members-8583.test.ts
@@ -7,10 +7,18 @@
  */
 
 /**
- * objectui#8583 (item 1) — `RecordDetailsComponentProps.sections[]` declares
- * the six member keys `@objectstack/spec` `RecordDetailsProps.sections[]`
- * declares and this type used to omit: `columns`, `icon`, `description`,
- * `showBorder`, `defaultCollapsed`, `headerColor`.
+ * objectui#8583 — `RecordDetailsComponentProps.sections[]` and the
+ * `@objectstack/spec` `RecordDetailsProps.sections[]` member set, both
+ * directions of the divergence the card measured.
+ *
+ * - **item 1** — this type declares the six keys the spec declares and it used
+ *   to omit: `columns`, `icon`, `description`, `showBorder`,
+ *   `defaultCollapsed`, `headerColor`.
+ * - **item 2** — this type no longer declares `collapsed`, the one key it
+ *   declared that the spec REFUSES by name. Retired outright, no transition
+ *   window (director seat, decision batch #101, applying batch #87 — this repo
+ *   does not declare a key the spec refuses by name — and the 2026-08-27
+ *   retirement-pacing ruling). `defaultCollapsed` is the surviving spelling.
  *
  * ## Why this pin is compile-time, and why a runtime pin alone measures nothing
  *
@@ -143,6 +151,53 @@ const refusedByTsc: RecordDetailsComponentProps = {
   ],
 };
 
+/* ── item 2: `collapsed` is RETIRED, `defaultCollapsed` is the spelling ───── */
+
+/**
+ * The retired key must NOT be a member. The directive is the assertion: re-add
+ * `collapsed?: boolean` to the section entry and `Expect<true>` compiles, the
+ * directive goes UNUSED, and TypeScript reports TS2578 here. That is the
+ * ablation this pin is built to fail on.
+ */
+// @ts-expect-error objectui#8583 (item 2) — `collapsed` is retired; it is not a member of the section entry.
+type _CollapsedIsRetired = Expect<IsSectionMember<'collapsed'>>;
+
+/**
+ * The surviving spelling, asserted with NO directive so it carries the other
+ * half: were `defaultCollapsed` ever dropped too, this line goes red on its own
+ * terms rather than leaving the pin above to pass over an entry that declares
+ * neither collapse key.
+ */
+type _DefaultCollapsedSurvives = Expect<IsSectionMember<'defaultCollapsed'>>;
+
+/**
+ * FROM `collapsed` TO `defaultCollapsed`, at the authoring site an external
+ * consumer actually writes. Excess-property checking on the section literal is
+ * what turns the retirement into a compile error (TS2353 naming the key) rather
+ * than a value that is silently carried and never read.
+ */
+const retiredSpellingRefusedByTsc: RecordDetailsComponentProps = {
+  sections: [
+    {
+      fields: ['phone'],
+      collapsible: true,
+      // @ts-expect-error objectui#8583 (item 2) — the retired spelling does not compile; write `defaultCollapsed`.
+      collapsed: true,
+    },
+  ],
+};
+
+/** The same literal in the surviving spelling — no directive, so it MUST compile. */
+const survivingSpellingAccepted: RecordDetailsComponentProps = {
+  sections: [
+    {
+      fields: ['phone'],
+      collapsible: true,
+      defaultCollapsed: true,
+    },
+  ],
+};
+
 /** Member keys of one `sections[]` entry, read off the INSTALLED spec. */
 const specSectionKeys = (): string[] =>
   listedShapeKeys(arrayElementSchema(resolvePropsShape(RecordDetailsProps)?.sections));
@@ -155,7 +210,7 @@ const specSectionKeys = (): string[] =>
 const refusedKeys = (issues: ReadonlyArray<unknown> | undefined): string[] =>
   (issues ?? []).flatMap((i) => (i as { keys?: string[] }).keys ?? []);
 
-describe('record:details `sections[]` declares the six spec members (objectui#8583, item 1)', () => {
+describe('record:details `sections[]` mirrors the spec member set, both directions (objectui#8583)', () => {
   it('the installed spec still declares all six on the section object', () => {
     // The premise the compile-time pins rest on: were the spec to drop one, the
     // `Equal` above would go red for the right reason, and this names it first.
@@ -215,10 +270,32 @@ describe('record:details `sections[]` declares the six spec members (objectui#85
     ]);
   });
 
+  it('item 2 — the installed spec does not declare `collapsed`, and refuses it BY NAME', () => {
+    // The premise batch #87 rests on, measured rather than recalled, and with
+    // its own control in the same read: a zero here is a reading only because
+    // the surviving spelling is present in the same key list.
+    const keys = specSectionKeys();
+    expect(keys).toContain('defaultCollapsed');
+    expect(keys).not.toContain('collapsed');
+
+    // And the refusal is attributable to the NAME, not to a parser that refuses
+    // everything: the same canonical section parsed clean in control A above.
+    const parsed = RecordDetailsProps.safeParse({
+      sections: [{ ...canonicalSection, collapsed: true }],
+    });
+    expect(parsed.success).toBe(false);
+    expect(parsed.error?.issues.map((i) => i.code)).toContain('unrecognized_keys');
+    expect(refusedKeys(parsed.error?.issues)).toEqual(['collapsed']);
+  });
+
   it('keeps the compile-time literals alive at runtime', () => {
     // Runtime shape is unaffected by the type-level assertions above; these
     // exist so the file also fails visibly if a literal is ever emptied.
     expect(canonicalSection.headerColor).toBe('muted');
     expect(refusedByTsc.sections).toHaveLength(1);
+    // item 2's pair. `collapsed` is read through `Object.keys` because the type
+    // no longer declares it — which is the whole point of the pin above.
+    expect(Object.keys(retiredSpellingRefusedByTsc.sections?.[0] ?? {})).toContain('collapsed');
+    expect(survivingSpellingAccepted.sections?.[0]?.defaultCollapsed).toBe(true);
   });
 });
diff --git a/packages/types/src/record-components.ts b/packages/types/src/record-components.ts
index 6a2617028b..a3c22d5e10 100644
--- a/packages/types/src/record-components.ts
+++ b/packages/types/src/record-components.ts
@@ -117,7 +117,6 @@ export interface RecordDetailsComponentProps {
       | 'secondary/10'
       | 'destructive/10';
     collapsible?: boolean;
-    collapsed?: boolean;
   }>;
   /** Specific fields to display (overrides auto-detection from object) */
   fields?: string[];