diff --git a/.changeset/21279-record-activity-host-feed-guidance.md b/.changeset/21279-record-activity-host-feed-guidance.md
new file mode 100644
index 00000000000..7f048f767c7
--- /dev/null
+++ b/.changeset/21279-record-activity-host-feed-guidance.md
@@ -0,0 +1,12 @@
+---
+'@objectstack/spec': patch
+---
+
+fix(spec): `record:activity`'s props row names `items` / `loading` as the host's feed slot when it refuses them
+
+Clause-②: no
+
+`ComponentPropsMap['record:activity']` (`RecordActivityProps`) refused an authored `properties.items` or `properties.loading` with only the generic "Unrecognized key(s) on this `record:activity`" line. Both are keys the objectui `record:activity` renderer reads, as a feed a host that composes the block in code already owns, so an author copying a TSX composition into a JSON page met no reason for the refusal.
+
+- The refusal now says who reads each key on each mount the row reaches. On a standalone `record:activity`, `items` is the host's data channel and `loading` the host's fetch state for that feed. On a `record:chatter` / `record:discussion` `feed`, which is the same object, nothing reads either. The remedy is the same on both: omit them. The block then presents the record page's discussion feed, and a standalone `record:activity` with no discussion context fetches the record's own `sys_activity` rows. This is the same `guidance` shape `record:history`'s row already uses for `entries` / `loading`.
+- The accept set does not change. Both keys stay refused, through the row and through `record:chatter` / `record:discussion`'s `feed`, which is the same object. Only the message text changes; `record:history` is unchanged.
diff --git a/packages/spec/src/ui/component-record-blocks.test.ts b/packages/spec/src/ui/component-record-blocks.test.ts
index d1eafce555c..1de5765b4f3 100644
--- a/packages/spec/src/ui/component-record-blocks.test.ts
+++ b/packages/spec/src/ui/component-record-blocks.test.ts
@@ -280,6 +280,78 @@ describe('ComponentPropsMap["record:history"] (#8744)', () => {
   });
 });
 
+describe('ComponentPropsMap["record:activity"] — the host feed slot is named, not declared', () => {
+  // `record:history`'s `entries` / `loading` pair, on the activity block: the
+  // standalone renderer takes `items` and `loading` as a host-supplied feed.
+  // Both keys stay REFUSED (the accept set is unchanged); only the refusal
+  // names them. The row is also `record:chatter` / `record:discussion`'s
+  // `feed`, where nothing reads either key, so each message states both mounts.
+  const row = ComponentPropsMap['record:activity'];
+  const refusalOf = (bag: Record<string, unknown>) => {
+    const result = row.safeParse(bag);
+    expect(result.success).toBe(false);
+    const issues = result.error!.issues;
+    expect(issues).toHaveLength(1);
+    return issues[0]!;
+  };
+
+  it('refuses `items` and `loading` at the bag, for every value — the key, not a value domain', () => {
+    for (const [key, values] of [
+      ['items', [[{ id: 'a1', type: 'comment' }], []]],
+      ['loading', [true, false]],
+    ] as const) {
+      for (const value of values) {
+        const issue = refusalOf({ [key]: value });
+        expect(issue.code).toBe('unrecognized_keys');
+        expect(issue.path).toEqual([]);
+        expect((issue as { keys?: string[] }).keys).toEqual([key]);
+      }
+    }
+  });
+
+  it('names `items` as the standalone block\'s host data channel, with the omit remedy', () => {
+    const { message } = refusalOf({ items: [{ id: 'a1', type: 'comment' }] });
+    expect(message).toContain('`items` is not authorable surface. On a standalone `record:activity` it is the HOST\'s data channel');
+    expect(message).toContain('Omit it');
+    // The self-fetch fallback is scoped to the standalone block — the chatter mount has none.
+    expect(message).toContain('a standalone `record:activity` with no discussion context self-fetches the record\'s own `sys_activity` rows');
+  });
+
+  it('names `loading` as the standalone block\'s host fetch state, with the omit remedy', () => {
+    const { message } = refusalOf({ loading: true });
+    expect(message).toContain('`loading` is not authorable surface. On a standalone `record:activity` it is the host\'s fetch state for its `items` feed');
+    expect(message).toContain('Omit it with `items`');
+  });
+
+  it('CONTROL: an unrelated unknown key keeps the generic refusal, with no host-channel line', () => {
+    const issue = refusalOf({ inventedFeedKey: [] });
+    expect((issue as { keys?: string[] }).keys).toEqual(['inventedFeedKey']);
+    expect(issue.message).toContain('Unrecognized key(s) on this `record:activity`');
+    expect(issue.message).not.toContain('data channel');
+    expect(issue.message).not.toContain('fetch state');
+  });
+
+  it('the `record:chatter` / `record:discussion` `feed` (the same object) refuses both keys and says nothing reads them there', () => {
+    // At the `.objectui-sha` pin the chatter renderer reads its rows and its
+    // loading flag off the discussion context and never reads `feed.items` /
+    // `feed.loading`, so the mount-true clause is the one asserted here.
+    for (const type of ['record:chatter', 'record:discussion'] as const) {
+      for (const key of ['items', 'loading'] as const) {
+        const result = ComponentPropsMap[type].safeParse({ feed: { [key]: key === 'items' ? [] : true } });
+        expect(result.success).toBe(false);
+        const issues = result.error!.issues;
+        expect(issues).toHaveLength(1);
+        const issue = issues[0]!;
+        expect(issue.code).toBe('unrecognized_keys');
+        expect(issue.path).toEqual(['feed']);
+        expect((issue as { keys?: string[] }).keys).toEqual([key]);
+        expect(issue.message).toContain('On a `record:chatter` / `record:discussion` `feed` nothing reads it.');
+        expect(issue.message).toContain('Omit it');
+      }
+    }
+  });
+});
+
 describe('the `record:discussion` / `record:chatter` pair (#8744)', () => {
   it('judges both names with one accept face', () => {
     const authored = { position: 'bottom', collapsible: false } as const;
diff --git a/packages/spec/src/ui/component.zod.ts b/packages/spec/src/ui/component.zod.ts
index 03fda207923..81692a690c7 100644
--- a/packages/spec/src/ui/component.zod.ts
+++ b/packages/spec/src/ui/component.zod.ts
@@ -1610,6 +1610,40 @@ export const RecordActivityProps = strictObject({
   surface: 'this `record:activity`',
   history: PROPS_HISTORY,
   guidanceSets: COMPONENT_LEVEL_GUIDANCE,
+  /**
+   * The host feed slot, NAMED rather than declared — `RecordHistoryProps`'
+   * `entries` / `loading` pair, on this block. Measured at the `.objectui-sha`
+   * pin `89cad75d5570` (objectui `plugin-detail/src/renderers/
+   * record-activity.tsx:161-166`, `:265-266`): the renderer takes `items` and
+   * `loading` off the node or out of this bag as a feed a host composing the
+   * block in code already owns, and the host flag wins over every other
+   * loading source. Neither is authorable: an authored feed is a snapshot that
+   * never updates, and an authored `loading: true` pins the loading state on.
+   * So both stay UNDECLARED — the accept set is unchanged — and this table only
+   * makes the refusal name them instead of the generic unrecognized-key line.
+   *
+   * The same object is `record:chatter` / `record:discussion`'s `feed`, so the
+   * entries answer there too. That renderer reads its rows and its loading
+   * flag off the host's discussion context, reads neither `feed.items` nor
+   * `feed.loading`, and never self-fetches (`renderers/record-chatter.tsx`,
+   * same pin). `guidance` is one flat table keyed by key name — it cannot be
+   * scoped to a mount — so each entry says who reads the key on EACH mount,
+   * and every clause is true wherever the refusal is raised.
+   */
+  guidance: {
+    items: '`items` is not authorable surface. On a standalone `record:activity` it is the HOST\'s '
+      + 'data channel: a host composing that block in code passes the feed it already owns, and the '
+      + 'renderer presents it in place of its own sources; hand-authored items would ship a static '
+      + 'snapshot of the feed that never updates. On a `record:chatter` / `record:discussion` `feed` '
+      + 'nothing reads it. Omit it: the block presents the record page\'s discussion feed, and a '
+      + 'standalone `record:activity` with no discussion context self-fetches the record\'s own '
+      + '`sys_activity` rows.',
+    loading: '`loading` is not authorable surface. On a standalone `record:activity` it is the host\'s '
+      + 'fetch state for its `items` feed and wins over the block\'s other loading sources, so '
+      + 'authored `true` pins the loading state on forever. On a `record:chatter` / '
+      + '`record:discussion` `feed` nothing reads it. Omit it with `items`: each block takes its '
+      + 'loading state from its own source.',
+  },
 }, {
   /**
    * Feed/activity kinds to show — an OPEN vocabulary (commit 1a6a19c31, executing the