@@ -137,18 +137,24 @@ jobs:
       contents: read
       pull-requests: read
     outputs:
-      # On merge_group — and on the `schedule` / `workflow_dispatch` full runs
-      # (#16467) — everything counts as changed: dorny/paths-filter resolves a
-      # diff only on `pull_request` and `push`, and each of these three events
-      # is a place a skipped job can never be the right answer (the queue build
-      # is the last validation before main; the hourly run IS the full battery).
-      # A skipped step's output is the empty string (falsy), so `|| 'true'`
-      # supplies their value without touching PR/push behavior.
-      docs: ${{ steps.changes.outputs.docs || 'true' }}
-      core: ${{ steps.changes.outputs.core || 'true' }}
+      # On the `schedule` / `workflow_dispatch` full runs (#16467) everything
+      # counts as changed: dorny/paths-filter is skipped there (no diff to
+      # resolve), a skipped step's output is the empty string (falsy), and
+      # `|| 'true'` supplies the value without touching PR/push behavior.
+      #
+      # On merge_group the step RUNS (the diff is the queue base_sha against
+      # the group head, see the step below), but only the three ui outputs
+      # read it. `docs`, `core` and `crosspkg` stay 'true' there through the
+      # `github.event_name != 'merge_group' &&` guard: the queue build is the
+      # last validation before main, and those jobs run on every entry, as
+      # they did before the step ran on this event. On any other event the
+      # guard is true and the expression reads exactly as `<output> ||
+      # 'true'` did.
+      docs: ${{ github.event_name != 'merge_group' && steps.changes.outputs.docs || 'true' }}
+      core: ${{ github.event_name != 'merge_group' && steps.changes.outputs.core || 'true' }}
       # Gates the `test` job ONLY, ORed with `core` — see the `crosspkg`
       # filter below and the `test` job's `if:` (#9829, #10015).
-      crosspkg: ${{ steps.changes.outputs.crosspkg || 'true' }}
+      crosspkg: ${{ github.event_name != 'merge_group' && steps.changes.outputs.crosspkg || 'true' }}
       # The objectui tree under `ui/` (objectstack#25677). Three outputs, read
       # together by the four `* UI` jobs at the bottom of this file:
       #   ui            — a path under ui/, or a root file the ui jobs build
@@ -158,13 +164,21 @@ jobs:
       #                   UI) and built (Build & E2E UI) against the new shape in
       #                   the PR that moves it;
       #   ui_tree       — `ui/package.json` exists in this checkout.
+      # On merge_group `ui` and `ui_consumers` come from the queue diff
+      # (objectstack#25867): an entry whose merged diff against
+      # `merge_group.base_sha` touches no ui path and none of the seven
+      # packages skips the four ui jobs, exactly as the same diff does on its
+      # PR head; an entry that touches one runs them. If the step fails (the
+      # base cannot be fetched, the diff errors), it is `continue-on-error`
+      # there, its outputs are empty and `|| 'true'` runs every ui job.
+      #
       # `ui_tree` is what keeps the ui jobs SKIPPED, not failed, on a tree that
       # does not carry ui/ yet: `ui` and `ui_consumers` fall back to 'true' on
-      # merge_group / schedule / dispatch (half 1 below), and on push to main a
-      # spec change sets `ui_consumers`, so neither path output alone can tell a
-      # job there is nothing to build. Its step runs on every event, so the
-      # `|| 'true'` fallback is reached only when this job died before it — and
-      # then THE FILTER CONTRACT's "run everything" is the intended reading.
+      # schedule / dispatch (half 1 below), and on push to main a spec change
+      # sets `ui_consumers`, so neither path output alone can tell a job there
+      # is nothing to build. Its step runs on every event, so the `|| 'true'`
+      # fallback is reached only when this job died before it — and then THE
+      # FILTER CONTRACT's "run everything" is the intended reading.
       ui: ${{ steps.changes.outputs.ui || 'true' }}
       ui_consumers: ${{ steps.changes.outputs.ui_consumers || 'true' }}
       ui_tree: ${{ steps.ui-tree.outputs.present || 'true' }}
@@ -216,13 +230,15 @@ jobs:
             echo 'ui/package.json is absent: every ui job skips on this run.'
           fi
 
-      # ⭐ SPELLED AS AN ALLOW-LIST, not as `!= 'merge_group'` (#16467). This
-      # action reads a diff, and the only two events that hand it one it can
-      # resolve unaided are `pull_request` (base vs head) and `push`
-      # (`event.before` vs `after`). `merge_group` was the first event with
-      # neither, and the exclusion was written as its name; `schedule` and
-      # `workflow_dispatch` are the second and third, and under the old
-      # spelling they would have RUN the action with no resolvable base.
+      # ⭐ SPELLED AS AN ALLOW-LIST, never as a deny-list (#16467). This
+      # action reads a diff, and only three events hand it one it can resolve
+      # unaided: `pull_request` (base vs head), `push` (`event.before` vs
+      # `after`) and `merge_group` (the payload's `base_sha` vs `head_sha`; at
+      # the pinned v4.0.3, src/main.ts reads both when `base` / `ref` are
+      # unset, and src/git.ts fetches the base sha at depth 1 and diffs the two
+      # trees, `base..head`). `schedule` and `workflow_dispatch` hand it
+      # neither, and under a deny-list spelling they would RUN the action with
+      # no resolvable base.
       #
       # That failure direction is the dangerous one: every filter output would
       # come back `false`, `|| 'true'` supplies nothing (the output exists and
@@ -231,9 +247,16 @@ jobs:
       # of nothing at all. An allow-list makes each new event fail toward
       # THE FILTER CONTRACT's half 1 instead: the step is skipped, its outputs
       # are the empty string, `|| 'true'` fills them in, and everything runs.
+      #
+      # `merge_group` joined the list for the ui outputs alone (#25867; the
+      # three core-side outputs above ignore the step on that event). Its
+      # `continue-on-error` is the fail-open half there: a queue build whose
+      # base cannot be fetched reads as "run every ui job", never as a red
+      # `filter` job. On pull_request and push a failure stays red, as before.
       - uses: dorny/paths-filter@ceb8a2b8f2d89434be7ff52d3de7ec3738c5cc9d # v4.0.3
         id: changes
-        if: github.event_name == 'pull_request' || github.event_name == 'push'
+        if: github.event_name == 'pull_request' || github.event_name == 'push' || github.event_name == 'merge_group'
+        continue-on-error: ${{ github.event_name == 'merge_group' }}
         with:
           filters: |
             docs:
 
@@ -42,12 +42,16 @@
  *
  * It does NOT answer the DIFF question — "should this ROSTERED job have run on
  * THIS diff?" — and the boundary is deliberate. For ci.yml's `filter`-gated
- * family the merge-queue build answers it: on `merge_group` every filter output
- * widens to `'true'` and the family runs on the merged tree before anything
- * reaches `main` (the `|| 'true'` half of ci.yml's filter contract, pinned by
- * `--self-test`) — except the full-battery-only jobs (ci.yml's THE TWO SPEEDS),
- * which skip on every PR head and queue build and are answered by `main`'s
- * push and hourly runs instead. For the event- and label-conditional advisory jobs the reason
+ * family the merge-queue build answers it: on `merge_group` the `docs`, `core`
+ * and `crosspkg` outputs widen to `'true'` and those jobs run on the merged
+ * tree before anything reaches `main` (the `|| 'true'` half of ci.yml's filter
+ * contract, pinned by `--self-test`). Two exceptions. The four ui jobs: on
+ * `merge_group` their outputs are the same paths filter read against the
+ * queue's `base_sha`, so a queue build skips them exactly when the merged diff
+ * touches no ui path, and main's hourly run widens them. And the
+ * full-battery-only jobs (ci.yml's THE TWO SPEEDS), which skip on every PR
+ * head and queue build and are answered by `main`'s push and hourly runs
+ * instead. For the event- and label-conditional advisory jobs the reason
  * column names the condition a reader checks by hand. Deriving expectedness
  * live from the workflows' `paths` filters would answer the diff question, at
  * the cost of a SECOND evaluator of the platform's own semantics — the
@@ -184,7 +188,9 @@ export const OWN_PROXY_REARM_GUARD = 'OS_EXPECTED_SKIPS_PROXY_REARMED';
  *
  *   filter-output  ci.yml's `filter` job said `false` for every named output —
  *                  the job's `if:` spells `needs.filter.outputs.<o> != 'false'`
- *                  and the filter widens each output with `|| 'true'` off PRs.
+ *                  and the filter widens each output with `|| 'true'` where
+ *                  its paths filter did not run (the ui outputs read it on
+ *                  merge_group too).
  *   event          the job's `if:` excludes `github.event.action` values, so a
  *                  `labeled` / `unlabeled` / `edited` run of the same head skips.
  *   label          the job's `if:` reads a label literal — an opt-out (skip when
@@ -216,6 +222,8 @@ const UI_GATE = Object.freeze({ kind: 'filter-output', outputs: ['ui_tree', 'ui'
 const UI_CONSUMERS_GATE = Object.freeze({ kind: 'filter-output', outputs: ['ui_tree', 'ui', 'ui_consumers'] });
 const UI_TREE_REASON =
   "gated on ci.yml's `filter` job `ui_tree` output: it skips on every event while `ui/package.json` is absent (before the objectui import), and is not a required context";
+const UI_QUEUE_REASON =
+  "the merge-queue build reads the same paths filter against the queue's `base_sha`, so a queue entry whose merged diff touches none of those paths skips it too; main's hourly run widens it to 'true'";
 const FULL_BATTERY_REASON =
   "full battery only (ci.yml's THE TWO SPEEDS): its `if:` excludes `pull_request` and `merge_group`, so it skips on every PR head and every queue build and runs on main's push, hourly schedule and dispatch runs";
 
@@ -335,28 +343,28 @@ export const EXPECTED_SKIPS = Object.freeze([
     workflow: 'ci.yml',
     job: 'type-check-ui',
     gate: UI_CONSUMERS_GATE,
-    reason: `${UI_TREE_REASON}; with ui/ present it runs on a ui/ path or one of the seven packages objectui consumes (the \`ui_consumers\` output)`,
+    reason: `${UI_TREE_REASON}; with ui/ present it runs on a ui/ path or one of the seven packages objectui consumes (the \`ui_consumers\` output); ${UI_QUEUE_REASON}`,
   },
   {
     name: 'Test UI (${{ matrix.shard }}/16)',
     workflow: 'ci.yml',
     job: 'test-ui',
     gate: UI_GATE,
-    reason: `${UI_TREE_REASON}; with ui/ present it runs on a ui/ path; the raw matrix template is the pre-expansion name`,
+    reason: `${UI_TREE_REASON}; with ui/ present it runs on a ui/ path; ${UI_QUEUE_REASON}; the raw matrix template is the pre-expansion name`,
   },
   {
     name: 'Lint UI',
     workflow: 'ci.yml',
     job: 'lint-ui',
     gate: UI_GATE,
-    reason: `${UI_TREE_REASON}; with ui/ present it runs on a ui/ path`,
+    reason: `${UI_TREE_REASON}; with ui/ present it runs on a ui/ path; ${UI_QUEUE_REASON}`,
   },
   {
     name: 'Build & E2E UI',
     workflow: 'ci.yml',
     job: 'build-e2e-ui',
     gate: UI_CONSUMERS_GATE,
-    reason: `${UI_TREE_REASON}; with ui/ present it runs on a ui/ path or one of the seven packages objectui consumes (the \`ui_consumers\` output)`,
+    reason: `${UI_TREE_REASON}; with ui/ present it runs on a ui/ path or one of the seven packages objectui consumes (the \`ui_consumers\` output); ${UI_QUEUE_REASON}`,
   },
 ]);
 
@@ -373,8 +381,9 @@ export const FAILED_CONCLUSIONS = Object.freeze(['failure', 'cancelled', 'timed_
  * exists in its workflow, carries the row's name, carries an `if:`, and that
  * `if:` spells the gate the row declares. For `filter-output` rows the
  * `filter` job's output must carry the `|| 'true'` widening, because the
- * reason column promises the queue build runs the job — a promise that is only
- * true while that half of the filter contract stands.
+ * reason column promises the job runs whenever the filter could not answer —
+ * the queue build for the core-side outputs, the hourly run for every output —
+ * a promise that is only true while that half of the filter contract stands.
  *
  * @param {readonly object[]} roster
  * @param {(file: string) => object|null} readWorkflow  parsed YAML, or null when unreadable
 
@@ -7502,7 +7502,8 @@ export function selfTest({ tier = 'full' } = {}) {
       'scripts/check-issue-citations.mjs:204 local-env',
       'scripts/cli-build-prerequisite.mjs:111 inherited-population',
       // 131 -> 133 (#22998): the header gained two lines naming the full-battery-only jobs.
-      'scripts/pm/check-expected-skips.mjs:133 self-test-reads',
+      // 133 -> 137 (#25867): the header gained four lines on the ui jobs' queue-diff outputs.
+      'scripts/pm/check-expected-skips.mjs:137 self-test-reads',
       'scripts/pm/dispatch-gates.mjs:736 inherited-population',
     ].join(' · '),
     censusRows.join(' · '),