diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index b3804271814..3a3803a59e7 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -1041,8 +1041,24 @@ jobs:
     # simply never created. The literal after `||` is the full roster, read
     # when the plan job published nothing (it died or timed out), and the gate
     # reads this same expression, so the two cannot count different rosters.
+    #
+    # ⭐ FAIL-FAST IN THE MERGE QUEUE ONLY (#31517, ruling record 6730013883).
+    # On a `merge_group` run the first shard to go red cancels its siblings,
+    # so `Test Core` reads red when that shard ends instead of when the
+    # slowest shard ends, and the queue ejects the entry and rebuilds the
+    # entries behind it that much sooner. On every other event the
+    # expression is false and every shard runs to its end: the pull_request
+    # run is where an author reads every red shard in one round (#23177
+    # ruling B, item A1, comment 6139629628), and push and schedule runs stay
+    # complete. The queue is the exception because its job is a fast
+    # mergeable-or-not answer, and the PR run of the same diff already
+    # showed every red. It is the matrix's own `fail-fast`, ⛔ never a
+    # run-level cancel: a real red dominates the matrix result (`failure`,
+    # never `cancelled`, measured on run 30271824408, a fail-fast matrix),
+    # and `Test Core` reads `failure` as a veto, whereas a matrix cancelled
+    # with zero attestations passes its aggregate (#3668).
     strategy:
-      fail-fast: false
+      fail-fast: ${{ github.event_name == 'merge_group' }}
       matrix:
         shard: ${{ fromJSON(needs.test-plan.outputs.shards || '[1, 2, 3, 4, 5, 6, 7, 8]') }}
 
@@ -1580,6 +1596,11 @@ jobs:
           # non-zero exit, and the step exits with it, so the shard is red
           # exactly when it was before — and now names every red package of the
           # shard in one run instead of the first one turbo reached.
+          # A1 holds inside a shard on every event. Across shards the merge
+          # queue is the exception (#31517, ruling record 6730013883): there
+          # the matrix is fail-fast, so the first red shard cancels the
+          # others, and the queue run names that shard's reds only. The
+          # pull_request run of the same diff names every shard's.
           #
           # Measured before this was written: a three-package probe (turbo
           # 2.11.5, `a#test` exits 1, `c` depends on `a`) ran `b` and `c` to
@@ -2716,8 +2737,13 @@ jobs:
     timeout-minutes: 30
     permissions:
       contents: read
+    # Fail-fast on `merge_group` only, as on Test Core (#31517, ruling record
+    # 6730013883; the reasons are on the `test` job's strategy). Push and
+    # schedule, the other events this matrix runs on, run all three shards to
+    # their end. A red shard dominates the matrix result over the siblings it
+    # cancels, and `Dogfood Regression Gate` reads `failure` as a veto.
     strategy:
-      fail-fast: false
+      fail-fast: ${{ github.event_name == 'merge_group' }}
       matrix:
         shard: [1, 2, 3]
 
@@ -3083,7 +3109,11 @@ jobs:
     # 30271824408, a fail-fast matrix with one real failure + one cancelled
     # sibling): a real shard failure DOMINATES the aggregate — it reads
     # "failure", never "cancelled" — so passing `cancelled` masks no
-    # regression. That dominance is what does NOT hold for `abandoned`, which
+    # regression. That run is the merge queue's own shape since #31517: the
+    # dogfood matrix is fail-fast on `merge_group`, and the sibling a red
+    # shard cancels records "The strategy configuration was canceled
+    # because …" on its check run, which the cause read classifies as no
+    # supersession: a partially attested matrix it cut short reads red. That dominance is what does NOT hold for `abandoned`, which
     # is why `abandoned` gets counting rather than a place in a word list.
     name: Dogfood Regression Gate
     needs: [dogfood, dogfood-verify, filter, queue-verdict]
@@ -4147,8 +4177,15 @@ jobs:
     timeout-minutes: 30
     permissions:
       contents: read
+    # Fail-fast on `merge_group` only, as on Test Core (#31517, ruling record
+    # 6730013883; the reasons are on the `test` job's strategy). Every other
+    # event runs all sixteen shards to their end. The `Test UI` aggregate
+    # passes only `success` or a skip `filter` explains, so the matrix of a
+    # red shard and the siblings it cancelled reads red there. A fail-fast queue run is red, so a push never
+    # borrows it: `queue-verdict` takes only a `success` queue run, and
+    # `ui_full_green` counts only shards that concluded `success`.
     strategy:
-      fail-fast: false
+      fail-fast: ${{ github.event_name == 'merge_group' }}
       matrix:
         shard: [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16]
 
@@ -4373,7 +4410,9 @@ jobs:
     # That is #4928's skipped-only-when-filter-succeeded guard, and it makes a
     # queue build or PR that touches no ui path read green here the way Lint
     # UI and Build & E2E UI read `skipped` there. Everything else is red:
-    # `failure`; `cancelled`, which is also what a shard killed by its own
+    # `failure`, which is also what the matrix reads when a red shard on a
+    # `merge_group` run cancelled its siblings by fail-fast (#31517);
+    # `cancelled`, which is also what a shard killed by its own
     # `timeout-minutes` reads (on a superseded push this paints the OLD sha
     # red, which no required check reads); a `skipped` that `filter` does not
     # explain; and any lifecycle value such as the `abandoned` test-gate met
diff --git a/scripts/check-shard-attestation.mjs b/scripts/check-shard-attestation.mjs
index d1614f3cd6f..53a60f370fd 100644
--- a/scripts/check-shard-attestation.mjs
+++ b/scripts/check-shard-attestation.mjs
@@ -80,6 +80,13 @@
  *     `cancelled`. So a superseded matrix masks no regression, and demanding
  *     credentials there would paint the false red on the superseded SHA that
  *     #3668 removed.
+ *     Since #31517 (ruling record 6730013883) the merge queue's `test` and
+ *     `dogfood` matrices are fail-fast, which is run 30271824408's own shape:
+ *     the red shard's `failure` dominates the leg, the leg's declared negative
+ *     vetoes the gate, and the shards the strategy cancelled are missing from
+ *     the count. Such a shard records `The strategy configuration was
+ *     canceled because "…" failed` on its check run, which
+ *     `classifyCancelAnnotations` reads as `other`, never a supersession.
  *     ⛔ That argument covers a shard that RAN AND FAILED. It does not cover a
  *     shard killed while still executing, which produces no verdict at all —
  *     there is no `failure` to dominate (#16157, run 34007386254: `Test Core
@@ -1740,11 +1747,12 @@ const SELF_TEST_BATTERIES = Object.freeze({
   '#26084 a planned roster: only the shards the plan started are counted': 16,
   '#26084 the static guard holds the planned matrix and the gate to one expression': 10,
   '#31519 a borrowed push: the gate skip and the seed-only credential guard': 8,
+  '#31517 merge-queue fail-fast: one shard red, the rest cancelled by the strategy': 8,
 });
 
 // DELETING an entry silences that battery's floor exactly as effectively as
 // zeroing it, so the roster's own size is pinned too.
-const SELF_TEST_BATTERY_FLOOR = 25;
+const SELF_TEST_BATTERY_FLOOR = 26;
 
 // The key an assertion is filed under when no battery is open. It is not a
 // declared battery, so it reds by the same set difference rather than silently
@@ -2293,6 +2301,40 @@ async function selfTest() {
     "#22653 control: the single-leg path keeps #22606's verdict and words — a killed CLI leg is red beside a superseded matrix shard",
   );
 
+  // ── #31517: the merge queue's matrices are fail-fast ─────────────────────
+  // On `merge_group` the `test` and `dogfood` matrices stop their remaining
+  // shards once one is red (ruling record 6730013883). The shape is run
+  // 30271824408's: the red shard concludes `failure`, the shards the strategy
+  // cancelled conclude `cancelled` and attest nothing, and the matrix result
+  // reads `failure`. The annotation below is that run's cancelled shard (check
+  // run 89995981782), read from the check-run annotations endpoint.
+  battery('#31517 merge-queue fail-fast: one shard red, the rest cancelled by the strategy');
+  const FAIL_FAST_ANNOTATIONS = [annotation('The strategy configuration was canceled because "matrix._1" failed'), annotation('The operation was canceled.')];
+  const failFast = (ids) => testCore(ids, undefined, { result: 'failure' });
+  const fastShardsFirst = failFast([1, 3].map((n) => attest('test', n, 6)));
+  assert(!fastShardsFirst.ok, '#31517: shard 2 red, 1 and 3 attested before it, 4-6 cancelled by fail-fast ⇒ red');
+  assert(
+    fastShardsFirst.errors.some((e) => e.includes("reported result 'failure'")) &&
+      fastShardsFirst.errors.some((e) => [2, 4, 5, 6].every((n) => e.includes(`test-${n}-of-6`))),
+    '#31517: …by the declared negative AND by the count, which names the red shard and every shard it cancelled',
+  );
+  assert(!failFast([]).ok, '#31517: the first shard red before any sibling attested, all cancelled ⇒ red — `failure` is never the zero-attestation #3668 pass');
+  assert(
+    shardsNeedingCancelCause(testLegs('failure'), new Map([1, 3].map((n) => attest('test', n, 6)))).length === 0,
+    '#31517: a fail-fast matrix reads `failure`, so the gate asks no cancel cause — no API read can turn it green',
+  );
+  assert(classifyCancelAnnotations(FAIL_FAST_ANNOTATIONS).kind === 'other', "#31517: a shard cancelled by its matrix's fail-fast is never read as a supersession");
+  const ifItEverReadCancelled = testCore(only5, new Map([1, 2, 3, 4, 6].map((n) => [attestationId('test', n, 6), classifyCancelAnnotations(FAIL_FAST_ANNOTATIONS)])));
+  assert(
+    !ifItEverReadCancelled.ok && ifItEverReadCancelled.errors.some((e) => e.includes('The strategy configuration was canceled')),
+    '#31517: were the matrix ever to read `cancelled` after a sibling attested, the fail-fast cause still reds it, quoting the record',
+  );
+  assert(
+    !judge({ gate: 'Test Core', legs: [{ job: 'test', total: 8, result: 'failure', shards: [2, 5, 7], planResult: 'success' }], filterResult: 'success', present: new Map([attest('test', 5, 8)]), runId: '99' }).ok,
+    '#31517 × #26084: a planned roster cut short by fail-fast ⇒ red',
+  );
+  assert(!dogfood('failure', 'success', [fullDogfood[0], fullDogfood[3]]).ok, '#31517: dogfood 2/3 red, 3/3 cancelled by fail-fast, the CLI leg green ⇒ the Dogfood Regression Gate reads red');
+
   // ── #22653: the per-shard cause read itself, offline ─────────────────────
   battery('#22653 the cause read, offline: one jobs list, then one annotations read per unattested shard');
   const CI_TEXT = readFileSync(join(scriptRepoRoot(), '.github', 'workflows', 'ci.yml'), 'utf8');
diff --git a/scripts/ci-shard-matrix.mjs b/scripts/ci-shard-matrix.mjs
index 48ada86067a..60b87708e1b 100644
--- a/scripts/ci-shard-matrix.mjs
+++ b/scripts/ci-shard-matrix.mjs
@@ -217,7 +217,9 @@ function selfTest() {
     check(`${label} ⇒ refused naming '${needle}' (got ${JSON.stringify(message)})`, message !== null && message.includes(needle) && message.includes(`job '${job}'`));
   };
   const workflow = (body) => `on: push\njobs:\n${body}`;
-  const matrixJob = (id, shardLine, extra = '') => `  ${id}:\n    runs-on: ubuntu-latest\n${extra}    strategy:\n      fail-fast: false\n      matrix:\n        ${shardLine}\n    steps:\n      - run: echo\n`;
+  // The strategy spells ci.yml's sharded jobs' `fail-fast` (merge-queue only, #31517), so the
+  // reader is exercised over an expression line between `strategy:` and `matrix:`.
+  const matrixJob = (id, shardLine, extra = '') => `  ${id}:\n    runs-on: ubuntu-latest\n${extra}    strategy:\n      fail-fast: \${{ github.event_name == 'merge_group' }}\n      matrix:\n        ${shardLine}\n    steps:\n      - run: echo\n`;
 
   battery('the live ci.yml');
   const live = readFileSync(join(REPO_ROOT, CI_WORKFLOW), 'utf8');