@@ -157,6 +157,14 @@ const TIMINGS_PATH = path.join(REPO_ROOT, 'scripts', 'test-shard-timings.json');
 // beside them. That last figure is the floor this count cannot move: a CLI
 // slice (~960-1010s measured) is serial, so past eight the slowest shard is
 // the slice and more shards only add runners.
+//
+// ⚠ The bins above were placed by their SUM. Since #25870 partition() places
+// them by that same wall model (WHOLE_LEG_PARALLELISM), so a slice shard no
+// longer carries a whole-package leg ahead of its slice; on the committed
+// dataset's full list the predicted steps move from 1277s on the three slice
+// shards (643s elsewhere) to 966s on the slice shards and 830s elsewhere. On
+// the 2026-10-26 refresh, with the CLI cut into 4 slices (pin 3c, below), all
+// eight read ~738s.
 export const SHARD_COUNT = 8;
 
 // The acceptance bound #10472 set: the slowest shard within ~1.3x the mean.
@@ -354,10 +362,23 @@ export function timingModeNote(event) {
 //   n = 2    869.44s   1.52x                  still the floor
 //   n = 3    579.63s   1.01x                  meets -- spec's task is the floor
 //
-// So n = 3, and pin 3c holds it there. Slices are placed at their PLAIN weight
-// (partition() is unchanged), so every bin keeps the density the whole-package
+// So n = 3, and pin 3c held it there. Slices were placed at their PLAIN weight
+// (partition() unchanged), so every bin kept the density the whole-package
 // split had -- see densityCap() for the hard limit on that and its pin.
 //
+// RE-DERIVED AT 4 (#25870), on the 14-run refresh of 2026-10-26 (newest run
+// 40272142430): the CLI at 2697.45s. `@objectstack/spec` (1168.98s) and
+// `@objectstack/runtime` (793.70s) each run as two tasks, so the heaviest
+// other serial task is `@objectstack/metadata-protocol`'s 670.12s, and a slice
+// must sit within 1.3 x 670.12 = 871.16s.
+//
+//   n = 3    899.15s   1.34x                  still the floor -- pin 3c refused 3
+//   n = 4    674.36s   1.01x                  meets -- metadata-protocol is the floor
+//
+// Since #25870 partition() places a slice at its wall share rather than its
+// plain weight (WHOLE_LEG_PARALLELISM); `total`, and so the density cap, still
+// reads plain sums.
+//
 // ⛔ THE COUNT IS CONFIGURED; WHETHER A RUN SLICES IS DECIDED BY THAT RUN. A
 // pull_request or merge_group run splits its own affected set, so planShards()
 // asks, of the run's own list, whether a configured package is that run's
@@ -378,7 +399,7 @@ export function timingModeNote(event) {
 // a change to what that generator can DECODE, which is why the map it replaced
 // is kept below.
 export const FILE_SHARDED_PACKAGES = Object.freeze({
-  '@objectstack/cli': 3,
+  '@objectstack/cli': 4,
 });
 
 // THE MAP AS IT STOOD BEFORE ITS LAST CHANGE, read only by the generator's
@@ -406,8 +427,10 @@ export const FILE_SHARDED_PACKAGES = Object.freeze({
 // `test-core-run-summary-*`, #25771), so four days after that PR lands no
 // retained summary predates the change and this map decodes nothing.
 // The outgoing map of #22075's third round was empty -- the runs before it
-// carried no slice -- so this one is empty too.
-export const PREVIOUS_FILE_SHARDED_PACKAGES = Object.freeze({});
+// carried no slice. #25870 raised the CLI from 3 slices to 4, so this holds 3.
+export const PREVIOUS_FILE_SHARDED_PACKAGES = Object.freeze({
+  '@objectstack/cli': 3,
+});
 
 // The item grammar. A shard item is a package (`@objectstack/cli`) or a SLICE
 // of one (`@objectstack/cli 1/2`), and this pair of functions is the only place
@@ -882,21 +905,58 @@ function readManifest(dir) {
   }
 }
 
-// LPT greedy: heaviest package into the currently lightest bin. Deterministic:
-// input order never matters because both the package sort and the bin choice
-// break ties explicitly (by name / by lowest bin index).
+// ── A SLICE IS A SERIAL LEG: BINS ARE PLACED BY WALL, NOT BY SUM (#25870) ────
+//
+// A shard's "Run this shard's tests" step runs its whole packages four at a
+// time (`--concurrency=4`) and THEN each file-level slice as a turbo run of its
+// own, alone on the runner (ci.yml). So a slice's seconds reach the wall one for
+// one while a whole package's are shared about 2.9 ways -- the wall model the
+// SHARD_COUNT header calibrated (#25376): step ~ wholeSum / 2.9 + slice. This
+// constant is that 2.9, not a new measurement.
+//
+// Placing bins by their SUM of seconds treated a slice second like a whole
+// second, so every slice shard was filled up to the mean with whole packages and
+// then ran its slice after them. On the committed dataset's full list that put
+// ~900s of whole packages in front of each 966s CLI slice: predicted step 1277s
+// on the three slice shards against 643s on the other five, and measured the
+// same way on the pull_request runs #25870 tabulates -- 3/8-5/8 at 25-29 min,
+// the rest at 7-17, the gap being each slice shard's whole-package leg
+// (`Run this shard's tests` minus the slice leg's own turbo `Time:`).
+//
+// So partition() places by LOAD -- a bin's predicted wall, kept in whole-package
+// seconds (a slice counts WHOLE_LEG_PARALLELISM times its weight) so that a list
+// with no slice is placed by exactly the same additions and comparisons as the
+// plain-sum LPT it replaced, byte for byte. `total` stays the plain sum of
+// predicted test windows: the density cap and the drift report are statements
+// about that quantity, and neither changes meaning here.
+export const WHOLE_LEG_PARALLELISM = 2.9;
+
+// One item's share of a bin's load, in whole-package seconds.
+export function loadOf(item) {
+  return item.slice ? item.weight * WHOLE_LEG_PARALLELISM : item.weight;
+}
+
+// A bin's predicted `Run this shard's tests` step under the wall model.
+export function wallOf(bin) {
+  return bin.load / WHOLE_LEG_PARALLELISM;
+}
+
+// LPT greedy on load (above): heaviest item into the currently lightest bin.
+// Deterministic: input order never matters because both the item sort and the
+// bin choice break ties explicitly (by name / by lowest bin index).
 export function partition(items, shardCount) {
   const sorted = [...items].sort(
-    (a, b) => b.weight - a.weight || a.name.localeCompare(b.name, 'en')
+    (a, b) => loadOf(b) - loadOf(a) || a.name.localeCompare(b.name, 'en')
   );
-  const bins = Array.from({ length: shardCount }, () => ({ total: 0, names: [] }));
+  const bins = Array.from({ length: shardCount }, () => ({ total: 0, load: 0, names: [] }));
   for (const it of sorted) {
     let best = 0;
     for (let i = 1; i < bins.length; i++) {
-      if (bins[i].total < bins[best].total) best = i;
+      if (bins[i].load < bins[best].load) best = i;
     }
     bins[best].names.push(it.name);
     bins[best].total += it.weight;
+    bins[best].load += loadOf(it);
   }
   return bins;
 }
@@ -905,13 +965,34 @@ export function partition(items, shardCount) {
 // `floor` is the heaviest single package: sharding is BY PACKAGE, so no split
 // at any shard count can put max below it, and comparing it to the mean says
 // whether the bound is even reachable before anyone tries to reach it.
+//
+// `walls` / `wallRatio` are the same statistic on what the bins were placed by
+// (wallOf, #25870). With no slice in the split they are the totals scaled by
+// one constant, so the two ratios agree; with a slice, only the wall ratio says
+// whether the shards finish together. A bin built by hand (no `load`) is read
+// as all whole packages.
 export function balanceOf(bins, items = null) {
   const totals = bins.map((b) => b.total);
   const sum = totals.reduce((a, b) => a + b, 0);
   const mean = sum / bins.length;
   const max = Math.max(...totals);
   const floor = items ? Math.max(0, ...items.map((i) => i.weight)) : null;
-  return { totals, sum, mean, max, min: Math.min(...totals), ratio: mean === 0 ? 1 : max / mean, floor };
+  const walls = bins.map((b) => wallOf({ load: b.load ?? b.total }));
+  const wallMean = walls.reduce((a, b) => a + b, 0) / bins.length;
+  const wallMax = Math.max(...walls);
+  return {
+    totals,
+    sum,
+    mean,
+    max,
+    min: Math.min(...totals),
+    ratio: mean === 0 ? 1 : max / mean,
+    floor,
+    walls,
+    wallMean,
+    wallMax,
+    wallRatio: wallMean === 0 ? 1 : wallMax / wallMean,
+  };
 }
 
 // ── THE DENSITY CAP: NO SHARD CARRIES MORE THAN THE SPLIT PROVEN GREEN (#22075) ─
@@ -971,7 +1052,7 @@ export function refineToCap(bins, items, cap) {
   const weightOf = new Map(items.map((i) => [i.name, i.weight]));
   const slices = new Set(items.filter((i) => i.slice).map((i) => i.name));
   const movable = (name) => !slices.has(name);
-  const out = bins.map((b) => ({ total: b.total, names: [...b.names] }));
+  const out = bins.map((b) => ({ total: b.total, load: b.load, names: [...b.names] }));
   const EPSILON = 1e-9;
   for (let step = 0; step < 10_000; step++) {
     let h = 0;
@@ -998,15 +1079,21 @@ export function refineToCap(bins, items, cap) {
     }
     if (best === null) break;
     const { to, give, take } = best;
+    // Only whole packages move, and a whole package's load is its weight, so
+    // `load` follows `total` move for move.
     out[h].names.splice(out[h].names.indexOf(give), 1);
     out[h].total -= weightOf.get(give);
+    out[h].load -= weightOf.get(give);
     out[to].names.push(give);
     out[to].total += weightOf.get(give);
+    out[to].load += weightOf.get(give);
     if (take !== null) {
       out[to].names.splice(out[to].names.indexOf(take), 1);
       out[to].total -= weightOf.get(take);
+      out[to].load -= weightOf.get(take);
       out[h].names.push(take);
       out[h].total += weightOf.get(take);
+      out[h].load += weightOf.get(take);
     }
   }
   return out;
@@ -1484,7 +1571,9 @@ const SELF_TEST_BATTERIES = Object.freeze({
   // 8 of these 25 are pins 3b and 3c, the file-level slicing's (#16173), and
   // every one of them runs unconditionally: 3c's fixtures and 3b's cut of the
   // heaviest package do not depend on what the live map slices.
-  'the balancing pins (#10472)': 25,
+  // 2 of these 27 are pin 2b, the wall placement's (#25870): a fixture, so it
+  // runs whatever the dataset holds.
+  'the balancing pins (#10472)': 27,
   'predicted-vs-measured drift (#16173)': 9,
   'drift warning tier under the red (#16465)': 12,
   'file-level slice items (#16173)': 20,
@@ -1731,16 +1820,55 @@ function selfTest() {
   });
   const real = datasetPlan.bins;
   const balance = balanceOf(real, datasetItems);
+  // Graded on predicted WALL, the quantity the bins are placed by (#25870): with
+  // a slice in the split, the bin SUMS are deliberately unequal -- a slice shard
+  // carries its serial slice and little else -- and only the walls say whether
+  // the shards finish together. With no slice the two ratios are the same number.
   check(() => {
-    if (balance.ratio > MAX_SHARD_OVER_MEAN) {
+    if (balance.wallRatio > MAX_SHARD_OVER_MEAN) {
       throw new Error(
-        `balance: at ${SHARD_COUNT} shards the heaviest bin is ${balance.ratio.toFixed(2)}x the mean ` +
-          `(${balance.max.toFixed(0)}s vs ${balance.mean.toFixed(0)}s), past the ${MAX_SHARD_OVER_MEAN}x bound. ` +
-          'Bins: ' + balance.totals.map((t) => t.toFixed(0)).join('/') + 's.'
+        `balance: at ${SHARD_COUNT} shards the slowest predicted test step is ${balance.wallRatio.toFixed(2)}x the mean ` +
+          `(${balance.wallMax.toFixed(0)}s vs ${balance.wallMean.toFixed(0)}s), past the ${MAX_SHARD_OVER_MEAN}x bound. ` +
+          'Walls: ' + balance.walls.map((t) => t.toFixed(0)).join('/') + 's; bins: ' +
+          balance.totals.map((t) => t.toFixed(0)).join('/') + 's.'
       );
     }
   });
 
+  // 2b. A SLICE IS PLACED AS THE SERIAL LEG IT IS (#25870). Three shards, a
+  //     package cut in two (100s a slice) and three whole packages (100s, 50s,
+  //     50s). Placed by SUM -- the LPT this replaced -- each slice shard also
+  //     takes a 50s whole package, which runs four-wide BEFORE the slice and
+  //     lengthens that shard's step to ~117s. Placed by wall, the slices run
+  //     alone and the three whole packages share the third shard (~69s). The
+  //     two placements differ, so a revert to the sum reds here.
+  const serial = partition(
+    [
+      { name: 's 1/2', weight: 100, slice: { index: 1, count: 2 } },
+      { name: 's 2/2', weight: 100, slice: { index: 2, count: 2 } },
+      mk('x', 100),
+      mk('y', 50),
+      mk('z', 50),
+    ],
+    3
+  );
+  check(() => {
+    const crowded = serial.filter((bin) => bin.names.some((n) => n.startsWith('s ')) && bin.names.length > 1);
+    if (crowded.length > 0) {
+      throw new Error(
+        `slice placement: a slice shard also carries whole packages (${crowded.map((b) => b.names.join('+')).join('; ')}) ` +
+          'that run before its serial slice -- are the bins placed by their plain SUM again?'
+      );
+    }
+  });
+  check(() => {
+    const walls = serial.map(wallOf);
+    const expected = [100, 100, 200 / WHOLE_LEG_PARALLELISM];
+    if (walls.some((w, i) => Math.abs(w - expected[i]) > 1e-9)) {
+      throw new Error(`slice placement: predicted walls ${walls.map((w) => w.toFixed(2)).join('/')}s, expected ${expected.map((w) => w.toFixed(2)).join('/')}s`);
+    }
+  });
+
   // 3. The bound is REACHABLE at this shard count -- the arithmetic #10472
   //    asked about when it floated 6 -> 8. No split can put the heaviest bin
   //    below the heaviest single package, so once that package exceeds
@@ -3014,7 +3142,8 @@ function selfTest() {
   console.log(
     `partition-test-shards: self-test OK (${datasetPackages.length} measured packages ` +
       `-> ${datasetItems.length} shard items, ${SHARD_COUNT} shards, ` +
-      `max/mean ${balance.ratio.toFixed(2)}x <= ${MAX_SHARD_OVER_MEAN}x, floor ${balance.floor.toFixed(0)}s, ` +
+      `predicted test-step walls ${balance.walls.map((t) => t.toFixed(0)).join('/')}s, ` +
+      `max/mean ${balance.wallRatio.toFixed(2)}x <= ${MAX_SHARD_OVER_MEAN}x, floor ${balance.floor.toFixed(0)}s, ` +
       `bins ${balance.totals.map((t) => t.toFixed(2)).join('/')}s within the ${datasetCap.toFixed(2)}s density cap, ` +
       `file-level slices: ` +
       `${datasetPlan.decisions.filter((d) => d.count > 1).map((d) => `${d.name} x${d.count}`).join(', ') || 'none'})`
@@ -3137,7 +3266,7 @@ function main() {
     { excluded }
   );
   const mine = bins[shardIndex - 1];
-  const { max, mean, ratio } = balanceOf(bins);
+  const { max, mean, ratio, walls, wallRatio } = balanceOf(bins);
   // Printed on every shard, not just the imbalanced one, and printed as the
   // RATIO the acceptance bound is written in: the per-shard numbers alone never
   // said whether the split was balanced, which is why #10472's imbalance had to
@@ -3151,6 +3280,13 @@ function main() {
       `density cap ${cap.toFixed(2)}s; ` +
       `${packages - estimated} measured, ${estimated} estimated from test-file count)`
   );
+  // What the bins were placed by (wallOf, #25870): the test step each shard is
+  // predicted to take, a slice counted as the serial leg it runs as.
+  console.error(
+    `  predicted test-step walls (whole / ${WHOLE_LEG_PARALLELISM} + slice): ` +
+      `${walls.map((w) => w.toFixed(0)).join('/')}s, this shard ${wallOf(mine).toFixed(0)}s; ` +
+      `max/mean ${wallRatio.toFixed(2)}x`
+  );
   // Only a document that states its own slicing prints this, so a Test Core
   // shard's log is unchanged; the nightly's says where its slice count came from.
   if (acrossShards !== null) {