@@ -913,16 +913,29 @@ jobs:
       # hash in one namespace (turbo.json `globalEnv`).
       #
       # ⛔ EXCEPT a shard carrying a package in DTS_READERS, whose tests compile
-      # against BUILT declarations and go red without them (TS7016). Measured
-      # on 830a63a, the whole Test Core list under `OS_SKIP_DTS=1`: red only in
+      # against BUILT declarations and go red without them (TS7016, or ENOENT
+      # on a `dist/*.d.ts`). Measured on 830a63a, the whole Test Core list under
+      # `OS_SKIP_DTS=1`, re-measured for the CLI on 43731031 (#25870): red only in
       #   @objectstack/cli       published-subpath-console/-hook-body.pin (its
       #                          own dist/*.d.ts), scaffold-emission-typechecks,
-      #                          scaffold-test-script-passes (spec's .d.ts)
+      #                          scaffold-test-script-passes,
+      #                          create-in-repo-scaffold-typechecks (spec's .d.ts)
       #   @objectstack/metadata  typescript-serializer-annotation (spec's .d.ts)
-      # A new test that reads a built `.d.ts` reds the same loud way on a
-      # skipping shard: add its package here. Every other package's counts
-      # matched the DTS-on run but one: spec's root-entry-type-nameability
-      # pin skips (by its own design) and runs in lint.yml's built-dist step.
+      # Every other package's counts matched the DTS-on run but one: spec's
+      # root-entry-type-nameability pin skips (by its own design) and runs in
+      # lint.yml's built-dist step.
+      #
+      # ⛔ A SLICED reader keeps declarations on ONE slice, DTS_READER_SLICE
+      # (#25870). The CLI's vitest config pins its reader files into slice 1/n
+      # (DECLARATION_READER_FILES, packages/cli/vitest.config.ts), so its other
+      # slices read no `.d.ts` and skip them like any other shard; the
+      # partitioner charges slice 1/n the closure and puts the whole readers
+      # here on its shard (DECLARATION_READER_SLICES / DECLARATION_READER_PACKAGES,
+      # scripts/partition-test-shards.mjs, whose self-test reads this step back).
+      # A whole reader, or the CLI unsliced, keeps them as before. A new test
+      # that reads a built `.d.ts` reds the same loud way on a skipping shard:
+      # add its package here, or, for the CLI, its file to
+      # DECLARATION_READER_FILES.
       #
       # Local cold closure, 65 packages, 4 cores, `--force`: 5m59s with
       # declarations, 1m24s without. The cost is replay: a skipping shard
@@ -931,10 +944,14 @@ jobs:
       - name: Choose this shard's declaration build
         run: |
           DTS_READERS='@objectstack/cli @objectstack/metadata'
+          DTS_READER_SLICE='1'
           KEEP=""
-          while read -r PKG _; do
+          while read -r PKG SLICE; do
             for R in $DTS_READERS; do
-              [ "$PKG" = "$R" ] && KEEP="$KEEP $PKG"
+              [ "$PKG" = "$R" ] || continue
+              case "$SLICE" in
+                ''|"$DTS_READER_SLICE"/*) KEEP="$KEEP $PKG${SLICE:+ $SLICE}" ;;
+              esac
             done
           done < "$RUNNER_TEMP/shard-packages.txt"
           if [ -n "$KEEP" ]; then
 
@@ -643,7 +643,8 @@
 // `node_modules` exclusion: an exact-path list matches nothing it does not name.
 import { defineConfig } from 'vitest/config';
 import path from 'path';
-import { parseCLI, type TestUserConfig } from 'vitest/node';
+import { createHash } from 'node:crypto';
+import { BaseSequencer, parseCLI, type TestSpecification, type TestUserConfig } from 'vitest/node';
 import {
   runFilterPreflight,
   runProjectCliOverridePreflight,
@@ -740,6 +741,103 @@ runProjectCliOverridePreflight({
 // lives at its use site in the `test` block below, not in a helper binding
 // that could outlive the spread.
 
+// #25870 — THE FILES THAT READ BUILT DECLARATIONS RUN IN SLICE 1/n, AND ONLY
+// THERE. Test Core builds a shard's dependency closure without `.d.ts`
+// (`OS_SKIP_DTS=1`, ci.yml "Choose this shard's declaration build") unless the
+// shard tests a package whose tests compile against BUILT declarations. In this
+// package exactly the files in `DECLARATION_READER_FILES` do: each runs `tsc`
+// over a packed tarball or a scaffolded project that resolves
+// `@objectstack/spec` (or this package) through `dist/*.d.ts`, and on a
+// declaration-less closure each reds with TS7016 or ENOENT. Measured, never
+// recalled: build the closure with `OS_SKIP_DTS=1`, run the whole suite, and
+// the red files are this list; with declarations they are green (the PR that
+// added this section carries both readings).
+//
+// vitest's own `shard()` scatters files by the sha1 of their path, so those
+// files sat in all four CLI slices and every slice shard paid the ~4.5 min
+// declaration closure for one or two files. `DeclarationReaderSequencer` puts
+// them first, then every other file in vitest's own sha1 order, and cuts that
+// one sequence into vitest's own equal-count ranges, so the readers take the
+// first places of slice 1/n and each other file moves at most a few places
+// along vitest's order. ci.yml then keeps declarations for slice 1/n only, and
+// `scripts/partition-test-shards.mjs` (`DECLARATION_READER_SLICES`) charges
+// that slice the closure so its shard carries less whole-package work. The
+// index is spelled in those three places; the partitioner's self-test reads the
+// other two back.
+//
+// ⛔ Checked at config load, both directions of drift:
+//   - a listed file that is not in this run's queue population (renamed,
+//     deleted, or moved to a nightly tier) refuses to load the config, naming
+//     it -- a pin on a file that no longer runs would keep slice 1 holding the
+//     closure for nothing;
+//   - a NEW reader that is not listed lands wherever sha1 puts it. On slices
+//     2..n it reds its own PR, loudly and deterministically, with TS7016 or
+//     ENOENT on a declaration-less closure: add it here. On slice 1 it passes
+//     until a later file moves the cut past it, and then reds the same way.
+const DECLARATION_READER_SLICE = 1;
+export const DECLARATION_READER_FILES = Object.freeze([
+  'test/create-in-repo-scaffold-typechecks.test.ts',
+  'test/published-subpath-console.pin.test.ts',
+  'test/published-subpath-hook-body.pin.test.ts',
+  'test/scaffold-emission-typechecks.test.ts',
+  'test/scaffold-test-script-passes.test.ts',
+]);
+
+// The run's queue population holds every listed reader; a nightly-tier run
+// holds none of them, and the pin is inert there (its closure builds
+// declarations on every shard).
+if (readTierMode() === 'queue') {
+  const population = new Set([...UNIT_FILES, ...INTEGRATION_FILES]);
+  const missing = DECLARATION_READER_FILES.filter((file) => !population.has(file));
+  if (missing.length > 0) {
+    throw new Error(
+      `packages/cli/vitest.config.ts: DECLARATION_READER_FILES names ${missing.join(', ')}, which this ` +
+        'package\'s queue test population does not hold. Remove or rename the entry: a pinned reader that ' +
+        'never runs keeps slice 1/n building declarations for nothing.'
+    );
+  }
+}
+
+// vitest's equal-count range for slice `index` of `count` over `total` files
+// (`BaseSequencer.calculateShardRange`, which vitest declares private).
+function shardRange(total: number, index: number, count: number): [number, number] {
+  const base = Math.floor(total / count);
+  const remainder = total % count;
+  if (remainder >= index) return [(base + 1) * (index - 1), (base + 1) * index];
+  const start = remainder * (base + 1) + (index - remainder - 1) * base;
+  return [start, start + base];
+}
+
+const READER_SET = new Set<string>(DECLARATION_READER_FILES);
+
+class DeclarationReaderSequencer extends BaseSequencer {
+  override async shard(files: TestSpecification[]): Promise<TestSpecification[]> {
+    const { root, shard } = this.ctx.config;
+    if (!shard) return files;
+    const { index, count } = shard;
+    const relative = (spec: TestSpecification) =>
+      path.relative(root, spec.moduleId).split(path.sep).join('/');
+    // vitest's own key: the path below the root, with its leading separator.
+    const key = (spec: TestSpecification) => createHash('sha1').update(`/${relative(spec)}`).digest('hex');
+    const readers = files.filter((spec) => READER_SET.has(relative(spec)));
+    const rest = files
+      .filter((spec) => !READER_SET.has(relative(spec)))
+      .map((spec) => ({ spec, hash: key(spec) }))
+      .sort((a, b) => (a.hash < b.hash ? -1 : a.hash > b.hash ? 1 : 0))
+      .map(({ spec }) => spec);
+    const ordered = [...readers.sort((a, b) => (relative(a) < relative(b) ? -1 : 1)), ...rest];
+    const [start, end] = shardRange(ordered.length, DECLARATION_READER_SLICE, count);
+    if (readers.length > end - start) {
+      throw new Error(
+        `packages/cli/vitest.config.ts: ${readers.length} declaration readers do not fit in slice ` +
+          `${DECLARATION_READER_SLICE}/${count}, which holds ${end - start} files. Pin fewer files, or slice fewer ways.`
+      );
+    }
+    const [from, to] = shardRange(ordered.length, index, count);
+    return ordered.slice(from, to);
+  }
+}
+
 // #23181 slice 14 — THE OCLIF MANIFEST SETUP. One `globalSetup` module writes
 // `oclif.manifest.json` from source when the `integration` project runs (the
 // tier every CLI-spawning file is derived into) and removes it at teardown, so
@@ -869,6 +967,10 @@ export default defineConfig({
     // The file-level slice, when Test Core runs one (#19278) — see the section
     // above `export default` for why it is spread and typed this way.
     ...({ shard: process.env.OS_TEST_SHARD } satisfies Pick<TestUserConfig, 'shard'>),
+    // The declaration readers into slice 1/n (#25870) -- see the section of
+    // that number above `export default`. Read by the root context only, which
+    // is the one that shards.
+    sequence: { sequencer: DeclarationReaderSequencer },
     // A late console.* must not redden a green suite (#10374): vitest's worker
     // forwards console output over RPC and discards the promise, and a write
     // landing after teardown's rpcDone() snapshot is rejected into an unhandled