diff --git a/.github/workflows/ci-cache-cleanup.yml b/.github/workflows/ci-cache-cleanup.yml
new file mode 100644
index 00000000000..d6426b250c5
--- /dev/null
+++ b/.github/workflows/ci-cache-cleanup.yml
@@ -0,0 +1,32 @@
+name: CI Cache Cleanup
+
+# test-build.yml keys the Next.js build cache sticky disk per branch, so every PR
+# leaves a ~5 GB volume behind. Branches are short-lived; the disks aren't. Only
+# the pull_request disks are reclaimed — the push disks belong to main/staging/dev
+# and must stay warm.
+
+on:
+  pull_request:
+    types: [closed]
+
+permissions:
+  contents: read
+
+jobs:
+  delete-nextjs-cache:
+    name: Delete Next.js build cache disk
+    # Sticky disks only exist on Blacksmith; the GitHub break-glass path uses
+    # actions/cache, which expires on its own.
+    if: vars.CI_PROVIDER == '' || vars.CI_PROVIDER == 'blacksmith'
+    runs-on: blacksmith-2vcpu-ubuntu-2404
+    timeout-minutes: 5
+
+    steps:
+      # Must stay byte-identical to the Mount Next.js build cache key in
+      # test-build.yml, or this deletes nothing and the disks accumulate.
+      # Non-blocking: PRs skipped by ci.yml's paths-ignore never made a disk.
+      - name: Delete sticky disk
+        uses: useblacksmith/stickydisk-delete@b41313d28b8647d72114c9ba3c96bb04061562b6 # v1
+        continue-on-error: true
+        with:
+          delete-key: ${{ github.repository }}-nextjs-cache-pull_request${{ github.event.pull_request.head.repo.fork && '-fork' || '' }}-${{ github.head_ref }}
diff --git a/.github/workflows/test-build.yml b/.github/workflows/test-build.yml
index c50e076a317..192337458aa 100644
--- a/.github/workflows/test-build.yml
+++ b/.github/workflows/test-build.yml
@@ -265,14 +265,21 @@ jobs:
 
       # Turbopack's persistent build cache (NEXT_TURBOPACK_BUILD_CACHE below)
       # writes ~5 GB into .next/cache — a sticky disk mounts it in ~1s where an
-      # actions/cache round-trip would eat the warm-build win. Same event/fork
-      # namespacing as the other mounts; the GitHub fallback inside cache-mount
-      # still uses actions/cache with a run_id-suffixed key.
+      # actions/cache round-trip would eat the warm-build win.
+      #
+      # Keyed per branch, not just per event. A sticky disk is one mutable volume
+      # per key: mounting clones the last committed snapshot, job end commits back
+      # last-write-wins. An event-only key had every open PR restoring a cache
+      # built from a different branch — 14.0 min vs 9.3 min for the single-writer
+      # push disk on the same commit. Branch scoping also keeps us off
+      # cross-commit restore, which turbopackFileSystemCacheForBuild (beta) does
+      # not document as supported (vercel/next.js#87283: stale HTML from a cache
+      # built at another commit). ci-cache-cleanup.yml reclaims the disks.
       - name: Mount Next.js build cache
         uses: ./.github/actions/cache-mount
         with:
           provider: ${{ vars.CI_PROVIDER }}
-          key: ${{ github.repository }}-nextjs-cache-${{ github.event_name }}${{ github.event.pull_request.head.repo.fork && '-fork' || '' }}
+          key: ${{ github.repository }}-nextjs-cache-${{ github.event_name }}${{ github.event.pull_request.head.repo.fork && '-fork' || '' }}-${{ github.head_ref || github.ref_name }}
           path: ./apps/sim/.next/cache
 
       # Running out of RAM kills the whole VM and surfaces only as "the runner
@@ -293,6 +300,13 @@ jobs:
       - name: Install dependencies
         run: bun install --frozen-lockfile --ignore-scripts
 
+      # The disk mounts successfully whether or not it carried anything and turbo
+      # buffers the build log, so cache warmth is otherwise unobservable — #5859
+      # shipped a cache that carried almost nothing and it took a PR to notice.
+      # Reported, never gated.
+      - name: Report Next.js cache size (pre-build)
+        run: du -sh apps/sim/.next/cache 2>/dev/null || echo 'cold — no cache restored'
+
       - name: Build application
         env:
           NODE_OPTIONS: '--no-warnings --max-old-space-size=8192'
@@ -304,7 +318,13 @@ jobs:
           AWS_REGION: 'us-west-2'
           ENCRYPTION_KEY: '7cf672e460e430c1fba707575c2b0e2ad5a99dddf9b7b7e3b5646e630861db1c' # dummy key for CI only
           TURBO_CACHE_DIR: .turbo
-          # Opt into Turbopack's persistent build cache (beta) for this CI
-          # check build only — measured 105s cold vs 22s warm locally.
+          # Opt into Turbopack's persistent build cache (beta) for this CI check
+          # build only. #5869's 105s-cold/22s-warm was measured locally and has
+          # never reproduced in CI (compile has ranged 3.5-17.8 min) — local
+          # numbers, not a CI target.
           NEXT_TURBOPACK_BUILD_CACHE: '1'
         run: bunx turbo run build --filter=sim
+
+      - name: Report Next.js cache size (post-build)
+        if: always()
+        run: du -sh apps/sim/.next/cache 2>/dev/null || echo 'no cache written'