diff --git a/.github/workflows/stickydisk-gc.yml b/.github/workflows/stickydisk-gc.yml
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+name: Sticky Disk GC
+
+# node_modules sticky disks are keyed on hashFiles('bun.lock') by design — see the
+# "Mount node_modules" comment in test-build.yml. A sticky disk is a mutable volume
+# and `bun install --frozen-lockfile` adds what the lockfile needs without pruning
+# what it dropped, so branches on different lockfiles must not share one. That
+# design is correct and is preserved here; its cost is a new 6-16 GB disk per
+# lockfile bump, created at ~4.7/day.
+#
+# Blacksmith already evicts any sticky disk after 7 days of inactivity, so this is
+# not a leak — it is a retention window that is far too generous for this key. The
+# median disk is only USED for ~0.16 days and then billed for another 7, so the
+# retention tail is almost the entire cost. This collects at 3 days instead.
+#
+# Age-based on purpose. The key contains NO PR identifier, so every open PR whose
+# checkout has the same bun.lock mounts the SAME disk — with ~180 open PRs sharing
+# on the order of 20 distinct lockfile hashes, the shared case is the common case.
+# Deleting on PR close would therefore destroy a disk that dozens of other open PRs
+# are actively using. Never add a pull_request or pull_request_target trigger here.
+
+on:
+  schedule:
+    - cron: '17 9 * * *'
+  workflow_dispatch:
+    inputs:
+      retention_days:
+        description: Delete node_modules disks unused for more than this many days.
+        required: false
+        default: '3'
+      dry_run:
+        description: List what would be deleted without deleting it.
+        type: boolean
+        required: false
+        default: false
+
+# Nothing in this job reads the repository.
+permissions: {}
+
+concurrency:
+  group: stickydisk-gc
+  cancel-in-progress: false
+
+jobs:
+  gc:
+    name: Reclaim idle node_modules sticky disks
+    # GitHub-hosted on purpose, not a Blacksmith runner: the CLI is a pure API
+    # client, and GC has to keep working during a Blacksmith outage or a
+    # CI_PROVIDER break-glass switch — exactly when disks sit idle and still bill.
+    runs-on: ubuntu-latest
+    timeout-minutes: 10
+    env:
+      # Pinned binary + checksum rather than `curl https://get.blacksmith.sh | sh`:
+      # this job holds an org-wide token, so it must not execute unpinned remote
+      # shell. Bump both values together; the vendor publishes a .sha256 sidecar
+      # next to the binary to check against.
+      BLACKSMITH_CLI_VERSION: v0.4.57
+      BLACKSMITH_CLI_SHA256: 7f60f3b9f8d4d7644d9743f5d962acb3b3dbf675f51676702e5f292e02060bca
+      # The CLI self-updates in the background on every invocation, which would
+      # silently defeat the pin above.
+      BLACKSMITH_DISABLE_AUTO_UPDATE: '1'
+      BLACKSMITH_ORG: simstudioai
+      TARGET_REPO: ${{ github.repository }}
+      RETENTION_DAYS: ${{ inputs.retention_days || '3' }}
+      DRY_RUN: ${{ inputs.dry_run || 'false' }}
+
+    steps:
+      - name: Install Blacksmith CLI
+        run: |
+          set -euo pipefail
+          url="https://clireleases.blacksmith.sh/cli/${BLACKSMITH_CLI_VERSION}/linux/amd64/blacksmith"
+          curl -fsSL "$url" -o /usr/local/bin/blacksmith
+          echo "${BLACKSMITH_CLI_SHA256}  /usr/local/bin/blacksmith" | sha256sum -c -
+          chmod +x /usr/local/bin/blacksmith
+
+      - name: Authenticate
+        env:
+          BLACKSMITH_CLI_TOKEN: ${{ secrets.BLACKSMITH_CLI_TOKEN }}
+        run: |
+          set -euo pipefail
+          printf '%s' "$BLACKSMITH_CLI_TOKEN" \
+            | blacksmith auth login --api-token - --non-interactive --organization "$BLACKSMITH_ORG"
+
+      - name: Delete node_modules disks idle beyond the retention window
+        run: |
+          set -euo pipefail
+
+          blacksmith stickydisk list \
+            --repo "$TARGET_REPO" \
+            --search '-node-modules-' \
+            --per-page 100 \
+            --format json > disks.json
+
+          # Two independent guards, because the blast radius of a wrong key is a
+          # cache every CI job depends on:
+          #   1. --search narrows server-side to the node_modules family.
+          #   2. The regex re-proves each key's full shape locally. The event
+          #      segment is [a-z_]+ rather than an enumerated list — the key
+          #      interpolates ${{ github.event_name }}, and a workflow_dispatch
+          #      disk already exists that an enumerated push|pull_request would
+          #      have silently skipped forever.
+          # Verified against the live account: this matches all 39 node_modules
+          # disks and none of the 19 bun/turbo/Docker disks, which are mounted
+          # every run, never idle, and must survive.
+          #
+          # Grouped by key before the staleness test because `delete` without
+          # --arch removes every architecture variant, so a key may only go when
+          # its NEWEST variant is stale.
+          jq -r --arg repo "$TARGET_REPO" --argjson days "$RETENTION_DAYS" '
+            (now - ($days * 86400)) as $cutoff
+            | .entries
+            | map(select(.type == "stickydisk"))
+            | map(select(.key | test("^" + ($repo | gsub("/"; "\\/")) + "-node-modules-[a-z_]+-[0-9a-f]{64}$")))
+            | group_by(.key)
+            | map({
+                key: .[0].key,
+                gb: (map(.size_bytes) | add / 1000000000 * 100 | round / 100),
+                last_used: (map(.last_used_at | sub("\\.[0-9]+Z$"; "Z") | fromdateiso8601) | max)
+              })
+            | map(select(.last_used < $cutoff))
+            | .[] | "\(.key)\t\(.gb)"
+          ' disks.json > stale.tsv
+
+          count=$(wc -l < stale.tsv | tr -d ' ')
+          reclaimed=$(awk -F'\t' '{s+=$2} END {printf "%.1f", s+0}' stale.tsv)
+          {
+            echo "### Sticky disk GC"
+            echo "Retention: ${RETENTION_DAYS}d · dry run: ${DRY_RUN} · candidates: ${count} (${reclaimed} GB)"
+          } >> "$GITHUB_STEP_SUMMARY"
+
+          failed=0
+          while IFS=$'\t' read -r key gb; do
+            [ -n "$key" ] || continue
+            if [ "$DRY_RUN" = "true" ]; then
+              echo "- would delete \`${key}\` (${gb} GB)" >> "$GITHUB_STEP_SUMMARY"
+              continue
+            fi
+            if blacksmith stickydisk delete --repo "$TARGET_REPO" --key "$key" --yes; then
+              echo "- deleted \`${key}\` (${gb} GB)" >> "$GITHUB_STEP_SUMMARY"
+            else
+              echo "- FAILED \`${key}\`" >> "$GITHUB_STEP_SUMMARY"
+              failed=1
+            fi
+          done < stale.tsv
+
+          # Fail loudly rather than continue-on-error: a revoked token or a changed
+          # CLI JSON shape would otherwise silently revert us to 7-day billing.
+          exit "$failed"