diff --git a/.github/workflows/guards-nightly.yml b/.github/workflows/guards-nightly.yml
index af5fe6b1d5..c4bf2e699d 100644
--- a/.github/workflows/guards-nightly.yml
+++ b/.github/workflows/guards-nightly.yml
@@ -192,6 +192,8 @@ jobs:
         run: bash scripts/perf_gate.sh --selftest
       - name: Book rust examples compile
         run: setsid --wait bash scripts/check_book_examples_compile.sh
+      - name: Harness fixture servers die with their harness (LAB, quarantined from the merge path, #5873)
+        run: setsid --wait bash scripts/check_harness_server_cleanup.sh
       - name: aprender-profile lib tests (was gated by nothing)
         run: cargo test -p aprender-profile --lib
       - name: model-tests falsification suites (was gated by nothing, aprender#2522) -- SATD ceiling measured on the comparand (BSE-03)
diff --git a/ci/explicit-test-commands.d/817-harness-server-cleanup.cmd b/ci/explicit-test-commands.d/817-harness-server-cleanup.cmd
deleted file mode 100644
index 18ede22b41..0000000000
--- a/ci/explicit-test-commands.d/817-harness-server-cleanup.cmd
+++ /dev/null
@@ -1 +0,0 @@
-bash scripts/check_harness_server_cleanup.sh
diff --git a/docs/roadmaps/entries/PMAT-5873.yaml b/docs/roadmaps/entries/PMAT-5873.yaml
new file mode 100644
index 0000000000..175bb979cb
--- /dev/null
+++ b/docs/roadmaps/entries/PMAT-5873.yaml
@@ -0,0 +1,20 @@
+- id: PMAT-5873
+  github_issue: 5873
+  item_type: task
+  title: 'LAB: check_harness_server_cleanup apr-stub row races; quarantine to guards-nightly, then fix'
+  status: in_progress
+  priority: high
+  assigned_to: aprender-w5382
+  created: 2026-10-21T15:20:00Z
+  updated: 2026-10-21T15:20:00Z
+  spec: null
+  acceptance_criteria:
+  - 'the check runs in guards-nightly (LAB), not in ci/explicit-test-commands.d'
+  - 'the apr-stub row no longer races (second PR)'
+  phases: []
+  subtasks: []
+  estimated_effort: null
+  labels:
+  - kind:code
+  notes: null
+  epic: 5510
diff --git a/docs/roadmaps/roadmap.yaml b/docs/roadmaps/roadmap.yaml
index ef47935f92..5530309f23 100644
--- a/docs/roadmaps/roadmap.yaml
+++ b/docs/roadmaps/roadmap.yaml
@@ -25359,3 +25359,23 @@ Not done, pending the ruling: moving the 31 fixture files to an obviously synthe
   - kind:code
   notes: null
   epic: 5510
+- id: PMAT-5873
+  github_issue: 5873
+  item_type: task
+  title: 'LAB: check_harness_server_cleanup apr-stub row races; quarantine to guards-nightly, then fix'
+  status: in_progress
+  priority: high
+  assigned_to: aprender-w5382
+  created: 2026-10-21T15:20:00Z
+  updated: 2026-10-21T15:20:00Z
+  spec: null
+  acceptance_criteria:
+  - 'the check runs in guards-nightly (LAB), not in ci/explicit-test-commands.d'
+  - 'the apr-stub row no longer races (second PR)'
+  phases: []
+  subtasks: []
+  estimated_effort: null
+  labels:
+  - kind:code
+  notes: null
+  epic: 5510
diff --git a/scripts/guards_nightly_manifest.txt b/scripts/guards_nightly_manifest.txt
index 423ddca459..be7f3ab4e3 100644
--- a/scripts/guards_nightly_manifest.txt
+++ b/scripts/guards_nightly_manifest.txt
@@ -49,3 +49,5 @@
 # #4709 (C303): the pv census ratchet, LAB from its first night (never on a PR path, C324).
 # Seconds are a local wall-clock sample (2026-10-14, pv prebuilt); the first nightly replaces them.
 6	The pv census bound share may only rise (#4709)
+# #5873: the harness fixture-server cleanup check, quarantined from ci/explicit-test-commands.d/817 (its apr-stub row races); LAB, never on a PR path (C324).
+60	Harness fixture servers die with their harness (LAB, quarantined from the merge path, #5873)