diff --git a/docs/design/track_b_insured_status.md b/docs/design/track_b_insured_status.md
index 4a1f68e4..601594d9 100644
--- a/docs/design/track_b_insured_status.md
+++ b/docs/design/track_b_insured_status.md
@@ -3954,6 +3954,30 @@ These tests execute the invariants this round adds:
   (`test_a_middle_completion_past_the_boundary_cap_refuses`,
   `test_the_boundary_guard_refuses_only_past_4096`).
 
+## Round 25 the whole-period proof keeps its test
+
+Verification class: statutory conformance on invented declarations only. This
+correction admits no population, behavioral, payment, or forecast claim.
+
+Since round 23, `_unknown_prior_joint_shortfall` drops an uncredited onset
+quarter from its clock when a prior can end in that quarter.
+`test_unknown_prior_history_keeps_its_row_on_the_whole_period` had an August
+1975 onset, so a prior could end in July. Its clock then held 41 quarters at
+1975Q3 with 20 QCs in the last 40, and the proof's own 40-quarter test kept the
+row on either reading. This test caught round 17's `proof_keeps_last_40`, and
+round 23's list didn't carry that mutation, so no run showed the loss. At
+`4ab97297`, that mutation's cap passes all 1,997 cases in the 41 insured test
+modules.
+
+The test now has a July onset. A prior can end in an onset's quarter only in an
+earlier month of that quarter, and July opens 1975Q3, so the clock keeps that
+quarter and holds 42 quarters. The 21 QCs meet half of them and 19 sit in the
+last 40. With the regulation's cap in `special_recent`, the rule raises
+`InvalidFamilyInput` on this record, and `proof_keeps_last_40` fails this test
+again. `insured_status.py` doesn't change. `round_25_mutations` in
+`track_b_insured_status_mutations.json` holds that run, and a later list takes
+round 24's 135 mutations and this one.
+
 ## Not in scope
 
 No real data, population evaluation, baseline claiming hazard, behavioral
diff --git a/docs/design/track_b_insured_status_mutations.json b/docs/design/track_b_insured_status_mutations.json
index e1fbea14..8eba2f0c 100644
--- a/docs/design/track_b_insured_status_mutations.json
+++ b/docs/design/track_b_insured_status_mutations.json
@@ -16686,5 +16686,35 @@
     "tests/test_track_b_insured_status_status_stress.py": "882e3b1f0a1c91fd126d8ba85da50fd7672b475b13b81eb48e132d470cd27bed"
   },
   "round_24_replay": "Write round_24_runner_source to scratch/review/run_round24_named.py. For each round_24_mutations record, write name/source_sha256/changes to its spec_path, then execute its exact command at d30e4426. Mutations apply only in memory.",
-  "round_24_final_source_sha256": "7d2d202b02bfcc14ee4d988cb35bef8706bf28de7af494fd6bad2f674ca80c89"
+  "round_24_final_source_sha256": "7d2d202b02bfcc14ee4d988cb35bef8706bf28de7af494fd6bad2f674ca80c89",
+  "round_25_mutations": [
+    {
+      "changes": [
+        {
+          "before": "                window, required = elapsed, max(6, len(elapsed) // 2)\n            return len(set(window) & assigned) >= required\n",
+          "after": "                window, required = elapsed[-40:], max(6, min(40, len(elapsed)) // 2)\n            return len(set(window) & assigned) >= required\n"
+        }
+      ],
+      "command": "env POPULACE_DYNAMICS_PSID_DIR=/nonexistent/trackb-round24 POPULACE_DYNAMICS_EPUF_DIR=/nonexistent/trackb-round24 OMP_NUM_THREADS=1 OMP_WAIT_POLICY=PASSIVE POLARS_MAX_THREADS=1 PYTHONPATH=$PWD/src:$PWD PYTHONDONTWRITEBYTECODE=1 TRACKB_ROUND24_MUTATION=scratch/review/round25-mutation-proof_keeps_last_40.json HYPOTHESIS_STORAGE_DIRECTORY=scratch/review/hyp25/proof_keeps_last_40 python scratch/review/run_round24_named.py -p no:xdist -p no:cacheprovider -q -x --tb=line -rf tests/test_track_b_insured_status_fix_round_17.py::test_unknown_prior_history_keeps_its_row_on_the_whole_period",
+      "exit_code": 1,
+      "failing_test_nodes": [
+        "tests/test_track_b_insured_status_fix_round_17.py::test_unknown_prior_history_keeps_its_row_on_the_whole_period"
+      ],
+      "log": "scratch/review/round25-mutation-proof_keeps_last_40.log",
+      "name": "proof_keeps_last_40",
+      "open_audit": {
+        "os.system": 0,
+        "restricted_opens": 0,
+        "subprocess.Popen": 0
+      },
+      "source_sha256": "7d2d202b02bfcc14ee4d988cb35bef8706bf28de7af494fd6bad2f674ca80c89",
+      "spec_path": "scratch/review/round25-mutation-proof_keeps_last_40.json"
+    }
+  ],
+  "round_25_note": "proof_keeps_last_40 fails the named test with the July onset. At 4ab97297, with the August onset, the same cap, written as (elapsed[-40:], 20) past 40 quarters, passed all 1,997 cases in the 41 insured test modules. A later round's list takes round 24's 135 mutations and this one.",
+  "round_25_test_sha256": {
+    "tests/test_track_b_insured_status_fix_round_17.py": "c6fa611f7ba7b031c97dc09c7fc937853d5f949d5449bfd1921d9c9b29dd1065"
+  },
+  "round_25_replay": "Write round_24_runner_source to scratch/review/run_round24_named.py. Write the round_25_mutations record's name/source_sha256/changes to its spec_path, then execute its exact command. Mutations apply only in memory.",
+  "round_25_final_source_sha256": "7d2d202b02bfcc14ee4d988cb35bef8706bf28de7af494fd6bad2f674ca80c89"
 }
diff --git a/tests/test_track_b_insured_status_fix_round_17.py b/tests/test_track_b_insured_status_fix_round_17.py
index e49dfaa9..a4775693 100644
--- a/tests/test_track_b_insured_status_fix_round_17.py
+++ b/tests/test_track_b_insured_status_fix_round_17.py
@@ -185,12 +185,15 @@ def test_an_age_62_fully_request_keeps_its_row(exit_month, last):
 def test_unknown_prior_history_keeps_its_row_on_the_whole_period():
     # Fixed QCs sit in 1965Q2, 1965Q3, 1967Q1 to Q3 and 1971Q3 to 1975Q2. The
     # joint proof's clock drops uncredited quarters after the first possible
-    # prior, which leaves 42 quarters at 1975Q3. The 21 QCs meet half of those
-    # and only 19 sit in the last 40, so the proof must not reject the row.
+    # prior. No prior can end in 1975Q3 before a July onset, so the clock keeps
+    # that quarter and holds 42 at 1975Q3. The 21 QCs meet half of those and
+    # only 19 sit in the last 40, so the proof must not reject the row. A prior
+    # can end in July before an August onset, which leaves 41 quarters with 20
+    # QCs in the last 40, and that record keeps its row on either reading.
     person = i.Person(
         date(1944, 2, 2),
         sex=i.Sex.FEMALE,
-        disability_onset=date(1975, 8, 15),
+        disability_onset=date(1975, 7, 15),
         current_disability_period_established=True,
         current_disability_period_provenance=OBSERVED,
         current_disability_period_end=date(1976, 6, 30),