diff --git a/tools/re-capture/check_refuted.py b/tools/re-capture/check_refuted.py
index 0683f1f8..51568964 100755
--- a/tools/re-capture/check_refuted.py
+++ b/tools/re-capture/check_refuted.py
@@ -19,6 +19,7 @@ means "no verbatim revival", not "no revival".
 
     check_refuted.py [--context N]
 """
+import os
 import re
 import sys
 from pathlib import Path
@@ -41,7 +42,65 @@ MARKERS = ("refuted", "REFUTED", "withdrawn", "WITHDRAWN", "retracted", "RETRACT
 CHRONOLOGICAL = {"BACKLOG.md"}
 CTX = int(sys.argv[sys.argv.index("--context") + 1]) if "--context" in sys.argv else 4
 
-root = Path("docs")
+# Overridable so the harness self-test can drive the REAL machinery over a
+# synthetic corpus instead of reasoning about what it would do.
+if "--selftest" in sys.argv:
+    # ── HARNESS SELF-TEST ────────────────────────────────────────────────────
+    # sylpheed-port closed this gap first: their controls asserted
+    # failure-on-perturbation but nothing asserted that a BROKEN harness reports
+    # broken. Their stub is a check that cannot fail; the equivalent here is a
+    # register with NO CLAIMS LOADED, which reports clean forever.
+    #
+    # These push synthetic corpora through this script as a SUBPROCESS and read
+    # its real exit code. An earlier version of their test reasoned about what
+    # the machinery would do instead of running it -- the error this whole thread
+    # is about, committed inside the tool built to prevent it.
+    #
+    # Exit convention, theirs: 0 fine · 1 a real check failed · 2 the HARNESS is
+    # broken and nothing it reports can be trusted.
+    import subprocess, tempfile, textwrap
+
+    def _corpus(d, refuted, other):
+        (d / "re").mkdir(parents=True, exist_ok=True)
+        (d / "re" / "REFUTED.md").write_text(refuted)
+        (d / "re" / "other.md").write_text(other)
+
+    CLAIM = 'the synthetic widget is on the disc nowhere'
+    REG = f'* "{CLAIM}"\n'
+    cases = [
+        ("clean corpus, claim not revived", REG, "Nothing to see here.\n", 0),
+        ("verbatim revival, no marker", REG,
+         f"A live assertion: {CLAIM} and that is that.\n", 1),
+        ("revival WITH a marker nearby", REG,
+         f"~~{CLAIM}~~ was refuted on 2026-01-01.\n", 0),
+        ("EMPTY REGISTER — must refuse, not pass", "no quoted claims at all\n",
+         f"A live assertion: {CLAIM}.\n", 2),
+    ]
+    bad = 0
+    print("── check_refuted harness self-test ──", flush=True)
+    with tempfile.TemporaryDirectory() as td:
+        for name, refuted, other, want in cases:
+            d = Path(td) / name.replace(" ", "_").replace(",", "")
+            _corpus(d, refuted, other)
+            env = dict(os.environ, CHECK_REFUTED_ROOT=str(d))
+            r = subprocess.run([sys.executable, __file__], env=env,
+                               capture_output=True, text=True)
+            n = 0
+            for line in r.stdout.splitlines():
+                if "quoted claims in REFUTED.md" in line:
+                    n = int(line.split()[0])
+            ok = (r.returncode == want)
+            bad += not ok
+            print(f"   {'✅' if ok else '🔴'} {name:38} exit={r.returncode} "
+                  f"(want {want}), claims={n}")
+    if bad:
+        print("🔴 HARNESS SELF-TEST FAILED — nothing this tool reports can be "
+              "trusted.", flush=True)
+        sys.exit(2)
+    print("   ✅ harness self-test passed", flush=True)
+    sys.exit(0)
+
+root = Path(os.environ.get("CHECK_REFUTED_ROOT", "docs"))
 ref = root / "re" / "REFUTED.md"
 claims = []
 seen_report = set()
@@ -51,6 +110,15 @@ for line in ref.read_text().splitlines():
         claims.append(m.group(1))
 
 print(f"{len(claims)} quoted claims in REFUTED.md\n")
+
+# 🔴 A register that loaded NOTHING cannot fail, and would report clean forever --
+# the same shape as sylpheed-port's stub that prints "everything is fine" and
+# asserts nothing. Refuse rather than pass. Found by this tool's own harness
+# self-test, which is the only reason it was visible.
+if not claims:
+    print("🔴 NO CLAIMS PARSED from REFUTED.md — this run asserts NOTHING. "
+          "Exiting 2 (harness broken), not 0.")
+    sys.exit(2)
 hits = 0
 suppressed = []
 seen_marked = set()