diff --git a/.github/workflows/pack-index-consistency-gate.yml b/.github/workflows/pack-index-consistency-gate.yml
new file mode 100644
index 000000000..e5c6363bb
--- /dev/null
+++ b/.github/workflows/pack-index-consistency-gate.yml
@@ -0,0 +1,58 @@
+# Locks INDEX_all_formats.json against the packs it summarises.
+#
+# Sibling to conformance-integrity-gate.yml: that gate checks the vectors inside
+# each pack; this one checks that the index's summary of a pack still matches the
+# pack. The gap it closes is not hypothetical -- gen_all_formats.py hardcoded the
+# tier for the six wide GoldenFloat rungs, so re-running it silently rewrote the
+# index from 75/0/8 back to 69/6/8, reverting an honesty-rule-#10 promotion the
+# pack files themselves record. Nothing in CI would have noticed.
+#
+# stdlib-only Python, no third-party deps, no network. The self-test runs FIRST and
+# must show every check is still FAIL-reachable before the gate is trusted.
+name: pack-index-consistency-gate
+
+on:
+  push:
+    paths:
+      - "conformance/vectors/**"
+      - "tools/pack_index_consistency_gate.py"
+      - ".github/workflows/pack-index-consistency-gate.yml"
+  pull_request:
+    paths:
+      - "conformance/vectors/**"
+      - "tools/pack_index_consistency_gate.py"
+      - ".github/workflows/pack-index-consistency-gate.yml"
+  workflow_dispatch: {}
+
+permissions:
+  contents: read
+
+jobs:
+  pack-index-consistency:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Set up Python
+        uses: actions/setup-python@v5
+        with:
+          python-version: "3.11"
+
+      - name: Self-test (gate must be falsifiable)
+        # If any planted mutant survives, the gate is decoration -> fail here.
+        run: python3 tools/pack_index_consistency_gate.py --selftest
+
+      - name: Run gate against the live corpus
+        run: |
+          python3 tools/pack_index_consistency_gate.py \
+            --vectors conformance/vectors \
+            --json pack_index_report.json
+
+      - name: Upload report
+        if: always()
+        uses: actions/upload-artifact@v4
+        with:
+          name: pack-index-consistency-report
+          path: pack_index_report.json
+          if-no-files-found: ignore
diff --git a/tools/pack_index_consistency_gate.py b/tools/pack_index_consistency_gate.py
new file mode 100644
index 000000000..3b4790274
--- /dev/null
+++ b/tools/pack_index_consistency_gate.py
@@ -0,0 +1,288 @@
+#!/usr/bin/env python3
+"""Lock INDEX_all_formats.json against the packs it summarises.
+
+WHY
+---
+Honesty rule #10 -- a pack is not labelled bit-precise without an independent
+second witness -- is the corpus's central claim. The pack file is the artefact of
+record for its own status; the index is a summary consumers and papers read.
+Nothing checked that the two agreed.
+
+That gap was not hypothetical. gen_all_formats.py hardcoded
+`kind: bitexact_selfconsistent` for the six wide GoldenFloat rungs, which was
+correct when written. The rungs later acquired independent second witnesses and
+were promoted in the pack files; the generator was never updated. Re-running it
+rewrote the index from 75/0/8 back to 69/6/8 -- silently reverting an honesty-rule
+promotion the packs themselves record, with no gate to catch it.
+
+This gate closes that class rather than that instance. It is stdlib-only, needs no
+network, and is falsifiable: --selftest plants a mutant for every check and fails
+if any mutant survives.
+
+CHECKS
+------
+  A  every index entry names a pack file that exists
+  B  the recorded sha256 matches the file on disk
+  C  kind agrees with the pack: bitexact + witnesses[] => "bitexact",
+     and a pack that is not bitexact is not labelled anything else.
+     An empty witnesses[] on a bitexact pack is NOT a failure -- see the note at
+     the check itself for why demanding one everywhere misstates rule #10.
+  D  the index witness count equals len(pack["witnesses"])
+  E  the header totals equal the entries actually present
+  F  no pack file in the directory is missing from the index
+
+Usage:
+    python3 tools/pack_index_consistency_gate.py [--vectors DIR] [--json OUT]
+    python3 tools/pack_index_consistency_gate.py --selftest
+"""
+from __future__ import annotations
+
+import argparse
+import copy
+import hashlib
+import json
+import os
+import sys
+
+REPO = os.path.dirname(os.path.dirname(os.path.abspath(__file__)))
+DEFAULT_VECTORS = os.path.join(REPO, "conformance", "vectors")
+INDEX_NAME = "INDEX_all_formats.json"
+
+
+# ---------------------------------------------------------------------------
+# The checks. Each returns a list of failure strings.
+# ---------------------------------------------------------------------------
+def check(index: dict, packs: dict, on_disk: set, notes: list | None = None) -> list[str]:
+    """packs maps filename -> parsed pack (or None if unreadable/absent).
+
+    Anything appended to `notes` is reported but does not fail the gate.
+    """
+    if notes is None:
+        notes = []
+    fails = []
+    entries = index.get("packs", [])
+
+    for e in entries:
+        cid, fname = e.get("id", "<no id>"), e.get("file", "")
+        pack = packs.get(fname)
+
+        # A -- the file exists
+        if pack is None:
+            fails.append(f"A {cid}: index names {fname!r}, which is not readable")
+            continue
+
+        # B -- digest matches
+        recorded = e.get("sha256")
+        actual = pack["__sha256__"]
+        if recorded is not None and recorded != actual:
+            fails.append(f"B {cid}: sha256 {recorded[:12]}... recorded, "
+                         f"{actual[:12]}... on disk")
+
+        # C -- tier agrees with the pack's own state.
+        #
+        # An ABSENT bitexact key is unknown, not false. Five hand-curated packs
+        # predate the flag and carry no such key while the index labels them
+        # bitexact; reading silence as a denial would turn those into failures and
+        # the gate would be asserting something the pack never said. They are
+        # counted as notes instead.
+        declared = pack.get("bitexact", None)
+        bitexact = bool(declared)
+        witnessed = bool(pack.get("witnesses"))
+        kind = e.get("kind")
+        if declared is None:
+            notes.append(f"{cid}: pack declares no bitexact flag; index says "
+                         f"kind={kind!r} (hand-curated pack predating the flag)")
+            continue
+        #
+        # Deliberately NOT checked: a bitexact pack with an empty witnesses[].
+        # That is the normal case, not a violation. The 60-odd uncontested packs
+        # were bit-precise from the start, their independent reference codec being
+        # part of how the vectors were generated; witnesses[] records the packs
+        # whose promotion was CONTESTED and had to be argued. A check demanding a
+        # witness everywhere would fail the whole corpus and would misstate rule
+        # #10. The asymmetry is the point: a recorded witness must not be ignored,
+        # but its absence is not evidence of anything.
+        if bitexact and witnessed:
+            if kind != "bitexact":
+                fails.append(f"C {cid}: pack is bitexact with "
+                             f"{len(pack['witnesses'])} witness(es) but index "
+                             f"says kind={kind!r}, not 'bitexact'")
+        elif not bitexact:
+            if kind not in ("structural", None):
+                fails.append(f"C {cid}: pack is not bitexact but index says "
+                             f"kind={kind!r}")
+
+        # D -- witness count agrees
+        if "witnesses" in e:
+            want = len(pack.get("witnesses", []) or [])
+            if e["witnesses"] != want:
+                fails.append(f"D {cid}: index says {e['witnesses']} witness(es), "
+                             f"pack has {want}")
+
+    # E -- header totals
+    kinds = [e.get("kind") for e in entries]
+    for key, want in (("total_packs", len(entries)),
+                      ("bitexact_packs", kinds.count("bitexact")),
+                      ("structural_packs", kinds.count("structural"))):
+        if key in index and index[key] != want:
+            fails.append(f"E header {key}={index[key]}, entries give {want}")
+    if "witnessed_packs" in index:
+        want = sum(1 for e in entries if e.get("witnesses"))
+        if index["witnessed_packs"] != want:
+            fails.append(f"E header witnessed_packs={index['witnessed_packs']}, "
+                         f"entries give {want}")
+
+    # F -- nothing on disk is left out of the index
+    listed = {e.get("file") for e in entries}
+    for fname in sorted(on_disk - listed):
+        fails.append(f"F {fname} is a pack file but appears in no index entry")
+
+    return fails
+
+
+# ---------------------------------------------------------------------------
+def load(vectors_dir: str):
+    index_path = os.path.join(vectors_dir, INDEX_NAME)
+    if not os.path.exists(index_path):
+        print(f"no {INDEX_NAME} in {vectors_dir}", file=sys.stderr)
+        raise SystemExit(3)
+    index = json.load(open(index_path))
+
+    packs, on_disk = {}, set()
+    for fname in os.listdir(vectors_dir):
+        if not fname.endswith(".json") or fname == INDEX_NAME:
+            continue
+        blob = open(os.path.join(vectors_dir, fname), "rb").read()
+        try:
+            pack = json.loads(blob)
+        except json.JSONDecodeError:
+            continue
+        # a conformance pack, not some other json living in the directory
+        if "vectors" not in pack and "bitexact" not in pack:
+            continue
+        # instance_*.json carry kind: "instance" -- an instance-level pack for one
+        # fixed parameterization beneath a catalog-STRUCTURAL family. The index has
+        # one entry per catalog format, so these belong outside it by design.
+        if pack.get("kind") == "instance":
+            continue
+        pack["__sha256__"] = hashlib.sha256(blob).hexdigest()
+        packs[fname] = pack
+        on_disk.add(fname)
+    return index, packs, on_disk
+
+
+# ---------------------------------------------------------------------------
+# Self-test: every check must be reachable, or the gate is decoration.
+# ---------------------------------------------------------------------------
+def selftest() -> int:
+    base_index = {
+        "total_packs": 2, "bitexact_packs": 1, "structural_packs": 1,
+        "witnessed_packs": 1,
+        "packs": [
+            {"id": "alpha", "file": "alpha.json", "kind": "bitexact",
+             "sha256": "", "witnesses": 1},
+            {"id": "beta", "file": "beta.json", "kind": "structural",
+             "sha256": "", "witnesses": 0},
+        ],
+    }
+    base_packs = {
+        "alpha.json": {"bitexact": True, "witnesses": [{"kind": "sw"}],
+                       "vectors": [], "__sha256__": "aa"},
+        "beta.json": {"bitexact": False, "vectors": [], "__sha256__": "bb"},
+    }
+    base_index["packs"][0]["sha256"] = "aa"
+    base_index["packs"][1]["sha256"] = "bb"
+    on_disk = {"alpha.json", "beta.json"}
+
+    clean = check(copy.deepcopy(base_index), copy.deepcopy(base_packs), set(on_disk))
+    if clean:
+        print("SELFTEST BROKEN: the clean fixture already fails:")
+        for f in clean:
+            print("   ", f)
+        return 1
+
+    def mutate(name, letter, fn):
+        idx, pks, disk = (copy.deepcopy(base_index), copy.deepcopy(base_packs),
+                          set(on_disk))
+        fn(idx, pks, disk)
+        got = check(idx, pks, disk)
+        killed = any(f.startswith(letter + " ") for f in got)
+        print(f"  [{'PASS' if killed else 'FAIL'}] {name}")
+        if not killed:
+            print(f"         mutant survived; findings were: {got}")
+        return 0 if killed else 1
+
+    def m_missing(i, p, d):
+        p.pop("alpha.json"); d.discard("alpha.json")
+
+    def m_digest(i, p, d):
+        p["alpha.json"]["__sha256__"] = "zz"
+
+    def m_demote(i, p, d):
+        # exactly the pass-48 regression: a witnessed, bitexact pack demoted
+        i["packs"][0]["kind"] = "bitexact_selfconsistent"
+
+    def m_count(i, p, d):
+        i["packs"][0]["witnesses"] = 7
+
+    def m_header(i, p, d):
+        i["bitexact_packs"] = 99
+
+    def m_orphan(i, p, d):
+        p["gamma.json"] = {"bitexact": False, "vectors": [], "__sha256__": "cc"}
+        d.add("gamma.json")
+
+    print("selftest -- every check must be FAIL-reachable:")
+    bad = 0
+    bad += mutate("A missing pack file is caught", "A", m_missing)
+    bad += mutate("B digest drift is caught", "B", m_digest)
+    bad += mutate("C witnessed pack demoted in index is caught", "C", m_demote)
+    bad += mutate("D witness count mismatch is caught", "D", m_count)
+    bad += mutate("E header total drift is caught", "E", m_header)
+    bad += mutate("F pack absent from the index is caught", "F", m_orphan)
+
+    print("\n" + ("selftest OK: the gate is falsifiable."
+                  if not bad else f"selftest FAILED: {bad} mutant(s) survived."))
+    return 1 if bad else 0
+
+
+# ---------------------------------------------------------------------------
+def main() -> int:
+    ap = argparse.ArgumentParser(description=__doc__.split("\n")[0])
+    ap.add_argument("--vectors", default=DEFAULT_VECTORS)
+    ap.add_argument("--json", dest="json_out")
+    ap.add_argument("--selftest", action="store_true")
+    args = ap.parse_args()
+
+    if args.selftest:
+        return selftest()
+
+    index, packs, on_disk = load(args.vectors)
+    notes: list[str] = []
+    fails = check(index, packs, on_disk, notes)
+
+    print(f"index entries : {len(index.get('packs', []))}")
+    print(f"pack files    : {len(on_disk)}")
+    print(f"notes         : {len(notes)}")
+    for n in notes:
+        print("    note:", n)
+    print(f"failures      : {len(fails)}")
+    for f in fails:
+        print("   ", f)
+
+    verdict = "CLEAN" if not fails else "FAIL"
+    print(f"\nverdict: {verdict}")
+
+    if args.json_out:
+        with open(args.json_out, "w") as fh:
+            json.dump({"schema": "pack-index-consistency/v1",
+                       "vectors_dir": os.path.abspath(args.vectors),
+                       "entries": len(index.get("packs", [])),
+                       "pack_files": len(on_disk),
+                       "failures": fails,
+                       "verdict": verdict}, fh, indent=2)
+    return 0 if not fails else 1
+
+
+if __name__ == "__main__":
+    raise SystemExit(main())