diff --git a/.claude/tools/append_only_gate.py b/.claude/tools/append_only_gate.py
new file mode 100755
index 000000000..654b83d28
--- /dev/null
+++ b/.claude/tools/append_only_gate.py
@@ -0,0 +1,275 @@
+#!/usr/bin/env python3
+"""Fail a PR that SHORTENS an append-only board file.
+
+The protected files below are append-only logs: every session PREPENDS its
+newest entry at the head. Concurrent sessions therefore collide at exactly the
+same lines, by construction -- and the tempting resolution ("take mine", "take
+theirs", `git checkout --ours`) silently DELETES the other session's entry. The
+loss is invisible in review: the file still parses, still reads well, and the
+only trace is that it got shorter.
+
+The workspace law (root `CLAUDE.md`; the full statement plus its incident
+receipts live in
+`.claude/knowledge/never-truncate-a-file-you-still-need-to-read.md`):
+
+    an append-only file that got SHORTER is always a defect.
+
+That check has been manual (`wc -l` after every ledger write). On 2026-09-04
+three sessions collided within one hour and only a hand-run `wc -l` prevented a
+loss. This makes it mechanical.
+
+WHAT IT IS NOT
+--------------
+This gate measures LINE COUNT ONLY. It cannot see a same-length rewrite, a
+reordering, or an entry replaced by another of equal size. It catches the one
+failure mode that is both the most common and the most silent -- a dropped
+prepend -- and claims nothing more. A green run is not proof the file is
+append-only; a red run is proof it is not.
+
+USAGE
+-----
+    python3 .claude/tools/append_only_gate.py [BASE_REF]     # default origin/main
+    python3 .claude/tools/append_only_gate.py --self-test
+
+Exit 0 = no protected file shrank. Exit 1 = at least one did (or the gate could
+not do its job -- an unresolvable base ref fails closed, never silently green).
+"""
+
+from __future__ import annotations
+
+import subprocess
+import sys
+
+DEFAULT_BASE = "origin/main"
+
+# The append-only board files, per root `CLAUDE.md` -- "The governance files are
+# APPEND-ONLY (prepend new entries; never edit past entries except the
+# `**Status:**` / `**Confidence:**` lines)".
+#
+# Deliberately NOT the whole of `.claude/board/`: several files there are
+# genuinely rewritable (the generated `SUPERSESSION-INDEX.md`, working
+# scratch/roadmap files), and protecting them would make the gate fire on
+# correct work -- a gate that objects to everything carries exactly as much
+# information as one that never fires.
+PROTECTED = (
+    ".claude/board/LATEST_STATE.md",
+    ".claude/board/EPIPHANIES.md",
+    ".claude/board/PR_ARC_INVENTORY.md",
+    ".claude/board/STATUS_BOARD.md",
+    ".claude/board/ISSUES.md",
+    ".claude/board/TECH_DEBT.md",
+    ".claude/board/AGENT_LOG.md",
+    ".claude/board/INTEGRATION_PLANS.md",
+)
+
+REMEDY = """
+  This is almost always a PREPEND CONFLICT resolved by picking a side.
+  Two sessions each added a new entry at the head of the same append-only
+  file; taking one side dropped the other session's entry entirely.
+
+  DO NOT resolve it by keeping "the newer" or "the bigger" side.
+  RESOLVE IT BY KEEPING BOTH ENTRIES -- put both new blocks at the head,
+  in whatever order reads correctly, and leave every pre-existing entry
+  below them untouched. Nothing below the head should have moved at all.
+
+  To see exactly what went missing:
+      git diff {base}...HEAD -- {paths}
+
+  If a shrink is genuinely intended (a deliberate, operator-sanctioned
+  removal), say so explicitly in the PR body -- and note that the storno
+  convention is to REGRADE an entry in place, never to delete it.
+"""
+
+
+class GateError(RuntimeError):
+    """The gate could not do its job. Fails closed, never silently green."""
+
+
+def _run(args: list[str]) -> subprocess.CompletedProcess:
+    return subprocess.run(args, capture_output=True, text=True)
+
+
+def resolve_base(base_ref: str) -> str:
+    """Merge-base of HEAD and the base ref.
+
+    A PR branch's base ref moves on after the branch was cut, so a straight
+    `git show <base>:<path>` would compare against work the branch never saw --
+    an unrelated session's later prepend would read as "grew", masking a real
+    shrink. The merge-base is the file as the branch actually inherited it.
+    """
+    proc = _run(["git", "merge-base", "HEAD", base_ref])
+    if proc.returncode != 0:
+        raise GateError(
+            f"cannot compute merge-base of HEAD and {base_ref!r}: "
+            f"{proc.stderr.strip()}\n"
+            "  (in CI this usually means a shallow checkout -- the workflow "
+            "needs `fetch-depth: 0`)"
+        )
+    return proc.stdout.strip()
+
+
+def line_count_at(rev: str, path: str) -> int | None:
+    """Line count of `path` at `rev`, or None when the file does not exist there."""
+    proc = _run(["git", "show", f"{rev}:{path}"])
+    if proc.returncode != 0:
+        return None
+    return count_lines(proc.stdout)
+
+
+def line_count_in_tree(path: str) -> int | None:
+    """Line count of the working-tree file, or None when absent.
+
+    Read from the tree rather than from HEAD so the gate is equally usable
+    locally on uncommitted work, which is where a session can still fix a
+    clobber cheaply.
+    """
+    try:
+        with open(path, "r", encoding="utf-8", errors="replace") as fh:
+            return count_lines(fh.read())
+    except FileNotFoundError:
+        return None
+
+
+def count_lines(text: str) -> int:
+    """Number of lines, counting a final unterminated line.
+
+    `str.count("\\n")` alone under-counts a file with no trailing newline by
+    one, which would read as a one-line shrink on an otherwise untouched file.
+    """
+    if not text:
+        return 0
+    return text.count("\n") + (0 if text.endswith("\n") else 1)
+
+
+def evaluate(before: int | None, after: int | None) -> tuple[str, str]:
+    """Classify one file. Returns (verdict, human note).
+
+    verdict is one of: "ok", "new", "shrank", "deleted".
+    """
+    if before is None and after is None:
+        return "ok", "absent at base and in head (nothing to check)"
+    if before is None:
+        return "new", f"new file, {after} lines (absent at base -- fine)"
+    if after is None:
+        return "deleted", f"DELETED (was {before} lines at base)"
+    if after < before:
+        return "shrank", f"{before} -> {after} lines ({after - before})"
+    if after == before:
+        return "ok", f"{before} lines, unchanged length"
+    return "ok", f"{before} -> {after} lines (+{after - before})"
+
+
+def check(base_ref: str, paths=PROTECTED) -> int:
+    base_sha = resolve_base(base_ref)
+    print(f"append-only gate: base {base_ref} -> merge-base {base_sha[:12]}")
+
+    failures = []
+    for path in paths:
+        before = line_count_at(base_sha, path)
+        after = line_count_in_tree(path)
+        verdict, note = evaluate(before, after)
+        marker = "FAIL" if verdict in ("shrank", "deleted") else "ok  "
+        print(f"  {marker}  {path}: {note}")
+        if verdict in ("shrank", "deleted"):
+            failures.append((path, verdict, before, after))
+
+    if not failures:
+        print(f"\nOK: no protected append-only file shrank ({len(paths)} checked).")
+        return 0
+
+    print("\n" + "=" * 72)
+    print("APPEND-ONLY VIOLATION: a board file got SHORTER.")
+    print("=" * 72)
+    for path, verdict, before, after in failures:
+        if verdict == "deleted":
+            print(f"\n  {path}\n      DELETED  (base: {before} lines, head: file absent)")
+        else:
+            print(
+                f"\n  {path}\n"
+                f"      base: {before} lines\n"
+                f"      head: {after} lines\n"
+                f"      delta: {after - before} lines LOST"
+            )
+    print(
+        REMEDY.format(
+            base=base_ref,
+            paths=" ".join(p for p, _, _, _ in failures),
+        )
+    )
+    return 1
+
+
+# --------------------------------------------------------------------------
+# Self-test. Both halves are required: a gate that cannot fire and a gate that
+# fires on everything carry the same information (zero). These run in-process
+# against `evaluate`, the single place the verdict is decided, so they need no
+# git repo and no fixtures.
+# --------------------------------------------------------------------------
+
+SELF_TEST_CASES = [
+    # (name, before, after, expected_verdict)
+    ("shortened file FIRES", 500, 480, "shrank"),
+    ("one line lost FIRES", 500, 499, "shrank"),
+    ("whole file deleted FIRES", 500, None, "deleted"),
+    ("grown file is SILENT", 500, 530, "ok"),
+    ("unchanged file is SILENT", 500, 500, "ok"),
+    ("newly created file is SILENT", None, 42, "new"),
+    ("absent both sides is SILENT", None, None, "ok"),
+]
+
+
+def self_test() -> int:
+    print("append-only gate self-test")
+    print("-" * 72)
+    failed = 0
+    for name, before, after, expected in SELF_TEST_CASES:
+        verdict, note = evaluate(before, after)
+        fires = verdict in ("shrank", "deleted")
+        want_fires = expected in ("shrank", "deleted")
+        ok = verdict == expected
+        print(
+            f"  [{'PASS' if ok else 'FAIL'}] {name:<32} "
+            f"before={str(before):>4} after={str(after):>4} "
+            f"-> {verdict:<7} ({'fires' if fires else 'silent'}) | {note}"
+        )
+        if not ok:
+            failed += 1
+            print(f"          expected verdict {expected!r}, got {verdict!r}")
+
+    # Anti-vacuity: the suite must contain BOTH a case that fires and a case
+    # that stays silent, or it proves nothing about discrimination.
+    fired = sum(1 for _, b, a, _ in SELF_TEST_CASES if evaluate(b, a)[0] in ("shrank", "deleted"))
+    silent = len(SELF_TEST_CASES) - fired
+    print("-" * 72)
+    print(f"  discrimination: {fired} case(s) fire, {silent} case(s) stay silent")
+    if fired == 0:
+        print("  [FAIL] no case fires -- a gate that cannot fire is not a gate")
+        failed += 1
+    if silent == 0:
+        print("  [FAIL] no case stays silent -- a gate that fires on everything is not a gate")
+        failed += 1
+
+    if failed:
+        print(f"\nSELF-TEST FAILED: {failed} problem(s).")
+        return 1
+    print(f"\nSELF-TEST PASSED: {len(SELF_TEST_CASES)} cases, both halves proven.")
+    return 0
+
+
+def main(argv: list[str]) -> int:
+    args = argv[1:]
+    if args and args[0] in ("--self-test", "--selftest"):
+        return self_test()
+    if args and args[0] in ("-h", "--help"):
+        print(__doc__)
+        return 0
+    base_ref = args[0] if args else DEFAULT_BASE
+    try:
+        return check(base_ref)
+    except GateError as exc:
+        print(f"append-only gate: ERROR: {exc}", file=sys.stderr)
+        return 1
+
+
+if __name__ == "__main__":
+    sys.exit(main(sys.argv))
diff --git a/.github/workflows/append-only-gate.yml b/.github/workflows/append-only-gate.yml
new file mode 100644
index 000000000..721a74295
--- /dev/null
+++ b/.github/workflows/append-only-gate.yml
@@ -0,0 +1,54 @@
+name: Append-only board files never shrink
+on:
+  pull_request:
+    paths:
+      # The protected files themselves. A PR that does not touch the board
+      # cannot shrink it, so there is nothing to check.
+      - .claude/board/LATEST_STATE.md
+      - .claude/board/EPIPHANIES.md
+      - .claude/board/PR_ARC_INVENTORY.md
+      - .claude/board/STATUS_BOARD.md
+      - .claude/board/ISSUES.md
+      - .claude/board/TECH_DEBT.md
+      - .claude/board/AGENT_LOG.md
+      - .claude/board/INTEGRATION_PLANS.md
+      # Broader than the eight above ON PURPOSE: the protected list lives in
+      # the script, and a future entry added there must be gated from the
+      # commit that adds it -- not from the next commit that happens to touch
+      # a file already on the list.
+      - .claude/board/**
+      # The gate's own two files, so a change to either is checked by itself.
+      - .claude/tools/append_only_gate.py
+      - .github/workflows/append-only-gate.yml
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}
+  cancel-in-progress: true
+
+permissions:
+  contents: read
+
+jobs:
+  no-shrink:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          # REQUIRED, not a tuning knob. The gate compares each protected file
+          # against the MERGE-BASE of this branch and the target -- the version
+          # the branch actually inherited, not wherever the target has moved
+          # since. A shallow checkout has no merge-base to compute, and the
+          # gate fails closed rather than passing on a comparison it could not
+          # make.
+          fetch-depth: 0
+
+      - name: Self-test the gate
+        # Runs first, and deliberately. If the gate cannot prove it both FIRES
+        # on a shortened file and STAYS SILENT on a grown one, its verdict on
+        # the real diff is worthless -- so the self-test gates the gate.
+        run: python3 .claude/tools/append_only_gate.py --self-test
+
+      - name: Check protected board files did not shrink
+        run: |
+          python3 .claude/tools/append_only_gate.py \
+            "origin/${{ github.event.pull_request.base.ref }}"