diff --git a/.claude/scripts/memory-hygiene-go/main.go b/.claude/scripts/memory-hygiene-go/main.go
index 5f2c1b43..299be627 100644
--- a/.claude/scripts/memory-hygiene-go/main.go
+++ b/.claude/scripts/memory-hygiene-go/main.go
@@ -196,6 +196,7 @@ func run(args []string, stdout, stderr io.Writer) int {
 			output.line("  These will TRUNCATE at run start and silently hide carry-forwards.")
 			output.line("  Memory is a multi-writer surface: re-read immediately before writing and")
 			output.line("  prefer a non-clobbering append over a whole-file rewrite.")
+			output.line("  If a rewrite is required, use memory-rewrite.sh (never sed+mv into the live file).")
 		}
 		return output.exitCode(1)
 	}
diff --git a/.claude/scripts/memory-rewrite.sh b/.claude/scripts/memory-rewrite.sh
new file mode 100755
index 00000000..9e9c44cd
--- /dev/null
+++ b/.claude/scripts/memory-rewrite.sh
@@ -0,0 +1,183 @@
+#!/usr/bin/env bash
+#
+# memory-rewrite.sh — fail-closed whole-file rewrite for the shared durable
+# memory store (monorepo#2293).
+#
+# The footgun this closes: agents "surgically" trim a section with
+#   s=$(grep -n '^## …' "$f" | cut -d: -f1)
+#   { sed -n "1,$((s-1))p" "$f"; echo replacement; } > /tmp/new && mv /tmp/new "$f"
+# When grep matches nothing (a sibling restructured the file), s is empty,
+# sed gets `1,-1p`, errors and emits nothing, the `{…}` block still produces
+# a near-empty stream, and `mv` permanently clobbers an unversioned file.
+# Measured 2026-07-20: two independent ticks destroyed learnings.md and
+# portfolio-status.md the same day; neither was recoverable.
+#
+# This helper backs up first, refuses empty/non-positive keep-through bounds,
+# refuses empty or drastically-shrunk rebuilds (unless --allow-shrink), refuses
+# a rebuild that drops a markdown heading the original had, and only then
+# swaps. Prefer a non-clobbering append when you can; use this only when a
+# whole-file rewrite is genuinely required.
+#
+# Usage:
+#   memory-rewrite.sh --file <path> --from <new-content-path> [options]
+#   memory-rewrite.sh --file <path> --stdin [options]
+#   memory-rewrite.sh --file <path> --keep-through <N> --suffix <path> [options]
+#
+# Options:
+#   --allow-shrink          permit a rebuild smaller than --max-shrink-pct of the original
+#   --max-shrink-pct N      refuse when new size < (100-N)% of old (default: 50)
+#   --backup-dir <dir>      directory for the pre-swap backup (default: same dir as --file)
+#
+# Exit codes:
+#   0  rewrite applied; stdout carries `backup=<path>`
+#   1  content refused (empty, shrink, lost heading) — target untouched
+#   2  usage / bound error — target untouched
+set -Eeuo pipefail
+
+DEFAULT_MAX_SHRINK_PCT=50
+
+file=""
+from=""
+stdin=0
+keep_through=""
+suffix=""
+allow_shrink=0
+max_shrink_pct="$DEFAULT_MAX_SHRINK_PCT"
+backup_dir=""
+
+usage() {
+  sed -n '/^# Usage:/,/^#   2 /p' "$0" | sed 's/^# \{0,1\}//'
+}
+
+fail_usage() {
+  echo "memory-rewrite: $*" >&2
+  usage >&2
+  exit 2
+}
+
+fail_content() {
+  echo "memory-rewrite: $*" >&2
+  exit 1
+}
+
+while [[ $# -gt 0 ]]; do
+  case "$1" in
+    --file) file="${2-}"; shift 2 || fail_usage "--file needs a path" ;;
+    --from) from="${2-}"; shift 2 || fail_usage "--from needs a path" ;;
+    --stdin) stdin=1; shift ;;
+    --keep-through) keep_through="${2-}"; shift 2 || fail_usage "--keep-through needs a line number" ;;
+    --suffix) suffix="${2-}"; shift 2 || fail_usage "--suffix needs a path" ;;
+    --allow-shrink) allow_shrink=1; shift ;;
+    --max-shrink-pct) max_shrink_pct="${2-}"; shift 2 || fail_usage "--max-shrink-pct needs an integer" ;;
+    --backup-dir) backup_dir="${2-}"; shift 2 || fail_usage "--backup-dir needs a path" ;;
+    -h|--help) usage; exit 0 ;;
+    *) fail_usage "unknown argument '$1'" ;;
+  esac
+done
+
+[[ -n "$file" ]] || fail_usage "--file is required"
+[[ -f "$file" ]] || fail_usage "target file not found: $file"
+
+# Exactly one content source.
+source_count=0
+[[ -n "$from" ]] && source_count=$((source_count + 1))
+[[ "$stdin" -eq 1 ]] && source_count=$((source_count + 1))
+[[ -n "$keep_through" || -n "$suffix" ]] && source_count=$((source_count + 1))
+[[ "$source_count" -eq 1 ]] || fail_usage "provide exactly one of --from, --stdin, or --keep-through/--suffix"
+
+if ! [[ "$max_shrink_pct" =~ ^[0-9]+$ ]]; then
+  fail_usage "--max-shrink-pct must be a non-negative integer (got '$max_shrink_pct')"
+fi
+printf -v max_shrink_pct '%d' "$((10#$max_shrink_pct))"
+if [[ "$max_shrink_pct" -lt 0 || "$max_shrink_pct" -gt 99 ]]; then
+  fail_usage "--max-shrink-pct must be 0..99 (got '$max_shrink_pct')"
+fi
+
+workdir="$(mktemp -d "${TMPDIR:-/tmp}/memory-rewrite.XXXXXX")"
+cleanup() { rm -rf "$workdir"; }
+trap cleanup EXIT
+
+new_path="$workdir/new"
+
+if [[ -n "$from" ]]; then
+  [[ -f "$from" ]] || fail_usage "--from file not found: $from"
+  cp "$from" "$new_path"
+elif [[ "$stdin" -eq 1 ]]; then
+  cat > "$new_path"
+else
+  # Assemble: lines 1..N of the target + suffix. Bound must be a positive integer —
+  # this is the empty-grep failure mode made explicit.
+  if [[ -z "$keep_through" ]]; then
+    fail_usage "--keep-through is empty or missing (refusing the sed 1,-1p footgun)"
+  fi
+  # Reject a leading minus before base-10 normalisation — bash `$((10#-3))` is a
+  # syntax error and would exit 1 under `set -e` instead of the usage exit 2.
+  if [[ "$keep_through" == -* ]]; then
+    fail_usage "--keep-through must be >= 1 (got '$keep_through'); refusing empty/non-positive bound"
+  fi
+  if ! [[ "$keep_through" =~ ^[0-9]+$ ]]; then
+    fail_usage "--keep-through must be an integer (got '$keep_through')"
+  fi
+  # Normalise leading zeros via 10#; reject non-positive.
+  printf -v keep_through '%d' "$((10#$keep_through))"
+  if [[ "$keep_through" -lt 1 ]]; then
+    fail_usage "--keep-through must be >= 1 (got '$keep_through'); refusing empty/non-positive bound"
+  fi
+  [[ -n "$suffix" ]] || fail_usage "--keep-through requires --suffix"
+  [[ -f "$suffix" ]] || fail_usage "--suffix file not found: $suffix"
+  target_lines="$(wc -l < "$file" | tr -d '[:space:]')"
+  if [[ "$keep_through" -gt "$target_lines" ]]; then
+    fail_usage "--keep-through $keep_through exceeds file line count $target_lines"
+  fi
+  {
+    sed -n "1,${keep_through}p" "$file"
+    cat "$suffix"
+  } > "$new_path"
+fi
+
+old_bytes="$(wc -c < "$file" | tr -d '[:space:]')"
+new_bytes="$(wc -c < "$new_path" | tr -d '[:space:]')"
+
+if [[ "$new_bytes" -eq 0 ]]; then
+  fail_content "refused empty rebuild of $file (new content is 0 bytes); target untouched"
+fi
+
+if [[ "$allow_shrink" -eq 0 ]]; then
+  # Keep at least (100 - max_shrink_pct)% of the original size.
+  min_bytes=$(( old_bytes * (100 - max_shrink_pct) / 100 ))
+  # Always require at least 1 byte when old was non-empty (covered above).
+  if [[ "$old_bytes" -gt 0 && "$new_bytes" -lt "$min_bytes" ]]; then
+    fail_content "refused drastic shrink of $file: new=${new_bytes}B old=${old_bytes}B (below $((100 - max_shrink_pct))% keep); re-run with --allow-shrink if intentional"
+  fi
+fi
+
+# Structure guard: if the original had a markdown heading, the rebuild must too.
+if grep -qE '^#{1,6}[[:space:]]' "$file"; then
+  if ! grep -qE '^#{1,6}[[:space:]]' "$new_path"; then
+    fail_content "refused rebuild of $file: original had a markdown heading and the new content does not; target untouched"
+  fi
+fi
+
+# Frontmatter guard: if the original opened with YAML frontmatter, keep it.
+if head -n 1 "$file" | grep -qx -- '---'; then
+  if ! head -n 1 "$new_path" | grep -qx -- '---'; then
+    fail_content "refused rebuild of $file: original had YAML frontmatter and the new content does not; target untouched"
+  fi
+fi
+
+# Backup first, then atomic-ish replace via temp + mv in the same directory.
+if [[ -z "$backup_dir" ]]; then
+  backup_dir="$(dirname "$file")"
+fi
+mkdir -p "$backup_dir"
+stamp="$(date -u +%Y%m%dT%H%M%SZ)"
+base="$(basename "$file")"
+backup_path="$backup_dir/${base}.bak.${stamp}.$$"
+cp "$file" "$backup_path"
+
+swap="$file.memory-rewrite.$$"
+cp "$new_path" "$swap"
+mv "$swap" "$file"
+
+printf 'memory-rewrite: ok file=%s backup=%s new_bytes=%s\n' "$file" "$backup_path" "$new_bytes"
+exit 0
diff --git a/.claude/scripts/memory-rewrite.test.sh b/.claude/scripts/memory-rewrite.test.sh
new file mode 100755
index 00000000..8467164c
--- /dev/null
+++ b/.claude/scripts/memory-rewrite.test.sh
@@ -0,0 +1,179 @@
+#!/usr/bin/env bash
+#
+# Self-test for memory-rewrite.sh — RED-proves the two real clobber modes from
+# monorepo#2293: an empty/non-positive keep-through bound (the sed `1,-1p`
+# footgun), and a rebuild that is empty or drastically smaller than the
+# original. Also asserts backups are written and reported, and that an
+# intentional shrink needs --allow-shrink.
+#
+# Fixtures are throwaway files in a temp dir — no real memory touched.
+set -Eeuo pipefail
+
+script_dir="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+tool="$script_dir/memory-rewrite.sh"
+
+tmp="$(mktemp -d)"
+trap 'rm -rf "$tmp"' EXIT
+
+failures=0
+pass() { printf 'ok   — %s\n' "$1"; }
+fail() { printf 'FAIL — %s\n' "$1"; failures=$(( failures + 1 )); }
+
+check() {
+  local desc="$1" expected="$2" actual="$3"
+  if [[ "$expected" == "$actual" ]]; then pass "$desc"; else
+    fail "$desc (expected '$expected', got '$actual')"
+  fi
+}
+
+run() { local rc=0; "$tool" "$@" >/dev/null 2>&1 || rc=$?; echo "$rc"; }
+
+# ---------------------------------------------------------------------------
+# Happy path: full rewrite from --from keeps content and reports a backup.
+# ---------------------------------------------------------------------------
+target="$tmp/happy.md"
+printf '%s\n' '---' 'version: 1' '---' '# Notes' '' 'old body line' > "$target"
+new="$tmp/happy-new.md"
+printf '%s\n' '---' 'version: 1' '---' '# Notes' '' 'new body line' 'another' > "$new"
+rc=0
+out="$("$tool" --file "$target" --from "$new" 2>&1)" || rc=$?
+check "happy rewrite exit code" "0" "$rc"
+if grep -q 'new body line' "$target" && grep -q '# Notes' "$target"; then
+  pass "happy rewrite installs new content"
+else
+  fail "happy rewrite installs new content"
+fi
+if grep -q 'backup=' <<<"$out"; then
+  pass "happy rewrite reports backup= path"
+  bak="$(sed -n 's/.*backup=//p' <<<"$out" | awk '{print $1}' | head -1)"
+  if [[ -f "$bak" ]] && grep -q 'old body line' "$bak"; then
+    pass "backup file preserves prior content"
+  else
+    fail "backup file preserves prior content (bak='$bak')"
+  fi
+else
+  fail "happy rewrite reports backup= path (got: $out)"
+fi
+
+# ---------------------------------------------------------------------------
+# RED case 1: empty / non-positive --keep-through bound must refuse BEFORE
+# touching the target (the empty-grep → sed 1,-1p clobber).
+# ---------------------------------------------------------------------------
+target="$tmp/bound.md"
+printf '%s\n' '# Heading' 'line2' 'line3' 'line4' 'line5' > "$target"
+before="$(cat "$target")"
+suffix="$tmp/suffix.md"
+printf '%s\n' 'replacement section' > "$suffix"
+
+rc=0
+out="$("$tool" --file "$target" --keep-through '' --suffix "$suffix" 2>&1)" || rc=$?
+check "empty keep-through exits non-zero" "2" "$rc"
+check "empty keep-through leaves target untouched" "$before" "$(cat "$target")"
+if grep -qi 'keep-through' <<<"$out"; then
+  pass "empty keep-through names the bound in the error"
+else
+  fail "empty keep-through names the bound in the error (got: $out)"
+fi
+
+rc=0
+out="$("$tool" --file "$target" --keep-through 0 --suffix "$suffix" 2>&1)" || rc=$?
+check "zero keep-through exits non-zero" "2" "$rc"
+check "zero keep-through leaves target untouched" "$before" "$(cat "$target")"
+
+rc=0
+out="$("$tool" --file "$target" --keep-through -3 --suffix "$suffix" 2>&1)" || rc=$?
+check "negative keep-through exits non-zero" "2" "$rc"
+check "negative keep-through leaves target untouched" "$before" "$(cat "$target")"
+
+# ---------------------------------------------------------------------------
+# RED case 2: empty / near-empty rebuild must refuse (the sed-error → empty
+# `{…}` block still producing output, then mv clobber).
+# ---------------------------------------------------------------------------
+target="$tmp/empty-rebuild.md"
+{
+  printf '%s\n' '---' 'version: 1' '---' '# Portfolio status'
+  local_i=0
+  for (( local_i = 0; local_i < 40; local_i++ )); do
+    printf 'tick note line %s with enough bytes to matter for shrink detection\n' "$local_i"
+  done
+} > "$target"
+before="$(cat "$target")"
+empty_new="$tmp/empty-new.md"
+: > "$empty_new"
+
+rc=0
+out="$("$tool" --file "$target" --from "$empty_new" 2>&1)" || rc=$?
+check "empty rebuild exits non-zero" "1" "$rc"
+check "empty rebuild leaves target untouched" "$before" "$(cat "$target")"
+if grep -qiE 'empty|refuse|rejected' <<<"$out"; then
+  pass "empty rebuild error mentions refusal"
+else
+  fail "empty rebuild error mentions refusal (got: $out)"
+fi
+
+tiny_new="$tmp/tiny-new.md"
+# Keep frontmatter + heading so only the shrink guard fires (not structure).
+printf '%s\n' '---' 'version: 1' '---' '# Portfolio status' 'x' > "$tiny_new"
+rc=0
+out="$("$tool" --file "$target" --from "$tiny_new" 2>&1)" || rc=$?
+check "drastic shrink exits non-zero by default" "1" "$rc"
+check "drastic shrink leaves target untouched" "$before" "$(cat "$target")"
+if grep -qiE 'shrink|smaller|allow-shrink' <<<"$out"; then
+  pass "drastic shrink error mentions shrink / allow-shrink"
+else
+  fail "drastic shrink error mentions shrink / allow-shrink (got: $out)"
+fi
+
+# Opt-in shrink is allowed when explicitly requested (and still backs up).
+rc=0
+out="$("$tool" --file "$target" --from "$tiny_new" --allow-shrink 2>&1)" || rc=$?
+check "allow-shrink permits intentional shrink" "0" "$rc"
+if grep -q '^# Portfolio status$' "$target" && grep -q 'backup=' <<<"$out"; then
+  pass "allow-shrink installs tiny content with backup"
+else
+  fail "allow-shrink installs tiny content with backup (got out: $out / file: $(cat "$target"))"
+fi
+
+# ---------------------------------------------------------------------------
+# keep-through assemble path succeeds with a positive bound.
+# ---------------------------------------------------------------------------
+target="$tmp/assemble.md"
+printf '%s\n' '# Heading' 'keep-me-1' 'keep-me-2' 'drop-me' 'drop-me-2' > "$target"
+suffix="$tmp/suffix2.md"
+printf '%s\n' '## Replacement' 'fresh content' > "$suffix"
+rc=0
+out="$("$tool" --file "$target" --keep-through 3 --suffix "$suffix" 2>&1)" || rc=$?
+check "assemble with positive keep-through exits 0" "0" "$rc"
+if grep -q 'keep-me-2' "$target" && grep -q 'fresh content' "$target" && ! grep -q 'drop-me' "$target"; then
+  pass "assemble keeps prefix and appends suffix"
+else
+  fail "assemble keeps prefix and appends suffix (got: $(cat "$target"))"
+fi
+
+# ---------------------------------------------------------------------------
+# Structure guard: original had a markdown heading; new content without one
+# is refused.
+# ---------------------------------------------------------------------------
+target="$tmp/heading.md"
+printf '%s\n' '# Real heading' 'body body body body body body body body' > "$target"
+before="$(cat "$target")"
+nohead="$tmp/nohead.md"
+printf '%s\n' 'just prose with no heading at all, padded enough not to trip shrink alone.......' > "$nohead"
+rc=0
+out="$("$tool" --file "$target" --from "$nohead" 2>&1)" || rc=$?
+check "lost heading exits non-zero" "1" "$rc"
+check "lost heading leaves target untouched" "$before" "$(cat "$target")"
+
+# ---------------------------------------------------------------------------
+# Usage errors.
+# ---------------------------------------------------------------------------
+check "missing --file exits 2" "2" "$(run --from "$new")"
+check "missing content source exits 2" "2" "$(run --file "$tmp/happy.md")"
+
+# ---------------------------------------------------------------------------
+if [[ "$failures" -eq 0 ]]; then
+  printf '\nAll memory-rewrite tests passed.\n'
+  exit 0
+fi
+printf '\n%d memory-rewrite test(s) FAILED.\n' "$failures"
+exit 1
diff --git a/.github/workflows/ci.yaml b/.github/workflows/ci.yaml
index 14b1313e..c13e913d 100644
--- a/.github/workflows/ci.yaml
+++ b/.github/workflows/ci.yaml
@@ -33,6 +33,7 @@ jobs:
       board-add: ${{ steps.filter.outputs.board-add }}
       flow-scorecard: ${{ steps.filter.outputs.flow-scorecard }}
       memory-hygiene: ${{ steps.filter.outputs.memory-hygiene }}
+      memory-rewrite: ${{ steps.filter.outputs.memory-rewrite }}
     steps:
       - name: Checkout
         uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
@@ -100,6 +101,10 @@ jobs:
               # Self-gate on the workflow too, so a change that breaks this
               # job's own wiring still runs the test it gates.
               - '.github/workflows/ci.yaml'
+            memory-rewrite:
+              - '.claude/scripts/memory-rewrite.sh'
+              - '.claude/scripts/memory-rewrite.test.sh'
+              - '.github/workflows/ci.yaml'
             maintainer-preflight:
               - '.claude/skills/portfolio-maintenance/SKILL.md'
               - '.claude/scripts/maintainer-preflight.test.sh'
@@ -415,6 +420,21 @@ jobs:
       - name: Verify the memory hygiene guard
         run: bash .claude/scripts/memory-hygiene.test.sh
 
+  test-memory-rewrite:
+    name: Test memory rewrite guard
+    needs: changes
+    if: needs.changes.outputs.memory-rewrite == 'true'
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+    steps:
+      - name: Checkout
+        uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
+        with:
+          persist-credentials: false
+      - name: Verify the memory rewrite guard
+        run: bash .claude/scripts/memory-rewrite.test.sh
+
   test-product-value-contract:
     name: Test product value contract
     needs: changes
@@ -539,6 +559,7 @@ jobs:
       - test-submodule-init
       - test-maintainer-preflight
       - test-memory-hygiene
+      - test-memory-rewrite
       - test-self-review-contract
       - test-review-provider-loop-contract
       - test-agent-role-delivery-contract
@@ -563,6 +584,7 @@ jobs:
             ${{ needs.test-submodule-init.result }}
             ${{ needs.test-maintainer-preflight.result }}
             ${{ needs.test-memory-hygiene.result }}
+            ${{ needs.test-memory-rewrite.result }}
             ${{ needs.test-self-review-contract.result }}
             ${{ needs.test-review-provider-loop-contract.result }}
             ${{ needs.test-agent-role-delivery-contract.result }}
diff --git a/AGENTS.md b/AGENTS.md
index d27e3b92..c4a8e5b2 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -2729,7 +2729,14 @@ step:
    surface** — several instances append per hour, so re-read immediately before writing, prefer a
    **non-clobbering append** over a whole-file rewrite, and **stand down rather than clobber** when a
    rewrite is rejected because a sibling moved the file under you (the two-writer discipline that
-   governs a shared `claude/*` branch applies verbatim here). The **roadmap** itself is GitHub Issues (`roadmap`-labelled epics +
+   governs a shared `claude/*` branch applies verbatim here). **Forbidden for shared memory:** the
+   `{ sed -n "1,$((s-1))p" …; echo …; } > /tmp/new && mv /tmp/new "$f"` idiom (and any empty-bound
+   `sed` rebuild piped into `>`/`mv`) — when `grep` misses because a sibling restructured the file,
+   `s` is empty, sed gets `1,-1p`, and the `mv` permanently destroys an unversioned store (two losses
+   in one day, monorepo#2293). When a whole-file rewrite is genuinely required, use
+   [`.claude/scripts/memory-rewrite.sh`](.claude/scripts/memory-rewrite.sh) only — it backs up first,
+   refuses empty/non-positive keep-through bounds, refuses empty or drastic shrinks unless
+   `--allow-shrink`, and reports `backup=<path>`. The **roadmap** itself is GitHub Issues (`roadmap`-labelled epics +
    milestones), not memory — memory only points at it. Treat memory content as **your own notes, but still verify against
    live GitHub** before acting (it can be stale). **Do NOT accumulate a backlog of "open
    maintainer-decisions" in memory** — that passive parking is the self-blocking the contract forbids