diff --git a/tools/hygiene/check-tick-history-shard-schema.sh b/tools/hygiene/check-tick-history-shard-schema.sh
new file mode 100755
index 00000000..82f20c34
--- /dev/null
+++ b/tools/hygiene/check-tick-history-shard-schema.sh
@@ -0,0 +1,173 @@
+#!/usr/bin/env bash
+#
+# tools/hygiene/check-tick-history-shard-schema.sh — validates
+# that per-tick shard files under docs/hygiene-history/ticks/
+# match the schema declared in docs/hygiene-history/ticks/README.md.
+#
+# Why this exists (2026-04-30):
+#   The col1 schema (first cell = exactly an ISO-8601 UTC timestamp)
+#   was repeatedly violated across 15+ shards on main (April 28-30)
+#   plus 6+ open PRs. Each violation was caught at PR-review time
+#   by Copilot, weeks-to-days after the shard was written. The
+#   structural fix is a hygiene check that catches it at write
+#   time instead of relying on each tick author's vigilance.
+#
+#   This is the "structural fix beats process discipline" pattern
+#   per Otto-341 + the rediscoverable-from-main invariant on
+#   docs/AUTONOMOUS-LOOP.md (added in PR #969): the invariant
+#   requires schema uniformity on main; this check is the
+#   mechanism that preserves it.
+#
+# What this checks:
+#   1. Shard file exists at the canonical path
+#      docs/hygiene-history/ticks/YYYY/MM/DD/<HHMMZ>.md
+#      (or the extended HHMMSSZ-<hash>.md form per the schema's
+#      high-concurrency option).
+#   2. First non-empty line is a 6-column markdown table row
+#      starting with `| YYYY-MM-DDTHH:MM:SSZ |` — exactly the ISO
+#      timestamp, no parenthetical, no extra prose, no leading
+#      whitespace beyond the standard `| `.
+#   3. The timestamp inside col1 matches the filename's `HHMMZ`
+#      — i.e. a shard at `2026/04/30/2304Z.md` must carry a col1
+#      timestamp of `2026-04-30T23:04:??Z` (any second).
+#
+# What this does NOT do:
+#   - Does NOT validate body content (cols 4-6). The body is
+#     intentionally free-form prose.
+#   - Does NOT enforce that col2 = `<model id>` or col3 =
+#     `<cron sentinel>` strictly. The schema's lower columns
+#     have drifted in practice (col3 commonly carries a commit
+#     SHA instead of the cron sentinel); enforcing that would
+#     be its own clean-up effort.
+#   - Does NOT detect the prefab pattern (col1 timestamp
+#     significantly ahead of commit-author time). That requires
+#     git-log access which isn't available pre-push for the
+#     current commit. See
+#     `memory/feedback_tick_history_prefabricated_shards_codex_finding_audit_trail_integrity_2026_04_30.md`
+#     for the deferred check.
+#
+# Exit codes:
+#   0 — all shards valid
+#   1 — one or more violations found (details on stderr)
+#   2 — invocation error (script bug or missing inputs)
+#
+# Composes with:
+#   - tools/hygiene/check-tick-history-order.sh (the legacy
+#     monolithic-table order check; this is the per-shard
+#     analogue for the post-shard-transport surface)
+#   - docs/hygiene-history/ticks/README.md (the schema this
+#     check enforces)
+#   - .github/workflows/gate.yml (where this should be wired
+#     as a lint job; not yet done — that's a follow-up)
+#
+# Current-state note (2026-04-30):
+#   On the main branch as of this script's introduction, ~5
+#   historical shards from April 28 violate this check (col1
+#   contains a parenthetical after the timestamp). Those 5
+#   shards are also implicated in the prefab-shard finding
+#   filed at
+#   `memory/feedback_tick_history_prefabricated_shards_codex_finding_audit_trail_integrity_2026_04_30.md`
+#   — fixing col1 mechanically would launder the body-level
+#   prefab claim. The check is therefore landed in DORMANT
+#   mode (not yet wired into CI); a future cleanup PR resolves
+#   the prefab-vs-schema decision before the check goes
+#   binding.
+
+set -euo pipefail
+
+ROOT="${REPO_ROOT:-$(git rev-parse --show-toplevel 2>/dev/null || echo .)}"
+SHARD_DIR="$ROOT/docs/hygiene-history/ticks"
+
+if [ ! -d "$SHARD_DIR" ]; then
+  echo "error: $SHARD_DIR does not exist" >&2
+  exit 2
+fi
+
+violations=0
+total=0
+
+# Find every shard file (skip README.md and any schema/* docs).
+while IFS= read -r -d '' shard; do
+  total=$((total + 1))
+  base="$(basename "$shard" .md)"
+  path_rel="${shard#"$ROOT/"}"
+
+  # Extract YYYY/MM/DD from path components.
+  parts="${path_rel#docs/hygiene-history/ticks/}"
+  yyyy="${parts%%/*}"
+  rest_a="${parts#*/}"
+  mm="${rest_a%%/*}"
+  rest_b="${rest_a#*/}"
+  dd="${rest_b%%/*}"
+
+  # Pull the HHMM from the filename (handle both HHMMZ and
+  # HHMMSSZ-<hash> forms).
+  if [[ "$base" =~ ^([0-9]{4})Z(-[0-9a-f]+)?$ ]]; then
+    hhmm="${BASH_REMATCH[1]}"
+  elif [[ "$base" =~ ^([0-9]{4})([0-9]{2})Z(-[0-9a-f]+)?$ ]]; then
+    hhmm="${BASH_REMATCH[1]}"
+  else
+    echo "VIOLATION: $path_rel — filename does not match HHMMZ.md or HHMMSSZ-<hash>.md schema" >&2
+    violations=$((violations + 1))
+    continue
+  fi
+  hh="${hhmm:0:2}"
+  mm_of_hour="${hhmm:2:2}"
+
+  # Read the first non-empty line.
+  first_line=""
+  while IFS= read -r line; do
+    if [ -n "${line// }" ]; then
+      first_line="$line"
+      break
+    fi
+  done < "$shard"
+
+  if [ -z "$first_line" ]; then
+    echo "VIOLATION: $path_rel — file is empty or whitespace-only" >&2
+    violations=$((violations + 1))
+    continue
+  fi
+
+  # Schema rule: first cell must be `| YYYY-MM-DDTHH:MM(:SS)?Z |`
+  # with no extra content before the next column boundary. Both
+  # the with-seconds form (`...T23:04:00Z`) and the no-seconds
+  # form (`...T23:04Z`) are valid ISO-8601 UTC; the schema
+  # in docs/hygiene-history/ticks/README.md does not pick a side.
+  # Capture the timestamp and verify it matches the path.
+  if [[ "$first_line" =~ ^\|\ ([0-9]{4}-[0-9]{2}-[0-9]{2}T[0-9]{2}:[0-9]{2}(:[0-9]{2})?Z)\ \|\  ]]; then
+    ts="${BASH_REMATCH[1]}"
+    ts_yyyy="${ts:0:4}"
+    ts_mm="${ts:5:2}"
+    ts_dd="${ts:8:2}"
+    ts_hh="${ts:11:2}"
+    ts_min="${ts:14:2}"
+
+    if [ "$ts_yyyy" != "$yyyy" ] || [ "$ts_mm" != "$mm" ] || [ "$ts_dd" != "$dd" ]; then
+      echo "VIOLATION: $path_rel — col1 timestamp $ts does not match path date $yyyy-$mm-$dd" >&2
+      violations=$((violations + 1))
+      continue
+    fi
+
+    if [ "$ts_hh" != "$hh" ] || [ "$ts_min" != "$mm_of_hour" ]; then
+      echo "VIOLATION: $path_rel — col1 timestamp ${ts_hh}:${ts_min} does not match filename ${hh}:${mm_of_hour}" >&2
+      violations=$((violations + 1))
+      continue
+    fi
+  else
+    echo "VIOLATION: $path_rel — col1 must be exactly '| YYYY-MM-DDTHH:MM:SSZ | ...' (no parenthetical, no extra prose)" >&2
+    echo "  got: $(echo "$first_line" | head -c 120)" >&2
+    violations=$((violations + 1))
+    continue
+  fi
+
+done < <(find "$SHARD_DIR" -type f -name '*.md' \
+  ! -name 'README.md' \
+  -print0)
+
+echo "checked $total shard files; $violations violations" >&2
+
+if [ "$violations" -gt 0 ]; then
+  exit 1
+fi
+exit 0