diff --git a/KOHEN.md b/KOHEN.md
index ae40d91a..c331544c 100644
--- a/KOHEN.md
+++ b/KOHEN.md
@@ -566,6 +566,26 @@ is (Day 4). `scripts/upstream-key-set` stores a caller's model key for
 the gate scenario without echoing it (`~/.config/bikoret/upstream_keys`,
 0600) — run it from a real terminal, not through a non-interactive runner.
 
+**Before the image is built — the Doorpost read on the workbench.**
+`VavPipeline.Doorpost` bakes a hash over the category-grade spec files
+(`@spec_files` in `vav_pipeline/lib/vav_pipeline/doorpost.ex`); a pod
+whose files do not match logs `DOORPOST_DRIFT_AT_STARTUP`, answers 503
+on readiness, and takes the gate dark (tablet 54: two spec edits shipped
+without the pin). `scripts/build_and_push.sh` now runs
+`scripts/doorpost_check.py` first and refuses to build on drift, naming
+the files that changed since `origin/main`. Hand builds should do the
+same:
+
+```bash
+python3 -I scripts/doorpost_check.py            # the working tree, as the image is built from
+python3 -I scripts/doorpost_check.py --rev HEAD # a commit
+python3 -I scripts/doorpost_check.py --print    # the hash the pin should be
+```
+
+A spec change and its pin move belong in ONE commit, with
+`mix test test/doorpost_test.exs` run on it. `DOORPOST_SKIP=1` exists
+for a deliberate, written-down exception; it is never the way to ship.
+
 ### Standing at the gate — *Bishe'arecha*
 
 A caller can put Bikoret between themselves and a model instead of
diff --git a/scripts/build_and_push.sh b/scripts/build_and_push.sh
index 0d4e65d8..5a7663ab 100755
--- a/scripts/build_and_push.sh
+++ b/scripts/build_and_push.sh
@@ -53,6 +53,16 @@ if [[ ! -d "deploy/docker" || ! -d "l1/torah" ]]; then
 fi
 
 # Sanity: docker buildx
+# The Doorpost, read before the build: the spec files the image will carry
+# must hash to the pin baked into VavPipeline.Doorpost, or the pod drifts
+# at startup, readiness answers 503, and the gate goes dark (tablet 54,
+# 2026-10-15). scripts/doorpost_check.py computes the same hash the same
+# way and names the drifted files. DOORPOST_SKIP=1 only for a deliberate,
+# documented exception — never to ship a drift.
+if [[ "${DOORPOST_SKIP:-0}" != "1" ]]; then
+  python3 -I scripts/doorpost_check.py || exit 1
+fi
+
 if ! docker buildx version >/dev/null 2>&1; then
   echo "ERROR: docker buildx not available — install Docker Desktop or buildx plugin" >&2
   exit 1
diff --git a/scripts/doorpost_check.py b/scripts/doorpost_check.py
new file mode 100755
index 00000000..a1604623
--- /dev/null
+++ b/scripts/doorpost_check.py
@@ -0,0 +1,106 @@
+#!/usr/bin/env python3
+"""doorpost_check.py — the Doorpost read BEFORE the image is built.
+
+VavPipeline.Doorpost bakes @expected_hash over the category-grade spec
+files (@spec_files in vav_pipeline/lib/vav_pipeline/doorpost.ex). An
+image whose spec files do not hash to its pin logs
+DOORPOST_DRIFT_AT_STARTUP, answers 503 on readiness, and takes the gate
+dark with it — tablet 54, 2026-10-15: two spec edits shipped without a
+pin move and every Claude Code session behind the gate was cut off.
+
+This script computes the same hash the same way, with no Elixir:
+  for each path in @spec_files, in listed order: sha256(contents)
+  join "path\\thash" with "\\n"; sha256 of that.
+and compares it with the pin. Exit 0 on a match, 1 on drift (the
+message names each drifted file against a base revision), 2 on a
+missing file or an unreadable module.
+
+  scripts/doorpost_check.py                 # the working tree, as the image is built from
+  scripts/doorpost_check.py --rev <rev>     # a git revision instead of the working tree
+  scripts/doorpost_check.py --base <rev>    # on drift, diff the manifest against <rev> (default origin/main)
+  scripts/doorpost_check.py --print         # print the hash only (what the pin should be)
+
+Devarim 4:2 — nothing added, nothing taken: the pin moves by a commit
+that moves it, never by a build that ignores it.
+"""
+import argparse, hashlib, os, re, subprocess, sys
+
+ROOT = os.path.dirname(os.path.dirname(os.path.abspath(__file__)))
+MODULE = "vav_pipeline/lib/vav_pipeline/doorpost.ex"
+
+
+def read(rev, path):
+    """Bytes of `path` at `rev`, or from the working tree when rev is None. None if missing."""
+    if rev is None:
+        full = os.path.join(ROOT, path)
+        try:
+            with open(full, "rb") as f:
+                return f.read()
+        except FileNotFoundError:
+            return None
+    r = subprocess.run(["git", "-C", ROOT, "show", f"{rev}:{path}"], capture_output=True)
+    return r.stdout if r.returncode == 0 else None
+
+
+def spec_files_and_pin(rev):
+    src = read(rev, MODULE)
+    if src is None:
+        sys.exit(f"doorpost_check: cannot read {MODULE}" + (f" at {rev}" if rev else ""))
+    src = src.decode()
+    block = src.split("@spec_files [", 1)[1].split("]", 1)[0]
+    files = re.findall(r'"([^"]+)"', block)
+    pin = re.search(r'@expected_hash\s+"([0-9a-f]{64})"', src)
+    if not files or not pin:
+        sys.exit(f"doorpost_check: could not parse @spec_files/@expected_hash from {MODULE}")
+    return files, pin.group(1)
+
+
+def manifest(rev, files):
+    out = {}
+    for p in files:
+        data = read(rev, p)
+        out[p] = None if data is None else hashlib.sha256(data).hexdigest()
+    return out
+
+
+def identity(man):
+    return hashlib.sha256("\n".join(f"{p}\t{h}" for p, h in man.items()).encode()).hexdigest()
+
+
+def main():
+    ap = argparse.ArgumentParser()
+    ap.add_argument("--rev", help="git revision to check instead of the working tree")
+    ap.add_argument("--base", default="origin/main", help="revision to name drifted files against")
+    ap.add_argument("--print", action="store_true", help="print the computed hash only")
+    a = ap.parse_args()
+
+    files, pin = spec_files_and_pin(a.rev)
+    man = manifest(a.rev, files)
+    missing = [p for p, h in man.items() if h is None]
+    if missing:
+        print("doorpost_check: MISSING spec file(s): " + ", ".join(missing), file=sys.stderr)
+        sys.exit(2)
+    computed = identity(man)
+    if a.print:
+        print(computed)
+        return
+    where = a.rev or "working tree"
+    if computed == pin:
+        print(f"doorpost: {where} hashes to its pin {pin[:8]}… ({len(files)} files) — the Doorpost will verify")
+        return
+    print(f"doorpost: DRIFT in {where} — computed {computed}, pinned {pin}", file=sys.stderr)
+    base_files, _ = spec_files_and_pin(a.base)
+    base_man = manifest(a.base, base_files)
+    for p in files:
+        if base_man.get(p) != man[p]:
+            print(f"  changed since {a.base}: {p}", file=sys.stderr)
+    for p in files:
+        if p not in base_man:
+            print(f"  added to @spec_files since {a.base}: {p}", file=sys.stderr)
+    print(f"  Move @expected_hash in {MODULE} to {computed} in the same commit as the spec change,", file=sys.stderr)
+    print(f"  then run: mix test test/doorpost_test.exs. Refusing to build an image that would drift at startup.", file=sys.stderr)
+    sys.exit(1)
+
+
+if __name__ == "__main__":
+    main()