diff --git a/web/scripts/drift/_bom.ts b/web/scripts/drift/_bom.ts
new file mode 100644
index 00000000..629a1b2a
--- /dev/null
+++ b/web/scripts/drift/_bom.ts
@@ -0,0 +1,56 @@
+/**
+ * The BOM as the drift checks read it. hardware/bom/bom_v3.9.csv is the
+ * source of truth (hard rule 2 in AGENTS.md): check:bom holds it against the
+ * build guide's parts tables, check:bom-schematic against the schematic PDF.
+ */
+import { read } from "./_lib";
+
+export const BOM = "hardware/bom/bom_v3.9.csv";
+
+export type Row = Record<string, string>;
+
+// RFC 4180: a quoted cell may hold commas, line breaks and "" for a quote.
+function parse(text: string): string[][] {
+  const rows: string[][] = [];
+  let row: string[] = [];
+  let cell = "";
+  let quoted = false;
+  for (let i = 0; i < text.length; i++) {
+    const c = text[i];
+    if (quoted) {
+      if (c === '"' && text[i + 1] === '"') cell += text[++i];
+      else if (c === '"') quoted = false;
+      else cell += c;
+    } else if (c === '"') quoted = true;
+    else if (c === "," || c === "\n") {
+      row.push(cell);
+      cell = "";
+      if (c === "\n") {
+        rows.push(row);
+        row = [];
+      }
+    } else if (c !== "\r") cell += c;
+  }
+  if (cell || row.length) rows.push([...row, cell]);
+  return rows;
+}
+
+/** The BOM's rows, each keyed by the CSV's header line. */
+export function bom(): Row[] {
+  const [head, ...rows] = parse(read(BOM));
+  return rows.filter((r) => r.some(Boolean)).map((r) => Object.fromEntries(head.map((h, i) => [h, r[i] ?? ""])));
+}
+
+/**
+ * The designators a ref cell names. "J1" is one, and "SW1-SW4" is four, with
+ * a hyphen or an en dash. Any other cell names none: "-" off the board, the
+ * guide's "—", and "U1 (sockets)", whose notes say it is not a refdes.
+ */
+export function designators(ref: string): string[] {
+  const m = ref.trim().match(/^([A-Z]+)(\d+)(?:\s*[-–]\s*([A-Z]+)(\d+))?$/);
+  if (!m) return [];
+  const [, prefix, from, prefixTo, to] = m;
+  if (!to) return [prefix + from];
+  if (prefixTo !== prefix || +to <= +from) throw new Error(`${ref}: a range keeps one prefix and counts up`);
+  return Array.from({ length: +to - +from + 1 }, (_, i) => `${prefix}${+from + i}`);
+}
diff --git a/web/scripts/drift/bom-schematic.ts b/web/scripts/drift/bom-schematic.ts
new file mode 100644
index 00000000..0e75e6c5
--- /dev/null
+++ b/web/scripts/drift/bom-schematic.ts
@@ -0,0 +1,37 @@
+/**
+ * check:bom-schematic: the BOM's designators against the schematic's.
+ * hardware/pcb/schematic.pdf is KiCad output in stroke fonts, and pdftotext
+ * (poppler) reads every designator out of its text layer all the same, so
+ * this is strict from its first day.
+ *
+ * A token in the PDF is a designator when it has a designator's prefix: U,
+ * SW, J and C are on the board, R, D, Q, L and F are what a part added later
+ * would carry, and USB is what the v3.0 board's USB-C footprint was (hard
+ * rule 6). Pin names (S1, IO4) and ESP32 carry none of them. Without
+ * pdftotext this skips.
+ */
+import { execFileSync } from "node:child_process";
+import path from "node:path";
+
+import { BOM, bom, designators } from "./_bom";
+import { ROOT, compare, done, skip } from "./_lib";
+
+const PDF = "hardware/pcb/schematic.pdf";
+const PREFIXES = ["U", "SW", "J", "C", "R", "D", "Q", "L", "F", "USB"];
+
+let text: string | undefined;
+try {
+  text = execFileSync("pdftotext", ["-layout", path.join(ROOT, PDF), "-"], { encoding: "utf8" });
+} catch (e) {
+  if ((e as NodeJS.ErrnoException).code !== "ENOENT") throw e;
+  skip("designators", "pdftotext is not installed (poppler-utils)");
+}
+if (text !== undefined) {
+  const token = new RegExp(`\\b(?:${PREFIXES.join("|")})\\d+\\b`, "g");
+  compare(
+    "designators",
+    { label: BOM, items: bom().flatMap((r) => designators(r.ref)) },
+    { label: PDF, items: new Set(text.match(token)) },
+  );
+}
+done();
diff --git a/web/scripts/drift/bom.ts b/web/scripts/drift/bom.ts
new file mode 100644
index 00000000..64d4e8af
--- /dev/null
+++ b/web/scripts/drift/bom.ts
@@ -0,0 +1,78 @@
+/**
+ * check:bom: the BOM against the two parts tables in section 1 of
+ * BUILD_GUIDE.md. Hard rule 2 derives the guide's tables from the BOM, so a
+ * designator, a part or a quantity on one side only is drift.
+ *
+ * On the board a row is its designators, its quantity and its MPN. The names
+ * are worded apart ("Rotary encoder w/ switch" for "Rotary encoder with push
+ * switch") and are not compared. The sockets row has no designator: the BOM
+ * writes "U1 (sockets)" and the guide "—". Off the board there is neither, so
+ * the rows pair in order, and the guide's part keeps every word of the BOM's:
+ * "Sacrificial USB cable (it gets cut)" for "USB cable (sacrificial)". A
+ * hyphen and an en dash are the same in a quantity, so 6-12 is 6–12.
+ */
+import { BOM, bom, designators, type Row } from "./_bom";
+import { compare, done, read } from "./_lib";
+
+const GUIDE = "BUILD_GUIDE.md";
+
+// Bold, code, emphasis and link targets out; a link keeps its text.
+const plain = (s: string) =>
+  s
+    .replace(/\[([^\]]*)\]\([^)]*\)/g, "$1")
+    .replace(/[*`]/g, "")
+    .trim();
+const dashes = (s: string) => s.replace(/[–—]/g, "-");
+
+/** The first table under a heading of the guide, each row keyed by the table's header line. */
+function table(heading: string, columns: string[]): Row[] {
+  const lines = read(GUIDE).split("\n");
+  const at = lines.findIndex((l) => l.trim() === heading);
+  if (at < 0) throw new Error(`${GUIDE} has no "${heading}"`);
+  const rows: string[][] = [];
+  for (const line of lines.slice(at + 1)) {
+    if (line.startsWith("|")) rows.push(line.trim().replace(/^\||\|$/g, "").split("|").map(plain));
+    else if (rows.length || line.startsWith("#")) break;
+  }
+  const [head, ...body] = rows.filter((r) => !r.every((c) => /^:?-+:?$/.test(c)));
+  for (const c of columns) if (!head?.includes(c)) throw new Error(`${GUIDE}, ${heading}: no "${c}" column`);
+  return body.map((r) => Object.fromEntries(head.map((h, i) => [h, r[i] ?? ""])));
+}
+
+const rows = bom();
+
+// On the board: designators, quantity, MPN. The guide's MPN cell ends in the maker, "(Bourns)".
+const ref = (cell: string) => {
+  const d = designators(dashes(cell));
+  return d.length > 1 ? `${d[0]}-${d[d.length - 1]}` : (d[0] ?? "—");
+};
+const part = (r: string, qty: string, mpn: string) => `${ref(r)}: ${dashes(qty)} × ${mpn}`;
+compare(
+  "on the board",
+  { label: BOM, items: rows.filter((r) => r.category === "pcb").map((r) => part(r.ref, r.qty, r.mpn)) },
+  {
+    label: GUIDE,
+    items: table("### On the board", ["Ref", "Qty", "MPN (Manufacturer)"]).map((g) =>
+      part(g.Ref, g.Qty, g["MPN (Manufacturer)"].replace(/\s*\([^)]*\)$/, "")),
+    ),
+  },
+);
+
+// Off the board: quantity and part, in order. A guide row is named as the BOM's
+// row in its place when it holds every word of that row's part.
+const words = (s: string) => s.toLowerCase().match(/[\p{L}\p{N}]+/gu) ?? [];
+const off = rows.filter((r) => r.category === "off-board");
+console.log("off the board, rows pair in order, and a guide row with every word of its BOM row's part takes that name");
+compare(
+  "off the board",
+  { label: BOM, items: off.map((r) => `${dashes(r.qty)} × ${r.part}`) },
+  {
+    label: GUIDE,
+    items: table("### Off the board", ["Qty", "Part"]).map((g, i) => {
+      const has = new Set(words(g.Part));
+      const same = off[i] && words(off[i].part).every((w) => has.has(w));
+      return `${dashes(g.Qty)} × ${same ? off[i].part : g.Part}`;
+    }),
+  },
+);
+done();