diff --git a/firmware/patternflow/src/core_shared.h b/firmware/patternflow/src/core_shared.h
new file mode 100644
index 00000000..4a7f65ae
--- /dev/null
+++ b/firmware/patternflow/src/core_shared.h
@@ -0,0 +1,45 @@
+// GENERATED by web/scripts/gen-shared.ts from shared/constants.json. Do not edit.
+// Change the JSON and run `npm run gen:shared` in web/. check:shared fails
+// while this file says anything else.
+//
+// License: MIT
+#pragma once
+
+// The geometries PANEL_GEOMETRY picks from in config.h. Two 64x64 modules in a
+// chain make the same 128x64 canvas as one 128x64 module.
+#define PF_SHARED_PANEL_GEOM_128x64_SINGLE 1
+#define PF_SHARED_PANEL_GEOM_128x64_SINGLE_PHYS_W 128
+#define PF_SHARED_PANEL_GEOM_128x64_SINGLE_PHYS_H 64
+#define PF_SHARED_PANEL_GEOM_128x64_SINGLE_CHAIN 1
+#define PF_SHARED_PANEL_GEOM_128x64_SINGLE_RES_W 128
+#define PF_SHARED_PANEL_GEOM_128x64_SINGLE_RES_H 64
+#define PF_SHARED_PANEL_GEOM_64x64_SINGLE 2
+#define PF_SHARED_PANEL_GEOM_64x64_SINGLE_PHYS_W 64
+#define PF_SHARED_PANEL_GEOM_64x64_SINGLE_PHYS_H 64
+#define PF_SHARED_PANEL_GEOM_64x64_SINGLE_CHAIN 1
+#define PF_SHARED_PANEL_GEOM_64x64_SINGLE_RES_W 64
+#define PF_SHARED_PANEL_GEOM_64x64_SINGLE_RES_H 64
+#define PF_SHARED_PANEL_GEOM_64x64_CHAIN2 3
+#define PF_SHARED_PANEL_GEOM_64x64_CHAIN2_PHYS_W 64
+#define PF_SHARED_PANEL_GEOM_64x64_CHAIN2_PHYS_H 64
+#define PF_SHARED_PANEL_GEOM_64x64_CHAIN2_CHAIN 2
+#define PF_SHARED_PANEL_GEOM_64x64_CHAIN2_RES_W 128
+#define PF_SHARED_PANEL_GEOM_64x64_CHAIN2_RES_H 64
+#define PF_SHARED_PANEL_GEOM_DEFAULT PF_SHARED_PANEL_GEOM_128x64_SINGLE
+
+// The PEC11R-4220F-S0024 has 24 detents a turn. Two turns cross a parameter's
+// whole range, so 48 detents do.
+#define PF_SHARED_KNOB_COUNT 4
+#define PF_SHARED_DETENTS_PER_TURN 24
+#define PF_SHARED_TURNS_PER_FULL_RANGE 2
+
+// Quadrature edges in one detent. A press held past LONG_PRESS_MS is a long
+// press, and button edges closer than BUTTON_DEBOUNCE_MS are ignored.
+#define PF_SHARED_EDGES_PER_DETENT 4
+#define PF_SHARED_LONG_PRESS_MS 1000
+#define PF_SHARED_BUTTON_DEBOUNCE_MS 50
+
+// The absolute bus speaks integers 0 to BUS_MAX. A pattern that reads only knob
+// deltas gets one click for every BUS_UNITS_PER_CLICK a held channel moves.
+#define PF_SHARED_BUS_MAX 1000
+#define PF_SHARED_BUS_UNITS_PER_CLICK 10
diff --git a/shared/constants.json b/shared/constants.json
new file mode 100644
index 00000000..7fce1275
--- /dev/null
+++ b/shared/constants.json
@@ -0,0 +1,29 @@
+{
+  "//": "Values the firmware and the site both carry. Change one here, then run npm run gen:shared in web/. It writes firmware/patternflow/src/core_shared.h and web/src/lib/shared/constants.ts, and check:shared fails while either is stale.",
+  "panel": {
+    "//": "The geometries PANEL_GEOMETRY picks from in config.h. Two 64x64 modules in a chain make the same 128x64 canvas as one 128x64 module.",
+    "PANEL_GEOM_DEFAULT": "128x64_SINGLE",
+    "PANEL_GEOMETRIES": {
+      "128x64_SINGLE": { "id": 1, "physW": 128, "physH": 64, "chain": 1 },
+      "64x64_SINGLE": { "id": 2, "physW": 64, "physH": 64, "chain": 1 },
+      "64x64_CHAIN2": { "id": 3, "physW": 64, "physH": 64, "chain": 2 }
+    }
+  },
+  "knobs": {
+    "//": "The PEC11R-4220F-S0024 has 24 detents a turn. Two turns cross a parameter's whole range, so 48 detents do.",
+    "KNOB_COUNT": 4,
+    "DETENTS_PER_TURN": 24,
+    "TURNS_PER_FULL_RANGE": 2
+  },
+  "encoder": {
+    "//": "Quadrature edges in one detent. A press held past LONG_PRESS_MS is a long press, and button edges closer than BUTTON_DEBOUNCE_MS are ignored.",
+    "EDGES_PER_DETENT": 4,
+    "LONG_PRESS_MS": 1000,
+    "BUTTON_DEBOUNCE_MS": 50
+  },
+  "bus": {
+    "//": "The absolute bus speaks integers 0 to BUS_MAX. A pattern that reads only knob deltas gets one click for every BUS_UNITS_PER_CLICK a held channel moves.",
+    "BUS_MAX": 1000,
+    "BUS_UNITS_PER_CLICK": 10
+  }
+}
diff --git a/web/package.json b/web/package.json
index ec4bc6b2..a5a0fa4b 100644
--- a/web/package.json
+++ b/web/package.json
@@ -42,6 +42,7 @@
     "check:ci": "npm run check:license && npm run check:moderation && npm run check:headermod && npm run check:hidemod && npm run check:retention && npm run check:deck && npm run check:pack && npm run check:sharing && npm run check:publish && npm run check:notify && npm run check:ports && npm run check:workshop && npm run check:sandbox && npm run check:capture && npm run check:thumbs && npm run check:flatten && npm run check:labedit && npm run check:performance && npm run check:pixelcode && npm run check:director && npm run check:pfstvectors && npm run check:ramp && npm run check:panels && npm run check:sandbox-sync && npm run check:modcache && npm run check:drift",
     "build:sandbox": "tsx scripts/build-sandbox.ts",
     "check:sandbox-sync": "tsx scripts/build-sandbox.ts --check",
+    "gen:shared": "tsx scripts/gen-shared.ts",
     "doc-map": "tsx scripts/doc-map.ts",
     "check:drift": "tsx scripts/drift.ts",
     "check:links-absolute": "tsx scripts/drift.ts links-absolute",
@@ -52,6 +53,7 @@
     "check:presets": "tsx scripts/drift.ts presets",
     "check:roadmap": "tsx scripts/drift.ts roadmap",
     "check:changelog": "tsx scripts/drift.ts changelog",
+    "check:shared": "tsx scripts/drift.ts shared",
     "check:human-lock": "tsx scripts/human-lock.ts",
     "check:voice": "tsx scripts/voice-lint.ts"
   },
diff --git a/web/scripts/drift/shared.ts b/web/scripts/drift/shared.ts
new file mode 100644
index 00000000..c8b32504
--- /dev/null
+++ b/web/scripts/drift/shared.ts
@@ -0,0 +1,58 @@
+/**
+ * check:shared: the two files gen:shared writes from shared/constants.json,
+ * against what it writes now. A hand edit to core_shared.h or constants.ts
+ * fails here, and so does a change to the JSON nobody regenerated.
+ *
+ * Two copies cannot be generated, so this reads them. abi/pf_params.h clamps
+ * the bus to a literal 1000 and is frozen: installed modules read it without
+ * recompiling. build_module.py names the panel's 128 and 64 itself, and the
+ * module cache hashes it. The copies written for other readers, the REST
+ * contract's 24 and the Ableton bridge's 48, stay by hand and are not read.
+ */
+import fs from "node:fs";
+import path from "node:path";
+
+import { render, SOURCE } from "../gen-shared";
+import { compare, done, read, ROOT } from "./_lib";
+
+let stale = 0;
+for (const [file, want] of Object.entries(render())) {
+  const have = fs.existsSync(path.join(ROOT, file)) ? read(file) : "";
+  if (have === want) {
+    console.log(`${file}: same as gen:shared writes`);
+    continue;
+  }
+  stale++;
+  console.log(`${file}: stale, run npm run gen:shared in web/`);
+  const [a, b] = [want.split("\n"), have.split("\n")];
+  const lines = [...Array(Math.max(a.length, b.length)).keys()].filter((i) => a[i] !== b[i]);
+  for (const i of lines.slice(0, 10)) {
+    console.log(`  line ${i + 1}, gen:shared writes: ${a[i] ?? "(no line)"}`);
+    console.log(`  line ${i + 1}, the file has:      ${b[i] ?? "(no line)"}`);
+  }
+  if (lines.length > 10) console.log(`  and ${lines.length - 10} more lines`);
+}
+
+const data = JSON.parse(read(SOURCE));
+
+const ABI = "firmware/patternflow/abi/pf_params.h";
+const body = read(ABI)
+  .replace(/\/\*[\s\S]*?\*\/|\/\/[^\n]*/g, "")
+  .match(/\bunit1000\s*\([^)]*\)\s*\{([^}]*)\}/)?.[1];
+if (body === undefined) console.log(`no unit1000() in ${ABI}`);
+compare(
+  "the bus maximum",
+  { label: `${SOURCE} BUS_MAX`, items: [String(data.bus.BUS_MAX)] },
+  { label: `the literals in unit1000(), ${ABI}`, items: new Set(body?.match(/\b\d+(\.\d+)?f?\b/g)?.map((n) => String(parseFloat(n)))) },
+);
+
+const BUILD = "firmware/toolchain/build_module.py";
+const panel = data.panel.PANEL_GEOMETRIES[data.panel.PANEL_GEOM_DEFAULT];
+compare(
+  "the default panel's canvas",
+  { label: `${SOURCE} ${data.panel.PANEL_GEOM_DEFAULT}`, items: [`PANEL_W ${panel.physW * panel.chain}`, `PANEL_H ${panel.physH}`] },
+  { label: BUILD, items: [...read(BUILD).matchAll(/^(PANEL_[WH]) = (\d+)/gm)].map((m) => `${m[1]} ${m[2]}`) },
+);
+
+done();
+if (stale) process.exitCode = 1;
diff --git a/web/scripts/gen-shared.ts b/web/scripts/gen-shared.ts
new file mode 100644
index 00000000..e28df517
--- /dev/null
+++ b/web/scripts/gen-shared.ts
@@ -0,0 +1,106 @@
+/**
+ * gen:shared: writes shared/constants.json out as the two files that read it,
+ * firmware/patternflow/src/core_shared.h and web/src/lib/shared/constants.ts
+ * (docs/rfc-cleanup.md §2.5). The lab counted 20 detents a turn until 31 July
+ * 2026, against the encoder's 24, and the community sandbox until 4 September.
+ * Each copy was found and fixed by hand.
+ *
+ * A key in the JSON is the constant's name on both sides: DETENTS_PER_TURN is
+ * PF_SHARED_DETENTS_PER_TURN in the header and DETENTS_PER_TURN in the module.
+ * The prefix keeps the macros clear of the names config.h already defines, and
+ * of every feature's PF_<NAME>_ flags, which check_boundaries.py refuses in
+ * src/. A key "//" is a note, and each group's note goes over its lines.
+ *
+ * check:shared (scripts/drift/shared.ts) imports render() and compares, so the
+ * files on disk must be exactly what this writes.
+ */
+import fs from "node:fs";
+import path from "node:path";
+import { fileURLToPath } from "node:url";
+
+export const ROOT = path.resolve(path.dirname(fileURLToPath(import.meta.url)), "../..");
+export const SOURCE = "shared/constants.json";
+export const HEADER = "firmware/patternflow/src/core_shared.h";
+export const MODULE = "web/src/lib/shared/constants.ts";
+
+type Group = Record<string, unknown>;
+
+const fail = (what: string): never => {
+  throw new Error(`${SOURCE}: ${what}`);
+};
+const whole = (v: unknown, at: string) =>
+  Number.isInteger(v) && (v as number) > 0 ? (v as number) : fail(`${at} is ${JSON.stringify(v)}, not a whole number above 0`);
+
+/** A note as comment lines, wrapped at 80 columns. */
+function comment(note: unknown): string[] {
+  if (typeof note !== "string") return [];
+  const lines: string[] = [];
+  let line = "//";
+  for (const word of note.split(/\s+/)) {
+    if (line !== "//" && line.length + 1 + word.length > 80) {
+      lines.push(line);
+      line = "//";
+    }
+    line += ` ${word}`;
+  }
+  return [...lines, line];
+}
+
+const BANNER = [
+  `// GENERATED by web/scripts/gen-shared.ts from ${SOURCE}. Do not edit.`,
+  ...comment("Change the JSON and run `npm run gen:shared` in web/. check:shared fails while this file says anything else."),
+];
+
+/** Both files as gen:shared writes them, keyed by their path from the repository root. */
+export function render(json = fs.readFileSync(path.join(ROOT, SOURCE), "utf8")): Record<string, string> {
+  const data = JSON.parse(json) as Record<string, Group>;
+  const h = [...BANNER, "//", "// License: MIT", "#pragma once"];
+  const ts = [...BANNER];
+  const names = new Set(["PANEL_GEOMETRIES", "PANEL_GEOM_DEFAULT"]);
+
+  const panel = data.panel ?? fail("no panel group");
+  const geometries = Object.entries((panel.PANEL_GEOMETRIES ?? fail("no panel.PANEL_GEOMETRIES")) as Record<string, Group>);
+  const fallback = panel.PANEL_GEOM_DEFAULT;
+  if (!geometries.some(([name]) => name === fallback)) fail(`panel.PANEL_GEOM_DEFAULT ${JSON.stringify(fallback)} is not in PANEL_GEOMETRIES`);
+  const ids = new Set<number>();
+  h.push("", ...comment(panel["//"]));
+  ts.push("", ...comment(panel["//"]), "export const PANEL_GEOMETRIES = {");
+  for (const [name, g] of geometries) {
+    if (!/^\w+$/.test(name)) fail(`the geometry ${JSON.stringify(name)} is not a C name`);
+    const at = `panel.PANEL_GEOMETRIES.${name}`;
+    const [id, physW, physH, chain] = (["id", "physW", "physH", "chain"] as const).map((k) => whole(g[k], `${at}.${k}`));
+    if (ids.has(id)) fail(`${at}.id ${id} is taken by another geometry`);
+    ids.add(id);
+    // config.h refuses a build where these two disagree with the physical size.
+    const fields = { physW, physH, chain, resW: physW * chain, resH: physH };
+    h.push(`#define PF_SHARED_PANEL_GEOM_${name} ${id}`);
+    for (const [k, v] of Object.entries(fields)) h.push(`#define PF_SHARED_PANEL_GEOM_${name}_${k.replace(/[A-Z]/g, "_$&").toUpperCase()} ${v}`);
+    ts.push(`  "${name}": { id: ${id}, ${Object.entries(fields).map(([k, v]) => `${k}: ${v}`).join(", ")} },`);
+  }
+  h.push(`#define PF_SHARED_PANEL_GEOM_DEFAULT PF_SHARED_PANEL_GEOM_${fallback}`);
+  ts.push("} as const;", `export const PANEL_GEOM_DEFAULT = "${fallback}";`);
+
+  for (const [group, values] of Object.entries(data)) {
+    if (group === "//" || group === "panel") continue;
+    h.push("", ...comment(values["//"]));
+    ts.push("", ...comment(values["//"]));
+    for (const [name, v] of Object.entries(values)) {
+      if (name === "//") continue;
+      if (!/^[A-Z][A-Z0-9_]*$/.test(name)) fail(`${group}.${name} is not a constant's name`);
+      if (names.has(name)) fail(`${group}.${name} is named twice`);
+      names.add(name);
+      const n = whole(v, `${group}.${name}`);
+      h.push(`#define PF_SHARED_${name} ${n}`);
+      ts.push(`export const ${name} = ${n};`);
+    }
+  }
+  return { [HEADER]: `${h.join("\n")}\n`, [MODULE]: `${ts.join("\n")}\n` };
+}
+
+if (path.resolve(process.argv[1] ?? "") === fileURLToPath(import.meta.url)) {
+  for (const [file, text] of Object.entries(render())) {
+    fs.mkdirSync(path.dirname(path.join(ROOT, file)), { recursive: true });
+    fs.writeFileSync(path.join(ROOT, file), text);
+    console.log(`wrote ${file}`);
+  }
+}
diff --git a/web/src/lib/shared/constants.ts b/web/src/lib/shared/constants.ts
new file mode 100644
index 00000000..e5211834
--- /dev/null
+++ b/web/src/lib/shared/constants.ts
@@ -0,0 +1,29 @@
+// GENERATED by web/scripts/gen-shared.ts from shared/constants.json. Do not edit.
+// Change the JSON and run `npm run gen:shared` in web/. check:shared fails
+// while this file says anything else.
+
+// The geometries PANEL_GEOMETRY picks from in config.h. Two 64x64 modules in a
+// chain make the same 128x64 canvas as one 128x64 module.
+export const PANEL_GEOMETRIES = {
+  "128x64_SINGLE": { id: 1, physW: 128, physH: 64, chain: 1, resW: 128, resH: 64 },
+  "64x64_SINGLE": { id: 2, physW: 64, physH: 64, chain: 1, resW: 64, resH: 64 },
+  "64x64_CHAIN2": { id: 3, physW: 64, physH: 64, chain: 2, resW: 128, resH: 64 },
+} as const;
+export const PANEL_GEOM_DEFAULT = "128x64_SINGLE";
+
+// The PEC11R-4220F-S0024 has 24 detents a turn. Two turns cross a parameter's
+// whole range, so 48 detents do.
+export const KNOB_COUNT = 4;
+export const DETENTS_PER_TURN = 24;
+export const TURNS_PER_FULL_RANGE = 2;
+
+// Quadrature edges in one detent. A press held past LONG_PRESS_MS is a long
+// press, and button edges closer than BUTTON_DEBOUNCE_MS are ignored.
+export const EDGES_PER_DETENT = 4;
+export const LONG_PRESS_MS = 1000;
+export const BUTTON_DEBOUNCE_MS = 50;
+
+// The absolute bus speaks integers 0 to BUS_MAX. A pattern that reads only knob
+// deltas gets one click for every BUS_UNITS_PER_CLICK a held channel moves.
+export const BUS_MAX = 1000;
+export const BUS_UNITS_PER_CLICK = 10;