diff --git a/Makefile b/Makefile
index 59e2422..c077817 100644
--- a/Makefile
+++ b/Makefile
@@ -1,4 +1,4 @@
-.PHONY: bootstrap setup install test test-e2e test-e2e-firefox lint fmt fmt-check check check-censored docker hooks build build-debug package vendor-blocklist clean dev
+.PHONY: bootstrap setup install test test-e2e test-e2e-firefox lint fmt fmt-check check check-censored docker hooks build build-debug package vendor-blocklist icons clean dev
 
 # Standard targets are thin shims; the implementations live in script/
 # per the scripts-to-rule-them-all pattern (see the Entrypoints section
@@ -104,6 +104,12 @@ build-debug:
 vendor-blocklist:
 	@script/vendor-blocklist
 
+# Redraw the toolbar icons in icons/ from script/lib/icons.js, at every size
+# manifest/chrome.json declares. Commit the result with the drawing:
+# tests/icons.test.js fails while the two disagree.
+icons:
+	@node script/lib/icons.js
+
 clean:
 	@rm -rf dist/ release/
 
diff --git a/README.md b/README.md
index 79505b3..d695ff1 100644
--- a/README.md
+++ b/README.md
@@ -317,6 +317,10 @@ The Makefile shims to those. It also carries a few targets that have no
 - `make build-debug` — the same build with `AUTISTMASK_DEBUG=1`, verified as a
   debug build, and keeping its `dist/` on failure (see
   [Debug Builds](#debug-builds))
+- `make icons` — redraw the toolbar icons in `icons/` from
+  `script/lib/icons.js`, at every size `manifest/chrome.json` declares. Commit
+  the files with the drawing: `make check` fails while they differ from what it
+  draws
 - `make clean` — remove `dist/` and `release/`
 - `make dev` — run the build `make build` runs, without the checks that follow
   it, then run it again after every change to a file under `src/`, `manifest/`
diff --git a/build.js b/build.js
index 2fc632a..0cf68e4 100644
--- a/build.js
+++ b/build.js
@@ -588,15 +588,10 @@ async function build() {
         copyIcons(distDir);
     }
 
-    // copy manifests
-    copyEmitted(
-        path.join(__dirname, "manifest", "chrome.json"),
-        path.join(DIST_CHROME, "manifest.json"),
-    );
-    copyEmitted(
-        path.join(__dirname, "manifest", "firefox.json"),
-        path.join(DIST_FIREFOX, "manifest.json"),
-    );
+    // copy manifests, the same files copyIcons() read
+    for (const [distDir, manifestPath] of MANIFEST_SOURCES) {
+        copyEmitted(manifestPath, path.join(distDir, "manifest.json"));
+    }
 
     assertForbiddenTableCovered(forbiddenRecord);
 
diff --git a/script/lib/icons.js b/script/lib/icons.js
new file mode 100644
index 0000000..7487ada
--- /dev/null
+++ b/script/lib/icons.js
@@ -0,0 +1,147 @@
+// Draws the toolbar icon and writes it to every file manifest/chrome.json
+// declares under "icons". Run by `make icons`; tests/icons.test.js checks that
+// the committed files are exactly what this draws.
+//
+// The icon is a dark-navy rounded square carrying a teal triangle with a
+// smaller triangle cut out of it. Every coordinate below is a fraction of the
+// icon's side, so one drawing serves every size. A pixel's colour is the
+// average of an 8x8 grid of samples, one at the centre of each cell, which
+// smooths the edges.
+//
+// The PNG is written here rather than by a library: RGBA at 8 bits per
+// channel, filter type 0 (none) on every row, one IDAT chunk compressed by
+// node's zlib at level 9. Changing any of that changes the bytes.
+
+"use strict";
+
+const fs = require("fs");
+const path = require("path");
+const zlib = require("zlib");
+
+const { icons } = require("../../manifest/chrome.json");
+
+const NAVY = [0x10, 0x1a, 0x2e];
+const TEAL = [0x35, 0xe0, 0xc2];
+const CORNER_RADIUS = 0.22;
+const OUTER_TRIANGLE = [
+    [0.5, 0.15],
+    [0.115, 0.855],
+    [0.885, 0.855],
+];
+const INNER_TRIANGLE = [
+    [0.5, 0.4],
+    [0.29, 0.7],
+    [0.71, 0.7],
+];
+const SAMPLES_PER_SIDE = 8;
+
+const PNG_SIGNATURE = Buffer.from([
+    0x89, 0x50, 0x4e, 0x47, 0x0d, 0x0a, 0x1a, 0x0a,
+]);
+
+// Points are in pixels from the icon's top-left corner.
+function insideRoundedSquare(x, y, size) {
+    const r = CORNER_RADIUS * size;
+    // How far the point is past the start of a corner's curve, on each axis.
+    const dx = Math.max(r - x, 0, x - (size - r));
+    const dy = Math.max(r - y, 0, y - (size - r));
+    return dx * dx + dy * dy <= r * r;
+}
+
+// Inside or on an edge: the point is not on opposite sides of two edges.
+function insideTriangle(x, y, [a, b, c]) {
+    const side = (p, q) =>
+        (q[0] - p[0]) * (y - p[1]) - (q[1] - p[1]) * (x - p[0]);
+    const sides = [side(a, b), side(b, c), side(c, a)];
+    return !(sides.some((s) => s < 0) && sides.some((s) => s > 0));
+}
+
+// Rounds to the nearest integer and a half to the even neighbour, as the
+// committed icons were made. Math.round takes a half up, which would change
+// some pixels by one.
+function roundHalfToEven(v) {
+    const down = Math.floor(v);
+    if (v - down !== 0.5) {
+        return Math.round(v);
+    }
+    return down % 2 === 0 ? down : down + 1;
+}
+
+// The RGBA bytes of the pixel whose top-left corner is (px, py).
+function pixel(px, py, size, outer, inner) {
+    let inSquare = 0;
+    let inTeal = 0;
+    for (let j = 0; j < SAMPLES_PER_SIDE; j++) {
+        const y = py + (j + 0.5) / SAMPLES_PER_SIDE;
+        for (let i = 0; i < SAMPLES_PER_SIDE; i++) {
+            const x = px + (i + 0.5) / SAMPLES_PER_SIDE;
+            if (!insideRoundedSquare(x, y, size)) {
+                continue;
+            }
+            inSquare++;
+            if (insideTriangle(x, y, outer) && !insideTriangle(x, y, inner)) {
+                inTeal++;
+            }
+        }
+    }
+    if (inSquare === 0) {
+        return [0, 0, 0, 0];
+    }
+    const colour = NAVY.map((navy, k) =>
+        roundHalfToEven(navy + ((TEAL[k] - navy) * inTeal) / inSquare),
+    );
+    const alpha = roundHalfToEven((255 * inSquare) / SAMPLES_PER_SIDE ** 2);
+    return [...colour, alpha];
+}
+
+// A PNG chunk: the data's length, the type, the data, then the CRC-32 of the
+// type and the data.
+function chunk(type, data) {
+    const typeAndData = Buffer.concat([Buffer.from(type, "ascii"), data]);
+    const length = Buffer.alloc(4);
+    length.writeUInt32BE(data.length);
+    const crc = Buffer.alloc(4);
+    crc.writeUInt32BE(zlib.crc32(typeAndData));
+    return Buffer.concat([length, typeAndData, crc]);
+}
+
+// The complete PNG file for the icon at `size` pixels square.
+function drawIcon(size) {
+    const toPixels = (corners) => corners.map(([x, y]) => [x * size, y * size]);
+    const outer = toPixels(OUTER_TRIANGLE);
+    const inner = toPixels(INNER_TRIANGLE);
+
+    const rows = [];
+    for (let py = 0; py < size; py++) {
+        const row = [0]; // filter type 0: the row's bytes are stored as they are
+        for (let px = 0; px < size; px++) {
+            row.push(...pixel(px, py, size, outer, inner));
+        }
+        rows.push(Buffer.from(row));
+    }
+
+    const header = Buffer.alloc(13); // compression, filter, interlace: all 0
+    header.writeUInt32BE(size, 0); // width
+    header.writeUInt32BE(size, 4); // height
+    header[8] = 8; // bits per channel
+    header[9] = 6; // colour type: RGBA
+
+    return Buffer.concat([
+        PNG_SIGNATURE,
+        chunk("IHDR", header),
+        chunk("IDAT", zlib.deflateSync(Buffer.concat(rows), { level: 9 })),
+        chunk("IEND", Buffer.alloc(0)),
+    ]);
+}
+
+if (require.main === module) {
+    for (const [size, file] of Object.entries(icons)) {
+        fs.writeFileSync(
+            path.join(__dirname, "..", "..", file),
+            drawIcon(Number(size)),
+        );
+        console.log(`icons: wrote ${file}`);
+    }
+}
+
+module.exports = { drawIcon };
diff --git a/tests/icons.test.js b/tests/icons.test.js
new file mode 100644
index 0000000..8015693
--- /dev/null
+++ b/tests/icons.test.js
@@ -0,0 +1,20 @@
+// The toolbar icons in icons/ are drawn by script/lib/icons.js (`make icons`).
+// Each committed file must be exactly what it draws, byte for byte, so the
+// drawing and the files cannot drift apart: change the drawing, then run
+// `make icons` and commit the files with it.
+
+const fs = require("fs");
+const path = require("path");
+
+const { icons } = require("../manifest/chrome.json");
+const { drawIcon } = require("../script/lib/icons");
+
+describe("toolbar icons", () => {
+    test.each(Object.entries(icons))(
+        "the %spx icon %s is what script/lib/icons.js draws",
+        (size, file) => {
+            const committed = fs.readFileSync(path.join(__dirname, "..", file));
+            expect(drawIcon(Number(size))).toEqual(committed);
+        },
+    );
+});