diff --git a/TODO.md b/TODO.md
index b5ad5c4..bb91c7b 100644
--- a/TODO.md
+++ b/TODO.md
@@ -45,6 +45,18 @@ but the review is broader than any of them.
 
 # Completed Steps
 
+- 2026-08-23: Both manifests declare toolbar icons, and real PNGs at
+  16/32/48/128 ship inside both archives
+  ([#371](https://git.eeqj.de/sneak/AutistMask/issues/371)). Neither manifest
+  had an `icons` block, so both browsers drew a generic puzzle piece — the first
+  thing the owner sees on every launch, and how a user tells a real extension
+  from a look-alike. The sizes `build.js` copies into each browser directory are
+  read out of the manifest that ships next to them rather than from a second
+  list, so a declared size `icons/` does not hold fails `make build`;
+  `script/lib/package.js` already resolves `.png` references, so an icon that
+  reached a manifest but not the archive fails packaging. The artwork is
+  original: a flat dark-navy rounded field with a teal triangular "A", drawn
+  from geometry and rasterised into PNG, nothing traced or downloaded.
 - 2026-08-23: A swap amount and the token it is counted in now always come from
   the same hop, on both sides of the approval screen
   ([#359](https://git.eeqj.de/sneak/AutistMask/issues/359) and
diff --git a/build.js b/build.js
index 90ac8f5..20d553f 100644
--- a/build.js
+++ b/build.js
@@ -51,6 +51,17 @@ const RECEIPT_ENV = "AUTISTMASK_BUILD_RECEIPT";
 // rather than writing a receipt that cannot be checked.
 const SAFE_EMITTED_PATH = /^dist\/[A-Za-z0-9._][A-Za-z0-9._/-]*$/;
 
+// Where each browser directory's manifest comes from, and — through its
+// "icons" — which image files ship inside that directory.
+const MANIFEST_SOURCES = new Map([
+    [DIST_CHROME, path.join(__dirname, "manifest", "chrome.json")],
+    [DIST_FIREFOX, path.join(__dirname, "manifest", "firefox.json")],
+]);
+
+// What an "icons" entry may name: a plain file under icons/, so a manifest
+// value is never joined into a path that leaves the repo.
+const ICON_REF_RE = /^icons\/[A-Za-z0-9._-]+\.png$/;
+
 function ensureDir(dir) {
     fs.mkdirSync(dir, { recursive: true });
 }
@@ -257,6 +268,42 @@ function copyEmitted(src, dest) {
     recordEmitted(dest);
 }
 
+// Copy the icons one browser directory ships. The sizes come from the manifest
+// that will sit next to them, not from a second list here: a size the manifest
+// declares and icons/ does not hold fails the build, rather than shipping a
+// manifest whose reference resolves to nothing. Relative to the browser
+// directory, so nothing points up and out of it the way dist/styles.css does.
+function copyIcons(distDir) {
+    const manifestPath = MANIFEST_SOURCES.get(distDir);
+    const manifest = JSON.parse(fs.readFileSync(manifestPath, "utf8"));
+    const refs = Object.values(manifest.icons || {});
+    if (refs.length === 0) {
+        throw new Error(
+            `${repoRelative(manifestPath)} declares no icons, so the browser ` +
+                `renders a generic placeholder for this extension`,
+        );
+    }
+    for (const ref of refs) {
+        if (!ICON_REF_RE.test(ref)) {
+            throw new Error(
+                `${repoRelative(manifestPath)} declares icon ` +
+                    `${JSON.stringify(ref)}, which is not a plain file under ` +
+                    `icons/`,
+            );
+        }
+        const src = path.join(__dirname, ref);
+        if (!fs.existsSync(src)) {
+            throw new Error(
+                `${repoRelative(manifestPath)} declares ${ref}, which is not ` +
+                    `in this tree`,
+            );
+        }
+        const dest = path.join(distDir, ref);
+        ensureDir(path.dirname(dest));
+        copyEmitted(src, dest);
+    }
+}
+
 function sha256File(absPath) {
     return crypto
         .createHash("sha256")
@@ -524,6 +571,8 @@ async function build() {
             tailwindOutput,
             path.join(distDir, "src", "popup", "styles.css"),
         );
+
+        copyIcons(distDir);
     }
 
     // copy manifests
diff --git a/icons/icon128.png b/icons/icon128.png
new file mode 100644
index 0000000..ba9f3fb
Binary files /dev/null and b/icons/icon128.png differ
diff --git a/icons/icon16.png b/icons/icon16.png
new file mode 100644
index 0000000..ab8969a
Binary files /dev/null and b/icons/icon16.png differ
diff --git a/icons/icon32.png b/icons/icon32.png
new file mode 100644
index 0000000..956fecc
Binary files /dev/null and b/icons/icon32.png differ
diff --git a/icons/icon48.png b/icons/icon48.png
new file mode 100644
index 0000000..ba06004
Binary files /dev/null and b/icons/icon48.png differ
diff --git a/manifest/chrome.json b/manifest/chrome.json
index 991618e..b46dd49 100644
--- a/manifest/chrome.json
+++ b/manifest/chrome.json
@@ -9,6 +9,12 @@
     "content_security_policy": {
         "extension_pages": "default-src 'self'; script-src 'self' 'wasm-unsafe-eval'; object-src 'self'; style-src 'self' 'unsafe-inline'; img-src 'self' data:; connect-src 'self' https: http:; frame-src 'none'; form-action 'none'; base-uri 'none'"
     },
+    "icons": {
+        "16": "icons/icon16.png",
+        "32": "icons/icon32.png",
+        "48": "icons/icon48.png",
+        "128": "icons/icon128.png"
+    },
     "action": {
         "default_popup": "src/popup/index.html"
     },
diff --git a/manifest/firefox.json b/manifest/firefox.json
index 90e64de..5d2c0e2 100644
--- a/manifest/firefox.json
+++ b/manifest/firefox.json
@@ -5,6 +5,12 @@
     "description": "Minimal Ethereum wallet for Firefox",
     "permissions": ["storage", "activeTab", "alarms", "<all_urls>"],
     "content_security_policy": "default-src 'self'; script-src 'self' 'wasm-unsafe-eval'; object-src 'self'; style-src 'self' 'unsafe-inline'; img-src 'self' data:; connect-src 'self' https: http:; frame-src 'none'; form-action 'none'; base-uri 'none'",
+    "icons": {
+        "16": "icons/icon16.png",
+        "32": "icons/icon32.png",
+        "48": "icons/icon48.png",
+        "128": "icons/icon128.png"
+    },
     "browser_action": {
         "default_popup": "src/popup/index.html"
     },
diff --git a/tests/manifest.test.js b/tests/manifest.test.js
index 7ebc1da..7e3b258 100644
--- a/tests/manifest.test.js
+++ b/tests/manifest.test.js
@@ -47,6 +47,12 @@ const fs = require("fs");
 const path = require("path");
 
 const MANIFEST_DIR = path.join(__dirname, "..", "manifest");
+const ROOT = path.join(__dirname, "..");
+
+// The sizes both stores and both toolbars ask for.
+const EXPECTED_ICON_SIZES = ["16", "32", "48", "128"];
+
+const PNG_SIGNATURE = Buffer.from("89504e470d0a1a0a", "hex");
 
 const EXPECTED_DIRECTIVES = {
     "default-src": ["'self'"],
@@ -115,6 +121,51 @@ function assertPolicy(policy) {
     }
 }
 
+// The declared icons, in both manifests.
+//
+// Without an "icons" block a browser draws a generic puzzle piece in the
+// toolbar for this extension, which is both the first thing the user sees and
+// how a real extension is told apart from a look-alike. Declaring one is not
+// enough on its own: an entry naming a file that is not in the tree ships a
+// reference to nothing, so the referenced bytes are read here and required to
+// be a PNG of the size the entry claims. build.js copies these into each
+// browser directory, relative to it, and script/lib/package.js then refuses to
+// build an archive that does not contain everything the manifest names.
+function assertIcons(target) {
+    const icons = readManifest(target).icons;
+    expect(Object.keys(icons).sort()).toEqual(EXPECTED_ICON_SIZES.sort());
+    for (const size of EXPECTED_ICON_SIZES) {
+        const ref = icons[size];
+        expect([size, ref]).toEqual([size, `icons/icon${size}.png`]);
+
+        const bytes = fs.readFileSync(path.join(ROOT, ref));
+        expect(bytes.subarray(0, 8)).toEqual(PNG_SIGNATURE);
+        // IHDR width and height, at fixed offsets right after the signature
+        // and the chunk header.
+        expect([ref, bytes.readUInt32BE(16), bytes.readUInt32BE(20)]).toEqual([
+            ref,
+            Number(size),
+            Number(size),
+        ]);
+    }
+}
+
+describe("declared icons", () => {
+    test("chrome declares real icons at every size", () => {
+        assertIcons("chrome");
+    });
+
+    test("firefox declares real icons at every size", () => {
+        assertIcons("firefox");
+    });
+
+    test("both targets declare the same icons", () => {
+        expect(readManifest("firefox").icons).toEqual(
+            readManifest("chrome").icons,
+        );
+    });
+});
+
 describe("shipped Content Security Policy", () => {
     // MV3 takes an object and applies extension_pages to the popup and the
     // background service worker, which is where libsodium runs.
diff --git a/tests/packaging.test.js b/tests/packaging.test.js
index cb87836..b60a64a 100644
--- a/tests/packaging.test.js
+++ b/tests/packaging.test.js
@@ -90,6 +90,26 @@ describe("archive self-containment", () => {
         );
     });
 
+    // Toolbar icons are the other file the manifest names and no bundler
+    // emits, so an archive built without them would carry a manifest whose
+    // "icons" resolve to nothing and a browser would fall back to a generic
+    // placeholder without saying so.
+    test("a manifest naming an icon that is not in the archive fails", () => {
+        const { members, read } = archiveOf({
+            "manifest.json": JSON.stringify({
+                ...MINIMAL_MANIFEST,
+                icons: { 16: "icons/icon16.png", 128: "icons/icon128.png" },
+            }),
+            "src/popup/index.html": "<html></html>",
+            "src/popup/index.js": "//",
+            "src/background/index.js": "//",
+            "icons/icon16.png": "PNG",
+        });
+        expect(() => checkSelfContained("chrome", members, read)).toThrow(
+            /would not be self-contained.*icons\/icon128\.png/s,
+        );
+    });
+
     test("an archive with no manifest.json at its root fails", () => {
         const { members, read } = archiveOf({ "src/popup/index.js": "//" });
         expect(() => checkSelfContained("chrome", members, read)).toThrow(