diff --git a/extension/lib/platforms.js b/extension/lib/platforms.js
new file mode 100644
index 0000000..9092c80
--- /dev/null
+++ b/extension/lib/platforms.js
@@ -0,0 +1,85 @@
+/**
+ * Platform definitions — mirrored from backend/app/services/platforms.py.
+ * Two copies of these regexes (here and in backend extension_service.py)
+ * is cheaper than building a shared definition file for two short
+ * tables in different runtimes. Keep in sync by hand; reviewers catch
+ * drift.
+ */
+
+const PLATFORMS = {
+  patreon: {
+    name: 'Patreon',
+    domains: ['.patreon.com', 'www.patreon.com', 'patreon.com'],
+    authType: 'cookies',
+    color: '#FF424D',
+    urlPattern: /^https?:\/\/(www\.)?patreon\.com/,
+  },
+  subscribestar: {
+    name: 'SubscribeStar',
+    domains: [
+      '.subscribestar.com',
+      '.subscribestar.adult',
+      'www.subscribestar.com',
+      'subscribestar.com',
+      'subscribestar.adult',
+    ],
+    authType: 'cookies',
+    color: '#FFD700',
+    urlPattern: /^https?:\/\/(www\.)?subscribestar\.(com|adult)/,
+  },
+  hentaifoundry: {
+    name: 'Hentai Foundry',
+    domains: ['.hentai-foundry.com', 'www.hentai-foundry.com', 'hentai-foundry.com'],
+    authType: 'cookies',
+    color: '#9C27B0',
+    urlPattern: /^https?:\/\/(www\.)?hentai-foundry\.com/,
+  },
+  discord: {
+    name: 'Discord',
+    domains: ['.discord.com', 'discord.com'],
+    authType: 'token',
+    color: '#5865F2',
+    urlPattern: /^https?:\/\/(www\.)?discord\.com/,
+    note: 'Open Discord in browser to capture token',
+  },
+  pixiv: {
+    name: 'Pixiv',
+    domains: ['.pixiv.net', 'www.pixiv.net', 'pixiv.net'],
+    authType: 'token',
+    color: '#0096FA',
+    urlPattern: /^https?:\/\/(www\.)?pixiv\.net/,
+    note: 'Click to authenticate via OAuth',
+  },
+  deviantart: {
+    name: 'DeviantArt',
+    domains: ['.deviantart.com', 'www.deviantart.com', 'deviantart.com'],
+    authType: 'cookies',
+    color: '#05CC47',
+    urlPattern: /^https?:\/\/(www\.)?deviantart\.com/,
+  },
+};
+
+/**
+ * Per-platform artist-page URL patterns — these are STRICTER than
+ * urlPattern (which matches any URL on the domain). Used by the content
+ * script to decide whether to show the floating "Add as source" button.
+ */
+const PLATFORM_ARTIST_PATTERNS = {
+  patreon:       /^https?:\/\/(www\.)?patreon\.com\/(?!home$|search\b|messages\b|notifications\b|library\b|settings\b|posts\b|c\/)[^/?#]+\/?$/i,
+  subscribestar: /^https?:\/\/(www\.)?subscribestar\.(com|adult)\/(?!feed$|messages$|library$)[^/?#]+\/?$/i,
+  hentaifoundry: /^https?:\/\/(www\.)?hentai-foundry\.com\/user\/[^/?#]+/i,
+  deviantart:    /^https?:\/\/(www\.)?deviantart\.com\/(?!home$|watch\b|tag\b|browse\b)[^/?#]+\/?$/i,
+  pixiv:         /^https?:\/\/(www\.)?pixiv\.net\/(en\/)?users\/\d+/i,
+};
+
+function getPlatformFromUrl(url) {
+  for (const [key, platform] of Object.entries(PLATFORMS)) {
+    if (platform.urlPattern.test(url)) return key;
+  }
+  return null;
+}
+
+function isArtistPage(url, platformKey) {
+  const pattern = PLATFORM_ARTIST_PATTERNS[platformKey];
+  return pattern ? pattern.test(url) : false;
+}