diff --git a/backend/app/services/platforms.py b/backend/app/services/platforms.py
new file mode 100644
index 0000000..bd3723a
--- /dev/null
+++ b/backend/app/services/platforms.py
@@ -0,0 +1,140 @@
+"""FC-3b platforms registry — the single source of truth for what
+FabledCurator supports.
+
+Lifted from GallerySubscriber's
+~/Nextcloud/Projects/GallerySubscriber/backend/app/api/platforms.py
+and ~/.../extension/lib/platforms.js. Six platforms; auth_type and
+URL patterns match GS exactly so the existing browser extension
+hits FC unmodified.
+"""
+
+from dataclasses import dataclass
+from typing import Literal
+
+
+@dataclass(frozen=True)
+class PlatformInfo:
+    key: str
+    name: str
+    description: str
+    auth_type: Literal["cookies", "token"]
+    requires_auth: bool
+    url_pattern: str
+    url_examples: list[str]
+    default_config: dict
+    notes: str | None = None
+
+
+# Common defaults used across most platforms; embedded per-platform
+# below so per-platform overrides remain explicit.
+_DEFAULTS = {
+    "sleep": 3.0,
+    "sleep_request": 1.5,
+    "skip_existing": True,
+    "save_metadata": True,
+    "timeout": 3600,
+}
+
+
+PLATFORMS: dict[str, PlatformInfo] = {
+    "patreon": PlatformInfo(
+        key="patreon",
+        name="Patreon",
+        description="Download posts from Patreon creators",
+        auth_type="cookies",
+        requires_auth=True,
+        url_pattern=r"^https?://(www\.)?patreon\.com/",
+        url_examples=[
+            "https://www.patreon.com/example_artist",
+            "https://www.patreon.com/user?u=12345678",
+        ],
+        default_config={**_DEFAULTS, "content_types": ["images", "attachments"]},
+    ),
+    "subscribestar": PlatformInfo(
+        key="subscribestar",
+        name="SubscribeStar",
+        description="Download posts from SubscribeStar creators",
+        auth_type="cookies",
+        requires_auth=True,
+        url_pattern=r"^https?://(www\.)?subscribestar\.(com|adult)/",
+        url_examples=[
+            "https://subscribestar.adult/example_artist",
+            "https://www.subscribestar.com/example_artist",
+        ],
+        default_config={**_DEFAULTS, "content_types": ["all"]},
+    ),
+    "hentaifoundry": PlatformInfo(
+        key="hentaifoundry",
+        name="Hentai Foundry",
+        description="Download artwork from Hentai Foundry artists",
+        auth_type="cookies",
+        requires_auth=False,
+        url_pattern=r"^https?://(www\.)?hentai-foundry\.com/",
+        url_examples=[
+            "https://www.hentai-foundry.com/user/example_artist",
+            "https://www.hentai-foundry.com/pictures/user/example_artist",
+        ],
+        default_config={**_DEFAULTS, "content_types": ["pictures"]},
+    ),
+    "discord": PlatformInfo(
+        key="discord",
+        name="Discord",
+        description="Download attachments from Discord channels",
+        auth_type="token",
+        requires_auth=True,
+        url_pattern=r"^https?://(www\.)?discord\.com/channels/",
+        url_examples=["https://discord.com/channels/123456789/987654321"],
+        default_config={**_DEFAULTS, "content_types": ["all"]},
+        notes="Requires Discord user token (not bot token).",
+    ),
+    "pixiv": PlatformInfo(
+        key="pixiv",
+        name="Pixiv",
+        description="Download artwork from Pixiv artists",
+        auth_type="token",
+        requires_auth=True,
+        url_pattern=r"^https?://(www\.)?pixiv\.net/",
+        url_examples=[
+            "https://www.pixiv.net/users/12345678",
+            "https://www.pixiv.net/en/users/12345678",
+        ],
+        default_config={**_DEFAULTS, "content_types": ["all"]},
+        notes="Requires OAuth refresh token. Run `gallery-dl oauth:pixiv` to obtain one.",
+    ),
+    "deviantart": PlatformInfo(
+        key="deviantart",
+        name="DeviantArt",
+        description="Download artwork from DeviantArt artists",
+        auth_type="cookies",
+        requires_auth=False,
+        url_pattern=r"^https?://(www\.)?deviantart\.com/",
+        url_examples=[
+            "https://www.deviantart.com/example-artist",
+            "https://www.deviantart.com/example-artist/gallery",
+        ],
+        default_config={**_DEFAULTS, "content_types": ["gallery"]},
+    ),
+}
+
+
+def known_platform_keys() -> frozenset[str]:
+    return frozenset(PLATFORMS.keys())
+
+
+def auth_type_for(platform: str) -> str | None:
+    info = PLATFORMS.get(platform)
+    return info.auth_type if info else None
+
+
+def to_dict(info: PlatformInfo) -> dict:
+    return {
+        "key": info.key,
+        "name": info.name,
+        "description": info.description,
+        "auth_type": info.auth_type,
+        "requires_auth": info.requires_auth,
+        "url_pattern": info.url_pattern,
+        "url_examples": info.url_examples,
+        "default_config": info.default_config,
+        "notes": info.notes,
+    }
diff --git a/tests/test_platforms_registry.py b/tests/test_platforms_registry.py
new file mode 100644
index 0000000..c100792
--- /dev/null
+++ b/tests/test_platforms_registry.py
@@ -0,0 +1,50 @@
+import re
+
+import pytest
+
+from backend.app.services.platforms import (
+    PLATFORMS,
+    PlatformInfo,
+    auth_type_for,
+    known_platform_keys,
+    to_dict,
+)
+
+
+def test_known_platform_keys_is_gs_six():
+    assert known_platform_keys() == frozenset({
+        "patreon", "subscribestar", "hentaifoundry",
+        "discord", "pixiv", "deviantart",
+    })
+
+
+def test_fanbox_not_in_registry():
+    # Sanity check — FC-3a added 'fanbox' by mistake; it's not a GS platform.
+    assert "fanbox" not in PLATFORMS
+
+
+def test_auth_type_for_known_and_unknown():
+    assert auth_type_for("patreon") == "cookies"
+    assert auth_type_for("discord") == "token"
+    assert auth_type_for("nope") is None
+
+
+def test_each_platform_has_required_fields():
+    for key, info in PLATFORMS.items():
+        assert isinstance(info, PlatformInfo)
+        assert info.key == key
+        assert info.name
+        assert info.description
+        assert info.auth_type in ("cookies", "token")
+        assert isinstance(info.requires_auth, bool)
+        re.compile(info.url_pattern)  # valid regex
+        assert info.url_examples
+        assert isinstance(info.default_config, dict)
+
+
+def test_to_dict_round_trip():
+    d = to_dict(PLATFORMS["patreon"])
+    assert d["key"] == "patreon"
+    assert d["auth_type"] == "cookies"
+    assert d["default_config"]["sleep"] == 3.0
+    assert "url_examples" in d