diff --git a/extension/test/artist-url-samples.json b/extension/test/artist-url-samples.json
new file mode 100644
index 0000000..712edf4
--- /dev/null
+++ b/extension/test/artist-url-samples.json
@@ -0,0 +1,138 @@
+{
+  "$comment": [
+    "THE SHARED ARTIFACT for the JS<->Py artist-pattern mirror (issue #3093).",
+    "",
+    "extension/lib/platforms.js PLATFORM_ARTIST_PATTERNS and",
+    "backend/app/services/extension_service.py _PLATFORM_PATTERNS are two hand-kept",
+    "copies of one table, and they gate OPPOSITE HALVES of a single interaction:",
+    "the JS copy decides whether the 'Add to FC' button appears, the Python copy",
+    "decides whether the resulting POST is accepted. Drift is therefore never",
+    "cosmetic -- JS looser than Py shows a button that 400s, Py looser than JS",
+    "silently never offers a button for a URL the backend would take. Issue #1485",
+    "was the second of those, and its fix had to be applied to both files by hand.",
+    "",
+    "Neither runtime imports the other. This file is the shared artifact instead:",
+    "both suites read it and assert it against their OWN copy of the patterns, so",
+    "a change to one copy alone fails the other runtime's suite.",
+    "  - extension/test/platforms.spec.js  (vitest)",
+    "  - tests/test_extension_artist_patterns.py  (pytest)",
+    "",
+    "Lives under extension/test/ because that path is excluded from BOTH the XPI",
+    "file set and the extension version derivation (see scripts/packaging.sh:",
+    "NOT_PACKAGED_TRACKED and NOT_VERSION_RELEVANT both carry 'test/**'), so",
+    "adding samples here never ships bytes and never forces a re-sign.",
+    "",
+    "Adding a case: put the URL in the platform's match/no_match list with a",
+    "'why'. Run both suites. If only one goes red, you have found drift, which is",
+    "the entire point of the file.",
+    "",
+    "'slug' is read by the Python side only -- its _derive returns (platform,",
+    "slug) where the JS isArtistPage returns a boolean. It is not optional on a",
+    "match entry; the backend deriving the WRONG slug from a URL both copies",
+    "agree on is its own defect class, and this pins it."
+  ],
+
+  "patreon": {
+    "match": [
+      {
+        "url": "https://www.patreon.com/maewix",
+        "slug": "maewix",
+        "why": "bare creator root"
+      },
+      {
+        "url": "https://patreon.com/maewix",
+        "slug": "maewix",
+        "why": "www is optional"
+      },
+      {
+        "url": "http://patreon.com/maewix",
+        "slug": "maewix",
+        "why": "http as well as https"
+      },
+      {
+        "url": "https://www.patreon.com/c/Atole",
+        "slug": "Atole",
+        "why": "#1485: the /c/ creator shape"
+      },
+      {
+        "url": "https://www.patreon.com/cw/Atole",
+        "slug": "Atole",
+        "why": "#1485: /cw/ is the 'creator workspace' URL Patreon serves once you are SUBSCRIBED -- exactly when the button matters most, and exactly what the pre-#1485 pattern missed"
+      },
+      {
+        "url": "https://www.patreon.com/cw/Atole/posts",
+        "slug": "Atole",
+        "why": "#1485: a creator inner page still derives the creator"
+      },
+      {
+        "url": "https://www.patreon.com/Atole/membership",
+        "slug": "Atole",
+        "why": "#1485: inner page on the bare shape"
+      }
+    ],
+    "no_match": [
+      { "url": "https://www.patreon.com/home", "why": "nav page, not a creator" },
+      { "url": "https://www.patreon.com/search", "why": "nav page" },
+      { "url": "https://www.patreon.com/messages", "why": "nav page" },
+      { "url": "https://www.patreon.com/notifications", "why": "nav page" },
+      { "url": "https://www.patreon.com/library", "why": "nav page" },
+      { "url": "https://www.patreon.com/settings", "why": "nav page" },
+      { "url": "https://www.patreon.com/posts", "why": "nav page; also the post-permalink prefix" },
+      { "url": "https://www.patreon.com/home/anything", "why": "a nav page's inner path is still not a creator" },
+      { "url": "https://www.patreon.com/settings/profile", "why": "as above" }
+    ]
+  },
+
+  "subscribestar": {
+    "match": [
+      {
+        "url": "https://www.subscribestar.com/foobar",
+        "slug": "foobar",
+        "why": "creator root on the .com TLD"
+      },
+      {
+        "url": "https://subscribestar.adult/foobar",
+        "slug": "foobar",
+        "why": "creator root on the .adult TLD"
+      },
+      {
+        "url": "https://subscribestar.adult/foobar/",
+        "slug": "foobar",
+        "why": "trailing slash is tolerated"
+      }
+    ],
+    "no_match": [
+      { "url": "https://subscribestar.adult/feed", "why": "nav page" },
+      { "url": "https://subscribestar.adult/messages", "why": "nav page" },
+      { "url": "https://subscribestar.adult/library", "why": "nav page" },
+      {
+        "url": "https://subscribestar.adult/foobar/posts",
+        "why": "SubscribeStar's pattern end-anchors on the creator root, unlike Patreon's -- an inner page does NOT derive. Pinned so the asymmetry between the two platforms stays deliberate rather than becoming a silently-fixed bug in one copy only."
+      }
+    ]
+  },
+
+  "hentaifoundry": {
+    "match": [
+      {
+        "url": "https://www.hentai-foundry.com/user/Foo",
+        "slug": "Foo",
+        "why": "user page"
+      },
+      {
+        "url": "https://www.hentai-foundry.com/user/Foo/profile",
+        "slug": "Foo",
+        "why": "inner page still derives the user"
+      },
+      {
+        "url": "https://hentai-foundry.com/user/Foo",
+        "slug": "Foo",
+        "why": "www is optional"
+      }
+    ],
+    "no_match": [
+      { "url": "https://www.hentai-foundry.com/pictures/popular", "why": "gallery listing, not a user" },
+      { "url": "https://www.hentai-foundry.com/", "why": "site root" }
+    ]
+  }
+}
diff --git a/extension/test/platforms.spec.js b/extension/test/platforms.spec.js
index f869751..bee61ab 100644
--- a/extension/test/platforms.spec.js
+++ b/extension/test/platforms.spec.js
@@ -190,3 +190,60 @@ describe('manifest.json agrees with the platform table', () => {
     }
   })
 })
+
+describe('the JS<->Py artist-pattern mirror (#3093)', () => {
+  // PLATFORM_ARTIST_PATTERNS here and extension_service._PLATFORM_PATTERNS in
+  // the backend are two hand-kept copies of one table, and they gate OPPOSITE
+  // halves of a single interaction: this copy decides whether the "Add to FC"
+  // button appears, the Python copy decides whether the resulting POST is
+  // accepted. So JS-looser-than-Py shows a button that 400s, and
+  // Py-looser-than-JS never offers a button for a URL the backend would take.
+  // #1485 was the second of those, and its fix had to be applied to both
+  // files by hand.
+  //
+  // "Keep in sync by hand; reviewers catch drift" is the same guarantee
+  // manifest.json had before #3069, where deviantart survived seven weeks.
+  //
+  // The two-runtimes objection to a shared SOURCE file is fair, so the shared
+  // artifact is the SAMPLES instead: both suites read this JSON and assert it
+  // against their own copy of the patterns, and neither imports the other.
+  // The sibling half is tests/test_extension_artist_patterns.py; adding a
+  // sample there covers it here for free, and vice versa.
+  const samples = Object.fromEntries(
+    Object.entries(
+      JSON.parse(readFileSync(path.join(EXT_DIR, 'test', 'artist-url-samples.json'), 'utf8'))
+    ).filter(([key]) => !key.startsWith('$'))
+  )
+
+  for (const [platform, spec] of Object.entries(samples)) {
+    // `slug` on a match entry is read by the Python half only — isArtistPage
+    // answers a boolean, while the backend's _derive returns (platform, slug).
+    for (const { url, why } of spec.match) {
+      it(`shows the button on ${url} — ${why}`, () => {
+        expect(isArtistPage(url, platform)).toBe(true)
+      })
+    }
+    for (const { url, why } of spec.no_match) {
+      it(`hides the button on ${url} — ${why}`, () => {
+        expect(isArtistPage(url, platform)).toBe(false)
+      })
+    }
+  }
+
+  it('has samples for every platform that has an artist pattern', () => {
+    // The guard's own coverage check: without it, deleting a platform's
+    // samples would make this block pass by testing less. Discord is
+    // deliberately in neither — it is channel-based, with no creator page to
+    // put a button on, so it has no artist pattern on either side.
+    expect(Object.keys(samples).sort()).toEqual(Object.keys(PLATFORM_ARTIST_PATTERNS).sort())
+  })
+
+  it('has samples in both directions for every platform', () => {
+    // A platform with only positive samples pins half the invariant. The
+    // no_match half is the one that catches a pattern quietly widening.
+    for (const [platform, spec] of Object.entries(samples)) {
+      expect(spec.match.length, `${platform} match samples`).toBeGreaterThan(0)
+      expect(spec.no_match.length, `${platform} no_match samples`).toBeGreaterThan(0)
+    }
+  })
+})
diff --git a/tests/test_extension_artist_patterns.py b/tests/test_extension_artist_patterns.py
new file mode 100644
index 0000000..68a2892
--- /dev/null
+++ b/tests/test_extension_artist_patterns.py
@@ -0,0 +1,121 @@
+"""The Python half of the JS<->Py artist-pattern mirror guard (#3093).
+
+`extension/lib/platforms.js` PLATFORM_ARTIST_PATTERNS and
+`extension_service._PLATFORM_PATTERNS` are two hand-kept copies of one table.
+Both files say "keep in sync by hand; reviewers catch drift" — the same
+guarantee `manifest.json` had before #3069, where deviantart sat in the
+manifest for seven weeks after the product dropped it.
+
+Drift here is worse than the manifest case, because the two copies gate
+opposite halves of ONE interaction:
+
+- the **JS** copy decides whether the "Add to FC" button appears;
+- the **Python** copy decides whether the resulting POST is accepted.
+
+So JS-looser-than-Py shows the operator a button that 400s, and
+Py-looser-than-JS silently never offers a button for a URL the backend would
+happily take. #1485 (Patreon's `/c/` and `/cw/` shapes) was exactly the
+second, and its fix had to be applied to both files by hand.
+
+The two-runtimes objection to a shared source file is fair, so this tests the
+INVARIANT rather than the source: one table of URL samples, read by both
+suites and asserted against each one's own copy of the patterns. Neither
+runtime imports the other. A change to one copy alone turns the other
+runtime's suite red.
+
+The sibling half is `extension/test/platforms.spec.js`, which reads the same
+file. Adding a sample there covers it here for free, and vice versa — which is
+the property that makes the guard cheap enough to keep using.
+"""
+
+from __future__ import annotations
+
+import json
+from pathlib import Path
+
+import pytest
+
+from backend.app.services.extension_service import (
+    ExtensionService,
+    UnknownPlatformError,
+)
+
+_SAMPLES_PATH = (
+    Path(__file__).resolve().parents[1]
+    / "extension" / "test" / "artist-url-samples.json"
+)
+
+
+def _load_samples() -> dict:
+    """The shared table, minus its `$comment` preamble."""
+    raw = json.loads(_SAMPLES_PATH.read_text(encoding="utf-8"))
+    return {k: v for k, v in raw.items() if not k.startswith("$")}
+
+
+_SAMPLES = _load_samples()
+
+_MATCH_CASES = [
+    pytest.param(platform, entry["url"], entry["slug"], entry["why"],
+                 id=f"{platform}-match-{i}")
+    for platform, spec in _SAMPLES.items()
+    for i, entry in enumerate(spec["match"])
+]
+
+_NO_MATCH_CASES = [
+    pytest.param(platform, entry["url"], entry["why"],
+                 id=f"{platform}-nomatch-{i}")
+    for platform, spec in _SAMPLES.items()
+    for i, entry in enumerate(spec["no_match"])
+]
+
+
+def _derive(url: str) -> tuple[str, str]:
+    """`_derive` needs no session — it is pure regex over the URL."""
+    return ExtensionService(session=None)._derive(url)
+
+
+@pytest.mark.parametrize("platform,url,slug,why", _MATCH_CASES)
+def test_creator_url_derives_the_expected_platform_and_slug(platform, url, slug, why):
+    """A URL the extension would show the button on must be one the backend
+    accepts, and it must derive the SAME creator. The slug is asserted, not
+    just the platform: deriving the wrong creator from a URL both copies agree
+    on is its own defect, and nothing else pins it."""
+    assert _derive(url) == (platform, slug), why
+
+
+@pytest.mark.parametrize("platform,url,why", _NO_MATCH_CASES)
+def test_non_creator_url_derives_nothing(platform, url, why):
+    """The other direction, and the one that fails silently. A URL the
+    extension refuses to show the button on must also be one the backend
+    refuses — otherwise the backend is quietly looser than the button, and
+    nobody finds out because nothing visibly breaks.
+
+    `_derive` is asserted to raise rather than merely to miss `platform`: it
+    tries every pattern in turn, so a nav page that some OTHER platform's
+    pattern happened to swallow would still be 'accepted by the backend',
+    which is the same defect wearing a different platform name.
+    """
+    with pytest.raises(UnknownPlatformError):
+        _derive(url)
+
+
+def test_the_sample_table_covers_every_platform_that_has_a_pattern():
+    """The guard's own coverage check. Without it, deleting a platform's
+    samples would make this file pass by testing less — the failure mode that
+    makes absence-based tests untrustworthy (snippet #3352).
+
+    Discord is deliberately absent from both: it has no artist pattern on
+    either side, because it is channel-based and has no creator page to put a
+    button on.
+    """
+    from backend.app.services.extension_service import _PLATFORM_PATTERNS
+
+    assert set(_SAMPLES) == {platform for platform, _ in _PLATFORM_PATTERNS}
+
+
+def test_every_platform_has_samples_in_both_directions():
+    """A platform with only positive samples pins half the invariant. The
+    no_match half is the one that catches a pattern quietly widening."""
+    for platform, spec in _SAMPLES.items():
+        assert spec["match"], f"{platform} has no match samples"
+        assert spec["no_match"], f"{platform} has no no_match samples"