diff --git a/backend/app/services/audits/__init__.py b/backend/app/services/audits/__init__.py
new file mode 100644
index 0000000..96e5198
--- /dev/null
+++ b/backend/app/services/audits/__init__.py
@@ -0,0 +1,6 @@
+"""Audit rule modules. Each module exposes evaluate(pil_image, **params) -> bool.
+
+The retroactive library-cleanup tab and (future) import-time filter logic
+both consume these. Importers should NOT inline rule logic going forward;
+add the rule here and call from both sides.
+"""
diff --git a/backend/app/services/audits/transparency.py b/backend/app/services/audits/transparency.py
new file mode 100644
index 0000000..f15f31f
--- /dev/null
+++ b/backend/app/services/audits/transparency.py
@@ -0,0 +1,27 @@
+"""Transparency audit: matches images whose transparent-pixel fraction
+exceeds the threshold. Animated images short-circuit (skipped) to avoid
+the multi-frame PIL decode that hits Celery's hard time limit."""
+
+
+def evaluate(pil_image, *, threshold: float) -> bool:
+    """True iff the image's transparent-pixel fraction exceeds threshold.
+
+    False for non-alpha modes and animated images. Mirrors the import-side
+    Importer._transparency_pct logic so retroactive enforcement matches
+    prospective filtering.
+    """
+    if getattr(pil_image, "is_animated", False):
+        return False
+    if pil_image.mode not in ("RGBA", "LA") and not (
+        pil_image.mode == "P" and "transparency" in pil_image.info
+    ):
+        return False
+    im = pil_image
+    if im.mode != "RGBA":
+        im = im.convert("RGBA")
+    alpha = im.getchannel("A")
+    histogram = alpha.histogram()
+    transparent = histogram[0]
+    total = sum(histogram)
+    pct = transparent / total if total else 0.0
+    return pct > threshold
diff --git a/tests/test_audits_transparency.py b/tests/test_audits_transparency.py
new file mode 100644
index 0000000..44d6abb
--- /dev/null
+++ b/tests/test_audits_transparency.py
@@ -0,0 +1,46 @@
+"""Tests for the transparency audit rule.
+
+The rule mirrors `Importer._transparency_pct` semantics for retroactive
+enforcement: returns True iff the fraction of fully-transparent pixels
+exceeds the threshold. Animated images short-circuit to False to avoid
+the multi-frame PIL decode that triggered SoftTimeLimitExceeded
+2026-05-26 against animated WebPs.
+"""
+
+from PIL import Image
+
+from backend.app.services.audits import transparency
+
+
+def test_transparency_evaluate_true_when_fully_transparent():
+    im = Image.new("RGBA", (10, 10), (0, 0, 0, 0))
+    assert transparency.evaluate(im, threshold=0.5) is True
+
+
+def test_transparency_evaluate_false_when_fully_opaque():
+    im = Image.new("RGBA", (10, 10), (200, 100, 50, 255))
+    assert transparency.evaluate(im, threshold=0.5) is False
+
+
+def test_transparency_evaluate_respects_threshold_boundary():
+    # Half-transparent image: 50% alpha=0 pixels, 50% alpha=255.
+    im = Image.new("RGBA", (10, 10), (0, 0, 0, 0))
+    for x in range(5):
+        for y in range(10):
+            im.putpixel((x, y), (0, 0, 0, 255))
+    # 50% transparent. threshold=0.4 → True; threshold=0.6 → False.
+    assert transparency.evaluate(im, threshold=0.4) is True
+    assert transparency.evaluate(im, threshold=0.6) is False
+
+
+def test_transparency_evaluate_false_for_rgb_image_without_alpha():
+    im = Image.new("RGB", (10, 10), (128, 128, 128))
+    assert transparency.evaluate(im, threshold=0.5) is False
+
+
+def test_transparency_evaluate_false_for_animated_image():
+    im = Image.new("RGBA", (10, 10), (0, 0, 0, 0))
+    # Mark as animated (mimics PIL's WebP/GIF multi-frame attribute).
+    im.is_animated = True  # type: ignore[attr-defined]
+    im.n_frames = 5  # type: ignore[attr-defined]
+    assert transparency.evaluate(im, threshold=0.5) is False