diff --git a/backend/app/facets.py b/backend/app/facets.py
new file mode 100644
index 0000000..a10b7d3
--- /dev/null
+++ b/backend/app/facets.py
@@ -0,0 +1,110 @@
+"""§22.4a SLICE-3 — faceted catalog filtering + counts (read).
+
+Pure functions over already-mirrored entries; no I/O, no DB. An "entry" here is
+a plain dict carrying at least:
+  - "state": the lifecycle state column,
+  - "metadata_malformed": bool,
+  - "meta": the per-entry metadata mapping (from cached_rfcs.meta_json).
+
+Facetable fields (§5.1, plan decision 1): a collection's declared `enum` and
+`tags` fields, in declaration order, plus the built-in `state` facet appended
+last — but only when the collection declares a schema (INV-5: a no-`fields:`
+collection has no facets at all, so the frontend keeps its legacy chips). `text`
+fields are not faceted in v1 (they get a detail control in SLICE-4).
+
+Counts use drill-down semantics (plan decision 2): the count for a value of
+field F is taken over entries matching every OTHER field's selection (and the
+malformed toggle), not F's own — so within-field values stay switchable (OR
+within a field, AND across fields). The returned items list applies ALL
+selections.
+"""
+from __future__ import annotations
+
+from typing import Any
+
+# enum + tags are facetable; text is rendered as a detail control (SLICE-4).
+FACETABLE_TYPES = {"enum", "tags"}
+
+
+def facet_fields(fields: dict[str, dict] | None) -> list[tuple[str, str]]:
+    """Ordered `[(name, type), ...]` facetable from the schema, `state` last.
+
+    Empty when the collection declares no schema (INV-5)."""
+    if not fields:
+        return []
+    out = [
+        (name, spec.get("type"))
+        for name, spec in fields.items()
+        if spec.get("type") in FACETABLE_TYPES
+    ]
+    out.append(("state", "enum"))
+    return out
+
+
+def allowed_filter_keys(fields: dict[str, dict] | None) -> set[str]:
+    """Query-param keys the collection-scoped list accepts (plan decision 6)."""
+    keys = {name for name, _ in facet_fields(fields)}
+    keys.update({"unreviewed", "malformed"})
+    return keys
+
+
+def _values_for(entry: dict[str, Any], name: str, ftype: str) -> list[str]:
+    """The facet value(s) an entry contributes for field `name` (str-cast)."""
+    if name == "state":
+        v = entry.get("state")
+        return [str(v)] if v else []
+    meta = entry.get("meta") or {}
+    v = meta.get(name)
+    if v is None:
+        return []
+    if ftype == "tags":
+        return [str(x) for x in v] if isinstance(v, list) else []
+    return [str(v)]
+
+
+def _matches(entry: dict[str, Any], name: str, ftype: str, selected: set[str]) -> bool:
+    if not selected:
+        return True
+    return bool(set(_values_for(entry, name, ftype)) & selected)  # OR within field
+
+
+def filter_and_count(
+    entries: list[dict[str, Any]],
+    fields: dict[str, dict] | None,
+    selections: dict[str, set[str]],
+    only_malformed: bool = False,
+) -> tuple[list[dict[str, Any]], dict[str, dict[str, int]]]:
+    """Filter `entries` by `selections` and compute drill-down facet counts.
+
+    `selections` maps a facet field name → the set of selected values (OR within
+    the field; AND across fields). `only_malformed` narrows items and counts to
+    entries flagged malformed (INV-3). Returns `(items, facets)` where
+    `facets = {field: {value: count}}`. With no schema → `([all passing], {})`.
+    """
+    facetable = facet_fields(fields)
+
+    def passes_malformed(e: dict[str, Any]) -> bool:
+        return (not only_malformed) or bool(e.get("metadata_malformed"))
+
+    items = [
+        e for e in entries
+        if passes_malformed(e)
+        and all(_matches(e, n, t, selections.get(n, set())) for n, t in facetable)
+    ]
+
+    facets: dict[str, dict[str, int]] = {}
+    for name, ftype in facetable:
+        counts: dict[str, int] = {}
+        for e in entries:
+            if not passes_malformed(e):
+                continue
+            if not all(
+                _matches(e, on, ot, selections.get(on, set()))
+                for on, ot in facetable
+                if on != name
+            ):
+                continue
+            for val in _values_for(e, name, ftype):
+                counts[val] = counts.get(val, 0) + 1
+        facets[name] = counts
+    return items, facets
diff --git a/backend/tests/test_facets.py b/backend/tests/test_facets.py
new file mode 100644
index 0000000..92d20a8
--- /dev/null
+++ b/backend/tests/test_facets.py
@@ -0,0 +1,87 @@
+"""§22.4a SLICE-3 — pure facet field-set + filter/count (PUC-3).
+
+Per docs/design/2026-06-06-configurable-collection-metadata.md §5.1, §6.4.
+"""
+from app import facets
+
+
+PRIORITY = {"priority": {"type": "enum", "values": ["P0", "P1", "P2"]}}
+SCHEMA = {
+    "priority": {"type": "enum", "values": ["P0", "P1", "P2"]},
+    "tags": {"type": "tags"},
+    "owner": {"type": "text"},
+}
+
+
+def _e(slug, state="active", malformed=False, **meta):
+    return {"slug": slug, "state": state, "metadata_malformed": malformed, "meta": meta}
+
+
+def test_facet_fields_orders_declared_then_state_skips_text():
+    # enum + tags in declaration order, text skipped, state appended last.
+    assert facets.facet_fields(SCHEMA) == [
+        ("priority", "enum"), ("tags", "tags"), ("state", "enum")]
+
+
+def test_no_schema_yields_no_facets():
+    # INV-5: a collection with no fields has no facets (frontend keeps chips).
+    assert facets.facet_fields(None) == []
+    assert facets.facet_fields({}) == []
+
+
+def test_filter_and_count_basic_counts():
+    entries = [
+        _e("a", priority="P0", tags=["checkout"]),
+        _e("b", priority="P0", tags=["cart"]),
+        _e("c", priority="P1", tags=["checkout", "cart"]),
+    ]
+    items, fac = facets.filter_and_count(entries, SCHEMA, {})
+    assert {i["slug"] for i in items} == {"a", "b", "c"}
+    assert fac["priority"] == {"P0": 2, "P1": 1}
+    assert fac["tags"] == {"checkout": 2, "cart": 2}
+    assert fac["state"] == {"active": 3}
+
+
+def test_filter_compose_or_within_and_across():
+    entries = [
+        _e("a", priority="P0", tags=["checkout"]),   # P0 + checkout
+        _e("b", priority="P0", tags=["cart"]),        # P0, no checkout
+        _e("c", priority="P1", tags=["checkout"]),    # checkout, not P0
+    ]
+    # priority=P0 AND tags=checkout → only "a".
+    items, _ = facets.filter_and_count(
+        entries, SCHEMA, {"priority": {"P0"}, "tags": {"checkout"}})
+    assert {i["slug"] for i in items} == {"a"}
+
+
+def test_drilldown_counts_exclude_own_field_selection():
+    entries = [
+        _e("a", priority="P0"),
+        _e("b", priority="P1"),
+        _e("c", priority="P1"),
+    ]
+    # With P0 selected, the priority facet still counts P1 over the set that
+    # ignores priority's own selection — so P1 stays switchable.
+    _, fac = facets.filter_and_count(entries, SCHEMA, {"priority": {"P0"}})
+    assert fac["priority"] == {"P0": 1, "P1": 2}
+
+
+def test_malformed_toggle_narrows_items_and_counts():
+    entries = [
+        _e("a", state="active", malformed=True, priority="P9"),
+        _e("b", state="active", malformed=False, priority="P0"),
+    ]
+    items, fac = facets.filter_and_count(entries, SCHEMA, {}, only_malformed=True)
+    assert {i["slug"] for i in items} == {"a"}
+    assert fac["priority"] == {"P9": 1}
+
+
+def test_missing_value_contributes_no_facet_value():
+    entries = [_e("a", priority="P0"), _e("b")]  # b has no priority
+    _, fac = facets.filter_and_count(entries, SCHEMA, {})
+    assert fac["priority"] == {"P0": 1}
+
+
+def test_allowed_filter_keys():
+    assert facets.allowed_filter_keys(PRIORITY) == {"priority", "state",
+                                                    "unreviewed", "malformed"}