Cap tag-filter lists at 100 entries, all spellings

Review finding: unbounded tag lists fan out into one correlated EXISTS
per tag on both page and count statements. Cap each list at 100
normalized entries with 400 INVALID_TAG_FILTER naming the parameter.

Applies to the legacy spellings as well — a deliberate, decided
exception to the old-names-behave-identically rule, since a cap only on
new names would leave the same fan-out reachable through the aliases.

app/assets/api/routes.py

@@ -125,6 +125,13 @@ class InvalidTagFilterError(Exception):
         self.details = details
 
 
+# Applies to every tag-filter list, legacy spellings included — unlike the
+# combination validation below, which only runs when a new-name parameter is
+# present. Deliberate exception to legacy byte-identity (decision 2026-08-05):
+# an unbounded list fans out into one EXISTS per tag on page and count queries.
+MAX_TAG_FILTER_TAGS = 100
+
+
 def _resolve_tag_filters(
     q: schemas_in.ListAssetsQuery | schemas_in.TagsRefineQuery,
 ) -> tuple[list[str], list[str], list[str]]:
@@ -146,6 +153,24 @@ def _resolve_tag_filters(
     tags_any = normalize_tags(q.tags_any)
     tags_none = normalize_tags(q.tags_none)
 
+    for param_name, values in (
+        ("include_tags", include_tags),
+        ("exclude_tags", exclude_tags),
+        ("tags_all", tags_all),
+        ("tags_any", tags_any),
+        ("tags_none", tags_none),
+    ):
+        if len(values) > MAX_TAG_FILTER_TAGS:
+            raise InvalidTagFilterError(
+                f"'{param_name}' lists {len(values)} tags; the maximum is "
+                f"{MAX_TAG_FILTER_TAGS}.",
+                {
+                    "parameter": param_name,
+                    "count": len(values),
+                    "max": MAX_TAG_FILTER_TAGS,
+                },
+            )
+
     if not (tags_all or tags_any or tags_none):
         return include_tags, [], exclude_tags
 

tests-unit/assets_test/test_list_filter.py

@@ -719,6 +719,28 @@ def test_list_assets_tag_values_case_sensitive(http, api_base, asset_factory, ma
     assert names_for({"tags_all": f"unit-tests,{scope},{upper}", "tags_none": lower}) == {a["name"]}
 
 
+def test_tag_list_cap_applies_to_all_spellings(http, api_base):
+    """The 100-tag cap covers legacy spellings too — the one deliberate
+    exception to legacy byte-identity (decision 2026-08-05)."""
+    big = ",".join(f"cap-{i}" for i in range(101))
+    for param in ("tags_any", "include_tags"):
+        r = http.get(api_base + "/api/assets", params={param: big}, timeout=120)
+        body = r.json()
+        assert r.status_code == 400, body
+        assert body["error"]["code"] == "INVALID_TAG_FILTER"
+        assert body["error"]["details"]["parameter"] == param
+        assert body["error"]["details"]["max"] == 100
+
+    exact = ",".join(f"cap-{i}" for i in range(100))
+    r = http.get(api_base + "/api/assets", params={"tags_any": exact}, timeout=120)
+    assert r.status_code == 200, r.json()
+
+    # The cap counts normalized (deduped) tags, not raw CSV items.
+    dups = ",".join("cap-dup" for _ in range(150))
+    r = http.get(api_base + "/api/assets", params={"tags_any": dups}, timeout=120)
+    assert r.status_code == 200, r.json()
+
+
 def test_resolve_tag_filters_no_deprecation_warning():
     """The deprecated-field warning is for API clients; the server's own remap
     shim must not fire it on every request."""