diff --git a/tests/chapter_filter_normalize_tests.rs b/tests/chapter_filter_normalize_tests.rs
new file mode 100644
index 0000000..9c1f829
--- /dev/null
+++ b/tests/chapter_filter_normalize_tests.rs
@@ -0,0 +1,46 @@
+//! Property: the `--chapters` CLI token list is normalized to the zero-padded
+//! chapter-tag form ("01".."22") that `Filter::accepts` compares against, so a
+//! bare number ("4") filters identically to its padded form ("04"). Any token
+//! that maps to no known ICD-10-GM chapter is surfaced as unrecognised (for a
+//! stderr warning) instead of silently collapsing the result set to empty.
+//!
+//! Regression for: `--chapters 4,9` / `--chapters E` silently returned zero
+//! suggestions because the CLI passed raw split+trim tokens straight into the
+//! filter, which compares by exact string equality against padded tags.
+
+use alpha_id::model::normalize_chapters;
+
+#[test]
+fn bare_and_padded_numbers_normalize_to_the_same_padded_tag() {
+    let (padded_bare, unknown_bare) = normalize_chapters("4,9");
+    let (padded_pad, unknown_pad) = normalize_chapters("04,09");
+
+    assert_eq!(padded_bare, vec!["04".to_string(), "09".to_string()]);
+    assert_eq!(padded_bare, padded_pad);
+    assert!(unknown_bare.is_empty());
+    assert!(unknown_pad.is_empty());
+}
+
+#[test]
+fn out_of_range_number_is_surfaced_as_unrecognised() {
+    // "99" is a syntactically-valid number but not one of the 22 chapters.
+    let (padded, unknown) = normalize_chapters("99");
+    assert!(padded.is_empty());
+    assert_eq!(unknown, vec!["99".to_string()]);
+}
+
+#[test]
+fn non_numeric_token_is_surfaced_as_unrecognised() {
+    // Under the contract a letter like "E" is simply an unknown token: it must
+    // be reported, not silently dropped into an empty-result filter.
+    let (padded, unknown) = normalize_chapters("E");
+    assert!(padded.is_empty());
+    assert_eq!(unknown, vec!["E".to_string()]);
+}
+
+#[test]
+fn mixed_known_and_unknown_tokens_split_correctly() {
+    let (padded, unknown) = normalize_chapters("4, 99, 9");
+    assert_eq!(padded, vec!["04".to_string(), "09".to_string()]);
+    assert_eq!(unknown, vec!["99".to_string()]);
+}