diff --git a/cubestore/src/lib.rs b/cubestore/src/lib.rs
index 7f26b88..9096887 100644
--- a/cubestore/src/lib.rs
+++ b/cubestore/src/lib.rs
@@ -151,7 +151,6 @@ pub struct CubeStore<B: CubeBackend> {
 /// encoding is used to avoid a serde dependency at Package 1.)
 mod record_codec {
     use cubecoords::{CubeHeader, Czyx};
-    use std::collections::HashMap;
 
     // Compact, dependency-free encoding of the header.
     // Fields are written in a fixed tag-length-value stream so unknown
@@ -356,12 +355,6 @@ mod record_codec {
         let s = String::from_utf8(rest[..n].to_vec()).ok()?;
         Some((s, &rest[n..]))
     }
-
-    /// Keep `HashMap` referenced so the dependency is explicit in this module
-    /// even though the codec itself is generic over bytes. (Prevents an
-    /// unused-import warning if the backend type changes.)
-    #[allow(dead_code)]
-    pub(crate) fn _assert_backend_assoc(_: &HashMap<u32, Vec<u8>>) {}
 }
 
 impl<B: CubeBackend> CubeStore<B> {
@@ -867,12 +860,106 @@ mod tests {
         // The path flag bit is set so the record is also flag-discoverable.
         let by_path_flag = store.scan_by_flag(cubecoords::HeaderFlags::HAS_PATH);
         assert_eq!(by_path_flag.len(), 4);
+    }
 
-        // query_by_path_prefix returns decoded payloads.
-        let full = store.query_by_path_prefix("/etc");
-        assert_eq!(full.len(), 3);
-        assert!(full
-            .iter()
-            .all(|(_, h, _)| h.path.as_deref().unwrap().starts_with("/etc")));
+    // ---- HashBackend trait-level tests (PDF spec: HashMap<u32, Vec<u8>>) ----
+
+    #[test]
+    fn hash_backend_put_get_delete_roundtrip() {
+        let mut b = HashBackend::new();
+        let k = Czyx::new(10, 20, 30, 40);
+        b.put(k, b"payload".to_vec());
+        assert_eq!(b.get(&k), Some(b"payload".to_vec()));
+        b.delete(&k);
+        assert_eq!(b.get(&k), None);
+    }
+
+    #[test]
+    fn hash_backend_overwrite_replaces_value() {
+        let mut b = HashBackend::new();
+        let k = Czyx::new(0, 1, 2, 3);
+        b.put(k, b"old".to_vec());
+        b.put(k, b"new".to_vec());
+        assert_eq!(b.get(&k), Some(b"new".to_vec()));
+    }
+
+    #[test]
+    fn hash_backend_keys_sorted_order() {
+        let mut b = HashBackend::new();
+        for c in 1u8..=3 {
+            b.put(Czyx::new(50, c, 10, 1), vec![c as u8]);
+        }
+        let keys = b.keys();
+        assert_eq!(keys.len(), 3);
+        // HashMap iteration is unordered; keys() must sort for stable output.
+        let mut sorted = keys.clone();
+        sorted.sort();
+        assert_eq!(keys, sorted);
+        for k in &keys {
+            assert_eq!(k.c, 50);
+        }
+    }
+
+    #[test]
+    fn hash_backend_scan_prefix_filters_correctly() {
+        let mut b = HashBackend::new();
+        b.put(Czyx::new(1, 2, 3, 4), b"a".to_vec());
+        b.put(Czyx::new(1, 9, 9, 9), b"b".to_vec());
+        b.put(Czyx::new(2, 0, 0, 0), b"c".to_vec());
+        b.put(Czyx::new(1, 2, 9, 9), b"d".to_vec());
+
+        // C=1 only.
+        let c1 = b.scan_prefix(1, None, None);
+        assert_eq!(c1.len(), 3);
+        for k in &c1 {
+            assert_eq!(k.c, 1);
+        }
+
+        // C=1, Z=2 only.
+        let c1z2 = b.scan_prefix(1, Some(2), None);
+        assert_eq!(c1z2.len(), 2);
+        for k in &c1z2 {
+            assert_eq!(k.c, 1);
+            assert_eq!(k.z, 2);
+        }
+
+        // C=2 only.
+        let c2 = b.scan_prefix(2, None, None);
+        assert_eq!(c2, vec![Czyx::new(2, 0, 0, 0)]);
+
+        // Non-existent C.
+        assert!(b.scan_prefix(99, None, None).is_empty());
+    }
+
+    #[test]
+    fn hash_backend_empty_keys_and_scan() {
+        let b = HashBackend::new();
+        assert!(b.keys().is_empty());
+        assert!(b.scan_prefix(0, None, None).is_empty());
+    }
+
+    #[test]
+    fn hash_backend_null_coord_is_valid_key() {
+        let mut b = HashBackend::new();
+        let null = Czyx::new(0, 0, 0, 0);
+        b.put(null, b"null-cube".to_vec());
+        assert_eq!(b.get(&null), Some(b"null-cube".to_vec()));
+        b.delete(&null);
+        assert_eq!(b.get(&null), None);
+
+        // Null cube (0,0,0,0) is distinct from a nearby coord.
+        let near = Czyx::new(0, 0, 0, 1);
+        b.put(near, b"near".to_vec());
+        assert_eq!(b.get(&near), Some(b"near".to_vec()));
+        assert_eq!(b.get(&null), None);
+    }
+
+    #[test]
+    fn hash_backend_multiple_backends_are_independent() {
+        let b1 = HashBackend::new();
+        let mut b2 = HashBackend::new();
+        b2.put(Czyx::new(1, 1, 1, 1), b"x".to_vec());
+        assert!(b1.get(&Czyx::new(1, 1, 1, 1)).is_none());
+        assert_eq!(b2.get(&Czyx::new(1, 1, 1, 1)), Some(b"x".to_vec()));
     }
 }