diff --git a/CHANGELOG.md b/CHANGELOG.md
index f070cd5..2516491 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -31,7 +31,11 @@
   - `resize`: grow a chunked dataset up to its maximum dimensions (h5py's
     `Dataset.resize`).
   - `set_attr`: add or replace an attribute in an object header, in free
-    space or in a new continuation chunk at the end of the file.
+    space or in a new continuation chunk at the end of the file. A
+    version-2 header (h5py `libver='v110'` and later) without an Attribute
+    Info message gets one, as libhdf5's `H5O__attr_create` adds it: libhdf5
+    counts such a header's attributes through that message, and without it
+    h5py reported `len(obj.attrs) == 0` while listing them.
   - Each edit is planned in memory and refused as a whole
     (`Error::Unsupported`, file untouched) when any part is not supported:
     new chunks in a version-2 B-tree index (two or more unlimited
diff --git a/crates/clawhdf5-tools/tests/edit_interop.rs b/crates/clawhdf5-tools/tests/edit_interop.rs
index fe44aa8..7587126 100644
--- a/crates/clawhdf5-tools/tests/edit_interop.rs
+++ b/crates/clawhdf5-tools/tests/edit_interop.rs
@@ -1315,3 +1315,90 @@ fn optional_filters_that_fail_are_skipped() {
         assert!(o.status.success(), "h5rs check --data {p}:\n{}", text(&o));
     }
 }
+
+/// libhdf5 counts a version-2 object header's attributes through its
+/// Attribute Info message and reports none without one. The editor adds
+/// that message, as `H5O__attr_create` does, when it gives a version-2
+/// header (h5py `libver='v110'`/`'latest'`) its first attribute: afterwards
+/// h5py lists, counts and reads every attribute, new and existing, h5dump
+/// agrees, and h5py `r+` can add more (past the compact limit, into dense
+/// storage) with the count still right.
+#[test]
+fn attribute_count_in_version_2_headers() {
+    if !tools_ok() {
+        return;
+    }
+    for (lv, dump) in [("'v110'", true), ("'latest'", false)] {
+        let dir = tmpdir();
+        let path = dir.path().join("acount.h5");
+        let p = path.to_str().unwrap();
+        py(&format!(
+            "import h5py, numpy as np\n\
+             with h5py.File({p:?}, 'w', libver={lv}) as f:\n\
+             \x20   f.create_group('g')\n\
+             \x20   f.create_group('has').attrs['old'] = 7\n\
+             \x20   f.create_dataset('d', data=np.arange(3, dtype='<i4'))\n\
+             \x20   f.create_dataset('e', data=np.arange(3, dtype='<i4')).attrs['old'] = 7\n"
+        ));
+        let mut ed = FileEditor::open(&path).unwrap();
+        for obj in ["/", "g", "has", "d", "e"] {
+            for i in 0..3 {
+                ed.set_attr(obj, &format!("a{i}"), &AttrValue::I64(i))
+                    .unwrap();
+            }
+            // Replacing one keeps the count.
+            ed.set_attr(obj, "a1", &AttrValue::F64(1.5)).unwrap();
+        }
+        drop(ed);
+        check_tools(&path, dump);
+        let check = |extra: usize| {
+            py(&format!(
+                "import h5py\n\
+                 f = h5py.File({p:?}, 'r')\n\
+                 for o in ['/', 'g', 'has', 'd', 'e']:\n\
+                 \x20   a = f[o].attrs\n\
+                 \x20   want = {{'a0': 0, 'a1': 1.5, 'a2': 2}}\n\
+                 \x20   if o in ('has', 'e'): want['old'] = 7\n\
+                 \x20   for i in range({extra}): want[f'h{{i}}'] = i\n\
+                 \x20   assert len(a) == len(want), (o, len(a), list(a))\n\
+                 \x20   assert h5py.h5o.get_info(f[o].id).num_attrs == len(want), o\n\
+                 \x20   assert sorted(a) == sorted(want), (o, list(a))\n\
+                 \x20   assert {{k: a[k] for k in a}} == want, (o, dict(a))\n"
+            ));
+            let f = File::open(&path).unwrap();
+            for (o, n) in [("/", 3), ("g", 3), ("has", 4), ("d", 3), ("e", 4)] {
+                let attrs = match o {
+                    "/" => f.root().attrs().unwrap(),
+                    "d" | "e" => f.dataset(o).unwrap().attrs().unwrap(),
+                    _ => f.group(o).unwrap().attrs().unwrap(),
+                };
+                assert_eq!(attrs.len(), n + extra, "{o}");
+                assert!(matches!(attrs.get("a1"), Some(AttrValue::F64(x)) if *x == 1.5));
+            }
+            if dump {
+                // Every object's attributes: 3 set by the editor on each of
+                // the five, 2 existing, `extra` from h5py on each.
+                let out = Command::new("h5dump").args(["-A", p]).output().unwrap();
+                assert!(out.status.success(), "h5dump -A:\n{}", text(&out));
+                let s = String::from_utf8_lossy(&out.stdout);
+                assert_eq!(
+                    s.matches("ATTRIBUTE \"").count(),
+                    5 * (3 + extra) + 2,
+                    "h5dump -A:\n{s}"
+                );
+            }
+        };
+        check(0);
+        // libhdf5 adds more: to 7 or 8 (compact), then past its limit.
+        for extra in [4usize, 10] {
+            py(&format!(
+                "import h5py\n\
+                 with h5py.File({p:?}, 'r+') as f:\n\
+                 \x20   for o in ['/', 'g', 'has', 'd', 'e']:\n\
+                 \x20       for i in range({extra}): f[o].attrs[f'h{{i}}'] = i\n"
+            ));
+            check(extra);
+            check_tools(&path, dump);
+        }
+    }
+}
diff --git a/crates/clawhdf5/src/edit/mod.rs b/crates/clawhdf5/src/edit/mod.rs
index c0a5d07..15237fc 100644
--- a/crates/clawhdf5/src/edit/mod.rs
+++ b/crates/clawhdf5/src/edit/mod.rs
@@ -47,6 +47,8 @@ const MSG_EXTERNAL: u16 = 0x07;
 const MSG_ATTR_INFO: u16 = 0x15;
 /// Message flag: the message is shared (stored elsewhere).
 const MSG_FLAG_SHARED: u8 = 0x02;
+/// Message flag: the message must not be shared (`H5O_MSG_FLAG_DONTSHARE`).
+const MSG_FLAG_DONTSHARE: u8 = 0x04;
 
 /// An HDF5 file opened for in-place modification.
 ///
@@ -815,7 +817,24 @@ impl FileEditor {
             if let Some(i) = existing {
                 hdr.delete(img, i)?;
             }
+            // libhdf5 counts a version-2 header's attributes through its
+            // Attribute Info message and reports none without one; like
+            // H5O__attr_create, add it when missing: before the attribute
+            // when free space holds both (libhdf5's order), else after it,
+            // so that a new continuation chunk made for the attribute has
+            // room for it too.
+            let ainfo = (hdr.version == 2 && hdr.find(MSG_ATTR_INFO).is_none())
+                .then(|| attr_info_message(hdr.flags, img.os));
+            let ainfo_first = ainfo
+                .as_ref()
+                .is_some_and(|a| hdr.has_free(a.len() + hdr.hsize() + body.len()));
+            if let Some(a) = ainfo.as_ref().filter(|_| ainfo_first) {
+                hdr.insert(img, MSG_ATTR_INFO, MSG_FLAG_DONTSHARE, a, None)?;
+            }
             hdr.insert(img, MSG_ATTRIBUTE, 0, &body, None)?;
+            if let Some(a) = ainfo.as_ref().filter(|_| !ainfo_first) {
+                hdr.insert(img, MSG_ATTR_INFO, MSG_FLAG_DONTSHARE, a, None)?;
+            }
             hdr.finish(img)
         })
     }
@@ -900,6 +919,26 @@ fn attr_name(d: &[u8]) -> Result<&[u8], Error> {
     Ok(name.split(|&b| b == 0).next().unwrap_or(name))
 }
 
+/// A new Attribute Info message for a version-2 header with flags
+/// `hdr_flags`, as `H5O__attr_create` makes it: version 0, creation order
+/// tracked / indexed as the header's flags say, maximum creation index 0,
+/// and no dense storage (undefined fractal heap and B-tree addresses).
+fn attr_info_message(hdr_flags: u8, os: u8) -> Vec<u8> {
+    let track = hdr_flags & 0x04 != 0;
+    let index = hdr_flags & 0x08 != 0;
+    let mut b = vec![0u8, u8::from(track) | (u8::from(index) << 1)];
+    if track {
+        b.extend_from_slice(&0u16.to_le_bytes());
+    }
+    let undef_addr = vec![0xffu8; os as usize];
+    b.extend_from_slice(&undef_addr);
+    b.extend_from_slice(&undef_addr);
+    if index {
+        b.extend_from_slice(&undef_addr);
+    }
+    b
+}
+
 /// A version-2 header's limit on compact attributes: stored when its flags
 /// say so, else libhdf5's default of 8.
 fn max_compact_attrs(img: &Image<'_>, hdr: &Header) -> Result<u16, Error> {
diff --git a/crates/clawhdf5/src/edit/ohdr.rs b/crates/clawhdf5/src/edit/ohdr.rs
index 5446702..f3abfe0 100644
--- a/crates/clawhdf5/src/edit/ohdr.rs
+++ b/crates/clawhdf5/src/edit/ohdr.rs
@@ -326,6 +326,11 @@ impl Header {
             .map(|(i, _)| i)
     }
 
+    /// Whether free space in the header can take a body of `len` bytes.
+    pub(crate) fn has_free(&self, len: usize) -> bool {
+        self.best_nil(self.padded(len)).is_some()
+    }
+
     /// Put a message into slot `i` (a NIL message, or a message being
     /// moved away), splitting off the rest as a NIL message.
     fn place(