diff --git a/crates/clawhdf5-format/src/file_writer.rs b/crates/clawhdf5-format/src/file_writer.rs
index 1574dc1..65181a0 100644
--- a/crates/clawhdf5-format/src/file_writer.rs
+++ b/crates/clawhdf5-format/src/file_writer.rs
@@ -316,6 +316,18 @@ pub(crate) fn build_single_block_fractal_heap(
     // Direct block layout: sig(4) + ver(1) + heap_addr(os) + block_offset(bo_bytes)
     //   + checksum(4) [when flags bit 1 set] + data...
     let dblock_header_size = 4 + 1 + os + block_offset_bytes + 4; // +4 for checksum
+
+    // An object must fit one direct block: the writer has no huge-object
+    // path, and libhdf5 cannot read an object that overruns its block.
+    let max_managed = max_direct_block_size as usize - dblock_header_size;
+    if let Some(big) = serialized.iter().find(|s| s.len() > max_managed) {
+        return Err(FormatError::SerializationError(format!(
+            "a {}-byte message cannot go in dense storage: a fractal heap \
+             object holds at most {max_managed} bytes (huge heap objects are \
+             not written)",
+            big.len()
+        )));
+    }
     let total_data_size: usize = serialized.iter().map(|s| s.len()).sum();
     let dblock_content_size = dblock_header_size + total_data_size;
     let starting_block_size = dblock_content_size.next_power_of_two().max(512) as u64;
@@ -373,8 +385,7 @@ pub(crate) fn build_single_block_fractal_heap(
     frhp.extend_from_slice(&heap_id_length.to_le_bytes());
     frhp.extend_from_slice(&0u16.to_le_bytes()); // io_filter_encoded_length
     frhp.push(0x02); // flags: bit 1 = checksum direct blocks
-    let max_managed = max_direct_block_size as u32 - dblock_header_size as u32;
-    frhp.extend_from_slice(&max_managed.to_le_bytes());
+    frhp.extend_from_slice(&(max_managed as u32).to_le_bytes());
     write_length(&mut frhp, 0, LENGTH_SIZE); // next_huge_object_id
     write_undef_offset(&mut frhp, OFFSET_SIZE); // btree_huge_objects_address
     write_length(&mut frhp, free_space as u64, LENGTH_SIZE); // free_space_managed_blocks
@@ -455,7 +466,8 @@ pub(crate) fn build_single_block_fractal_heap(
 /// in turn hold indirect blocks. Objects are packed into direct blocks in
 /// heap-offset order and never span blocks; a block too small for the next
 /// object is left unallocated (an undefined address), as libhdf5 skips rows
-/// when it needs a bigger block. There is no huge-object path.
+/// when it needs a bigger block. The caller has checked that every object
+/// fits a maximum-size direct block (there is no huge-object path).
 fn build_multiblock_fractal_heap(
     serialized: &[Vec<u8>],
     base_address: u64,
@@ -730,7 +742,17 @@ impl HeapPacker<'_> {
                 } else if row < geom.max_direct_rows() {
                     slots.push(self.fill_direct(off, size));
                 } else {
-                    let child = self.fill(off, Some(geom.rows_for_size(size)))?;
+                    let child_rows = geom.rows_for_size(size);
+                    // A child whose biggest direct block cannot hold the
+                    // next object is skipped whole, not walked.
+                    let biggest = geom.row_size(child_rows.min(geom.max_direct_rows()) - 1);
+                    if self.objects[self.next].len() > (biggest as usize - geom.dblock_header_size)
+                    {
+                        slots.push(HeapSlot::Empty);
+                        off += size;
+                        continue;
+                    }
+                    let child = self.fill(off, Some(child_rows))?;
                     let used = child.slots.iter().any(|s| !matches!(s, HeapSlot::Empty));
                     slots.push(if used {
                         HeapSlot::Indirect(child)
diff --git a/crates/clawhdf5/tests/writer_groups_interop.rs b/crates/clawhdf5/tests/writer_groups_interop.rs
index 3b8727d..770b835 100644
--- a/crates/clawhdf5/tests/writer_groups_interop.rs
+++ b/crates/clawhdf5/tests/writer_groups_interop.rs
@@ -765,3 +765,45 @@ fn dense_attributes_past_the_direct_blocks_of_the_root() {
         }
     }
 }
+
+#[test]
+fn a_link_too_big_for_dense_storage_is_an_error() {
+    // A link message must fit one fractal heap direct block (64 KiB less
+    // its header); the writer has no huge-object path. It used to be
+    // written anyway, cut off, and libhdf5 could not list the group.
+    let mut b = FileBuilder::new();
+    for i in 0..10 {
+        b.create_dataset(&format!("d{i}")).with_i32_data(&[i]);
+    }
+    b.add_soft_link("s", &"/y".repeat(40_000));
+    let err = b.finish().unwrap_err().to_string();
+    assert!(err.contains("fractal heap object holds at most"), "{err}");
+    // The same for a dense attribute.
+    let mut b = FileBuilder::new();
+    let x = b.create_dataset("x");
+    x.with_i32_data(&[1]);
+    for i in 0..9 {
+        x.set_attr(&format!("a{i}"), AttrValue::I64(i));
+    }
+    x.set_attr("big", AttrValue::F64Array(vec![0.5; 9_000]));
+    let err = b.finish().unwrap_err().to_string();
+    assert!(err.contains("fractal heap object holds at most"), "{err}");
+
+    // Just under the limit is fine, and libhdf5 reads it back.
+    skip_if_no_python!();
+    let dir = tempfile::tempdir().unwrap();
+    let mut b = FileBuilder::new();
+    for i in 0..10 {
+        b.create_dataset(&format!("d{i}")).with_i32_data(&[i]);
+    }
+    let target = format!("/{}", "y".repeat(65_000));
+    b.add_soft_link("s", &target);
+    let path = write(&dir, "long_soft.h5", b);
+    let out = h5py(
+        &path,
+        "with h5py.File(path, 'r') as f:\n\
+         \x20   print(json.dumps([len(f), len(f.get('s', getlink=True).path)]))",
+    );
+    assert_eq!(out, "[11, 65001]");
+    h5dump_ok(&path);
+}