diff --git a/gemma/model_store.cc b/gemma/model_store.cc
index ca454bd..2aab1f5 100644
--- a/gemma/model_store.cc
+++ b/gemma/model_store.cc
@@ -463,7 +463,7 @@ void WriteSingleFile(const ModelConfig& config, const GemmaTokenizer& tokenizer,
 
   AddBlob(kMatPtrsName, serialized_mat_ptrs, writer);
 
-  writer.WriteAll();
+  writer.Finalize();
 }
 
 }  // namespace gcpp
diff --git a/io/blob_store.cc b/io/blob_store.cc
index 00d4fff..9ce7b09 100644
--- a/io/blob_store.cc
+++ b/io/blob_store.cc
@@ -99,7 +99,10 @@ struct BlobIO {
 // For V2: the file is represented as
 //   Header + PadToBlobAlign + Payload + PadToEndAlign + Directory + Header
 // The Header at the beginning has num_blobs == 0; and the Header at the end has
-// the correct num_blobs.
+// the correct num_blobs. This allows writing blobs without knowing the total
+// number of them, nor holding all them in memory. As of 2025-07-31, we support
+// reading both, but always write V2. Note that its num_blobs == 0 was
+// previously disallowed. To read V2, pull the latest code from the dev branch.
 //
 // Actual payload is indexed by the directory with keys, offset and bytes; keys
 // are unique, opaque 128-bit keys.
@@ -108,10 +111,6 @@ struct BlobIO {
 // Additional data may be added only inside new blobs. Changes to the blob
 // contents or type should be handled by renaming keys.
 //
-// The file format deliberately omits a version number because it is unchanging.
-// Additional data may be added only inside new blobs. Changes to the blob
-// contents or type should be handled by renaming keys.
-//
 // This class is for internal use by `BlobReader` and `BlobWriter`. Its
 // interface is more low-level: fixed-size keys instead of strings.
 class BlobStore {
@@ -473,7 +472,8 @@ static void EnqueueChunks(size_t key_idx, uint64_t offset, uint64_t bytes,
 BlobWriter::BlobWriter(const Path& filename, hwy::ThreadPool& pool)
     : file_(OpenFileOrNull(filename, "w+")), pool_(pool) {
   if (!file_) HWY_ABORT("Failed to open for writing %s", filename.path.c_str());
-  // Write a fake header to the beginning of the file.
+  // Write a placeholder header to the beginning of the file. If append-only,
+  // we will later write a footer, else we will update the header.
   std::vector<uint8_t> bytes_before_blobs = BlobStore::BytesBeforeBlobsV2();
   file_->Write(bytes_before_blobs.data(), bytes_before_blobs.size(), 0);
 }
@@ -503,13 +503,15 @@ void BlobWriter::Add(const std::string& key, const void* data, size_t bytes) {
       });
 }
 
-void BlobWriter::WriteAll() {
+void BlobWriter::Finalize() {
   const BlobStore bs = BlobStore(keys_, blob_sizes_);
 
   // Write the rest of the bytes, which contains: paddings + directory + header.
   const auto bytes_after_blobs = bs.BytesAfterBlobs();
   file_->Write(bytes_after_blobs.data(), bytes_after_blobs.size(),
                file_->FileSize());
+
+  file_.reset();  // closes the file
 }
 
 }  // namespace gcpp
diff --git a/io/blob_store.h b/io/blob_store.h
index b7dba0f..916091c 100644
--- a/io/blob_store.h
+++ b/io/blob_store.h
@@ -111,21 +111,19 @@ class BlobReader {
   std::unordered_map<std::string, size_t> key_idx_for_key_;
 };
 
-// Collects references to blobs and writes them all at once with parallel I/O.
+// Writes blobs immediately using parallel I/O, and collects their metadata for
+// writing the file footer.
 // Thread-compatible: independent instances can be used concurrently, but it
 // does not make sense to call the methods concurrently.
 class BlobWriter {
  public:
-  explicit BlobWriter(const Path& filename, hwy::ThreadPool& pool);
+  BlobWriter(const Path& filename, hwy::ThreadPool& pool);
 
+  // Writes the blob to disk with padding for alignment. Aborts on error.
   void Add(const std::string& key, const void* data, size_t bytes);
 
-  // For `ModelStore`: this is the `key_idx` of the next blob to be added.
-  size_t NumAdded() const { return keys_.size(); }
-
-  // Stores all blobs to disk in the given order with padding for alignment.
-  // Aborts on error.
-  void WriteAll();
+  // Appends a footer and closes the file. Must be called once after all `Add`.
+  void Finalize();
 
  private:
   std::unique_ptr<File> file_;
diff --git a/io/blob_store_test.cc b/io/blob_store_test.cc
index 36ba27f..078a974 100644
--- a/io/blob_store_test.cc
+++ b/io/blob_store_test.cc
@@ -55,7 +55,7 @@ TEST(BlobStoreTest, TestReadWrite) {
   BlobWriter writer(path, pool);
   writer.Add(keyA, "DATA", 5);
   writer.Add(keyB, buffer.data(), sizeof(buffer));
-  writer.WriteAll();
+  writer.Finalize();
   HWY_ASSERT_ARRAY_EQ(kOriginalData.data(), buffer.data(), buffer.size());
 
   std::fill(buffer.begin(), buffer.end(), 0);
@@ -126,7 +126,7 @@ TEST(BlobStoreTest, TestNumBlobs) {
     }
     HWY_ASSERT(keys.size() == num_blobs);
     HWY_ASSERT(blobs.size() == num_blobs);
-    writer.WriteAll();
+    writer.Finalize();
 
     BlobReader reader(path);
     HWY_ASSERT_EQ(reader.Keys().size(), num_blobs);