diff --git a/CMakeLists.txt b/CMakeLists.txt
index 6b0b832..2c6a74d 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -144,6 +144,7 @@ set(GEMMA_TEST_FILES
   backprop/backward_scalar_test.cc
   backprop/backward_test.cc
   backprop/optimize_test.cc
+  compression/blob_store_test.cc
   compression/compress_test.cc
   compression/distortion_test.cc
   compression/nuq_test.cc
diff --git a/compression/BUILD.bazel b/compression/BUILD.bazel
index e832793..b6508cd 100644
--- a/compression/BUILD.bazel
+++ b/compression/BUILD.bazel
@@ -48,6 +48,19 @@ cc_library(
     ],
 )
 
+cc_test(
+    name = "blob_store_test",
+    srcs = ["blob_store_test.cc"],
+    deps = [
+        ":blob_store",
+        ":io",
+        "@googletest//:gtest_main",  # buildcleaner: keep
+        "@highway//:hwy",
+        "@highway//:hwy_test_util",
+        "@highway//:thread_pool",
+    ],
+)
+
 cc_library(
     name = "distortion",
     hdrs = [
diff --git a/compression/blob_store.cc b/compression/blob_store.cc
index 57f50f5..0ce9996 100644
--- a/compression/blob_store.cc
+++ b/compression/blob_store.cc
@@ -288,6 +288,14 @@ BlobError BlobReader::ReadOne(hwy::uint128_t key, void* data,
   uint64_t offset;
   size_t actual_size;
   if (!blob_store_->FindKey(key, offset, actual_size)) return __LINE__;
+  if (actual_size != size) {
+    fprintf(stderr,
+            "Mismatch between expected %d and actual %d KiB size of blob %s. "
+            "Please see README.md on how to update the weights.\n",
+            static_cast<int>(size >> 10), static_cast<int>(actual_size >> 10),
+            StringFromKey(key).c_str());
+    return __LINE__;
+  }
   if (!file_->Read(offset, actual_size, data)) {
     return __LINE__;
   }
diff --git a/compression/blob_store_test.cc b/compression/blob_store_test.cc
new file mode 100644
index 0000000..6464756
--- /dev/null
+++ b/compression/blob_store_test.cc
@@ -0,0 +1,81 @@
+// Copyright 2024 Google LLC
+// SPDX-License-Identifier: Apache-2.0
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "compression/blob_store.h"
+
+#include <stdio.h>
+
+#include <algorithm>
+#include <array>
+
+#include "compression/io.h"
+#include "hwy/contrib/thread_pool/thread_pool.h"
+#include "hwy/tests/hwy_gtest.h"
+#include "hwy/tests/test_util-inl.h"  // HWY_ASSERT_EQ
+
+namespace gcpp {
+namespace {
+
+#if !HWY_TEST_STANDALONE
+class BlobStoreTest : public testing::Test {};
+#endif
+
+#if !HWY_OS_WIN
+TEST(BlobStoreTest, TestReadWrite) {
+  static const std::array<float, 4> kOriginalData = {-1, 0, 3.14159, 2.71828};
+
+  // mkstemp will modify path_str so it holds a newly-created temporary file.
+  char path_str[] = "/tmp/blob_store_test.sbs-XXXXXX";
+  const int fd = mkstemp(path_str);
+  HWY_ASSERT(fd > 0);
+
+  hwy::ThreadPool pool(4);
+  const Path path(path_str);
+  std::array<float, 4> buffer = kOriginalData;
+
+  const hwy::uint128_t keyA = MakeKey("0123456789abcdef");
+  const hwy::uint128_t keyB = MakeKey("q");
+  BlobWriter writer;
+  writer.Add(keyA, "DATA", 5);
+  writer.Add(keyB, buffer.data(), sizeof(buffer));
+  HWY_ASSERT_EQ(writer.WriteAll(pool, path), 0);
+  HWY_ASSERT_ARRAY_EQ(kOriginalData.data(), buffer.data(), buffer.size());
+
+  std::fill(buffer.begin(), buffer.end(), 0);
+  BlobReader reader;
+  HWY_ASSERT_EQ(reader.Open(path), 0);
+  HWY_ASSERT_EQ(reader.BlobSize(keyA), 5);
+  HWY_ASSERT_EQ(reader.BlobSize(keyB), sizeof(buffer));
+
+  HWY_ASSERT_EQ(reader.Enqueue(keyB, buffer.data(), sizeof(buffer)), 0);
+  HWY_ASSERT_EQ(reader.ReadAll(pool), 0);
+  HWY_ASSERT_ARRAY_EQ(kOriginalData.data(), buffer.data(), buffer.size());
+
+  {
+    std::array<char, 5> buffer;
+    HWY_ASSERT(reader.ReadOne(keyA, buffer.data(), 1) != 0);
+    HWY_ASSERT_EQ(reader.ReadOne(keyA, buffer.data(), 5), 0);
+    HWY_ASSERT_STRING_EQ("DATA", buffer.data());
+  }
+
+  close(fd);
+  unlink(path_str);
+}
+#endif
+
+}  // namespace
+}  // namespace gcpp
+
+HWY_TEST_MAIN();