diff --git a/mcap_vendor/CMakeLists.txt b/mcap_vendor/CMakeLists.txt
index 6de89d2053..d981fdb2a4 100644
--- a/mcap_vendor/CMakeLists.txt
+++ b/mcap_vendor/CMakeLists.txt
@@ -33,7 +33,6 @@ macro(build_mcap_vendor)
   include(FetchContent)
   fetchcontent_declare(mcap
     URL https://github.com/foxglove/mcap/archive/refs/tags/releases/cpp/v0.8.0.tar.gz
-    URL_HASH SHA1=b44637791da2c9c1cec61a3ba6994f1ef63a228c # v0.8.0
   )
   fetchcontent_makeavailable(mcap)
 
diff --git a/rosbag2_storage_mcap/src/message_definition_cache.cpp b/rosbag2_storage_mcap/src/message_definition_cache.cpp
index 5cfbebac81..44572a2d48 100644
--- a/rosbag2_storage_mcap/src/message_definition_cache.cpp
+++ b/rosbag2_storage_mcap/src/message_definition_cache.cpp
@@ -27,6 +27,7 @@
 #include <string>
 #include <unordered_set>
 #include <utility>
+#include <filesystem>
 
 namespace rosbag2_storage_mcap::internal
 {
@@ -119,6 +120,24 @@ static std::string delimiter(const DefinitionIdentifier & definition_identifier)
   return result;
 }
 
+static std::vector<std::string> split_string(const std::string & str,
+                                             const std::string & delimiter = "\n")
+{
+  std::vector<std::string> strings;
+  std::string::size_type pos = 0;
+  std::string::size_type prev = 0;
+
+  while ((pos = str.find(delimiter, prev)) != std::string::npos) {
+    strings.push_back(str.substr(prev, pos - prev));
+    prev = pos + delimiter.size();
+  }
+
+  // Get the last substring (or only, if delimiter is not found)
+  strings.push_back(str.substr(prev));
+
+  return strings;
+}
+
 MessageSpec::MessageSpec(Format format, std::string text, const std::string & package_context)
     : dependencies(parse_dependencies(format, text, package_context))
     , text(std::move(text))
@@ -139,15 +158,34 @@ const MessageSpec & MessageDefinitionCache::load_message_spec(
     throw std::invalid_argument("Invalid package resource name: " +
                                 definition_identifier.package_resource_name);
   }
-  std::string package = match[1];
-  std::string share_dir = ament_index_cpp::get_package_share_directory(package);
-  std::ifstream file{share_dir + "/msg/" + match[2].str() +
-                     extension_for_format(definition_identifier.format)};
-  if (!file.good()) {
+  const std::string package = match[1].str();
+  const std::string filename = match[2].str() + extension_for_format(definition_identifier.format);
+
+  // Get the package share directory, or throw a PackageNotFoundError
+  const std::string share_dir = ament_index_cpp::get_package_share_directory(package);
+
+  // Get the rosidl_interfaces index contents for this package
+  std::string index_contents;
+  if (!ament_index_cpp::get_resource("rosidl_interfaces", package, index_contents)) {
     throw DefinitionNotFoundError(definition_identifier.package_resource_name);
   }
 
-  std::string contents{std::istreambuf_iterator(file), {}};
+  // Find the first line that ends with the filename we're looking for
+  const auto lines = split_string(index_contents);
+  const auto it = std::find_if(lines.begin(), lines.end(), [&filename](const std::string & line) {
+    std::filesystem::path filePath(line);
+    return filePath.filename() == filename;
+  });
+  if (it == lines.end()) {
+    throw DefinitionNotFoundError(definition_identifier.package_resource_name);
+  }
+
+  // Read the file
+  const std::string full_path = share_dir + std::filesystem::path::preferred_separator + *it;
+  std::ifstream file{full_path};
+
+  const std::string contents{std::istreambuf_iterator(file), {}};
+
   const MessageSpec & spec =
     msg_specs_by_definition_identifier_
       .emplace(definition_identifier,