diff --git a/lib/rbs/collection/config/lockfile_generator.rb b/lib/rbs/collection/config/lockfile_generator.rb
index ebf4ab71c3..3426c31e32 100644
--- a/lib/rbs/collection/config/lockfile_generator.rb
+++ b/lib/rbs/collection/config/lockfile_generator.rb
@@ -4,18 +4,22 @@ module RBS
   module Collection
     class Config
       class LockfileGenerator
+        # Stdlibs that used to be bundled in rbs-gem but are now maintained in their own gems.
+        #
+        # The value is the version requirement of the gem that ships the type definitions in its `sig/` directory,
+        # or `nil` if no released version of the gem ships them yet.
         ALUMNI_STDLIBS = {
           "mutex_m" => ">= 0.3.0",
           "abbrev" => nil,
-          "base64" => nil,
+          "base64" => ">= 0.3.0",
           "benchmark" => nil,
-          "bigdecimal" => nil,
+          "bigdecimal" => ">= 4.1.0",
           "csv" => nil,
           "kconv" => nil,
           "logger" => nil,
           "minitest" => nil,
           "net-smtp" => nil,
-          "nkf" => nil,
+          "nkf" => ">= 0.3.0",
           "observer" => nil,
           "cgi" => nil,
           "pstore" => nil,
@@ -190,11 +194,14 @@ def generate
 
           case name
           when 'bigdecimal-math', 'kconv'
-            # These gems are never released as a gem.
-            # Therefore, `assign_gem` should not be called.
+            # These libraries are never released as a gem, so `assign_gem` should not be called.
+            # Their type definitions are shipped in the `sig/` directory of the parent gem
+            # (`bigdecimal` for `bigdecimal-math`, `nkf` for `kconv`) instead.
+            parent_gem = name == 'kconv' ? 'nkf' : 'bigdecimal'
+            parent_version = ALUMNI_STDLIBS.fetch(parent_gem) or raise
             RBS.logger.info {
               from = from_gem || "rbs_collection.yaml"
-              "`#{name}` is included in the RBS dependencies of `#{from}`, but the type definition as a stdlib in rbs-gem is deprecated. Delete `#{name}` from the RBS dependencies of `#{from}`."
+              "`#{name}` is included in the RBS dependencies of `#{from}`, but the type definition as a stdlib in rbs-gem is deprecated. Delete `#{name}` from the RBS dependencies of `#{from}` and add `#{parent_gem}` (#{parent_version}) to the dependency of your Ruby program to use the gem-bundled type definition."
             }
             source = find_source(name: name)
             if source&.is_a?(Sources::Stdlib)
diff --git a/sig/collection/config/lockfile_generator.rbs b/sig/collection/config/lockfile_generator.rbs
index 066f9f4fc3..ead55b19e8 100644
--- a/sig/collection/config/lockfile_generator.rbs
+++ b/sig/collection/config/lockfile_generator.rbs
@@ -4,6 +4,8 @@ module RBS
       class LockfileGenerator
         # Name of stdlibs that was rbs-bundled stdlib but is now a gem.
         #
+        # The value is the version requirement of the gem that ships the type definitions, or `nil` if not shipped yet.
+        #
         ALUMNI_STDLIBS: Hash[String, String?]
 
         NONGEM_STDLIBS: Set[String]
diff --git a/test/rbs/collection/config_test.rb b/test/rbs/collection/config_test.rb
index bc524dfa1b..9df0b4fced 100644
--- a/test/rbs/collection/config_test.rb
+++ b/test/rbs/collection/config_test.rb
@@ -401,6 +401,61 @@ def test_generate_lock_from_rubygems
     end
   end
 
+  EMPTY_GEMFILE_LOCK = <<~GEMFILE_LOCK
+    GEM
+      remote: https://rubygems.org/
+      specs:
+
+    PLATFORMS
+      x86_64-linux
+
+    DEPENDENCIES
+
+    BUNDLED WITH
+       2.2.0
+  GEMFILE_LOCK
+
+  def test_generate_lock_from_alumni_stdlib__kconv
+    # `kconv` is never released as a gem; its types are shipped with the `nkf` gem.
+    mktmpdir do |tmpdir|
+      config_path = tmpdir / 'rbs_collection.yaml'
+      config_path.write <<~YAML
+        sources:
+          - name: ruby/gem_rbs_collection
+            remote: https://github.com/ruby/gem_rbs_collection.git
+            revision: cde6057e7546843ace6420c5783dd945c6ccda54
+            repo_dir: gems
+
+        path: /path/to/somewhere
+
+        gems:
+          - name: kconv
+      YAML
+      gemfile_path = tmpdir / 'Gemfile'
+      gemfile_path.write 'source "https://rubygems.org"'
+      gemfile_lock_path = tmpdir / 'Gemfile.lock'
+      gemfile_lock_path.write EMPTY_GEMFILE_LOCK
+
+      definition = Bundler::Definition.build(gemfile_path, gemfile_lock_path, false)
+      lockfile, log = with_logger_output(level: Logger::INFO) do
+        _config, lockfile = RBS::Collection::Config.generate_lockfile(config_path: config_path, definition: definition)
+        lockfile
+      end
+
+      assert_config <<~YAML, YAML.dump(lockfile.to_lockfile)
+        path: "/path/to/somewhere"
+        gemfile_lock_path: 'Gemfile.lock'
+        gems:
+          - name: kconv
+            version: "0"
+            source:
+              type: stdlib
+      YAML
+      assert_include log, '`kconv` is included in the RBS dependencies of `rbs_collection.yaml`, but the type definition as a stdlib in rbs-gem is deprecated.'
+      assert_include log, 'add `nkf` (>= 0.3.0) to the dependency of your Ruby program'
+    end
+  end
+
   def test_generate_lock_with_empty_gemfile_lock
     mktmpdir do |tmpdir|
       config_path = tmpdir / 'rbs_collection.yaml'
@@ -870,4 +925,18 @@ def test_generate_lockfile_from_mismatched_lockfiles
       yield Pathname(path)
     end
   end
+
+  # Runs the block with `RBS.logger` writing to a string, and returns `[block_result, logged_string]`.
+  private def with_logger_output(level:)
+    orig_output = RBS.logger_output
+    orig_level = RBS.logger_level
+    io = StringIO.new
+    RBS.logger_output = io
+    RBS.logger_level = level
+    result = yield
+    [result, io.string]
+  ensure
+    RBS.logger_output = orig_output
+    RBS.logger_level = orig_level
+  end
 end