diff --git a/app/lib/language_utils.rb b/app/lib/language_utils.rb
index 46a0941a7..ddc1ddbef 100644
--- a/app/lib/language_utils.rb
+++ b/app/lib/language_utils.rb
@@ -43,7 +43,13 @@ def self.detect_from_extension(entity)
   end
 
   def self.detect_from_entity(entity) = detect_from_filename(entity) || detect_from_extension(entity)
-  def self.fill_missing_language(raw, entity:) = blank_or_unknown?(raw) ? detect_from_entity(entity) : raw
+  def self.fill_missing_language(raw, entity:)
+    if blank_or_unknown?(raw) || File.extname(entity.to_s).downcase == ".asm"
+      detect_from_entity(entity) || raw
+    else
+      raw
+    end
+  end
 
   # Canonical display name: "js" → "JavaScript", "cpp" → "C++"
   def self.display_name(raw) = raw.blank? ? "Unknown" : (find_name(raw) || raw)
diff --git a/app/services/heartbeat_ingest.rb b/app/services/heartbeat_ingest.rb
index 49c68bc3c..34c2fa109 100644
--- a/app/services/heartbeat_ingest.rb
+++ b/app/services/heartbeat_ingest.rb
@@ -75,10 +75,7 @@ def normalize_direct_heartbeat(heartbeat, last_language:)
         .where.not(language: [ nil, "", LAST_LANGUAGE_SENTINEL ]).order(time: :desc).pick(:language)
     end
 
-    if attrs[:language].blank? || attrs[:language] == "Unknown"
-      inferred = LanguageUtils.detect_from_extension(attrs[:entity])
-      attrs[:language] = inferred if inferred
-    end
+    attrs[:language] = LanguageUtils.fill_missing_language(attrs[:language], entity: attrs[:entity])
 
     attrs[:user_agent] ||= attrs.delete(:plugin)
     parsed_ua = WakatimeService.parse_user_agent(attrs[:user_agent])
diff --git a/config/languages_custom.yml b/config/languages_custom.yml
index e7de2d090..cf3b6922a 100644
--- a/config/languages_custom.yml
+++ b/config/languages_custom.yml
@@ -11,3 +11,10 @@ Lapse:
 AsciiDoc:
   extensions:
   - ".ad"
+
+Motorola 68K Assembly:
+  extensions:
+  - ".i"
+  - ".inc"
+  - ".s"
+  - ".x68"
diff --git a/test/lib/language_utils_test.rb b/test/lib/language_utils_test.rb
new file mode 100644
index 000000000..c05992809
--- /dev/null
+++ b/test/lib/language_utils_test.rb
@@ -0,0 +1,96 @@
+require "test_helper"
+
+class LanguageUtilsTest < Minitest::Test
+  def setup
+    ActiveRecord::FixtureSet.reset_cache
+  end
+
+  def test_detect_asm_extension_returns_assembly
+    assert_equal "Assembly", LanguageUtils.detect_from_extension("test.asm")
+  end
+
+  def test_detect_known_extension
+    assert_equal "Ruby", LanguageUtils.detect_from_extension("test.rb")
+    assert_equal "Python", LanguageUtils.detect_from_extension("test.py")
+    assert_equal "JavaScript", LanguageUtils.detect_from_extension("test.js")
+  end
+
+  def test_detect_motorola_68k_specific_extensions
+    assert_equal "Motorola 68K Assembly", LanguageUtils.detect_from_extension("test.x68")
+  end
+
+  def test_detect_unknown_extension_returns_nil
+    assert_nil LanguageUtils.detect_from_extension("test.xyzzy")
+  end
+
+  def test_blank_or_unknown_returns_true_for_blank
+    assert LanguageUtils.blank_or_unknown?(nil)
+    assert LanguageUtils.blank_or_unknown?("")
+    assert LanguageUtils.blank_or_unknown?("Unknown")
+    assert LanguageUtils.blank_or_unknown?("unknown")
+  end
+
+  def test_blank_or_unknown_returns_false_for_known
+    refute LanguageUtils.blank_or_unknown?("Assembly")
+    refute LanguageUtils.blank_or_unknown?("Ruby")
+  end
+
+  def test_find_name_resolves_aliases
+    assert_equal "Assembly", LanguageUtils.find_name("asm")
+    assert_equal "Assembly", LanguageUtils.find_name("nasm")
+    assert_equal "Motorola 68K Assembly", LanguageUtils.find_name("m68k")
+  end
+
+  def test_find_name_returns_nil_for_unknown
+    assert_nil LanguageUtils.find_name("NotARealLanguage")
+  end
+
+  def test_display_name_returns_canonical
+    assert_equal "Assembly", LanguageUtils.display_name("asm")
+    assert_equal "Assembly", LanguageUtils.display_name("Assembly")
+  end
+
+  def test_display_name_returns_unknown_for_nil
+    assert_equal "Unknown", LanguageUtils.display_name(nil)
+    assert_equal "Unknown", LanguageUtils.display_name("")
+  end
+
+  def test_fill_missing_language_returns_raw_when_present
+    assert_equal "Assembly", LanguageUtils.fill_missing_language("Assembly", entity: "test.asm")
+  end
+
+  def test_fill_missing_language_detects_from_extension_when_blank
+    assert_equal "Assembly", LanguageUtils.fill_missing_language(nil, entity: "test.asm")
+    assert_equal "Assembly", LanguageUtils.fill_missing_language("", entity: "test.asm")
+    assert_equal "Assembly", LanguageUtils.fill_missing_language("Unknown", entity: "test.asm")
+  end
+
+  def test_fill_missing_language_overrides_asm_variants
+    assert_equal "Assembly", LanguageUtils.fill_missing_language("RGBDS Assembly", entity: "test.asm")
+    assert_equal "Assembly", LanguageUtils.fill_missing_language("TASM", entity: "test.asm")
+    assert_equal "Assembly", LanguageUtils.fill_missing_language("ca65 assembler", entity: "test.asm")
+    assert_equal "Assembly", LanguageUtils.fill_missing_language("Assembly", entity: "test.asm")
+  end
+
+  def test_fill_missing_language_does_not_override_non_asm
+    assert_equal "Python", LanguageUtils.fill_missing_language("Python", entity: "test.py")
+    assert_equal "RGBDS Assembly", LanguageUtils.fill_missing_language("RGBDS Assembly", entity: "test.py")
+  end
+
+  def test_color_returns_hex_for_known_language
+    assert_equal "#6E4C13", LanguageUtils.color("Assembly")
+    assert_equal "#701516", LanguageUtils.color("Ruby")
+  end
+
+  def test_color_returns_default_for_unknown
+    assert_equal LanguageUtils::DEFAULT_COLOR, LanguageUtils.color("NotARealLanguage")
+  end
+
+  def test_extension_map_overrides_asm_to_assembly
+    assert_equal "Assembly", LanguageUtils.extension_map[".asm"]
+  end
+
+  def test_extension_map_still_has_motorola_specific_extensions
+    assert_equal "Motorola 68K Assembly", LanguageUtils.extension_map[".x68"]
+  end
+end