diff --git a/AGENTS.md b/AGENTS.md
index ed915c7..9080c69 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -24,6 +24,7 @@ lua test/run_tests.lua       # Run all tests
 lua test/test_udt.lua        # Run UDT parser tests
 lua test/test_db.lua         # Run DB parser tests
 lua test/test_fb.lua         # Run FB parser tests
+lua test/test_formatter.lua  # Run formatter tests
 lua test/test_plc_json.lua   # Run plc_json tests
 lua test/test_integration.lua # Run integration tests
 
diff --git a/test/run_tests.lua b/test/run_tests.lua
index ff5f701..4f622ee 100644
--- a/test/run_tests.lua
+++ b/test/run_tests.lua
@@ -28,6 +28,7 @@ local function run_all_tests()
   run_test_file("test_udt.lua")
   run_test_file("test_db.lua")
   run_test_file("test_fb.lua")
+  run_test_file("test_formatter.lua")
   run_test_file("test_plc_json.lua")
   run_test_file("test_integration.lua")
 
diff --git a/test/test_formatter.lua b/test/test_formatter.lua
new file mode 100644
index 0000000..872a446
--- /dev/null
+++ b/test/test_formatter.lua
@@ -0,0 +1,296 @@
+local test = dofile("test/test_harness.lua")
+
+package.path = package.path .. ";src/?.lua"
+
+local script_path = debug.getinfo(1, "S").source:gsub("^@", ""):match("(.*/)") or "."
+script_path = script_path:gsub("^test/", ""):gsub("/test$", "")
+if script_path == "" then script_path = "." end
+
+local formatter = dofile(script_path .. "/src/formatter.lua")
+
+local function test_formatter_basic()
+  local content = [[FUNCTION "TestFunc" : Int
+VAR_INPUT
+   a : Int;
+END_VAR
+BEGIN
+
+END_FUNCTION
+]]
+  local result = formatter.format_document(content, {})
+  test.assert_not_nil(result, "should return formatting result")
+  test.assert(result[1].newText, "should have newText")
+end
+
+local function test_formatter_function_block()
+  local content = [[FUNCTION_BLOCK "TestFB"
+VAR_INPUT
+   enable : Bool;
+   value : Int;
+END_VAR
+
+VAR_OUTPUT
+   done : Bool;
+END_VAR
+
+BEGIN
+   #done := #enable;
+END_FUNCTION_BLOCK
+]]
+  local result = formatter.format_document(content, {})
+  test.assert_not_nil(result, "should return formatting result")
+  local formatted = result[1].newText
+  test.assert(formatted:match("FUNCTION_BLOCK"), "should preserve FUNCTION_BLOCK")
+  test.assert(formatted:match("VAR_INPUT"), "should preserve VAR_INPUT")
+end
+
+local function test_formatter_if_statement()
+  local content = [[IF #x > 5 THEN
+#y := 1;
+ELSIF #x > 3 THEN
+#y := 2;
+ELSE
+#y := 0;
+END_IF;]]
+  local result = formatter.format_document(content, {})
+  local formatted = result[1].newText
+  test.assert(formatted:match("IF"), "should preserve IF")
+  test.assert(formatted:match("THEN"), "should preserve THEN")
+  test.assert(formatted:match("END_IF"), "should preserve END_IF")
+end
+
+local function test_formatter_for_loop()
+  local content = [[FOR #i := 0 TO 10 BY 1 DO
+#arr[#i] := #i;
+END_FOR;]]
+  local result = formatter.format_document(content, {})
+  local formatted = result[1].newText
+  test.assert(formatted:match("FOR"), "should preserve FOR")
+  test.assert(formatted:match("END_FOR"), "should preserve END_FOR")
+end
+
+local function test_formatter_case_statement()
+  local content = [[CASE #x OF
+1: #y := 10;
+2: #y := 20;
+ELSE
+#y := 0;
+END_CASE;]]
+  local result = formatter.format_document(content, {})
+  local formatted = result[1].newText
+  test.assert(formatted:match("CASE"), "should preserve CASE")
+  test.assert(formatted:match("END_CASE"), "should preserve END_CASE")
+end
+
+local function test_formatter_comments()
+  local content = [[// This is a comment
+(* This is a block comment *)
+(* Another block comment *)
+// Another line comment
+#x := 1;]]
+  local result = formatter.format_document(content, {})
+  local formatted = result[1].newText
+  test.assert(formatted:match("This is a comment"), "should preserve line comments")
+end
+
+local function test_formatter_multiline_assignment_simple()
+  local content = [[#x := 1 AND
+2 AND
+3;]]
+  local result = formatter.format_document(content, {})
+  local formatted = result[1].newText
+  test.assert(formatted:match("#x := 1 AND"), "should preserve assignment start")
+end
+
+local function test_formatter_multiline_assignment_with_hash()
+  local content = [[#statWarning := #instTurn.warning OR
+#instPusher.warning OR
+#instClamp.warning;]]
+  local result = formatter.format_document(content, {})
+  local formatted = result[1].newText
+  test.assert(formatted:match("#statWarning :="), "should preserve assignment start")
+  test.assert(formatted:match("#instClamp.warning;$"), "should have continuation")
+end
+
+local function test_formatter_multiline_assignment_complex()
+  local content = [[#interface.status.main.inBasePosition := #equipmentData.turn.status.homePos AND
+#equipmentData.pusher.status.homePos AND
+NOT #equipmentData.conveyor.status.forward AND
+NOT #equipmentData.conveyor.status.backward;]]
+  local result = formatter.format_document(content, {})
+  local formatted = result[1].newText
+  test.assert(formatted:match("#interface.status.main.inBasePosition :="), "should preserve first line")
+  test.assert(formatted:match("NOT #equipmentData"), "should preserve NOT continuation")
+end
+
+local function test_formatter_multiline_assignment_ends_properly()
+  local content = [[#first := 1 AND
+2 AND
+3;
+
+#second := 10;]]
+  local result = formatter.format_document(content, {})
+  local formatted = result[1].newText
+  test.assert(formatted:match("#first := 1"), "should preserve first assignment")
+  test.assert(formatted:match("#second := 10"), "should start new assignment properly")
+end
+
+local function test_formatter_fb_call_single_line()
+  local content = [[#instMyFB(enable := #x, done => #y);]]
+  local result = formatter.format_document(content, {})
+  local formatted = result[1].newText
+  test.assert(formatted:match("#instMyFB"), "should preserve FB call")
+end
+
+local function test_formatter_fb_call_multiline()
+  local content = [[#instMyFB(
+enable := #x,
+done => #y
+);]]
+  local result = formatter.format_document(content, {})
+  local formatted = result[1].newText
+  test.assert(formatted:match("#instMyFB"), "should preserve FB call")
+end
+
+local function test_formatter_fb_call_with_long_params()
+  local content = [[#instPartCounter(enable := #unitStatus.currentSubMode <> "DRY_RUN",
+incPartIn := #equipmentData.partCounter.commands.incPartIn,
+incGoodPart := #equipmentData.partCounter.commands.incGoodPart,
+resetCntr := #equipmentData.partCounter.commands.resetCntr,
+status => #equipmentData.partCounter.status);]]
+  local result = formatter.format_document(content, {})
+  local formatted = result[1].newText
+  test.assert(formatted:match("#instPartCounter"), "should preserve FB call")
+  test.assert(formatted:match("status =>"), "should preserve output param")
+end
+
+local function test_formatter_region()
+  local content = [[REGION MyRegion
+// code
+#x := 1;
+END_REGION;]]
+  local result = formatter.format_document(content, {})
+  local formatted = result[1].newText
+  test.assert(formatted:match("REGION"), "should preserve REGION")
+  test.assert(formatted:match("END_REGION"), "should preserve END_REGION")
+end
+
+local function test_formatter_struct()
+  local content = [[TYPE MyStruct
+STRUCT
+   x : Int;
+   y : Bool;
+END_STRUCT;
+END_TYPE
+]]
+  local result = formatter.format_document(content, {})
+  local formatted = result[1].newText
+  test.assert(formatted:match("TYPE"), "should preserve TYPE")
+  test.assert(formatted:match("STRUCT"), "should preserve STRUCT")
+end
+
+local function test_formatter_var_sections()
+  local content = [[VAR_INPUT
+   in1 : Bool;
+   in2 : Int;
+END_VAR
+
+VAR_OUTPUT
+   out1 : Bool;
+END_VAR
+
+VAR
+   stat1 : Int;
+END_VAR
+]]
+  local result = formatter.format_document(content, {})
+  local formatted = result[1].newText
+  test.assert(formatted:match("VAR_INPUT"), "should preserve VAR_INPUT")
+  test.assert(formatted:match("VAR_OUTPUT"), "should preserve VAR_OUTPUT")
+  test.assert(formatted:match("\nVAR\n"), "should preserve VAR")
+end
+
+local function test_formatter_attribute_block()
+  local content = [[VAR
+   stat1{ S7_SetPoint := 'True' } : Int;
+   stat2 : Bool;
+END_VAR
+]]
+  local result = formatter.format_document(content, {})
+  local formatted = result[1].newText
+  test.assert(formatted:match("stat1"), "should preserve variable name")
+end
+
+local function test_formatter_while_loop()
+  local content = [[WHILE #x < 10 DO
+#x := #x + 1;
+END_WHILE;]]
+  local result = formatter.format_document(content, {})
+  local formatted = result[1].newText
+  test.assert(formatted:match("WHILE"), "should preserve WHILE")
+  test.assert(formatted:match("END_WHILE"), "should preserve END_WHILE")
+end
+
+local function test_formatter_repeat_loop()
+  local content = [[REPEAT
+#x := #x + 1;
+UNTIL #x >= 10;]]
+  local result = formatter.format_document(content, {})
+  local formatted = result[1].newText
+  test.assert(formatted:match("REPEAT"), "should preserve REPEAT")
+  test.assert(formatted:match("UNTIL"), "should preserve UNTIL")
+end
+
+local function test_formatter_assignment_with_string()
+  local content = [[#str := "Hello World";]]
+  local result = formatter.format_document(content, {})
+  local formatted = result[1].newText
+  test.assert(formatted:match('"Hello World"'), "should preserve string")
+end
+
+local function test_formatter_assignment_with_array()
+  local content = [[#arr[0] := 1;
+#arr[1] := 2;]]
+  local result = formatter.format_document(content, {})
+  local formatted = result[1].newText
+  test.assert(formatted:match("#arr%[0%]"), "should preserve array access")
+end
+
+local function run_formatter_tests()
+  test.run_suite("Formatter Tests", function()
+    print("\n--- Basic Tests ---")
+    test_formatter_basic()
+    test_formatter_function_block()
+    test_formatter_if_statement()
+    test_formatter_for_loop()
+    test_formatter_case_statement()
+    test_formatter_comments()
+    test_formatter_while_loop()
+    test_formatter_repeat_loop()
+    test_formatter_region()
+    test_formatter_struct()
+    test_formatter_var_sections()
+    test_formatter_attribute_block()
+    test_formatter_assignment_with_string()
+    test_formatter_assignment_with_array()
+
+    print("\n--- Multiline Assignment Tests ---")
+    test_formatter_multiline_assignment_simple()
+    test_formatter_multiline_assignment_with_hash()
+    test_formatter_multiline_assignment_complex()
+    test_formatter_multiline_assignment_ends_properly()
+
+    print("\n--- FB Call Tests ---")
+    test_formatter_fb_call_single_line()
+    test_formatter_fb_call_multiline()
+    test_formatter_fb_call_with_long_params()
+  end)
+end
+
+if vim then
+  vim.api.nvim_create_user_command("TestFormatter", function()
+    run_formatter_tests()
+  end, {})
+end
+
+run_formatter_tests()