diff --git a/AGENTS.md b/AGENTS.md
index 9080c69..6317527 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -104,7 +104,7 @@ return M
 - No trailing whitespace
 
 ### Comments
-**DO NOT ADD COMMENTS** in code unless explicitly requested by the user.
+Add comments to explain non-obvious code sections, complex logic, and public API functions. Keep comments concise and relevant.
 
 ### Parser Module API
 All parser modules must implement:
diff --git a/src/builtin_instructions.lua b/src/builtin_instructions.lua
index 0d3c730..73f0417 100644
--- a/src/builtin_instructions.lua
+++ b/src/builtin_instructions.lua
@@ -1,5 +1,9 @@
+-- TIA Portal built-in instructions for SCL LSP
+-- Defines functions, function blocks, data types, and their parameters
+
 local M = {}
 
+-- Built-in functions (return values, no state)
 M.FUNCTIONS = {
   ADD = true,
   SUB = true,
@@ -169,6 +173,7 @@ M.FUNCTIONS = {
   DCP_CLIENT = true,
 }
 
+-- Built-in function blocks (have state, need instance)
 M.FUNCTION_BLOCKS = {
   TON = true,
   TOF = true,
@@ -226,6 +231,8 @@ M.DATA_TYPES = {
   DTL = true,
 }
 
+-- Parameter definitions for built-in instructions
+-- Format: { inputs = { { name, type }, ... }, outputs = { { name, type }, ... } }
 M.PARAMETERS = {
   TON = { inputs = { { name = "IN", type = "BOOL" }, { name = "PT", type = "TIME" } }, outputs = { { name = "Q", type = "BOOL" }, { name = "ET", type = "TIME" } } },
   TOF = { inputs = { { name = "IN", type = "BOOL" }, { name = "PT", type = "TIME" } }, outputs = { { name = "Q", type = "BOOL" }, { name = "ET", type = "TIME" } } },
@@ -283,10 +290,48 @@ M.PARAMETERS = {
   I_BCD = { inputs = { { name = "IN" } }, outputs = { { name = "OUT" } } },
 }
 
+-- Get parameter info for a built-in instruction
+-- @param name string The instruction name
+-- @return table|nil Parameter definition with inputs and outputs arrays
 function M.get_parameters(name)
   return M.PARAMETERS[name:upper()]
 end
 
+-- Check if name is a known function block
+-- @param name string The name to check
+-- @return boolean True if known function block
+function M.is_known_function_block(name)
+  return M.FUNCTION_BLOCKS[name:upper()] or false
+end
+
+-- Check if name is a known function
+-- @param name string The name to check
+-- @return boolean True if known function
+function M.is_known_function(name)
+  return M.FUNCTIONS[name:upper()] or false
+end
+
+-- Check if name is a known data type
+-- @param name string The name to check
+-- @return boolean True if known data type
+function M.is_known_data_type(name)
+  return M.DATA_TYPES[name:upper()] or false
+end
+
+-- Check if name is a known instruction (function or function block)
+-- @param name string The name to check
+-- @return boolean True if known instruction
+function M.is_known_instruction(name)
+  return M.is_known_function(name) or M.is_known_function_block(name) or false
+end
+
+-- Check if name is a known type or instruction
+-- @param name string The name to check
+-- @return boolean True if known type or instruction
+function M.is_known_type_or_instruction(name)
+  return M.is_known_data_type(name) or M.is_known_instruction(name) or false
+end
+
 function M.is_known_function(name)
   return M.FUNCTIONS[name:upper()] or false
 end
diff --git a/src/diagnostics.lua b/src/diagnostics.lua
index 02e5517..506e913 100644
--- a/src/diagnostics.lua
+++ b/src/diagnostics.lua
@@ -1,4 +1,6 @@
 -- SCL Diagnostics - Linter functionality for the LSP server
+-- Validates SCL code for undefined variables, unknown types, and syntax errors
+
 local M = {}
 
 -- Get script path for loading parser (same pattern as main.lua)
@@ -18,6 +20,7 @@ package.path = package.path .. ";" .. script_path .. "/?.lua"
 local parser = dofile(script_path .. "/parser.lua")
 local builtin = dofile(script_path .. "/builtin_instructions.lua")
 
+-- LSP diagnostic severity levels
 local DiagnosticSeverity = {
   Error = 1,
   Warning = 2,
@@ -25,6 +28,10 @@ local DiagnosticSeverity = {
   Hint = 4,
 }
 
+--- Convert line/character to LSP position format
+--- @param line number Zero-based line number
+--- @param character number Zero-based character offset
+--- @return table LSP Position object
 local function position_to_lsp(line, character)
   return {
     line = line,
@@ -32,6 +39,12 @@ local function position_to_lsp(line, character)
   }
 end
 
+--- Create LSP Range object from start and end positions
+--- @param start_line number Start line (zero-based)
+--- @param start_char number Start character (zero-based)
+--- @param end_line number End line (zero-based)
+--- @param end_char number End character (zero-based)
+--- @return table LSP Range object
 local function range_to_lsp(start_line, start_char, end_line, end_char)
   return {
     start = position_to_lsp(start_line, start_char),
@@ -39,6 +52,12 @@ local function range_to_lsp(start_line, start_char, end_line, end_char)
   }
 end
 
+--- Validate SCL content and generate diagnostics
+--- Checks for: undefined variables, invalid # prefix, unknown data types
+--- @param content string The SCL source code to validate
+--- @param workspace_types table Known types from workspace scanning
+--- @param root_dir string Project root directory for additional type lookup
+--- @return table Array of LSP Diagnostic objects
 function M.validate_diagnostics(content, workspace_types, root_dir)
   local diagnostics = {}
   local lines = {}
diff --git a/src/formatter.lua b/src/formatter.lua
index 4a2b96b..b0bbc30 100644
--- a/src/formatter.lua
+++ b/src/formatter.lua
@@ -1,5 +1,6 @@
 -- SCL Formatter - Document formatting for the LSP server
 -- Stack-based implementation for proper nested structure handling
+
 local M = {}
 
 -- Default patterns for attribute blocks to collapse
@@ -10,10 +11,20 @@ local DEFAULT_COLLAPSE_PATTERNS = {
   "^%s*{%s*%w+%s*:=",
 }
 
+--- Create LSP Position object
+--- @param line number Line number
+--- @param character number Character offset
+--- @return table Position object
 local function position(line, character)
   return { line = line, character = character }
 end
 
+--- Create LSP Range object
+--- @param start_line number Start line
+--- @param start_char number Start character
+--- @param end_line number End line
+--- @param end_char number End character
+--- @return table Range object
 local function range(start_line, start_char, end_line, end_char)
   return {
     start = position(start_line, start_char),
@@ -89,6 +100,11 @@ local CONTINUATION_OPERATORS = {
   ["MOD"] = true,
 }
 
+--- Format SCL document content
+--- Handles indentation, attribute collapsing, and multiline FB calls
+--- @param content string The SCL source code
+--- @param options table Formatting options (insertSpaces, tabSize, collapseAttributes)
+--- @return table Array of TextEdit objects with range and newText
 function M.format_document(content, options)
   options = options or {}
   local use_spaces = options.insertSpaces or false
@@ -147,6 +163,15 @@ function M.format_document(content, options)
   end
 
   local function is_fb_call_start(trimmed)
+    local first_word = trimmed:match("^(#?[%w_]+)")
+    if not first_word then return false end
+    
+    -- Don't treat control structure keywords as FB calls
+    local control_keywords = { IF = true, ELSIF = true, FOR = true, WHILE = true, CASE = true, WITH = true }
+    if control_keywords[first_word:upper()] then
+      return false
+    end
+    
     if not (trimmed:match("^#?%w+") or trimmed:match('^"')) then
       return false
     end
@@ -341,10 +366,14 @@ function M.format_document(content, options)
 
     -- Check if continuing condition (IF/ELSIF condition before THEN)
     if in_condition and not ends_assignment then
-      local starts_with_operator = is_assignment_continuation(trimmed)
       local first_word = trimmed:match("^([%w_]+)")
       local is_then = first_word and first_word:upper() == "THEN"
-      if starts_with_operator or is_then then
+      local is_else = first_word and first_word:upper() == "ELSE"
+      local is_elsif = first_word and first_word:upper() == "ELSIF"
+      local is_ender = trimmed:match("^END_") ~= nil
+      local starts_new_statement = is_then or is_else or is_elsif or is_ender
+      
+      if not starts_new_statement then
         table.insert(formatted_lines, condition_indent .. trimmed)
         if is_then then
           in_condition = false
@@ -474,19 +503,20 @@ function M.format_document(content, options)
     if SAME_LEVEL[kw] then
       local formatted
       if kw == "THEN" then
-        local if_block = nil
-        for i = #stack, 1, -1 do
-          if stack[i].type == "IF" then
-            if_block = stack[i]
-            break
-          end
-        end
-        if if_block then
-          formatted = string.rep(indent_char, if_block.indent) .. trimmed
+        local then_indent
+        if in_condition and condition_indent ~= "" then
+          then_indent = condition_indent
         else
-          formatted = get_indent() .. trimmed
+          local if_block = nil
+          for i = #stack, 1, -1 do
+            if stack[i].type == "IF" then
+              if_block = stack[i]
+              break
+            end
+          end
+          then_indent = if_block and string.rep(indent_char, if_block.indent) or get_indent()
         end
-        table.insert(formatted_lines, formatted)
+        table.insert(formatted_lines, then_indent .. trimmed)
         table.insert(stack, { type = "THEN_CONTENT", indent = #stack, no_indent = true })
         in_condition = false
       elseif kw == "ELSE" or kw == "ELSIF" then
@@ -507,6 +537,11 @@ function M.format_document(content, options)
         end
         table.insert(formatted_lines, formatted)
         table.insert(stack, { type = "THEN_CONTENT", indent = #stack, no_indent = true })
+        -- Check if this is a multi-line condition (doesn't end with THEN on same line)
+        if not trimmed:match("THEN%s*$") then
+          in_condition = true
+          condition_indent = string.rep(indent_char, (if_block and if_block.indent or 0) + 1)
+        end
       else
         formatted = get_indent() .. trimmed
         table.insert(formatted_lines, formatted)
@@ -540,8 +575,12 @@ function M.format_document(content, options)
       table.insert(formatted_lines, formatted)
       table.insert(stack, { type = kw, indent = current_level })
       if kw == "IF" or kw == "ELSIF" then
-        in_condition = true
-        condition_indent = string.rep(indent_char, current_level)
+        -- Check if this is a multi-line condition (doesn't end with THEN on same line)
+        if not trimmed:match("THEN%s*$") then
+          in_condition = true
+          -- Continuation lines should be indented one level deeper than IF/ELSIF
+          condition_indent = string.rep(indent_char, current_level + 1)
+        end
       end
       goto continue
     end
@@ -587,6 +626,8 @@ function M.format_document(content, options)
   }
 end
 
+--- Get default formatting options
+--- @return table Default options table
 function M.get_formatting_options()
   return {
     insertSpaces = false,
diff --git a/src/json.lua b/src/json.lua
index 818d637..072b8be 100644
--- a/src/json.lua
+++ b/src/json.lua
@@ -1,5 +1,11 @@
+-- JSON Encoder/Decoder for LSP server
+-- Minimal implementation supporting basic JSON parsing and encoding
+
 local json = {}
 
+--- Check if a table is an array (sequential numeric keys starting from 1)
+--- @param t table Table to check
+--- @return boolean True if table is an array
 local function is_array(t)
   local i = 0
   for _ in pairs(t) do
@@ -11,6 +17,9 @@ local function is_array(t)
   return true
 end
 
+--- Escape special characters in string for JSON
+--- @param s string String to escape
+--- @return string Escaped string
 local function escape(s)
   s = s:gsub("\\", "\\\\")
   s = s:gsub('"', '\\"')
@@ -20,6 +29,9 @@ local function escape(s)
   return s
 end
 
+--- Encode Lua value to JSON string
+--- @param data any Value to encode
+--- @return string JSON string
 function json.encode(data)
   local t = type(data)
   if t == "nil" then
@@ -48,6 +60,10 @@ function json.encode(data)
   end
 end
 
+--- Skip whitespace in string
+--- @param s string JSON string
+--- @param i number Current position
+--- @return number New position after whitespace
 local function skip_whitespace(s, i)
   while i <= #s and s:sub(i, i):match("%s") do
     i = i + 1
@@ -55,6 +71,11 @@ local function skip_whitespace(s, i)
   return i
 end
 
+--- Parse JSON string literal
+--- @param s string JSON string
+--- @param i number Current position (at opening quote)
+--- @return string|nil Parsed string
+--- @return number New position
 local function parse_string(s, i)
   local result = {}
   i = i + 1
@@ -86,6 +107,11 @@ local function parse_string(s, i)
   return nil, i
 end
 
+--- Parse JSON number
+--- @param s string JSON string
+--- @param i number Current position
+--- @return number Parsed number
+--- @return number New position
 local function parse_number(s, i)
   local start = i
   if s:sub(i, i) == "-" then
@@ -114,6 +140,11 @@ end
 
 local parse_value
 
+--- Parse JSON array
+--- @param s string JSON string
+--- @param i number Current position (at opening bracket)
+--- @return table|nil Parsed array
+--- @return number New position
 local function parse_array(s, i)
   local arr = {}
   i = i + 1
@@ -136,6 +167,11 @@ local function parse_array(s, i)
   return nil, i
 end
 
+--- Parse JSON object
+--- @param s string JSON string
+--- @param i number Current position (at opening brace)
+--- @return table|nil Parsed object
+--- @return number New position
 local function parse_object(s, i)
   local obj = {}
   i = i + 1
@@ -205,6 +241,10 @@ parse_value = function(s, i)
   end
 end
 
+--- Decode JSON string to Lua value
+--- @param s string JSON string
+--- @return any|nil Parsed value
+--- @return number|nil Position after parsing or error
 function json.decode(s)
   if type(s) ~= "string" then
     return nil, "invalid input"
diff --git a/src/main.lua b/src/main.lua
index 7232070..a025492 100644
--- a/src/main.lua
+++ b/src/main.lua
@@ -154,6 +154,9 @@ end
 
 local handlers = {}
 
+--- Handle LSP initialize request
+--- @param params table Initialization parameters from client
+--- @return table Server capabilities and info
 function handlers.initialize(params)
 	return {
 		capabilities = capabilities,
@@ -161,8 +164,13 @@ function handlers.initialize(params)
 	}
 end
 
+--- Handle initialized notification (no response needed)
+--- @param params table Parameters (usually empty)
 function handlers.initialized(params) end
 
+--- Handle shutdown request
+--- @param params table Parameters (usually empty)
+--- @return nil
 function handlers.shutdown(params)
 	return nil
 end
@@ -206,10 +214,15 @@ local function find_project_root(start_path)
 	return nil
 end
 
+--- Handle exit notification - terminates the server
+--- @param params table Parameters (usually empty)
 function handlers.exit(params)
 	os.exit(0)
 end
 
+--- Handle textDocument/didOpen notification
+--- Parses document content, extracts symbols, and stores in documents cache
+--- @param params table Contains textDocument with uri, text, version
 function handlers.textDocument_didOpen(params)
 	local uri = params.textDocument.uri
 	local content = params.textDocument.text
@@ -247,6 +260,9 @@ function handlers.textDocument_didOpen(params)
 	}
 end
 
+--- Handle textDocument/didChange notification
+--- Re-parses document content after edits
+--- @param params table Contains textDocument with uri and contentChanges array
 function handlers.textDocument_didChange(params)
 	local uri = params.textDocument.uri
 	local content = params.contentChanges[1].text
@@ -285,11 +301,18 @@ function handlers.textDocument_didChange(params)
 	clear_symbol_cache()
 end
 
+--- Handle textDocument/didClose notification
+--- Removes document from cache
+--- @param params table Contains textDocument with uri
 function handlers.textDocument_didClose(params)
 	documents[params.textDocument.uri] = nil
 	clear_symbol_cache()
 end
 
+--- Handle textDocument/hover request
+--- Provides hover information for variables, types, and built-in instructions
+--- @param params table Contains textDocument with uri and position
+--- @return table|nil Hover result with contents and range, or nil
 function handlers.textDocument_hover(params)
 	local uri = params.textDocument.uri
 	local doc = documents[uri]
@@ -699,6 +722,10 @@ function handlers.textDocument_hover(params)
 	return nil
 end
 
+--- Handle textDocument/completion request
+--- Provides completion items based on context (VAR section, member access, etc.)
+--- @param params table Contains textDocument with uri and position
+--- @return table Completion list with items array
 function handlers.textDocument_completion(params)
 	local uri = params.textDocument.uri
 	local doc = documents[uri]
@@ -879,6 +906,10 @@ function handlers.textDocument_completion(params)
 	return { isIncomplete = false, items = items }
 end
 
+--- Handle textDocument/definition request
+--- Finds definition location for symbol at position
+--- @param params table Contains textDocument with uri and position
+--- @return table|nil Location with uri and range, or nil
 function handlers.textDocument_definition(params)
 	local uri = params.textDocument.uri
 	local doc = documents[uri]
@@ -921,6 +952,10 @@ function handlers.textDocument_definition(params)
 	return nil
 end
 
+--- Handle textDocument/declaration request
+--- Finds declaration locations for symbol at position across workspace
+--- @param params table Contains textDocument with uri and position
+--- @return table|nil Array of locations, or nil
 function handlers.textDocument_declaration(params)
 	local uri = params.textDocument.uri
 	local doc = documents[uri]
@@ -1141,6 +1176,10 @@ function handlers.textDocument_declaration(params)
 	return nil
 end
 
+--- Handle textDocument/references request
+--- Finds all references to symbol at position in current document
+--- @param params table Contains textDocument with uri and position
+--- @return table|nil Array of reference locations, or nil
 function handlers.textDocument_references(params)
 	local uri = params.textDocument.uri
 	local doc = documents[uri]
@@ -1202,6 +1241,10 @@ function handlers.textDocument_references(params)
 	return nil
 end
 
+--- Handle textDocument/documentSymbol request
+--- Returns all symbols in document for outline/navigation
+--- @param params table Contains textDocument with uri
+--- @return table Array of DocumentSymbol objects
 function handlers.textDocument_documentSymbol(params)
 	local uri = params.textDocument.uri
 	local doc = documents[uri]
@@ -1286,6 +1329,10 @@ function handlers.textDocument_documentSymbol(params)
 	return symbols
 end
 
+--- Handle textDocument/diagnostic request
+--- Validates document and returns diagnostics
+--- @param params table Contains textDocument with uri
+--- @return table Diagnostic report with items array
 function handlers.textDocument_diagnostic(params)
 	local uri = params.textDocument.uri
 	local doc = documents[uri]
@@ -1302,6 +1349,10 @@ function handlers.textDocument_diagnostic(params)
 	return { items = diag_results }
 end
 
+--- Handle textDocument/semanticTokens/full request
+--- Returns semantic tokens for syntax highlighting (currently disabled)
+--- @param params table Contains textDocument with uri
+--- @return table Semantic tokens with data array
 function handlers.textDocument_semanticTokensFull(params)
 	local uri = params.textDocument.uri
 	local doc = documents[uri]
@@ -1399,6 +1450,10 @@ function handlers.textDocument_semanticTokensFull(params)
 	return { data = tokens }
 end
 
+--- Handle textDocument/semanticTokens/range request
+--- Returns semantic tokens for a range (delegates to full)
+--- @param params table Contains textDocument with uri and range
+--- @return table Semantic tokens with data array
 function handlers.textDocument_semanticTokensRange(params)
 	local uri = params.textDocument.uri
 	local doc = documents[uri]
@@ -1412,6 +1467,10 @@ end
 handlers["textDocument_semanticTokens_full"] = handlers.textDocument_semanticTokensFull
 handlers["textDocument_semanticTokens_range"] = handlers.textDocument_semanticTokensRange
 
+--- Handle textDocument/inlayHint request
+--- Provides inlay hints for variable types in VAR sections
+--- @param params table Contains textDocument with uri and range
+--- @return table Array of InlayHint objects
 function handlers.textDocument_inlayHint(params)
 	local uri = params.textDocument.uri
 	local doc = documents[uri]
@@ -1449,6 +1508,10 @@ function handlers.textDocument_inlayHint(params)
 	return hints
 end
 
+--- Handle workspace/symbol request
+--- Searches workspace for symbols matching query
+--- @param params table Contains query string and optional maxResults
+--- @return table Array of SymbolInformation objects
 function handlers.workspace_symbol(params)
 	local query = params.query or ""
 	local max_results = params.maxResults or 50
@@ -1586,6 +1649,10 @@ function handlers.workspace_symbol(params)
 	return final_results
 end
 
+--- Handle textDocument/formatting request
+--- Formats entire document using SCL formatter
+--- @param params table Contains textDocument with uri and options
+--- @return table|nil Array of TextEdit objects, or nil
 function handlers.textDocument_formatting(params)
 	local uri = params.textDocument.uri
 	local doc = documents[uri]
diff --git a/src/parser.lua b/src/parser.lua
index 836eaa6..7fd83ee 100644
--- a/src/parser.lua
+++ b/src/parser.lua
@@ -1,6 +1,12 @@
 -- SCL Parser utilities for the LSP server
+-- Extracts variables, types, and function definitions from SCL source code
+
 local M = {}
 
+--- Extract variables from SCL content
+--- @param content string The SCL source code
+--- @return table variables Table of variable definitions keyed by name
+--- @return table variable_positions Table of variable positions keyed by name
 function M.extract_variables(content)
   local variables = {}
   local variable_positions = {}
@@ -164,6 +170,9 @@ function M.extract_variables(content)
   return variables, variable_positions
 end
 
+--- Extract type definitions from SCL content
+--- @param content string The SCL source code
+--- @return table types Table of type definitions keyed by name
 function M.extract_types(content)
   local types = {}
 
@@ -285,6 +294,9 @@ function M.extract_types(content)
   return types
 end
 
+--- Extract function/block definitions from SCL content
+--- @param content string The SCL source code
+--- @return table functions Array of function definitions with name, kind, start, final
 function M.extract_functions(content)
   local functions = {}
 
diff --git a/src/plc_json.lua b/src/plc_json.lua
index f03c7bb..0e0ece9 100644
--- a/src/plc_json.lua
+++ b/src/plc_json.lua
@@ -1,10 +1,14 @@
 -- Load UDT types from external .plc.json files
--- Auto-generate from data_types folder
+-- Auto-generate from data_types folder and .scl/.udt/.db files
 
 local M = {}
 
+-- Cache for loaded types: { [root_dir] = { [type_name] = type_info } }
 local cached_types = {}
 
+--- Execute shell command and capture output
+--- @param cmd string Command to execute
+--- @return string|nil Command output, or nil on error
 local function run_command(cmd)
   local handle = io.popen(cmd)
   if not handle then return nil end
@@ -26,6 +30,9 @@ else
   script_path = "."
 end
 
+--- Load and parse JSON content
+--- @param content string JSON string
+--- @return table|nil Parsed data, or nil on error
 local function load_json(content)
   local json = dofile(script_path .. "/json.lua")
   local ok, data = pcall(function() return json.decode(content) end)
@@ -33,6 +40,10 @@ local function load_json(content)
   return nil
 end
 
+--- Scan directory recursively for files matching pattern
+--- @param dir string Directory to scan
+--- @param pattern string Glob pattern for file names
+--- @return table Array of matching file paths
 local function scan_files_recursive(dir, pattern)
   local files = {}
   local result = run_command("find " .. dir .. " -type f -name \"" .. pattern .. "\" 2>/dev/null")
@@ -44,6 +55,9 @@ local function scan_files_recursive(dir, pattern)
   return files
 end
 
+--- Parse a PLC type definition from JSON structure
+--- @param dt table Type definition from JSON
+--- @return table|nil Parsed type info, or nil if invalid
 local function parse_plc_type(dt)
   local type_name = dt.name or dt.Name or dt.baseType or dt.dataTypeName
   if not type_name then return nil end
@@ -78,6 +92,9 @@ local function parse_plc_type(dt)
   return typ
 end
 
+--- Parse SCL type file content (TYPE...END_TYPE, FUNCTION_BLOCK, etc.)
+--- @param content string SCL source code
+--- @return table types Parsed types keyed by name
 local function parse_scl_type_file(content)
   local types = {}
   local in_type = false
@@ -244,6 +261,9 @@ local function parse_scl_type_file(content)
   return types
 end
 
+--- Parse DB file content (DATA_BLOCK...END_DATA_BLOCK)
+--- @param content string DB source code
+--- @return table types Parsed DB types keyed by name
 local function parse_db_file(content)
   local types = {}
   local in_var = false
@@ -288,6 +308,10 @@ local function parse_db_file(content)
   return types
 end
 
+--- Load all types from workspace directory
+--- Scans .plc.json, .scl, .udt, and .db files
+--- @param root_dir string Project root directory
+--- @return table Types keyed by name
 function M.load_types_from_workspace(root_dir)
   local types = {}
 
@@ -375,6 +399,9 @@ function M.load_types_from_workspace(root_dir)
   return types
 end
 
+--- Get types for a workspace (cached)
+--- @param root_dir string Project root directory
+--- @return table Types keyed by name
 function M.get_types(root_dir)
   if not root_dir or root_dir == "" then
     root_dir = "workspace"
@@ -389,6 +416,10 @@ function M.get_types(root_dir)
   return types
 end
 
+--- Generate .plc.json file from workspace types
+--- @param root_dir string Project root directory
+--- @param output_file string Output file path
+--- @return boolean True if file was written successfully
 function M.generate_plc_json(root_dir, output_file)
   local json = dofile(script_path .. "/json.lua")
 
@@ -435,6 +466,7 @@ function M.generate_plc_json(root_dir, output_file)
   return false
 end
 
+--- Clear all cached types
 function M.clear_cache()
   cached_types = {}
 end
diff --git a/src/treesitter.lua b/src/treesitter.lua
index e2d2f04..1e4c5b7 100644
--- a/src/treesitter.lua
+++ b/src/treesitter.lua
@@ -1,10 +1,13 @@
 -- Tree-sitter integration for SCL LSP
--- Uses tree-sitter to parse SCL files more accurately
+-- Uses tree-sitter to parse SCL files more accurately than regex parsing
 
 local M = {}
 
 local ts = nil
 
+--- Initialize tree-sitter parser
+--- Attempts to load vim.treesitter module
+--- @return boolean True if tree-sitter is available
 function M.init()
   local ok, treesitter = pcall(require, "vim.treesitter")
   if ok then
@@ -14,6 +17,10 @@ function M.init()
   return false
 end
 
+--- Parse SCL content using tree-sitter
+--- @param content string The SCL source code
+--- @return table|nil Parse tree, or nil on error
+--- @return string|nil Error message if parsing failed
 function M.parse_scl(content)
   if not ts then
     return nil, "Tree-sitter not available"
@@ -32,6 +39,11 @@ function M.parse_scl(content)
   return tree, nil
 end
 
+--- Extract variables from tree-sitter parse tree
+--- @param tree table Parse tree from parse_scl
+--- @param bufnr number Buffer number for text retrieval
+--- @return table variables Table of variable definitions
+--- @return table variable_positions Table of variable positions
 function M.extract_variables_ts(tree, bufnr)
   local variables = {}
   local variable_positions = {}
@@ -79,6 +91,10 @@ function M.extract_variables_ts(tree, bufnr)
   return variables, variable_positions
 end
 
+--- Extract function/block definitions from tree-sitter parse tree
+--- @param tree table Parse tree from parse_scl
+--- @param bufnr number Buffer number for text retrieval
+--- @return table functions Array of function definitions
 function M.extract_functions_ts(tree, bufnr)
   local functions = {}
 
@@ -109,6 +125,11 @@ function M.extract_functions_ts(tree, bufnr)
   return functions
 end
 
+--- Get syntax node at position
+--- @param tree table Parse tree
+--- @param row number Row number (zero-based)
+--- @param col number Column number (zero-based)
+--- @return table|nil Node at position, or nil if not found
 function M.get_node_at_position(tree, row, col)
   if not tree then return nil end
 
@@ -124,6 +145,12 @@ function M.get_node_at_position(tree, row, col)
   return nil
 end
 
+--- Get variable type at cursor position
+--- @param tree table Parse tree
+--- @param bufnr number Buffer number
+--- @param row number Row number (zero-based)
+--- @param col number Column number (zero-based)
+--- @return string|nil Variable type, or nil if not found
 function M.get_variable_type_at_cursor(tree, bufnr, row, col)
   if not tree or not ts then return nil end