feat: add configurable debug logging

Add opt-in debug logging following Neovim plugin conventions. Lua and
shell scripts write to the same log file for unified debugging.

- Create lua/code-preview/log.lua (DEBUG/INFO to file, WARN/ERROR also via vim.notify)
- Add `debug = false` config option, log file at stdpath("log")/code-preview.log
- Wire logging into diff.lua, neo_tree.lua, and both shell hook scripts
- Expose debug flag and log path in hook_context() for shell scripts

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: Cannon07

bin/core-post-tool.sh

@@ -22,6 +22,19 @@ TOOL_NAME="$(echo "$INPUT" | jq -r '.tool_name // empty' 2>/dev/null || true)"
 source "$SCRIPT_DIR/nvim-socket.sh" "$CWD" 2>/dev/null
 source "$SCRIPT_DIR/nvim-send.sh"
 
+# Set up logging — query debug config from nvim
+log_post() { :; }
+if [[ -n "${NVIM_SOCKET:-}" ]]; then
+  _POST_CTX=$(nvim --server "$NVIM_SOCKET" --remote-expr "luaeval(\"vim.json.encode({debug=require('code-preview.log').is_enabled(),log_file=require('code-preview.log').get_log_path() or ''})\")" 2>/dev/null || echo '{}')
+  _POST_DEBUG=$(echo "$_POST_CTX" | jq -r '.debug // false')
+  _POST_LOG_FILE=$(echo "$_POST_CTX" | jq -r '.log_file // ""')
+  if [[ "$_POST_DEBUG" == "true" && -n "$_POST_LOG_FILE" ]]; then
+    log_post() { printf '[%s] [INFO] core-post-tool.sh: %s\n' "$(date '+%Y-%m-%d %H:%M:%S')" "$*" >> "$_POST_LOG_FILE"; }
+  fi
+fi
+
+log_post "tool=$TOOL_NAME"
+
 # For Bash tool (rm detection), only clear deletion markers — don't touch edit markers or diff tab
 if [[ "$TOOL_NAME" == "Bash" ]]; then
   nvim_send "require('code-preview.changes').clear_by_status('deleted')" || true
@@ -36,6 +49,7 @@ FILE_PATH_ESC="$(escape_lua "${FILE_PATH:-}")"
 # Tell Lua to handle this file's close — tolerates out-of-order post-hooks
 # (OpenCode may fire them in a different order than pre-hooks).
 if [[ -n "$FILE_PATH" ]]; then
+  log_post "closing diff for file=$FILE_PATH"
   nvim_send "require('code-preview.diff').close_for_file('$FILE_PATH_ESC')" || true
   # neo_tree.refresh() is handled inside close_for_file() via vim.schedule()
 fi

bin/core-pre-tool.sh

@@ -31,6 +31,19 @@ if [[ -z "${NVIM_SOCKET:-}" ]]; then
   HAS_NVIM=false
 fi
 
+# Set up logging early so all code paths can use it
+log_pre() { :; }
+if [[ "$HAS_NVIM" == "true" ]]; then
+  _PRE_CTX=$(nvim --server "$NVIM_SOCKET" --remote-expr "luaeval(\"vim.json.encode({debug=require('code-preview.log').is_enabled(),log_file=require('code-preview.log').get_log_path() or ''})\")" 2>/dev/null || echo '{}')
+  _PRE_DEBUG=$(echo "$_PRE_CTX" | jq -r '.debug // false')
+  _PRE_LOG_FILE=$(echo "$_PRE_CTX" | jq -r '.log_file // ""')
+  if [[ "$_PRE_DEBUG" == "true" && -n "$_PRE_LOG_FILE" ]]; then
+    log_pre() { printf '[%s] [INFO] core-pre-tool.sh: %s\n' "$(date '+%Y-%m-%d %H:%M:%S')" "$*" >> "$_PRE_LOG_FILE"; }
+  fi
+fi
+
+log_pre "tool=$TOOL_NAME has_nvim=$HAS_NVIM"
+
 TMPDIR="${TMPDIR:-/tmp}"
 # Use unique temp files per hook invocation so rapid-fire pre-hooks
 # (OpenCode fires all before-hooks before any after-hooks) don't clobber
@@ -164,14 +177,18 @@ if [[ "$HAS_NVIM" == "true" ]]; then
   FILE_VISIBLE=$(echo "$HOOK_CTX" | jq -r '.file_visible // false')
   DEFER_PERMISSIONS=$(echo "$HOOK_CTX" | jq -r 'if .defer_claude_permissions == true then "true" else "false" end')
 
+  log_pre "file=$FILE_PATH visible_only=$VISIBLE_ONLY file_visible=$FILE_VISIBLE"
+
   # Decide whether to show the diff — skip nvim UI entirely when visible_only
   # is on and the file isn't in any visible window.
   SHOULD_SHOW="1"
   if [[ "$VISIBLE_ONLY" == "true" && "$FILE_VISIBLE" != "true" ]]; then
     SHOULD_SHOW="0"
+    log_pre "skipping diff: visible_only=true, file not visible"
   fi
 
   if [[ "$SHOULD_SHOW" == "1" ]]; then
+    log_pre "sending diff to nvim (layout via config)"
     nvim_send "require('code-preview.diff').show_diff('$ORIG_ESC', '$PROP_ESC', '$DISPLAY_ESC', '$FILE_PATH_ESC')" || true
   fi
 fi

lua/code-preview/diff.lua

@@ -1,5 +1,7 @@
 local M = {}
 
+local log = require("code-preview.log")
+
 -- Active diffs keyed by absolute file path.
 -- Each entry: { tab, bufs, augroup, inline_win }
 local active_diffs = {}
@@ -32,6 +34,7 @@ local function mark_change_and_reveal(abs_file_path)
   end
 
   local status = vim.loop.fs_stat(abs_file_path) and "modified" or "created"
+  log.debug(log.fmt("mark_change_and_reveal: %s → %s", abs_file_path, status))
   pcall(function() require("code-preview.changes").set(abs_file_path, status) end)
   pcall(function() require("code-preview.neo_tree").refresh() end)
 
@@ -371,16 +374,21 @@ end
 
 function M.show_diff(original_path, proposed_path, real_file_path, abs_file_path)
   local file_key = abs_file_path or real_file_path
+  local cfg = require("code-preview").config
+  log.info(log.fmt("show_diff: file=%s layout=%s active=%d",
+    file_key or "nil",
+    (cfg.diff and cfg.diff.layout) or "tab",
+    active_count()))
+
   -- If a diff for this SAME file is already open, close it first (re-edit)
   if file_key and active_diffs[file_key] then
+    log.debug(log.fmt("show_diff: re-edit detected, closing existing diff for %s", file_key))
     M.close_for_file(file_key)
   end
 
   -- Set the neo-tree indicator + reveal
   mark_change_and_reveal(abs_file_path)
 
-  local cfg = require("code-preview").config
-
   -- Inline layout
   if cfg.diff.layout == "inline" then
     local result = show_inline_diff(original_path, proposed_path, real_file_path, cfg)
@@ -478,9 +486,12 @@ end
 function M.close_for_file(file_path)
   local entry = active_diffs[file_path]
   if not entry then
+    log.debug(log.fmt("close_for_file: no active diff for %s, skipping", file_path))
     return
   end
 
+  log.info(log.fmt("close_for_file: closing diff for %s (remaining=%d)", file_path, active_count() - 1))
+
   -- Clear neo-tree indicator (refresh is deferred until after the tab is closed
   -- to avoid neo-tree walking a stale tabpage id)
   pcall(function() require("code-preview.changes").clear(file_path) end)
@@ -545,6 +556,7 @@ end
 
 -- Close ALL diffs and clear neo-tree indicators (for manual close via <leader>dq)
 function M.close_diff_and_clear()
+  log.info(log.fmt("close_diff_and_clear: closing all diffs (count=%d)", active_count()))
   -- Collect keys first to avoid modifying table during iteration
   local files = {}
   for file_path, _ in pairs(active_diffs) do

lua/code-preview/init.lua

@@ -4,6 +4,7 @@ local M = {}
 M.config = {}
 
 local default_config = {
+  debug = false,  -- enable debug logging to stdpath("log")/code-preview.log
   diff = {
     layout = "tab",        -- "tab", "vsplit", or "inline"
     labels = { current = "CURRENT", proposed = "PROPOSED" },
@@ -80,6 +81,9 @@ end
 function M.setup(user_config)
   M.config = deep_merge(default_config, user_config or {})
 
+  -- Initialise logging
+  require("code-preview.log").init({ debug = M.config.debug })
+
   -- ── New commands ──────────────────────────────────────────────
 
   vim.api.nvim_create_user_command("CodePreviewInstallClaudeCodeHooks", function()
@@ -162,12 +166,16 @@ function M.hook_context(file_path)
     end
   end
 
+  local log = require("code-preview.log")
+
   return vim.json.encode({
     neo_tree_reveal = neo_tree_reveal,
     reveal_root = reveal_root,
     visible_only = visible_only,
     file_visible = file_visible,
     defer_claude_permissions = defer_claude_permissions,
+    debug = log.is_enabled(),
+    log_file = log.get_log_path() or "",
   })
 end
 

lua/code-preview/log.lua

@@ -0,0 +1,70 @@
+--- code-preview.nvim — Logging module
+---
+--- Opt-in debug logging following Neovim plugin conventions.
+--- - WARN/ERROR: shown to user via vim.notify()
+--- - DEBUG/INFO: written to log file only (when enabled)
+--- - Log file: vim.fn.stdpath("log") .. "/code-preview.log"
+
+local M = {}
+
+local log_file_path = nil
+local enabled = false
+
+--- Initialise logging. Called once from setup().
+--- @param opts { debug: boolean }
+function M.init(opts)
+  enabled = opts and opts.debug or false
+  if enabled then
+    log_file_path = vim.fn.stdpath("log") .. "/code-preview.log"
+  end
+end
+
+--- Write a line to the log file. No-op when debug is disabled.
+--- @param level string "DEBUG"|"INFO"|"WARN"|"ERROR"
+--- @param msg string
+local function write(level, msg)
+  if not enabled or not log_file_path then
+    return
+  end
+  local f = io.open(log_file_path, "a")
+  if not f then
+    return
+  end
+  f:write(string.format("[%s] [%s] %s\n", os.date("%Y-%m-%d %H:%M:%S"), level, msg))
+  f:close()
+end
+
+function M.debug(msg) write("DEBUG", msg) end
+function M.info(msg) write("INFO", msg) end
+
+function M.warn(msg)
+  write("WARN", msg)
+  vim.notify("[code-preview] " .. msg, vim.log.levels.WARN)
+end
+
+function M.error(msg)
+  write("ERROR", msg)
+  vim.notify("[code-preview] " .. msg, vim.log.levels.ERROR)
+end
+
+--- Format helper for structured messages.
+--- @param template string format string
+--- @param ... any format arguments
+--- @return string
+function M.fmt(template, ...)
+  return string.format(template, ...)
+end
+
+--- Check whether debug logging is enabled.
+--- @return boolean
+function M.is_enabled()
+  return enabled
+end
+
+--- Return the log file path (for shell scripts via hook_context).
+--- @return string|nil
+function M.get_log_path()
+  return log_file_path
+end
+
+return M

lua/code-preview/neo_tree.lua

@@ -1,6 +1,7 @@
 local M = {}
 
 local changes = require("code-preview.changes")
+local log = require("code-preview.log")
 
 -- Guard: all neo-tree interaction goes through pcall
 local has_neo_tree = false
@@ -260,6 +261,7 @@ local function inject_virtual_nodes(state, pending)
     pcall(function()
       state.tree:add_node(file_node, parent_path)
       virtual_nodes[filepath] = true
+      log.debug(log.fmt("neo_tree: injected virtual node for %s", filepath))
     end)
     changed = true
 
@@ -276,10 +278,12 @@ function M.setup(cfg)
 
   local ok, neo_tree_events = pcall(require, "neo-tree.events")
   if not ok then
+    log.debug("neo_tree.setup: neo-tree not found, skipping integration")
     return
   end
   has_neo_tree = true
   setup_done = true
+  log.info("neo_tree.setup: neo-tree integration enabled")
 
   local symbols = cfg.neo_tree.symbols
   local highlights = cfg.neo_tree.highlights
@@ -340,6 +344,7 @@ function M.refresh()
   if not has_neo_tree then
     return
   end
+  log.debug("neo_tree.refresh: triggering filesystem refresh")
   pcall(function()
     require("neo-tree.sources.manager").refresh("filesystem")
   end)
@@ -349,6 +354,7 @@ function M.reveal(filepath, dir)
   if not has_neo_tree then
     return
   end
+  log.debug(log.fmt("neo_tree.reveal: %s (dir=%s)", filepath, dir or "nil"))
   pcall(function()
     local cfg = require("code-preview").config
     local position = cfg.neo_tree.position or "right"