feat: add prompt_guard callback mechanism (#78)

This PR adds a plugin option to run a callback before sending promts to the LLM or before opening the opencode window.

In my use case I want to be able to exclude some directories of my computer with sensitive information in order to not mistakenly send them to the LLM.
I've mostly vibe-coded this and then went back on the code to check what it did, I'm not super well versed in lua so I might have missed things.

Implementation details:

Add prompt_guard configuration option (function that returns boolean)
Check guard before sending prompts (ERROR notification if denied)
Check guard before opening buffer first time (WARN notification if denied)
Add util.check_prompt_allowed() helper functions
Guard has no parameters, users can access vim state directly
Proper error handling for guard callback failures

Co-authored-by: Guillaume BOEHM <[email protected]>

Author: guillaumeboehm

README.md

@@ -240,6 +240,7 @@ require('opencode').setup({
   debug = {
     enabled = false, -- Enable debug messages in the output window
   },
+  prompt_guard = nil, -- Optional function that returns boolean to control when prompts can be sent (see Prompt Guard section)
 })
 ```
 
@@ -537,6 +538,31 @@ The plugin defines several highlight groups that can be customized to match your
 - `OpencodeInputLegend`: Highlight for input window legend (default: #CCCCCC background)
 - `OpencodeHint`: Highlight for hinting messages in input window and token info in output window footer (linked to `Comment`)
 
+## 🛡️ Prompt Guard
+
+The `prompt_guard` configuration option allows you to control when prompts can be sent to Opencode. This is useful for preventing accidental or unauthorized AI interactions in certain contexts.
+
+### Configuration
+
+Set `prompt_guard` to a function that returns a boolean:
+
+```lua
+require('opencode').setup({
+  prompt_guard = function()
+    -- Your custom logic here
+    -- Return true to allow, false to deny
+    return true
+  end,
+})
+```
+
+### Behavior
+
+- **Before sending prompts**: The guard is checked before any prompt is sent to the AI. If denied, an ERROR notification is shown and the prompt is not sent.
+- **Before opening UI**: The guard is checked when opening the Opencode buffer for the first time. If denied, a WARN notification is shown and the UI is not opened.
+- **No parameters**: The guard function receives no parameters. Access vim state directly (e.g., `vim.fn.getcwd()`, `vim.bo.filetype`).
+- **Error handling**: If the guard function throws an error or returns a non-boolean value, the prompt is denied with an appropriate error message.
+
 ## 🔧 Setting up Opencode
 
 If you're new to opencode:

lua/opencode/config.lua

@@ -167,6 +167,7 @@ M.defaults = {
     enabled = false,
     capture_streamed_events = false,
   },
+  prompt_guard = nil,
 }
 
 M.values = vim.deepcopy(M.defaults)

lua/opencode/core.lua

@@ -48,6 +48,14 @@ function M.open(opts)
   local are_windows_closed = state.windows == nil
 
   if are_windows_closed then
+    -- Check if whether prompting will be allowed
+    local context_module = require('opencode.context')
+    local mentioned_files = context_module.context.mentioned_files or {}
+    local allowed, err_msg = util.check_prompt_allowed(config.prompt_guard, mentioned_files)
+    if not allowed then
+      vim.notify(err_msg or 'Prompts will be denied by prompt_guard', vim.log.levels.WARN)
+    end
+
     state.windows = ui.create_windows()
   end
 
@@ -81,6 +89,16 @@ end
 --- @param prompt string The message prompt to send.
 --- @param opts? SendMessageOpts
 function M.send_message(prompt, opts)
+  -- Check if prompt is allowed
+  local context_module = require('opencode.context')
+  local mentioned_files = context_module.context.mentioned_files or {}
+  local allowed, err_msg = util.check_prompt_allowed(config.prompt_guard, mentioned_files)
+
+  if not allowed then
+    vim.notify(err_msg or 'Prompt denied by prompt_guard', vim.log.levels.ERROR)
+    return
+  end
+
   opts = opts or {}
   opts.context = opts.context or config.context
   opts.model = opts.model or state.current_model

lua/opencode/types.lua

@@ -141,6 +141,7 @@
 ---@field ui OpencodeUIConfig
 ---@field context OpencodeContextConfig
 ---@field debug OpencodeDebugConfig
+---@field prompt_guard? fun(mentioned_files: string[]): boolean
 
 ---@class MessagePartState
 ---@field input TaskToolInput|BashToolInput|FileToolInput|TodoToolInput|GlobToolInput|GrepToolInput|WebFetchToolInput|ListToolInput Input data for the tool

lua/opencode/ui/highlight.lua

@@ -22,6 +22,7 @@ function M.setup()
     vim.api.nvim_set_hl(0, 'OpencodeContextualActions', { bg = '#90A4AE', fg = '#1976D2', bold = true, default = true })
     vim.api.nvim_set_hl(0, 'OpencodeInputLegend', { bg = '#757575', fg = '#424242', bold = false, default = true })
     vim.api.nvim_set_hl(0, 'OpencodeHint', { link = 'Comment', default = true })
+    vim.api.nvim_set_hl(0, 'OpencodeGuardDenied', { fg = '#F44336', bold = true, default = true })
   else
     vim.api.nvim_set_hl(0, 'OpencodeBorder', { fg = '#616161', default = true })
     vim.api.nvim_set_hl(0, 'OpencodeBackground', { link = 'Normal', default = true })
@@ -41,6 +42,7 @@ function M.setup()
     vim.api.nvim_set_hl(0, 'OpencodeContextualActions', { bg = '#3b4261', fg = '#61AFEF', bold = true, default = true })
     vim.api.nvim_set_hl(0, 'OpencodeInputLegend', { bg = '#616161', fg = '#CCCCCC', bold = false, default = true })
     vim.api.nvim_set_hl(0, 'OpencodeHint', { link = 'Comment', default = true })
+    vim.api.nvim_set_hl(0, 'OpencodeGuardDenied', { fg = '#EF5350', bold = true, default = true })
   end
 end
 

lua/opencode/ui/icons.lua

@@ -25,6 +25,7 @@ local presets = {
     -- statuses
     status_on = '🟢',
     status_off = '⚫',
+    guard_on = '🚫',
     -- borders and misc
     border = '▌',
   },
@@ -49,6 +50,7 @@ local presets = {
     -- statuses
     status_on = ' ',
     status_off = ' ',
+    guard_on = '',
     -- borders and misc
     border = '▌',
   },
@@ -73,6 +75,7 @@ local presets = {
     -- statuses
     status_on = 'ON',
     status_off = 'OFF',
+    guard_on = 'X',
     -- borders and misc
     border = '▌',
   },

lua/opencode/ui/prompt_guard_indicator.lua

@@ -0,0 +1,36 @@
+local M = {}
+
+local config = require('opencode.config')
+local util = require('opencode.util')
+local icons = require('opencode.ui.icons')
+local context = require('opencode.context')
+
+---Get the current prompt guard status
+---@return boolean allowed
+---@return string|nil error_message
+function M.get_status()
+  local mentioned_files = context.context.mentioned_files or {}
+  return util.check_prompt_allowed(config.prompt_guard, mentioned_files)
+end
+
+---Check if guard will deny prompts
+---@return boolean denied
+function M.is_denied()
+  local allowed, _ = M.get_status()
+  return not allowed
+end
+
+---Get formatted indicator string with highlight (empty if allowed)
+---@return string formatted_indicator
+function M.get_formatted()
+  if not M.is_denied() then
+    -- Prompts are allowed - don't show anything
+    return ''
+  end
+
+  -- Prompts will be denied - show red indicator
+  local icon = icons.get('guard_on')
+  return string.format('%%#OpencodeGuardDenied#%s%%*', icon)
+end
+
+return M

lua/opencode/ui/timer.lua

@@ -48,6 +48,12 @@ function Timer:start()
   end
 end
 
+--- Start the timer and immediately execute the callback
+function Timer:start_and_tick()
+  self:start()
+  self.on_tick(unpack(self.args))
+end
+
 function Timer:stop()
   if not self._uv_timer then
     return

lua/opencode/ui/topbar.lua

@@ -2,6 +2,7 @@ local M = {}
 
 local state = require('opencode.state')
 local config_file = require('opencode.config_file')
+local prompt_guard_indicator = require('opencode.ui.prompt_guard_indicator')
 
 local LABELS = {
   NEW_SESSION_TITLE = 'New session',
@@ -40,17 +41,40 @@ local function get_mode_highlight()
   end
 end
 
-local function create_winbar_text(description, model_info, mode_info, win_width)
-  local available_width = win_width - 2 -- 2 padding spaces
+local function create_winbar_text(description, model_info, mode_info, show_guard_indicator, win_width)
+  -- Calculate how many visible characters we have
+  -- Format: " [GUARD ]description padding model_info MODE "
+  -- Where [GUARD ] is optional
 
-  -- If total length exceeds available width, truncate description
-  if #description + 1 + #model_info + #mode_info + 1 > available_width then
-    local space_for_desc = available_width - (#model_info + #mode_info + 1) - 4 -- -4 for "... "
-    description = description:sub(1, space_for_desc) .. '... '
+  local guard_info = ''
+  local guard_visible_width = 0
+  if show_guard_indicator then
+    guard_info = prompt_guard_indicator.get_formatted()
+    guard_visible_width = 2 -- icon + space
   end
 
-  local padding = string.rep(' ', available_width - #description - #model_info - #mode_info - 1)
-  return string.format(' %s%s%s %s ', description, padding, model_info, get_mode_highlight() .. mode_info .. '%*')
+  -- Calculate used width: leading space + guard + trailing space + model + mode
+  local mode_info_str = get_mode_highlight() .. mode_info .. '%*'
+  local mode_visible_width = #mode_info
+  local model_visible_width = #model_info
+
+  -- Reserve space: 1 (padding) + guard_visible_width (with padding) + model + 1 (space before mode) + mode + 1 (padding)
+  local reserved_width = 1 + guard_visible_width + model_visible_width + 1 + mode_visible_width + 1
+
+  -- Available width for description and padding
+  local available_for_desc = win_width - reserved_width
+
+  -- Truncate description if needed
+  if #description > available_for_desc then
+    local space_for_desc = available_for_desc - 4 -- -4 for "... "
+    description = description:sub(1, space_for_desc) .. '...'
+  end
+
+  -- Calculate padding to right-align model and mode
+  local padding_width = available_for_desc - #description
+  local padding = string.rep(' ', math.max(0, padding_width))
+
+  return string.format(' %s %s%s%s %s ', guard_info, description, padding, model_info, mode_info_str)
 end
 
 local function update_winbar_highlights(win_id)
@@ -96,8 +120,10 @@ function M.render()
     end
     -- topbar needs to at least have a value to make sure footer is positioned correctly
     vim.wo[win].winbar = ' '
+
+    local show_guard_indicator = prompt_guard_indicator.is_denied()
     vim.wo[win].winbar =
-      create_winbar_text(get_session_desc(), format_model_info(), format_mode_info(), vim.api.nvim_win_get_width(win))
+      create_winbar_text(get_session_desc(), format_model_info(), format_mode_info(), show_guard_indicator, vim.api.nvim_win_get_width(win))
 
     update_winbar_highlights(win)
   end)
@@ -111,12 +137,14 @@ function M.setup()
   state.subscribe('current_mode', on_change)
   state.subscribe('current_model', on_change)
   state.subscribe('active_session', on_change)
+  state.subscribe('is_opencode_focused', on_change)
   M.render()
 end
 
 function M.close()
   state.unsubscribe('current_mode', on_change)
   state.unsubscribe('current_model', on_change)
   state.unsubscribe('active_session', on_change)
+  state.unsubscribe('is_opencode_focused', on_change)
 end
 return M

lua/opencode/util.lua

@@ -360,4 +360,32 @@ function M.parse_dot_args(args_str)
   return result
 end
 
+--- Check if prompt is allowed via guard callback
+--- @param guard_callback? function
+--- @param mentioned_files? string[] List of mentioned files in the context
+--- @return boolean allowed
+--- @return string|nil error_message
+function M.check_prompt_allowed(guard_callback, mentioned_files)
+  if not guard_callback then
+    return true, nil -- No guard = always allowed
+  end
+
+  if not type(guard_callback) == 'function' then
+    return false, 'prompt_guard must be a function'
+  end
+
+  mentioned_files = mentioned_files or {}
+  local success, result = pcall(guard_callback, mentioned_files)
+
+  if not success then
+    return false, 'prompt_guard error: ' .. tostring(result)
+  end
+
+  if type(result) ~= 'boolean' then
+    return false, 'prompt_guard must return a boolean'
+  end
+
+  return result, nil
+end
+
 return M