feat: add prompt_guard callback mechanism

- 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

Author: guillaumeboehm

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 opening buffer is allowed
+    local allowed, err_msg = util.check_prompt_allowed(config.prompt_guard)
+
+    if not allowed then
+      vim.notify(err_msg or 'Opening opencode buffer denied by prompt_guard', vim.log.levels.WARN)
+      return
+    end
+
     state.windows = ui.create_windows()
   end
 
@@ -81,6 +89,14 @@ end
 --- @param prompt string The message prompt to send.
 --- @param opts? SendMessageOpts
 function M.send_message(prompt, opts)
+  -- Check if prompt is allowed
+  local allowed, err_msg = util.check_prompt_allowed(config.prompt_guard)
+
+  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(): boolean
 
 ---@class MessagePartState
 ---@field input TaskToolInput|BashToolInput|FileToolInput|TodoToolInput|GlobToolInput|GrepToolInput|WebFetchToolInput|ListToolInput Input data for the tool

lua/opencode/util.lua

@@ -360,4 +360,30 @@ function M.parse_dot_args(args_str)
   return result
 end
 
+--- Check if prompt is allowed via guard callback
+--- @param guard_callback? function
+--- @return boolean allowed
+--- @return string|nil error_message
+function M.check_prompt_allowed(guard_callback)
+  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
+
+  local success, result = pcall(guard_callback)
+
+  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