fix: Respect user set default_mode and improve agent management

- The default_mode option was not being respected when setting the
  state current_mode. The current_mode is now set when first opening the
  opencode window.

- The mode switching is now wrapped in a "switch_to_mode" function to
  ensure the mode exists in the available agents for more control.

- The build and plan agents were added to the available agents even if
  explicitly disabled in the opencode config -> leading to a hang when
  sending messages to opencode. They are now prepended if they are not
  disabled.

- The agent list was always randomly returned, it is now sorted before
  using it.

Author: guillaumeboehm

lua/opencode/api.lua

@@ -367,11 +367,11 @@ function M.initialize()
 end
 
 function M.agent_plan()
-  state.current_mode = 'plan'
+  require('opencode.core').switch_to_mode('plan')
 end
 
 function M.agent_build()
-  state.current_mode = 'build'
+  require('opencode.core').switch_to_mode('build')
 end
 
 function M.select_agent()
@@ -383,7 +383,7 @@ function M.select_agent()
       return
     end
 
-    state.current_mode = selection
+    require('opencode.core').switch_to_mode(selection)
   end)
 end
 
@@ -399,7 +399,7 @@ function M.switch_mode()
   -- Calculate next index, wrapping around if necessary
   local next_index = (current_index % #modes) + 1
 
-  state.current_mode = modes[next_index]
+  require('opencode.core').switch_to_mode(modes[next_index])
 end
 
 function M.with_header(lines, show_welcome)

lua/opencode/config_file.lua

@@ -77,13 +77,23 @@ function M.get_opencode_agents()
   end
   local agents = {}
   for agent, opts in pairs(cfg.agent or {}) do
-    if opts.mode == 'primary' or opts.mode == 'all' then
+    -- Only include agents that are enabled and have the right mode
+    if opts.disable ~= true and (opts.mode == 'primary' or opts.mode == 'all') then
       table.insert(agents, agent)
     end
   end
-  for _, mode in ipairs({ 'build', 'plan' }) do
+
+  -- Sort the agents before prepending the default agents
+  table.sort(agents)
+
+  -- Only add build/plan as fallbacks if they're not explicitly disabled in config
+  for _, mode in ipairs({ 'plan', 'build' }) do
     if not vim.tbl_contains(agents, mode) then
-      table.insert(agents, mode)
+      local mode_config = cfg.agent and cfg.agent[mode]
+      -- Only add if not explicitly disabled or if no config exists (default behavior)
+      if mode_config == nil or (mode_config.disable ~= true) then
+        table.insert(agents, 1, mode)
+      end
     end
   end
   return agents

lua/opencode/core.lua

@@ -45,6 +45,8 @@ function M.open(opts)
     state.opencode_server = server_job.ensure_server() --[[@as OpencodeServer]]
   end
 
+  M.ensure_current_mode()
+
   local are_windows_closed = state.windows == nil
 
   if are_windows_closed then
@@ -272,6 +274,58 @@ local function on_opencode_server()
   state.current_permission = nil
 end
 
+--- Switches the current mode to the specified agent.
+--- @param mode string The agent/mode to switch to
+--- @return boolean success Returns true if the mode was switched successfully, false otherwise
+function M.switch_to_mode(mode)
+  if not mode or mode == '' then
+    vim.notify('Mode cannot be empty', vim.log.levels.ERROR)
+    return false
+  end
+
+  local config_file = require('opencode.config_file')
+  local available_agents = config_file.get_opencode_agents()
+
+  if not vim.tbl_contains(available_agents, mode) then
+    vim.notify(
+      string.format('Invalid mode "%s". Available modes: %s', mode, table.concat(available_agents, ', ')),
+      vim.log.levels.ERROR
+    )
+    return false
+  end
+
+  state.current_mode = mode
+  ui.render_output()
+  return true
+end
+
+--- Ensure the current_mode is set using the config.default_mode or falling back to the first available agent.
+--- @return boolean success Returns true if current_mode is set
+function M.ensure_current_mode()
+  if state.current_mode == nil then
+    local config_file = require('opencode.config_file')
+    local available_agents = config_file.get_opencode_agents()
+
+    if not available_agents or #available_agents == 0 then
+      vim.notify('No available agents found', vim.log.levels.ERROR)
+      return false
+    end
+
+    local default_mode = config.default_mode
+
+    -- Try to use the configured default mode if it's available
+    if default_mode and vim.tbl_contains(available_agents, default_mode) then
+      state.current_mode = default_mode
+    else
+      -- Fallback to first available agent
+      state.current_mode = available_agents[1]
+    end
+
+    ui.render_output()
+  end
+  return true
+end
+
 function M.setup()
   state.subscribe('opencode_server', on_opencode_server)
 

lua/opencode/state.lua

@@ -53,7 +53,7 @@ local _state = {
   last_output_window_position = nil,
   last_code_win_before_opencode = nil,
   display_route = nil,
-  current_mode = config.default_mode,
+  current_mode = nil,
   last_output = 0,
   -- context
   last_sent_context = nil,