refactor: carefully set model from session

When loading a session, clear state.current_model and set it to the
model used by the most recent message.

Topbar shouldn't set a model as a side effect, it should just display
what's been selected.

Add `core.initialize_current_model()` which will initialize
state.current_model to the default from the config_file if it hasn't
been set

Author: cameronr

lua/opencode/api.lua

@@ -355,7 +355,7 @@ function M.initialize()
     vim.notify('Failed to create new session', vim.log.levels.ERROR)
     return
   end
-  if not state.current_model then
+  if not core.initialize_current_model() or not state.current_model then
     vim.notify('No model selected', vim.log.levels.ERROR)
     return
   end

lua/opencode/core.lua

@@ -25,6 +25,9 @@ function M.select_session(parent_id)
       end
       return
     end
+    -- clear the model so it can be set by the session. If it doesn't get set
+    -- then core.get_model() will reset it to the default
+    state.current_model = nil
     state.active_session = selected_session
     if state.windows then
       state.restore_points = {}
@@ -67,6 +70,8 @@ function M.open(opts)
   if opts.new_session then
     state.active_session = nil
     state.last_sent_context = nil
+    -- clear current_model here so it can be reset to the default (if one is set)
+    state.current_model = nil
     state.active_session = M.create_new_session()
   else
     if not state.active_session then
@@ -107,7 +112,7 @@ function M.send_message(prompt, opts)
   opts.context = vim.tbl_deep_extend('force', state.current_context_config or {}, opts.context or {})
   state.current_context_config = opts.context
   context.load()
-  opts.model = opts.model or state.current_model
+  opts.model = opts.model or M.initialize_current_model()
   opts.agent = opts.agent or state.current_mode or config.default_mode
 
   local params = {}
@@ -325,6 +330,22 @@ function M.ensure_current_mode()
   return true
 end
 
+---Initialize current model if it's not already set.
+---@return string|nil The current model (or the default model, if configured)
+function M.initialize_current_model()
+  if state.current_model then
+    return state.current_model
+  end
+
+  local config_file = require('opencode.config_file').get_opencode_config()
+
+  if config_file and config_file.model and config_file.model ~= '' then
+    state.current_model = config_file.model
+  end
+
+  return state.current_model
+end
+
 function M.setup()
   state.subscribe('opencode_server', on_opencode_server)
 

lua/opencode/state.lua

@@ -1,5 +1,3 @@
-local config = require('opencode.config')
-
 ---@class OpencodeWindowState
 ---@field input_win integer|nil
 ---@field output_win integer|nil

lua/opencode/ui/renderer.lua

@@ -5,14 +5,14 @@ local output_window = require('opencode.ui.output_window')
 local Promise = require('opencode.promise')
 local RenderState = require('opencode.ui.render_state')
 
-local M = {}
-
-M._subscriptions = {}
-M._prev_line_count = 0
-M._render_state = RenderState.new()
-M._last_part_formatted = {
-  part_id = nil,
-  formatted_data = nil --[[@as Output|nil]],
+local M = {
+  _subscriptions = {},
+  _prev_line_count = 0,
+  _render_state = RenderState.new(),
+  _last_part_formatted = {
+    part_id = nil,
+    formatted_data = nil --[[@as Output|nil]],
+  },
 }
 
 local trigger_on_data_rendered = require('opencode.util').debounce(function()
@@ -135,6 +135,10 @@ function M._render_full_session_data(session_data)
 
   -- local event_manager = state.event_manager
 
+  -- if we're loading a session and there's no currently selected model, set it
+  -- from the messages
+  local set_mode_from_messages = not state.current_model
+
   for i, msg in ipairs(session_data) do
     if state.active_session.revert and state.active_session.revert.messageID == msg.info.id then
       revert_index = i
@@ -153,6 +157,9 @@ function M._render_full_session_data(session_data)
     M._write_formatted_data(formatter._format_revert_message(state.messages, revert_index))
   end
 
+  if set_mode_from_messages then
+    M._set_model_from_messages()
+  end
   M.scroll_to_bottom()
 end
 
@@ -182,6 +189,25 @@ function M.on_emit_events_finished()
   M.scroll_to_bottom()
 end
 
+---Find the most recently used model from the messages
+function M._set_model_from_messages()
+  if not state.messages then
+    return
+  end
+
+  for i = #state.messages, 1, -1 do
+    local message = state.messages[i]
+
+    if message and message.info and message.info.modelID and message.info.providerID then
+      state.current_model = message.info.providerID .. '/' .. message.info.modelID
+      return
+    end
+  end
+
+  -- we didn't find a model from any messages, set it to the default
+  require('opencode.core').initialize_current_model()
+end
+
 ---Auto-scroll to bottom if user was already at bottom
 ---Respects cursor position if user has scrolled up
 function M.scroll_to_bottom()
@@ -919,14 +945,6 @@ function M.get_actions_for_line(line)
   return M._render_state:get_actions_at_line(line)
 end
 
----Update stats from all messages in session
----@param messages OpencodeMessage[]
-function M._update_stats_from_messages(messages)
-  for _, msg in ipairs(messages) do
-    M._update_stats_from_message(msg)
-  end
-end
-
 ---Update display stats from a single message
 ---@param message OpencodeMessage
 function M._update_stats_from_message(message)

lua/opencode/ui/topbar.lua

@@ -1,3 +1,4 @@
+local config = require('opencode.config')
 local util = require('opencode.util')
 
 local M = {}
@@ -10,16 +11,10 @@ local LABELS = {
 }
 
 local function format_model_info()
-  if not state.current_model or state.current_model == '' then
-    local info = config_file.get_opencode_config()
-    state.current_model = info and info.model
-  end
-
-  local config = require('opencode.config')
   local parts = {}
 
-  if config.ui.display_model and state.current_model then
-    if state.current_model ~= '' then
+  if config.ui.display_model then
+    if state.current_model then
       table.insert(parts, state.current_model)
     end
   end