feat(timeline-picker): add fork session action

Author: sudo-tee

lua/opencode/api.lua

@@ -683,7 +683,9 @@ end
 function M.timeline()
   local user_messages = {}
   for _, msg in ipairs(state.messages or {}) do
-    if msg.info.role == 'user' then
+    local parts = msg.parts or {}
+    local is_summary = #parts == 1 and parts[1].synthetic == true
+    if msg.info.role == 'user' and not is_summary then
       table.insert(user_messages, msg)
     end
   end
@@ -700,6 +702,39 @@ function M.timeline()
   end)
 end
 
+function M.fork_session(message_id)
+  if not state.active_session then
+    vim.notify('No active session to fork', vim.log.levels.WARN)
+    return
+  end
+
+  local message_to_fork = message_id or state.last_user_message and state.last_user_message.info.id
+  if not message_to_fork then
+    vim.notify('No user message to fork from', vim.log.levels.WARN)
+    return
+  end
+
+  state.api_client
+    :fork_session(state.active_session.id, {
+      messageID = message_to_fork,
+    })
+    :and_then(function(response)
+      vim.schedule(function()
+        if response and response.id then
+          vim.notify('Session forked successfully. New session ID: ' .. response.id, vim.log.levels.INFO)
+          core.switch_session(response.id)
+        else
+          vim.notify('Session forked but no new session ID received', vim.log.levels.WARN)
+        end
+      end)
+    end)
+    :catch(function(err)
+      vim.schedule(function()
+        vim.notify('Failed to fork session: ' .. vim.inspect(err), vim.log.levels.ERROR)
+      end)
+    end)
+end
+
 -- Returns the ID of the next user message after the current undo point
 -- This is a port of the opencode tui logic
 -- https://github.com/sst/opencode/blob/dev/packages/tui/internal/components/chat/messages.go#L1199

lua/opencode/api_client.lua

@@ -213,6 +213,15 @@ function OpencodeApiClient:summarize_session(id, summary_data, directory)
   return self:_call('/session/' .. id .. '/summarize', 'POST', summary_data, { directory = directory })
 end
 
+--- Fork an existing session at a specific message
+--- @param id string Session ID (required)
+--- @param fork_data {messageID?: string}|nil Fork data
+--- @param directory string|nil Directory path
+--- @return Promise<Session>
+function OpencodeApiClient:fork_session(id, fork_data, directory)
+  return self:_call('/session/' .. id .. '/fork', 'POST', fork_data, { directory = directory })
+end
+
 -- Message endpoints
 
 --- List messages for a session

lua/opencode/core.lua

@@ -25,21 +25,26 @@ 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 = {}
-      -- Don't need to update either renderer because they subscribe to
-      -- session changes
-      ui.focus_input()
-    else
-      M.open()
-    end
+    M.switch_session(selected_session.id)
   end)
 end
 
+function M.switch_session(session_id)
+  local selected_session = session.get_by_id(session_id)
+  -- 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 = {}
+    -- Don't need to update either renderer because they subscribe to
+    -- session changes
+    ui.focus_input()
+  else
+    M.open()
+  end
+end
+
 ---@param opts? OpenOpts
 function M.open(opts)
   opts = opts or { focus = 'input', new_session = false }

lua/opencode/ui/timeline_picker.lua

@@ -1,6 +1,7 @@
 local M = {}
 local picker = require('opencode.ui.picker')
 local config = require('lua.opencode.config')
+local api = require('opencode.api')
 
 local picker_title = function()
   local config = require('opencode.config') --[[@as OpencodeConfig]]
@@ -329,8 +330,7 @@ function M.pick(messages, callback)
   end
 
   local function on_fork(msg)
-    -- TODO: Implement fork functionality
-    vim.notify('Fork functionality not yet implemented', vim.log.levels.WARN)
+    api.fork_session(msg.info.id)
   end
 
   vim.schedule(function()