feat(tmux): add fallback server discovery for split panes

lua/opencode/cli/server.lua

@@ -158,6 +158,15 @@ local function find_server_inside_nvim_cwd()
       end
     end
   end
+
+  -- Fallback: try provider-specific discovery
+  if not found_server then
+    local provider = require("opencode.config").provider
+    if provider and provider.find_server then
+      found_server = provider:find_server()
+    end
+  end
+
   if not found_server then
     error("No `opencode` servers inside Neovim's CWD", 0)
   end
@@ -200,7 +209,8 @@ end
 ---Attempt to get the `opencode` server's port. Tries, in order:
 ---1. A process responding on `opts.port`.
 ---2. Any `opencode` process running inside Neovim's CWD. Prioritizes embedded.
----3. Calling `opts.provider.start` and polling for the port.
+---3. Provider-specific discovery (e.g., tmux sibling panes).
+---4. Calling `opts.provider.start` and polling for the port.
 ---
 ---@param launch boolean? Whether to launch a new server if none found. Defaults to true.
 function M.get_port(launch)

lua/opencode/provider/init.lua

@@ -1,8 +1,8 @@
 ---@module 'snacks.terminal'
 
 ---Provide an integrated `opencode`.
----Providers should ignore manually-started `opencode` instances,
----operating only on those they start themselves.
+---`start`/`stop`/`toggle` should only operate on provider-managed instances.
+---`find_server` may attach to any existing instance for connection purposes.
 ---@class opencode.Provider
 ---
 ---The name of the provider.
@@ -33,6 +33,11 @@
 ---Should return `true` if the provider is available,
 ---else a reason string and optional advice (for `vim.health.warn`).
 ---@field health? fun(): boolean|string, ...string|string[]
+---
+---Find an existing `opencode` server via provider-specific discovery.
+---Unlike other methods, may return servers not started by the provider.
+---Called as a fallback when CWD-based discovery fails.
+---@field find_server? fun(self: opencode.Provider): opencode.cli.server.Server|nil
 
 ---Configure and enable built-in providers.
 ---@class opencode.provider.Opts

lua/opencode/provider/tmux.lua

@@ -113,4 +113,40 @@ function Tmux:stop()
   end
 end
 
+---Find an `opencode` server running in a sibling pane of the current tmux window.
+---@return opencode.cli.server.Server|nil
+function Tmux:find_server()
+  if self.health() ~= true then
+    return nil
+  end
+
+  local session_window = vim.fn.system("tmux display-message -p '#{session_name}:#{window_index}'"):gsub("\n", "")
+  local panes_output = vim.fn.system(string.format("tmux list-panes -t '%s' -F '#{pane_tty}'", session_window))
+
+  for tty in panes_output:gmatch("[^\r\n]+") do
+    local tty_short = tty:gsub("^/dev/", "")
+    local ps_output = vim.fn.system(string.format("ps -t %s -o pid,command", tty_short))
+
+    for line in ps_output:gmatch("[^\r\n]+") do
+      local pid = line:match("^%s*(%d+).*opencode")
+      if pid then
+        local lsof = vim.fn.system(string.format("lsof -w -iTCP -sTCP:LISTEN -P -n -a -p %s", pid))
+        local port = lsof:match(":(%d+) %(LISTEN%)")
+        if port then
+          local ok, path = pcall(require("opencode.cli.client").get_path, tonumber(port))
+          if ok then
+            return {
+              pid = tonumber(pid),
+              port = tonumber(port),
+              cwd = path.directory or path.worktree,
+            }
+          end
+        end
+      end
+    end
+  end
+
+  return nil
+end
+
 return Tmux