-// newCompactor returns the compactor of the agent name, which sends its calls
-// through client with model.
-func newCompactor(client *Client, name string, model ModelConfig) *Compactor {
+// newCompactor returns the compactor of an agent, which sends its calls through
+// client with model. instructions gives the system prompt of the agent at each
+// build, so a build sends the same prompt as the turns of that time.
+func newCompactor(client *Client, instructions func() (string, error), model ModelConfig) *Compactor {