A system reminder is a block of text wrapped in a <system-reminder> tag that
Claude Code attaches to the user message during a session, as observed in
Agent design lessons from Claude Code. According to that post, the reminders are
generated statically, depending on the tool that was just called and on the state of the agent’s TODO
list, and they exist to counter an agent forgetting its instructions or plan after many steps.
Usage
The post describes three kinds of reminder seen in Claude Code’s API requests: one at the beginning of the conversation restating general instructions (such as doing only what was asked and not creating files unless necessary), noting that the context may or may not be relevant; one after the first user message noting that the todo list is empty and suggesting the TodoWrite tool if the task would benefit from it, with an instruction not to mention the reminder to the user; and one attached after various steps giving the latest contents of the todo list and telling the agent to continue with the tasks at hand. The author presents them alongside a related technique — fixed instruction text appended to tool results — and reasons that repeating instructions in this way likely produces higher adherence than placing them only in the system prompt. He regards periodic reminders of the main control flow as important because Claude Code frequently takes hundreds of steps in one go.