CopilotKit

Sub-Agents

Decompose work across multiple specialized agents with a visible delegation log.

What is this?#

Sub-agents are the canonical multi-agent pattern: a top-level supervisor LLM orchestrates one or more specialized sub-agents by exposing each of them as a tool. The supervisor decides what to delegate, the sub-agents do their narrow job, and their results flow back up to the supervisor's next step.

This is fundamentally the same shape as tool-calling, but each "tool" is itself a full-blown agent with its own system prompt and (often) its own tools, memory, and model.

When should I use this?#

Reach for sub-agents when a task has distinct specialized sub-tasks that each benefit from their own focus:

  • Research → Write → Critique pipelines, where each stage needs a different system prompt and temperature.
  • Router + specialists, where one agent classifies the request and dispatches to the right expert.
  • Divide-and-conquer — any problem that fits cleanly into parallel or sequential sub-problems.

The example below uses the Research → Write → Critique shape as the canonical example.

Setting up sub-agents#

Each sub-agent is a full create_agent(...) call with its own model, its own system prompt, and (optionally) its own tools. They don't share memory or tools with the supervisor; the supervisor only ever sees what the sub-agent returns.

Keep sub-agent system prompts narrow and focused. The point of this pattern is that each one does one thing well. If a sub-agent needs to know the whole user context to do its job, that's a signal the boundary is wrong.

Exposing sub-agents as tools#

The supervisor delegates by calling tools. Each tool is a thin wrapper around sub_agent.invoke(...) that:

  1. Runs the sub-agent synchronously on the supplied task string.
  2. Records the delegation into a delegations slot in shared agent state (so the UI can render a live log).
  3. Returns the sub-agent's final message as a ToolMessage, which the supervisor sees as a normal tool result on its next turn.

This is where CopilotKit's shared-state channel earns its keep: the supervisor's tool calls mutate delegations as they happen, and the frontend renders every new entry live.

Rendering a live delegation log#

On the frontend, the delegation log is just a reactive render of the delegations slot. Subscribe with useAgent({ updates: [UseAgentUpdate.OnStateChanged, UseAgentUpdate.OnRunStatusChanged] }), read agent.state.delegations, and render one card per entry.

delegation-log.tsx
/** * Live delegation log — renders the `delegations` slot of agent state. * * Each entry corresponds to one invocation of a sub-agent. The list * grows in real time as the supervisor fans work out to its children. * The parent header shows how many sub-agents have been called and * whether the supervisor is still running. */// Fixed list of the three sub-agent roles the supervisor can call.// Rendered as always-visible indicator chips at the top of the log// (regardless of whether the supervisor has delegated yet) so the user// — and the e2e suite — can see at a glance which sub-agents exist and// which are currently active.const INDICATOR_ROLES: ReadonlyArray<{  role: "researcher" | "writer" | "critic";  subAgent: SubAgentName;}> = [  { role: "researcher", subAgent: "research_agent" },  { role: "writer", subAgent: "writing_agent" },  { role: "critic", subAgent: "critique_agent" },];export function DelegationLog({ delegations, isRunning }: DelegationLogProps) {  const calledRoles = new Set<SubAgentName>(    delegations.map((d) => d.sub_agent),  );  return (    <div      data-testid="delegation-log"      className="w-full h-full flex flex-col bg-white rounded-2xl shadow-sm border border-[#DBDBE5] overflow-hidden"    >      <div className="flex items-center justify-between px-6 py-3 border-b border-[#E9E9EF] bg-[#FAFAFC]">        <div className="flex items-center gap-3">          <span className="text-lg font-semibold text-[#010507]">            Sub-agent delegations          </span>          {isRunning && (            <span              data-testid="supervisor-running"              className="inline-flex items-center gap-1.5 px-2 py-0.5 rounded-full border border-[#BEC2FF] bg-[#BEC2FF1A] text-[#010507] text-[10px] font-semibold uppercase tracking-[0.12em]"            >              <span className="w-1.5 h-1.5 rounded-full bg-[#010507] animate-pulse" />              Supervisor running            </span>          )}        </div>        <span          data-testid="delegation-count"          className="text-xs font-mono text-[#838389]"        >          {delegations.length} calls        </span>      </div>      <div        data-testid="subagent-indicators"        className="flex items-center gap-2 border-b border-[#E9E9EF] bg-white px-6 py-2"      >        {INDICATOR_ROLES.map(({ role, subAgent }) => {          const style = SUB_AGENT_STYLE[subAgent];          const fired = calledRoles.has(subAgent);          return (            <span              key={role}              data-testid={`subagent-indicator-${role}`}              data-role={role}              data-fired={fired ? "true" : "false"}              className={`inline-flex items-center gap-1 px-2 py-0.5 rounded-full text-[10px] font-semibold uppercase tracking-[0.1em] border ${style.color} ${                fired ? "" : "opacity-60"              }`}            >              <span aria-hidden>{style.emoji}</span>              <span>{style.label}</span>            </span>          );        })}      </div>      <div className="flex-1 overflow-y-auto p-4 space-y-3">        {delegations.length === 0 ? (          <p className="text-[#838389] italic text-sm">            Ask the supervisor to complete a task. Every sub-agent it calls will            appear here.          </p>        ) : (          delegations.map((d, idx) => {            const style = SUB_AGENT_STYLE[d.sub_agent];            return (              <div                key={d.id}                data-testid="delegation-entry"                className="border border-[#E9E9EF] rounded-xl p-3 bg-[#FAFAFC]"              >                <div className="flex items-center justify-between mb-2">                  <div className="flex items-center gap-2">                    <span className="text-xs font-mono text-[#AFAFB7]">                      #{idx + 1}                    </span>                    <span                      className={`inline-flex items-center gap-1 px-2 py-0.5 rounded-full text-[10px] font-semibold uppercase tracking-[0.1em] border ${style.color}`}                    >                      <span>{style.emoji}</span>                      <span>{style.label}</span>                    </span>                  </div>                  <span className="text-[10px] uppercase tracking-[0.12em] font-semibold text-[#189370]">                    {d.status}                  </span>                </div>                <div className="text-xs text-[#57575B] mb-2">                  <span className="font-semibold text-[#010507]">Task: </span>                  {d.task}                </div>                <div className="text-sm text-[#010507] whitespace-pre-wrap bg-white rounded-lg p-2.5 border border-[#E9E9EF]">                  {d.result}                </div>              </div>            );          })        )}      </div>    </div>  );}

The result: as the supervisor fans work out to its sub-agents, the log grows in real time, giving the user visibility into a process that would otherwise be a long opaque spinner.

Related#

  • Shared State — the channel that makes the delegation log live.
  • State streaming — stream individual sub-agent outputs token-by-token inside each log entry.