Subagen Tidak Mewarisi Riwayat

You build a hub-and-spoke multi-agent system. The coordinator has had a long conversation with the user: requirements, constraints, prior decisions. Then it delegates a task to a subagent and assumes the subagent already "knows" all of that.

It does not. This is the single most common multi-agent bug, and it appears directly on the Claude Certified Architect exam.

The rule: subagents do NOT inherit the coordinator's conversation history. Every piece of context a subagent needs must be passed explicitly in its prompt.

The Claude API is stateless. The model keeps NO server-side memory between requests. On every turn you resend the FULL messages history yourself.

A subagent runs as its own independent loop, with its own messages array. The coordinator's history lives in the coordinator's request, not in some shared global memory. Nothing copies it across.

So when a subagent starts, its context is exactly what you put in its system prompt and first messages entry — and nothing more.

In the hub-and-spoke pattern the coordinator decomposes the work, delegates to subagents, then aggregates and routes the results. It owns orchestration and error handling.

But delegation is a one-way handoff of explicit instructions, not a shared brain. Think of each subagent as a brand-new contractor who has never seen your earlier emails. You must brief them fully in the work order itself.

• Coordinator: decompose, delegate, aggregate, route, handle errors.
• Subagent: receives a self-contained brief, does one focused job, returns a result.

Here is what the bug looks like in practice. The user told the coordinator the target is the checkout service and the deadline is strict. The coordinator then spawns a reviewer subagent with a vague brief.

The subagent has no idea which service, which constraints, or what "the file" refers to. It hallucinates a target, reviews the wrong thing, or asks a question the coordinator can't relay back cleanly.

Rewrite the brief so it stands entirely on its own. Pass the target, the constraints, the prior decisions, and the exact output you expect.

A good subagent prompt answers: What is the task? On what exact input? Under what constraints? In what output format? No reference to "earlier" or "as we said" survives the handoff.

Don't summarize transactional facts into mush before handing them off. Progressive summarization makes numbers, percentages, and dates vague — and a subagent acting on "roughly last quarter" instead of "2026-Q1" will be wrong.

Keep the hard facts a subagent needs in a verbatim "context" block: IDs, thresholds, file paths, exact dates. Summarize prose for flavor; never summarize the load-bearing details.

An AgentDefinition carries: name, description, system_prompt, and allowed_tools. Because the subagent is isolated, its tools and its system prompt ARE its whole world — scope them to the role.

Give a subagent the 4-5 tools it actually needs (4-5 per agent is optimal; 18+ degrades tool selection). And remember: for the coordinator to spawn subagents at all, the coordinator's allowedTools must include "Task".

Issuing multiple Task calls in a single response runs them in parallel. That is powerful — but it doubles down on the isolation rule.

Parallel subagents cannot see each other's history OR the coordinator's. Each must be briefed independently and completely. There is no implicit ordering and no shared scratchpad between them; if subagent B needs subagent A's output, the coordinator must collect A's result and feed it into B's prompt explicitly.

Isolation also shapes the return trip. The coordinator only gets back what the subagent emits — so make that emission machine-usable.

For aggregation, force structured output: a subagent with tool_choice="any" MUST call a tool, which guarantees the coordinator receives parseable JSON instead of free prose it has to scrape. And when a subagent fails, it should return structured context (failure type, attempted query, partial results) — not a generic "operation failed" that blocks recovery.

You might hope a resumed session smuggles history into a subagent. Be careful. --resume <name> continues a named session and fork_session branches from a shared point — but these resume a session's own state, they do not retroactively inject the coordinator's chat into a fresh subagent.

And resumed tool results can be stale if the codebase changed underneath them. Sometimes a fresh session seeded with a structured summary beats resuming — which is exactly the explicit-context discipline again.

Before you spawn any subagent, confirm its prompt is self-contained. Walk this checklist:

• Target: the exact file / record / id it operates on.
• Constraints: policies, thresholds, prior decisions — verbatim.
• Task: one focused job, stated with explicit criteria.
• Output: the exact shape to return (JSON schema / fields).
• Tools: only the 4-5 it needs, least privilege.

If you removed the coordinator entirely and handed this prompt to a stranger, could they do the job? If yes, you've briefed it correctly.

Key takeaways:

• The API is stateless and subagents are isolated — they inherit none of the coordinator's history.
• Every subagent prompt must be self-contained: target, constraints, task, output format, tools.
• Pass transactional facts (ids, thresholds, dates, paths) verbatim; don't let summarization blur them.
• Parallel Task calls are independent — brief each one separately; the coordinator feeds one subagent's output into another explicitly.
• Scope allowed_tools to the role (4-5 optimal); the coordinator needs "Task" to delegate.
• Use tool_choice="any" for structured returns and structured errors for recoverable aggregation.

Brief the stranger, not the friend. That mindset passes both the exam and production.