dsh-subagent-error-details

When a background subagent dies mid-turn, the core DSH settlement notice prints a single fixed sentence and throws away the real failure reason entirely. Whether the cause is a 429 rate-limit, a timeout, or a logic error, the parent agent only sees "subagent finished" with no diagnostic detail and no way to make a meaningful retry or fallback decision. dsh-subagent-error-details exists to close that information gap.

The plugin subscribes to the public subagent/end event as a host-plane bundle. When stopReason is error, it resolves the child session terminal failure detail through sessionPersistence.load(), reading from the live session store first and the durable JSONL log second, then routes a short companion message back to the owning parent via the session-header parentSession chain and the agent registry. Every code path is wrapped defensively: a plugin bug degrades to "no details" rather than breaking the parent loop, and all service lookups are deferred inside the handlers, so activation can never fail a session mount. There are zero runtime dependencies.

This plugin is aimed at developers running DSH multi-agent workflows where background subagents do the heavy lifting and the parent needs to know why a child failed. It works across every agent preset with no composition editing required. Once the upstream core fix ships the error text directly in the official notice, the plugin auto-detects it and suppresses the companion message, so no manual removal is needed.