Subagent-bound command: prompt to freshly-created child session can be silently dropped (no error, no log) #59135

Summary

When a custom command is bound to a subagent-mode agent (agent: <subagent> frontmatter), the command plugin creates the child session and prompts it immediately — with no delivery verification and no error surfacing if the prompt is dropped:

// packages/core/src/config/plugin/command.ts
const child = yield* sessions.create({ parentID: parent.id, title: ..., agent: selected.id, model: ... })
yield* sessions.prompt({ ...input.prompt, sessionID: child.id, text: [...], resume: false })
const recovery = { kind: "subagent", ... }
yield* subagents.start(recovery)
yield* subagents.background(recovery)
return

In a fraction of runs the prompt never materializes in the child session: the session sits with zero messages, the subagent never executes, the parent never receives a synthetic completion, and nothing is logged (no WARN/ERROR lines in the service log). The command silently no-ops.

Reproduction

Observed on OpenCode 2.1.4 (desktop app background service, macOS 15 arm64) with a global command in ~/.config/opencode/commands/ bound via agent: to a subagent agent in ~/.config/opencode/agents/:

1. POST /api/session (create a session at some directory)
2. POST /api/session/{id}/command with {"name": "<cmd>", "text": ""}
3. Poll GET /api/session?parentID={id} and export the child

Over ~10 dispatches in one evening, 1 left the child with 0 messages indefinitely. The same drop reproduces on the direct path: POST /api/session immediately followed by POST /api/session/{id}/prompt can silently lose the message — the session shows no user message, no error is returned, and re-POSTing the identical body seconds later delivers instantly (the message materializes immediately). This suggests the inbox delivery races the session/location becoming ready, and a lost prompt is never retried or surfaced.

Expected

Either the prompt is reliably queued for the new session, or a drop surfaces as an error (to the command executor and the logs).

Additional note

Passing "text": "." to a command breaks template delivery entirely: the child session is created but the template never arrives (and the command falls through to the parent session in some code paths). Empty string or a real argument works.

Environment

• OpenCode 2.1.4 (app-bundled service), CLI 2.1.2, macOS Sequoia arm64
• Related: subagent child sessions that DO run return empty exports (see companion issue)