prompt_async does not wake idle sessions (noReply: false returns 204 but no assistant turn)

[CommanderCrowCode]

Summary

POST /session/{id}/prompt_async with noReply: false returns HTTP 204 but does not reliably trigger a new assistant turn when the session is idle. The message is accepted at the HTTP layer but the orchestration loop does not engage.

Steps to Reproduce

1. Start an OpenCode session, let it complete its current task and go idle (waiting for user input)
2. POST to /session/{sessionID}/prompt_async with:

   {"noReply": false, "parts": [{"type": "text", "text": "Please respond to this message."}]}

3. Observe: HTTP 204 returned, but the session stays idle. No assistant response is generated.
4. Manually type into the session → the injected message appears in history but was never acted on.

Expected Behavior

With noReply: false, the injected prompt should trigger a new assistant turn, just as if the user had typed it.

Context

We're building a multi-agent messaging system (relay-mesh) where agents running in different OpenCode sessions communicate via MCP tools. When Agent A sends a message to Agent B, we use prompt_async to notify B. This works when B is actively working (mid-tool-call), but fails when B is idle.

The behavior is intermittent — sometimes the session does wake up, sometimes it doesn't. This suggests a race condition in the session state machine rather than a fundamental design limitation.

Related Issues

• TUI doesn't render messages from prompt_async endpoint #8564 — TUI doesn't render messages from prompt_async
• /session/status potentially not reporting properly #12860 — /session/status not reporting properly after prompt_async
• Sessions hang indefinitely when Task tool spawns subagents via REST API (opencode serve) #6573 — Sessions hang with REST API subagents

Environment

• OpenCode v1.3.17
• Linux (Ubuntu 24.04)
• Multiple concurrent sessions via shared OpenCode server

[github-actions]

This issue might be a duplicate of existing issues. Please check:

• Background agents ignore initial prompt - stuck until manually messaged #21211: Background agents also get stuck and don't respond to prompts until manually messaged — very similar symptom of idle sessions not waking up
• [BUG] UnknownError: prompt_async failed — race condition between session cancel and Todo Continuation / Question dismiss #20250: Closed issue about a race condition in prompt_async where session cancel races caused lost/failed prompts (related root cause area)