Background agents ignore initial prompt - stuck until manually messaged #21211

Bug Description

Background agents launched with run_in_background=true ignore their initial prompt and wait indefinitely until manually messaged via session_id.

Steps to Reproduce

1. Launch a background agent with a prompt:

task(
  run_in_background=true,
  subagent_type="explore",
  prompt="List files in /tmp"
)

2. Wait for agent completion (it will never complete on its own)

3. Notice the agent shows status "running" but produces no output

4. Only when sending a continuation message via session_id does the agent actually work:

task(
  session_id="ses_xxx",
  prompt="Execute your task now"
)

Expected Behavior

• Agent should process the initial prompt immediately upon launch
• No manual continuation message should be required

Actual Behavior

• Agent launches, shows "running" status
• Never processes initial prompt
• Stays stuck until explicitly messaged again

Workaround

Use run_in_background=false for synchronous execution, which works correctly.

Environment

• OpenCode Desktop app (macOS)
• Model: opencode/big-pickle (parent), nvidia/gemma-4-31b-it (subagent)

Additional Context

This makes background agents unusable for parallel workloads as they effectively become synchronous anyway.