Gemini Sub Agent

Gemini Sub-Agent Skill

Launch gemini as a headless sub-agent in a tmux pane with live JSONL monitoring. Depends on the tmux skill for pane interaction.

When to use

• Delegating a scoped task to Gemini CLI running autonomously
• Parallel execution alongside other sub-agents (copilot-sub-agent, claude-sub-agent, pi-sub-agent)
• Tasks where Gemini's explicit exit codes (41–53) are useful for error handling in scripts

Output format

Gemini streams newline-delimited JSON via --output-format stream-json. Key event types:

• init — startup, contains session_id, model
• message with role: user — echoes the prompt
• message with role: assistant, delta: true — streaming text chunks
• result — final line, contains status, stats (tokens, durationms, toolcalls)

Completion signal: "type":"result" line in the stream.

{"type":"result","status":"success","stats":{"total_tokens":11386,"input_tokens":10774,"output_tokens":71,"cached":6250,"duration_ms":5815,"tool_calls":0}}

Exit codes (unique to Gemini — use these in scripts):

• 0 — success
• 41 — authentication error
• 42 — invalid input
• 44 — sandbox error
• 52 — configuration error
• 53 — turn limit exceeded

Non-interactive invocation

Lean / headless invocation

cat /tmp/task-prompt.md | gemini \
  --yolo \
  --output-format stream-json \
  --model flash \
  > /tmp/gemini-output.jsonl 2>&1

What each flag does:

| Flag | Why | |---|---| | --yolo | Auto-approve all tool calls — required for headless | | --output-format stream-json | JSONL stream for monitoring; completion signalled by "type":"result" line | | --model flash | Use flash (fast) instead of auto or pro — significantly faster for agentic tasks |

Gemini's startup footprint is controlled by what's in ~/.gemini/settings.json and GEMINI.md. There is no flag to suppress MCP loading — configure allowed-mcp-server-names with an empty list if you need to exclude all MCP servers:

cat /tmp/task-prompt.md | gemini \
  --yolo \
  --output-format stream-json \
  --model flash \
  --allowed-mcp-server-names "" \
  > /tmp/gemini-output.jsonl 2>&1

Gemini's baseline system prompt is ~10–11k tokens and is cached by default on warm runs. There are no flags to reduce this further — the lean knob is model selection (flash vs pro) and avoiding GEMINI.md bloat when injecting personas.

Other useful flags:

• --model flash-lite — fastest/cheapest option
• --temperature 0 — deterministic output for automation
• --timeout — execution timeout in milliseconds
• --include-directories /path — add workspace directories

Workflow

1. Write the task prompt to a file

cat > /tmp/task-prompt.md  /tmp/gemini-output.jsonl 2>&1 &" Enter
sleep 3
tmux send-keys -t "$TARGET" \
  "python3 {SKILLS_DIR}/gemini-sub-agent/scripts/monitor.py /tmp/gemini-output.jsonl" Enter

4. Poll for completion

python3 {SKILLS_DIR}/gemini-sub-agent/scripts/poll.py "$TARGET" --interval 30

5. Check exit code and verify

tmux send-keys -t "$TARGET" C-c   # kill monitor

# Extract exit code from result line
python3 -c "
import json
for line in open('/tmp/gemini-output.jsonl'):
    d = json.loads(line.strip())
    if d.get('type') == 'result':
        print('Status:', d['status'])
        print('Stats:', json.dumps(d['stats'], indent=2))
"
git log --oneline -5

Full copy-paste pattern

cat > /tmp/task-prompt.md  /tmp/gemini-output.jsonl 2>&1 &" Enter
sleep 3
tmux send-keys -t "$TARGET" \
  "python3 {SKILLS_DIR}/gemini-sub-agent/scripts/monitor.py /tmp/gemini-output.jsonl" Enter

python3 {SKILLS_DIR}/gemini-sub-agent/scripts/poll.py "$TARGET" --interval 30

tmux send-keys -t "$TARGET" C-c
git log --oneline -5

Agent / persona injection

Gemini has no --system-prompt CLI flag. Persona injection works via GEMINI.md in the current working directory — Gemini automatically loads it as system context before any prompt.

# Write the persona to GEMINI.md in the working directory
cat {SKILLS_DIR}/bart-adversarial-reviewer/references/bart.md > /path/to/repo/GEMINI.md

# Run gemini from that directory — it will load GEMINI.md automatically
cd /path/to/repo
cat /tmp/task.md | gemini --yolo --output-format stream-json > /tmp/gemini-output.jsonl 2>&1

# Clean up after (or leave if you want the persona persistent)
rm /path/to/repo/GEMINI.md

The GEMINI.md file is a plain markdown system prompt — no frontmatter required. Gemini loads it from $CWD/GEMINI.md (also checks ~/.gemini/GEMINI.md as a user-level default).

For headless sub-agent use, write the persona before launching:

cat > /path/to/repo/GEMINI.md  /tmp/gemini-output.jsonl 2>&1 &" Enter

Model selection

gemini "task" --model flash-lite --yolo   # fastest, cheapest
gemini "task" --model flash --yolo        # fast, balanced (default)
gemini "task" --model pro --yolo          # most capable

Script integration (using exit codes)

cat /tmp/task-prompt.md | gemini --yolo --output-format stream-json > /tmp/out.jsonl 2>&1
case $? in
  0)  echo "Success" ;;
  41) echo "Auth error — check GEMINI_API_KEY" ; exit 1 ;;
  42) echo "Invalid input" ; exit 1 ;;
  52) echo "Config error" ; exit 1 ;;
  53) echo "Turn limit exceeded" ; exit 1 ;;
  *)  echo "Unknown error $?" ; exit 1 ;;
esac

Troubleshooting

Prompt truncated by shell: Use cat /tmp/file | gemini ... (stdin pipe) for long prompts.

Stuck with no result line: Check if --yolo is set; without it, Gemini pauses for tool approval.

tool_calls: 0 but task needed tools: Gemini may need --approval-mode yolo explicitly if --yolo alone doesn't propagate.

Parse the result stats:

grep '"type":"result"' /tmp/gemini-output.jsonl | python3 -c "import json,sys; d=json.loads(sys.stdin.read()); print(json.dumps(d['stats'], indent=2))"

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: shalomb
• Source: shalomb/agent-skills
• License: MIT

Install and usage instructions live in the source repository linked above.