How do I create a custom sub-agent in Line CLI?

Create a Markdown file:

• Project: .line/agents/<name>.md
• Yours, every project: ~/.line/agents/<name>.md

---
name: test-writer
description: Writes unit tests for a given module without touching source files.
tools: read_file, write_file, grep, bash
model: supercosmos
effort: flash
max_steps: 30
---
Write focused unit tests for the files you are given. Never edit files outside tests/. Run the tests before reporting.

The body is the sub-agent's instructions. Line's agent can then start it with the task tool, kind test-writer. A custom sub-agent pinned to the retired Line Code model runs on SuperCosmos.