Did you know you can completely rewrite OpenCode’s default system prompt directly from your config? All you have to do is drop a prompt key into your opencode.json file and point it to a local markdown file. You can target the build agent, the plan agent, or use this exact architecture to spin up entirely custom sub-agents inside your repo. Here is what the configuration looks like: from: Agents | OpenCode { "$schema": "https://opencode.ai/config.json", "agent": { "build": { "mode": "primary", "model": "anthropic/claude-sonnet-4-20250514", "prompt": "{file:./prompts/build.txt}", "permission": { "edit": "allow", "bash": "allow" } }, "plan": { "mode": "primary", "model": "anthropic/claude-haiku-4-20250514", "permission": { "edit": "deny", "bash": "deny" } }, "code-reviewer": { "description": "Reviews code for best practices and potential issues", "mode": "subagent", "model": "anthropic/claude-sonnet-4-20250514", "prompt": "You are a code reviewer. Focus on security, performance, and maintainability.", "permission": { "edit": "deny" } } } }