OpenCode: a reviewer subagent gated to exactly the git commands you allow

The Stack

Steps

1. 1

   Write the gated reviewer agent and validate the frontmatter

   Drop review.md in .opencode/agents/ (per project) or ~/.config/opencode/agents/ (global). Keep the "*" catch-all first and the specific allows after it, since the last matching rule wins. CI parses the YAML frontmatter and asserts edit=deny, wildcard-first, and git diff allowed.

   mkdir -p .opencode/agents
   cat > .opencode/agents/review.md <<'MD'
   ---
   description: Reviews code without making changes
   mode: subagent
   model: provider/your-cheap-model
   permission:
     edit: deny
     webfetch: deny
     bash:
       "*": ask
       "git diff": allow
       "git log*": allow
       "grep *": allow
   ---
   
   You are in review mode. Inspect the diff and flag bugs, security issues, and
   risky changes. Do not modify files. Suggest fixes as comments only.
   MD
   ruby -ryaml -e '
   lines = File.read(".opencode/agents/review.md").lines
   i1 = lines.index { |l| l.strip == "---" }
   abort "BAD: no frontmatter start" if i1.nil?
   rest = lines[(i1 + 1)..]
   i2 = rest.index { |l| l.strip == "---" }
   abort "BAD: no frontmatter end" if i2.nil?
   fm = YAML.load(rest[0...i2].join)
   perm = fm["permission"] || {}
   abort "BAD: edit not deny" unless perm["edit"] == "deny"
   bash = perm["bash"] || {}
   abort "BAD: wildcard not first" unless bash.keys.first == "*"
   abort "BAD: git diff not allowed" unless bash["git diff"] == "allow"
   puts "config OK: @review frontmatter parses, edit=deny, bash wildcard-first then git diff=allow"
   '

2. 2

   Use @review on a diff (the model step, not checked by CI)

   Mention @review in a message; it can run git diff/log and grep to inspect, but never write. It suggests fixes as comments only. The review runs the model, so CI never claims its content.