skill: add inherit directive for tool inheritance (#18)

Port of the `tools: inherit` directive from llm.rb's skill
implementation to mruby-llm. When a skill sets `tools: inherit` in
its SKILL.md frontmatter, the sub-agent spawned by the skill
inherits tools from its parent context or agent instead of
declaring a fixed list.

Author: bsdrobert

mrblib/mruby-llm/skill.rb

@@ -56,6 +56,7 @@ def initialize(path)
       @instructions = ""
       @frontmatter = LLM::Object.from({})
       @tools = []
+      @inherit_tools = false
     end
 
     ##
@@ -74,7 +75,8 @@ def load!
     # @param [LLM::Context] ctx
     # @return [Hash]
     def call(ctx)
-      instructions, tools, tracer = self.instructions, self.tools, ctx.llm.tracer
+      instructions, tracer = self.instructions, ctx.llm.tracer
+      tools = @inherit_tools ? [*ctx.params[:tools]].reject(&:skill?) : self.tools
       params = ctx.params.merge(mode: ctx.mode).reject { |key, _| [:tools, :schema].include?(key) }
       concurrency = params[:stream].extra[:concurrency] if LLM::Stream === params[:stream]
       params[:concurrency] = concurrency if concurrency
@@ -110,6 +112,13 @@ def to_tool(ctx)
       end
     end
 
+    ##
+    # Returns true when a skill should inherit tools from its parent
+    # @return [Boolean]
+    def inherit_tools?
+      @inherit_tools
+    end
+
     private
 
     def messages_for(ctx)
@@ -131,8 +140,25 @@ def parse(content)
       @frontmatter = LLM::Object.from(LLM::YAML.safe_load(match[1]) || {})
       @name = @frontmatter.name || @name
       @description = @frontmatter.description || @description
-      @tools = [*@frontmatter.tools].map { LLM::Tool.find_by_name!(_1) }
+      @inherit_tools, @tools = parse_tools(@frontmatter.tools)
       @instructions = content[match.end(0)..-1] || ""
     end
+
+    def parse_tools(tools)
+      case tools
+      when String
+        tools == "inherit" ? [true, []] : raise_invalid_error!(tools)
+      when Array
+        [false, tools.map { LLM::Tool.find_by_name!(_1) }]
+      when NilClass
+        [false, []]
+      else
+        raise_invalid_error!(tools)
+      end
+    end
+
+    def raise_invalid_error!(tools)
+      raise LLM::Error, "invalid value for tools key: '#{tools}'"
+    end
   end
 end

spec/fixtures/skills/inherit-skill/SKILL.md

@@ -0,0 +1,7 @@
+---
+  name: inherit-skill
+  description: Inherit skill
+  tools: inherit
+---
+
+Use inherited tools.

spec/skill_spec.rb

@@ -2,8 +2,6 @@
 
 describe "LLM::Skill" do
   let(:root) { File.join(File.dirname(__FILE__), "fixtures") }
-  let(:skill_dir) { File.join(root, "skills", "frontmatter-skill") }
-  let(:skill) { LLM::Skill.load(skill_dir) }
 
   before do
     Class.new(LLM::Tool) do
@@ -12,20 +10,50 @@
   end
 
   describe ".load" do
-    it "parses indented frontmatter names" do
-      expect(skill.name).must_equal "frontmatter-skill"
-    end
+    describe "with a tool list" do
+      let(:skill_dir) { File.join(root, "skills", "frontmatter-skill") }
+      let(:skill) { LLM::Skill.load(skill_dir) }
 
-    it "parses indented frontmatter descriptions" do
-      expect(skill.description).must_equal "Frontmatter skill"
-    end
+      it "parses indented frontmatter names" do
+        expect(skill.name).must_equal "frontmatter-skill"
+      end
+
+      it "parses indented frontmatter descriptions" do
+        expect(skill.description).must_equal "Frontmatter skill"
+      end
+
+      it "parses indented frontmatter tools" do
+        expect(skill.tools.first.name).must_equal "frontmatter-read-file"
+      end
+
+      it "keeps the body as instructions" do
+        expect(skill.instructions).must_equal "Use frontmatter.\n"
+      end
 
-    it "parses indented frontmatter tools" do
-      expect(skill.tools.first.name).must_equal "frontmatter-read-file"
+      it "does not inherit tools" do
+        expect(skill.inherit_tools?).must_equal false
+      end
     end
 
-    it "keeps the body as instructions" do
-      expect(skill.instructions).must_equal "Use frontmatter.\n"
+    describe "with tools: inherit" do
+      let(:skill_dir) { File.join(root, "skills", "inherit-skill") }
+      let(:skill) { LLM::Skill.load(skill_dir) }
+
+      it "parses indented frontmatter names" do
+        expect(skill.name).must_equal "inherit-skill"
+      end
+
+      it "parses indented frontmatter descriptions" do
+        expect(skill.description).must_equal "Inherit skill"
+      end
+
+      it "returns an empty tool list" do
+        expect(skill.tools).must_be_empty
+      end
+
+      it "inherits tools from parent" do
+        expect(skill.inherit_tools?).must_equal true
+      end
     end
   end
 end