Extending Loopy

Loopy has five extension points, from zero-code Markdown files to full Java SPIs.

Skills

Domain knowledge the agent reads on demand

Slash Commands

Markdown-defined commands from files

Subagents

Specialist agents for delegation

Tool Profiles

New tools via Java SPI

Skills

Skills are the fastest way to make Loopy smarter. A skill is a Markdown file with YAML frontmatter. The agent sees skill names and descriptions upfront but only loads full content when relevant — no tokens wasted.

Create a skill

Place a SKILL.md in your project:

.claude/skills/my-conventions/SKILL.md

---
name: my-conventions
description: Team coding conventions for our Spring Boot services
---

# Instructions

When working on this codebase:
- Use constructor injection, never field injection
- All REST endpoints return ProblemDetail for errors (RFC 9457)
- Tests use @WebMvcTest with MockMvc, not @SpringBootTest
- Entity IDs are UUIDs, never auto-increment

Next time you run Loopy in that directory, the agent discovers the skill automatically.

Where skills live

Location	Path	Use case
Project	`.claude/skills/*/SKILL.md`	Team conventions, checked into the repo
Global	`~/.claude/skills/*/SKILL.md`	Personal skills across all projects
Classpath	`META-INF/skills/*/SKILL.md` in JARs	Published skill packages (Maven dep)

Install from the catalog

Loopy ships with 23+ curated skills from 8 publishers:

/skills search testing
/skills info systematic-debugging
/skills add systematic-debugging

Publish skills as a JAR (SkillsJars)

Package skills as a Maven dependency so teams get them automatically:

my-skills.jar
└── META-INF/skills/
    └── my-org/my-repo/api-design/
        └── SKILL.md

Add the JAR to pom.xml and Loopy discovers it on the classpath. Skills follow the agentskills.io spec — they work in 40+ agentic CLIs, not just Loopy.

Slash Commands

Slash commands let you define reusable prompts as Markdown files, following Claude Code’s ~/.claude/commands/ convention. Type /command-name in the chat and the agent executes the expanded prompt.

Create a slash command

Create a .md file in your project or home directory:

.claude/commands/review.md

---
name: review
description: "Code review the current diff"
---

Review the staged changes (`git diff --cached`). For each file:

1. Check for bugs, security issues, and performance problems
2. Verify test coverage for new code paths
3. Flag any style violations

Summarize findings as a table. $ARGUMENTS

How it works

YAML front matter — name and description fields. If no front matter, the filename becomes the name.
$ARGUMENTS substitution — replaced with whatever the user types after the command. /review focus on error handling substitutes “focus on error handling”.
Agent delegation — the expanded prompt is sent to the agent as if the user typed it.

Where commands are discovered

Location	Path
Project	`.claude/commands/*.md`
Global	`~/.claude/commands/*.md`

Markdown commands override built-in Java commands with the same name.

Subagents

Subagents are specialist agents the main agent delegates to via the Task tool. Define them as Markdown files — no Java required.

Create a subagent

Create .claude/agents/test-runner.md:

---
name: test-runner
description: Runs tests and reports pass/fail summary
tools: Bash, Read
---

You are a testing specialist. When invoked:

1. Run `./mvnw test` in the working directory
2. Parse output for pass/fail/skip counts
3. If tests fail, read the relevant test source files
4. Report a concise summary: what passed, what failed, and why

The main agent delegates testing tasks to this subagent automatically based on the description field.

Frontmatter fields

Field	Required	Description
`name`	Yes	Unique identifier (lowercase, hyphens)
`description`	Yes	When to use — the main agent reads this to decide
`tools`	No	Allowed tools (comma-separated). Inherits all if omitted
`model`	No	`haiku`, `sonnet`, or `opus`

Tips

Keep description specific — “Runs tests and reports results” routes better than “helps with testing”
Restrict tools to what the subagent needs. A test runner doesn’t need Edit
Subagents run in isolated context windows — they can’t see the main conversation
Subagents cannot spawn other subagents (the Task tool is excluded automatically)

Tool Profiles (Java SPI)

Add new tools the agent can call. Implement a Java interface, package as a JAR, and Loopy discovers it at startup via ServiceLoader.

Implement ToolProfileContributor

public class DatabaseToolProfile implements ToolProfileContributor {

    @Override
    public String profileName() {
        return "database-tools";
    }

    @Override
    public List<ToolCallback> tools(ToolFactoryContext ctx) {
        return List.of(
            ToolCallbacks.from(new QueryTool(ctx.workingDirectory())),
            ToolCallbacks.from(new SchemaTool(ctx.workingDirectory()))
        );
    }
}

Register via ServiceLoader

Create META-INF/services/io.github.markpollack.loopy.tools.ToolProfileContributor:

com.example.tools.DatabaseToolProfile

What ToolFactoryContext provides

Field	Type	Description
`workingDirectory`	`Path`	Agent’s working directory
`chatModel`	`ChatModel`	The active LLM (for tools that need AI)
`commandTimeout`	`Duration`	Tool execution timeout (default 120s)
`interactive`	`boolean`	True in TUI mode, false in print/REPL

Built-in profiles

Profile	Description
`dev`	Full interactive toolset (bash, file I/O, search, skills, subagents)
`boot`	Spring Boot scaffolding tools
`headless`	Same as `dev` minus `AskUserQuestion` (for CI/CD)
`readonly`	Read-only: file read, grep, glob, list directory

Custom profiles load alongside built-in profiles, not replacing them.

Listeners

Observe what the agent does without changing its behavior.

ToolCallListener

Fires around every tool execution:

public class CostTracker implements ToolCallListener {

    @Override
    public void onToolExecutionCompleted(String runId, int turn,
            AssistantMessage.ToolCall toolCall, String result,
            Duration duration) {
        log.info("Tool {} took {}ms", toolCall.name(), duration.toMillis());
    }
}

AgentLoopListener

Fires at loop lifecycle boundaries:

public class ProgressReporter implements AgentLoopListener {

    @Override
    public void onLoopCompleted(String runId, LoopState state,
            TerminationReason reason) {
        System.err.printf("Done: %s (%d turns)%n", reason, state.turns());
    }
}

Wire listeners through the MiniAgent builder:

var agent = MiniAgent.builder()
    .config(config)
    .model(chatModel)
    .toolCallListener(new CostTracker())
    .loopListener(new ProgressReporter())
    .build();

Listener methods should not throw exceptions — exceptions are logged and swallowed to avoid crashing the agent loop.

Programmatic API

Embed Loopy’s agent in other Java applications:

LoopyAgent agent = LoopyAgent.builder()
    .workingDirectory(Path.of("/path/to/project"))
    .build();

LoopyResult result = agent.run("add input validation to UserController");

Multi-step with session memory

Context is preserved across run() calls by default:

LoopyAgent agent = LoopyAgent.builder()
    .workingDirectory(workspace)
    .maxTurns(80)
    .build();

agent.run("plan a refactoring of the service layer");
agent.run("now execute the plan");  // sees the previous conversation

Builder options

Method	Description	Default
`.model(String)`	Model ID	`claude-sonnet-4-6`
`.workingDirectory(Path)`	Agent’s working directory	required
`.systemPrompt(String)`	Custom system prompt	Built-in coding prompt
`.maxTurns(int)`	Max loop iterations	`80`
`.costLimit(double)`	Max cost in dollars	`$5.00`
`.sessionMemory(boolean)`	Preserve context across `run()` calls	`true`
`.timeout(Duration)`	Overall loop timeout	`10 min`
`.disabledTools(Set<String>)`	Tools to exclude	none

Custom endpoints (vLLM, LM Studio)

LoopyAgent agent = LoopyAgent.builder()
    .baseUrl("http://localhost:1234/v1")
    .apiKey("lm-studio")
    .model("local-model")
    .workingDirectory(workspace)
    .build();

Projects

AgentWorks

Agento

Supporting Projects

Migration

Skills

Slash Commands

Subagents

Tool Profiles

Skills

Create a skill

Where skills live

Install from the catalog

Publish skills as a JAR (SkillsJars)

Slash Commands

Create a slash command

How it works

Where commands are discovered

Subagents

Create a subagent

Frontmatter fields

Tips

Tool Profiles (Java SPI)

Implement ToolProfileContributor

Register via ServiceLoader

What ToolFactoryContext provides

Built-in profiles

Listeners

ToolCallListener

AgentLoopListener

Programmatic API

Multi-step with session memory

Builder options

Custom endpoints (vLLM, LM Studio)

Skills

Slash Commands

Subagents

Tool Profiles

​Skills

​Create a skill

​Where skills live

​Install from the catalog

​Publish skills as a JAR (SkillsJars)

​Slash Commands

​Create a slash command

​How it works

​Where commands are discovered

​Subagents

​Create a subagent

​Frontmatter fields

​Tips

​Tool Profiles (Java SPI)

​Implement ToolProfileContributor

​Register via ServiceLoader

​What ToolFactoryContext provides

​Built-in profiles

​Listeners

​ToolCallListener

​AgentLoopListener

​Programmatic API

​Multi-step with session memory

​Builder options

​Custom endpoints (vLLM, LM Studio)

Skills

Create a skill

Where skills live

Install from the catalog

Publish skills as a JAR (SkillsJars)

Slash Commands

Create a slash command

How it works

Where commands are discovered

Subagents

Create a subagent

Frontmatter fields

Tips

Tool Profiles (Java SPI)

Implement ToolProfileContributor

Register via ServiceLoader

What ToolFactoryContext provides

Built-in profiles

Listeners

ToolCallListener

AgentLoopListener

Programmatic API

Multi-step with session memory

Builder options

Custom endpoints (vLLM, LM Studio)