Creating Your First Custom Agent Skill (Hello World)

Building your first custom agent skill should feel simple and repeatable, not mysterious. The goal of a “Hello World” skill is to validate the full lifecycle: a clear use case, a predictable interface, and a clean local install path. Once that pipeline works, you can scale up to more complex skills without rewriting your process every time.

This guide walks through a practical, minimal workflow for shipping a first skill that works locally, integrates cleanly, and is testable. The emphasis is on correctness and clarity, not cleverness.

1. Start with a focused use case

Pick a single action your agent should perform reliably. Examples:

• Format a file with a known style.
• Summarize a local document.
• Query a known data source and return a structured response.

Scope matters. If your first skill tries to solve five problems at once, you will end up with ambiguous inputs and brittle outputs. A narrow scope forces you to design clean parameters and consistent responses.

2. Define inputs and outputs early

Before you write any code, write down:

• Required inputs (strings, file paths, IDs).
• Optional inputs (filters, limits, formatting options).
• Output format (plain text, JSON, or structured objects).

This is your contract. Your agent needs to know what to send and how to interpret the result. A stable interface also makes future refactors safe, because you can update internals without breaking integrations.

3. Use MCP as the transport layer

If your skill communicates with an AI client or agent runtime, use the Model Context Protocol (MCP) to standardize the integration. According to the MCP repository README, the project provides a protocol specification, a schema, and official documentation hosted at modelcontextprotocol.io. The schema is defined in TypeScript and also published as JSON Schema for broad compatibility. That means you can validate requests and responses before they hit your core logic.

For a first skill, focus on a single tool function. Treat it like an API:

• name: stable identifier
• description: short sentence explaining the behavior
• input_schema: explicit JSON schema
• output: predictable structure

MCP encourages contract-first thinking, which is exactly what a first skill needs.

4. Build the simplest possible server

Your first implementation should have:

• A server process that accepts input
• A single handler function
• Minimal dependencies

Whether you write it in Node, Python, or Go, keep the structure flat. Avoid layering, plug-ins, or advanced routing until the pipeline is stable. The “Hello World” skill should be easy to run, easy to stop, and easy to debug.

5. Add local validation before external calls

Defensive programming is non-negotiable. Validate inputs right away:

• Check required fields exist.
• Ensure types match the schema.
• Reject invalid paths or unsupported values.

This prevents failures deep in your logic and makes errors easier to understand. A skill that fails loudly and clearly is far easier to integrate and maintain.

6. Test with a minimal harness

Create a tiny local harness that sends example requests and prints responses. This can be a short script or a small test suite. The goal is to validate:

• The server starts and responds.
• The handler returns the expected structure.
• Errors are returned with helpful messages.

Avoid manual testing in a GUI or IDE at this stage. You want deterministic, repeatable checks.

7. Package with metadata

Every skill should ship with a metadata file (for example, skill.json) that includes:

• id, name, description
• author, version, updated_at
• github_url or source

This file is crucial for discovery and installation. Your marketplace or local indexer should be able to read it without guessing from filenames.

8. Document the usage path

Write a short SKILL.md that covers:

• What the skill does
• Required inputs
• Example usage
• Known limitations

Treat this as part of the deliverable. A skill without clear documentation will be ignored, even if it technically works.

9. Add a basic release checklist

Before you publish:

• Run your local harness tests.
• Verify metadata and version fields.
• Confirm the README and SKILL.md are consistent.
• Ensure the directory structure matches your local install rules.

These small checks prevent most of the “it works on my machine” problems later.

10. Plan the next iteration

Once the Hello World skill works, expand carefully:

• Add one additional option.
• Add one integration point.
• Improve error messages.

Your goal is to grow complexity without sacrificing the stability you just built.

Final thoughts

A first skill should be boring in the best way: predictable, testable, and easy to explain. MCP provides the structure for interoperability, and a strict contract keeps your skill reliable across environments. Start with a tiny scope, validate early, and ship a skill that you can evolve without fear. That foundation will save you far more time than any shortcut later.