Merlin – A Lightweight MCP v0.3 HTTP Server

Merlin is a minimal, HTTP‑only implementation of the Model Context Protocol (MCP) designed to bridge AI assistants with external tooling. It focuses on delivering a reliable, authenticated interface for tool invocation while remaining lightweight enough to run in constrained environments or as part of a larger service stack. Though it currently supports only MCP v0.3, Merlin provides the core capabilities that developers need to expose custom tools to an AI client without the overhead of a full‑fledged MCP server.

Why Merlin Matters

Developers building AI assistants often need to integrate external services—databases, APIs, or custom business logic—into the agent’s reasoning loop. MCP defines a standard for this integration, but many existing implementations are either incomplete or require complex deployment setups. Merlin solves this by offering a ready‑to‑use, OAuth‑protected HTTP endpoint that conforms to the MCP specification. This allows teams to:

• Expose tools securely without exposing raw API keys or credentials.
• Keep the integration simple by using standard HTTP request/response patterns, avoiding the need for streaming protocols or WebSocket support.
• Rapidly prototype MCP‑enabled agents in a containerized environment, with minimal configuration.

Core Features and Capabilities

• HTTP Request–Response: Merlin handles MCP calls over plain HTTP, simplifying network communication and compatibility with most infrastructure.
• OAuth Authentication: Built‑in support for OAuth (e.g., Google OAuth) ensures that only authenticated clients can invoke tools, aligning with modern security practices.
• Tool Exposure: The server implements the MCP “tools” endpoint, allowing clients to discover and call custom tool functions defined by the server.
• Falcon Framework: Leveraging Falcon’s minimalist ASGI/WSGI architecture keeps the server lightweight and highly performant.
• Container‑Ready: Merlin is designed to run inside Docker containers, making it easy to deploy in CI/CD pipelines or cloud environments.

Use Cases and Real‑World Scenarios

• Custom Business Logic: A financial advisory AI can call a Merlin server to fetch real‑time market data or execute trades through a secure, authenticated channel.
• Data Retrieval: A knowledge‑base assistant can query a Merlin server that interfaces with internal databases or document stores, keeping sensitive data off the AI’s direct memory.
• Task Automation: Workflow automation bots can invoke Merlin‑exposed tools to trigger downstream processes, such as sending emails or updating tickets.
• Rapid Prototyping: Startups can spin up a Merlin instance quickly to test MCP‑enabled agents before investing in a full production server.

Integration with AI Workflows

Merlin fits naturally into existing MCP client workflows. An AI assistant that supports MCP can treat Merlin as any other tool provider: it sends a JSON request describing the desired function call, receives a JSON response, and continues its reasoning. Because Merlin adheres to MCP v0.3, it is compatible with client SDKs that target this version, ensuring a smooth developer experience.

Unique Advantages

• Simplicity: By omitting streaming and advanced features, Merlin reduces complexity for developers who only need basic tool invocation.
• Security: Built‑in OAuth means authentication is handled out of the box, eliminating the need for custom auth logic.
• Performance: Falcon’s lightweight nature results in low latency, which is critical for real‑time agent interactions.
• Future‑Proofing: While it currently lacks streaming or resource support, Merlin’s architecture allows for easy extension as MCP evolves.

Merlin is not a production‑grade server yet, but it serves as an excellent starting point for developers looking to experiment with MCP‑enabled AI assistants in a controlled, secure environment.