Building a Custom AI Coding Agent with the Claude API and MCP Tools

Fazm Team··3 min read

Building a Custom AI Coding Agent with the Claude API and MCP Tools

Claude Code is impressive out of the box. But if you want an agent that fits your exact workflow - one that knows your deployment process, integrates with your internal tools, and follows your team's conventions - you need to build on the API directly.

Direct API vs Wrapper Approach

Using Claude Code as a black box means accepting its decisions about context management, tool selection, and workflow. Building on the API directly means you control:

  • Context loading - decide exactly what files and documentation enter the context window
  • Tool definitions - create MCP tools that map to your specific infrastructure
  • Conversation flow - manage how the agent plans, executes, and verifies work
  • Cost management - control token usage at a granular level

Custom MCP Tools Are the Key

MCP - Model Context Protocol - lets you define tools that the agent can call. Instead of generic file read/write operations, you create tools specific to your workflow:

  • A deploy tool that runs your exact deployment pipeline
  • A check-ci tool that queries your CI system for build status
  • A query-docs tool that searches your internal documentation
  • A run-migration tool that handles database schema changes safely

Each tool has a defined schema, clear descriptions, and controlled permissions. The agent understands what each tool does and when to use it.

The Architecture That Works

The pattern is straightforward: a thin orchestration layer that manages conversations with the Claude API, a set of MCP tools for your infrastructure, and a memory system that persists context across sessions.

The orchestration layer handles the loop - send a message, receive tool calls, execute them, return results. The MCP tools handle the integration with your actual systems. The memory system ensures the agent does not start from scratch every time.

This is exactly how Fazm works under the hood. Direct API access, custom MCP tools for macOS automation, and persistent memory. The result is an agent that fits the workflow instead of forcing you to adapt to it.


Fazm is an open source macOS AI agent. Open source on GitHub.

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