Stop Wasting Tokens: Building Deterministic Custom Agents with Google ADK [GDE]

In the world of AI orchestration, it's tempting to use a Large Language Model (LLM) for every step of a workflow. However, as applications scale, the "LLM-first" approach can introduce unnecessary latency, costs, and unpredictability. The Google Agent Development Kit (ADK) provides a powerful alternative: the BaseAgent. This post explores how to create a custom, programmatic agent—specifically an Email Agent—that handles deterministic tasks while still participating seamlessly in an AI-driven ecosystem. Why is the Email Agent a Custom Agent? The agent is 100% deterministic and relies on external APIs to format text and send emails. It uses the marked library to convert a Markdown string into HTML and the nodemailer library to send mail to an SMTP server. On the contrary, if I created a LlmAgent, the instruction, tool, and structured output would introduce LLM latency, input, and output tokens. The Benefits of Programmatic Execution Using a custom agent instead of an LLM-wrapped prompt