Build your own AI agent: three routes, but ChatGPT GPTs end December 11, 2026

The guide frames tool choice as the deciding factor, warning that mismatched picks lead to days of setup that still does not work or continued billing on a paid plan the user does not need. It groups agents by what the user wants to hand off: conversation-bound work like summarizing or drafting, recurring work that fires on a time or event, and work that touches files or custom processing. ChatGPT's custom GPTs, once a default starting point, can no longer be created on personal accounts and are set to shut down on December 11, 2026; Gemini's Gems on personal accounts are scheduled to migrate to Skills from November 2026. The author argues the agent's value lives in the written instructions rather than the tool, so prompts and design notes should be kept locally as text and reused if a platform changes.

Before building, the piece says to narrow the job to one task, split what the AI handles from what a person checks, and fix the input and output format in advance. For irreversible actions like sending mail or deleting files, it recommends keeping a human approval step, and it warns that agents reading web pages or email can be steered by hidden instructions, an attack known as prompt injection.