In 2025, everyone wants to build agents. Software development still happens but the demand has shifted away from just apps and websites.
So Replit built the first agent that builds agents.
And Replit's Agent 3 builds Mastra agents specifically.
Matt Palmer from Replit recently explained that Agent3 can build tools like:
- An inbox manager that automatically sorts and prioritizes emails
- A support chatbot that queries your internal databases
- A daily tech digest that fetches RSS feeds, summarizes articles with AI, and emails formatted newsletters
- Slack bots that retrieve and query information from your Notion database
- Telegram bots that schedule appointments directly on your calendar
- Automated meeting prep emails that research your guests and save notes to Google Drive
How It Works
When you select the "Agents & Automations" option from the Replit homepage, you describe what you want in natural language. Unlike building regular apps in Replit, there's no preview—you'll deploy Agents & Automations to use them in another surface via an integration (e.g., in Telegram, Slack, or email).
On one side of the workspace you'll see the Agent pane, and on the other side, instead of a preview, you'll see an admin dashboard where you can test your chatbot or workflow automation.
Agent 3 also has a self-testing and debugging loop. Once it generates code, the agent executes it, identifies errors, applies fixes, and reruns the code until it passes tests or meets the specified requirements. Replit says this process is up to 3x faster and 10x more cost-effective than comparable models.
How Mastra powers Agent 3
As Palmer explains in his video, Replit Agent 3 creates a clean Mastra project structure with three components:
- Agents: AI assistants with specific prompts and access to tools (defined as simple TypeScript objects with name, description, instructions, and model)
- Tools: Functions that agents can call, which can themselves call other agents in a composable architecture
- Workflows: Collections of steps that orchestrate agents and tools with durable execution via Inngest
Palmer's explains how he built a Hacker News digest automation in just three prompts. The workflow fetches RSS feeds, uses Jina Reader API to extract markdown from articles, passes content to GPT-5 for summarization, and sends formatted emails via Replit Mail — all without setting up any email infrastructure.
As for the deployment process? Just click "publish automation," set your schedule (like "every Friday at 9am PST"), and Replit bundles everything to run in the cloud.
The veterans who have been along the agentic AI ride for a while now (and even a few months is long enough) will tell you that Replit’s new release is a huge milestone.
If you're still skeptical, look at this chart and then get back to me…