Build an AI Agent with Mastra (TypeScript Agent Framework, 2026)

TL;DR — Mastra (from the Gatsby founders, $13M YC W25) shipped v1.0 in January 2026 with 300K weekly npm downloads. It gives you Agents, Workflows, RAG, and Evals as separate primitives over 3,300+ models from 94 providers — all type-safe.

What you'll build

A "support concierge" agent that classifies a customer message, routes to the right specialist agent (billing, tech, scheduling), calls a tool against your CRM, and emits an eval score for every run — all from one TypeScript file.

Prerequisites

1. Node 20+ or Bun 1.3, @mastra/core@^1, @mastra/memory, @ai-sdk/openai@^1.
2. mastra dev CLI for the local Studio.

Architecture

flowchart TD
  IN[User message] --> CLF[Classifier agent]
  CLF -->|billing| B[Billing agent + Stripe tool]
  CLF -->|tech|    T[Tech agent + KB RAG]
  CLF -->|book|    S[Scheduling agent + cal tool]
  B & T & S --> EV[Eval scorer] --> OUT[Reply]

Step 1 — Define an agent

```ts import { Mastra } from "@mastra/core"; import { Agent } from "@mastra/core/agent"; import { openai } from "@ai-sdk/openai";

const billing = new Agent({ name: "billing", instructions: "You handle invoice + refund questions. Always call lookupInvoice first.", model: openai("gpt-4o-mini"), tools: { lookupInvoice }, }); ```

Step 2 — Type-safe tools

```ts import { createTool } from "@mastra/core/tools"; import { z } from "zod";

export const lookupInvoice = createTool({ id: "lookupInvoice", description: "Find an invoice by id", inputSchema: z.object({ id: z.string() }), outputSchema: z.object({ amount: z.number(), status: z.string() }), execute: async ({ context }) => fetch(https://api.stripe.com/v1/invoices/${context.id}, { headers: { Authorization: Bearer ${process.env.STRIPE_KEY} }, }).then((r) => r.json()), }); ```

Step 3 — Workflow with branching

```ts import { createWorkflow, createStep } from "@mastra/core/workflows";

const classify = createStep({ id: "classify", inputSchema: z.object({ message: z.string() }), outputSchema: z.object({ route: z.enum(["billing","tech","book"]) }), execute: async ({ inputData, mastra }) => { const r = await mastra.getAgent("router").generate(inputData.message, { output: z.object({ route: z.enum(["billing","tech","book"]) }) }); return r.object; }, });

export const support = createWorkflow({ id: "support", inputSchema: z.object({ message: z.string() }), outputSchema: z.object({ reply: z.string() }), }) .then(classify) .branch([ [async ({ inputData }) => inputData.route === "billing", billingStep], [async ({ inputData }) => inputData.route === "tech", techStep], [async ({ inputData }) => inputData.route === "book", bookStep], ]) .commit(); ```

Step 4 — Wire RAG + memory

```ts import { Memory } from "@mastra/memory"; import { LibSQLStore } from "@mastra/libsql";

const memory = new Memory({ storage: new LibSQLStore({ url: "file:./mastra.db" }), options: { lastMessages: 10, semanticRecall: { topK: 5, messageRange: 2 } }, }); ```

Step 5 — Add evals

```ts import { createScorer } from "@mastra/core/scores";

const helpfulness = createScorer({ name: "helpfulness", judge: { model: openai("gpt-4o-mini"), instructions: "Score 0-1 on helpfulness." }, }).generateScore(({ run }) => parseFloat(run.text)); ```

Step 6 — Run + observe

```ts const mastra = new Mastra({ agents: { billing, tech, scheduling, router }, workflows: { support }, scorers: { helpfulness }, }); const result = await mastra.getWorkflow("support") .createRun().start({ inputData: { message: "Where is invoice in_123?" } }); ```

Pitfalls

• Apache 2 core, commercial enterprise — RBAC/SSO/ACL need a paid license at scale.
• Mastra Cloud pricing TBA — public pricing was promised Q1 2026 but wasn't published as of May 2026; budget conservatively.
• mastra dev ports — defaults to 4111; collisions with Vite + Next dev servers are common.

How CallSphere does this in production

Explore a live demo and compare current plans to find the right fit for your business.

FAQ

Mastra vs LangGraph? Mastra is opinionated TypeScript-first; LangGraph is graph-first and has a Python sibling. Mastra has cleaner DX for serverless deploys.

Does it support tool streaming? Yes — agent.streamVNext returns deltas including tool-call and tool-result events.

Can I run on Cloudflare Workers? Yes via the @mastra/deployer-cloudflare adapter (1.0+).

RAG store options? LibSQL (local), Postgres + pgvector, Pinecone, Chroma — all behind MastraVector.

Sources

• Mastra docs - https://mastra.ai/docs
• Mastra GitHub - https://github.com/mastra-ai/mastra
• Generative.inc - Mastra Complete Guide 2026 - https://www.generative.inc/mastra-ai-the-complete-guide-to-the-typescript-agent-framework-2026
• Firecrawl - Mastra Tutorial - https://www.firecrawl.dev/blog/mastra-tutorial