# MCP vs. REST API for email marketing: Which workflow should you use?

Canonical: https://brew.new/blog/mcp-vs-rest-api-for-email-marketing-which-workflow-should-you-use
Author: Thomas Park
Published: 2026-09-23
Updated: 2026-09-29

A developer on your team connects the email platform to Claude and drafts next week's campaign in a chat window. The next morning someone asks whether the signup welcome email could run the same way, through the assistant. The platform's docs list an MCP server and a REST API side by side.

Most email platforms that support agents now offer these two ways in. An MCP server lets an AI assistant call the platform's tools in conversation. A REST API exposes operations your code can call directly. They often reach the same endpoints underneath, so the choice is about who is doing the work and when.

Use MCP when a person is working through an assistant, and use the REST API when your product needs to send email without anyone watching. Most teams end up using both.

## Key takeaways

- MCP is for interactive, judgment-heavy work: drafting a campaign, building an audience, asking why a send underperformed.
- The REST API is for production paths: firing a trigger when a user signs up, syncing contacts, sending receipts.
- A CLI sits between the two. It is scriptable like the API and readable by coding agents.
- Brew's MCP requires live-send confirmation on OAuth and organization connections. Brand API-key clients need their own approval policy. Your API code has to supply its own review, idempotency and retries.

## What each one is

MCP, the Model Context Protocol, is an open standard for exposing tools to AI clients such as Claude, ChatGPT and Cursor. The client reads a list of tools with descriptions and schemas, then picks which to call based on your request. Brew's server is at `https://brew.new/api/mcp` and supports OAuth, so you sign in and pick a brand instead of handling keys.

A REST API is a set of HTTP endpoints your code calls. Brew publishes an OpenAPI spec, a TypeScript SDK, and a plain-text [agent guide](https://brew.new/api/v1/llms.txt) to its API. Every call carries an API key, and your code decides what to call and when.

Other vendors follow the same pattern. Customer.io's MCP server wraps its API in [separate read, write and delete tools](https://docs.customer.io/ai/mcp/get-started/). Brevo generates its MCP tools from its [OpenAPI spec](https://developers.brevo.com/docs/mcp-protocol).

## A side-by-side comparison

| | MCP | REST API |
| --- | --- | --- |
| Who drives | A person, through an assistant | Your code |
| Typical tasks | Draft and revise emails, build audiences, schedule a campaign, read analytics | Fire event triggers, upsert contacts, send transactional email, poll send status |
| Authentication | OAuth sign-in, or an API key for clients without OAuth | API key, scoped to a brand or an organization |
| Safety | Brew pauses live sends for confirmation on OAuth and organization connections | Your code decides; use idempotency keys and your own review steps |
| Failure handling | The assistant reads the error and tries again or asks you | You branch on stable error codes and retry with backoff |
| Response size | Limited by the assistant's context window | Limited only by pagination |
| Best when | The task needs judgment or iteration | The task must run the same way every time |

## When MCP is the right choice

Use MCP when the work is a conversation:

- Campaign production. "Write the October product update from this changelog, send me a test, and schedule it for Tuesday at 9." The assistant calls `create_email`, `edit_email`, `send_email` with `test: true`, then `send_email` with `scheduledAt`, and stops for your approval.
- Audience questions. "How many Pro customers signed up before July and opened anything last month?" The assistant combines `search_contacts`, field lookups and `create_audience`.
- Analysis. "Which automation has the worst click rate, and what changed?" The assistant reads `get_email_analytics` and automation performance.
- One-off setup. Creating a trigger and an automation once, with a person reviewing each step.

In Brew, emails generated over MCP use the same brand document as emails generated in the app. The [Brew MCP post](/blog/brew-mcp) covers the client setup.

MCP responses have to fit in the assistant's context. Brew's [flows post](/blog/email-flows) notes that asking for full HTML over MCP returns only the first step or two of a long sequence, while the REST route returns every body.

## When the REST API is the right choice

Use the API when email is part of your product:

- Event triggers. When a user signs up, your backend calls `POST /v1/automations/triggers/{triggerEventId}/fire` with the user's email and fields. The published automation handles the rest.
- Transactional email. Receipts and password resets fire the same way, through an automation on a transactional-purpose domain. See [transactional emails](/blog/transactional-emails).
- Contact sync. `POST /v1/contacts` upserts up to 1,000 contacts per request.
- Monitoring. `GET /v1/sends/{sendId}` returns status and stats for a dashboard or an alert.

On endpoints that support it, reuse the same `Idempotency-Key` and request body for retries within the documented 24-hour window. Use a different key for a different logical operation; read the endpoint contract before assuming a write is covered. Rate-limit headers tell you when to back off. API error responses include a `code`, such as `CONSENT_REQUIRED`, that you can branch on. Handle network failures separately. Brew's [API guide](https://brew.new/api/v1/llms.txt) describes all of these.

## Where the CLI fits

Coding agents in a terminal often do better with a CLI than with either option. Brew's [CLI](https://docs.brew.new/api-reference/cli/agents) returns the API response as one JSON line when it is not attached to a terminal, and uses exit codes an agent can act on. Code 4 means confirmation is required, and the CLI prints the exact command to re-run with `--yes` after approval. It also generates typed payload contracts with `brew-cli types`, which keeps your backend's trigger calls in step with the trigger's schema.

## A combined workflow

Here is one way a SaaS team might split the work:

1. In Cursor or Claude, a developer uses MCP to create the `user_signed_up` trigger and a welcome automation, tests it with `test_automation`, and publishes it.
2. The backend fires the trigger through the REST API on every signup, with an idempotency key built from the user ID.
3. Each month, a marketer asks Claude over MCP to draft and schedule the product update.
4. A scheduled application job checks current activation or billing state before firing a separate eligible-reminder event. Syncing contact fields alone does not update a running flow's original event payload.

In steps 1 and 3 a person reviews the assistant's work. Steps 2 and 4 run on every signup or every night without anyone watching.

## Frequently asked questions

### Is MCP slower or less reliable than the API?

It adds an assistant between you and the endpoint, so a request takes more steps, and the assistant can choose the wrong tool. That is fine for interactive work and wrong for a signup event that must fire every time.


### Which is safer?

Brew requires live-send confirmation on OAuth and organization connections. With brand API-key clients, configure your own approval policy. With the API, protect keys in environment variables, use brand-scoped keys where you can, and add idempotency keys to every retried write.

### Can an agent in my own product use the API as tools?

Yes. Brew's [agent integration guide](https://docs.brew.new/integrations/ai-agents/use-brew-with-ai-agents) describes wiring API endpoints as function-calling tools, with the SDK handling authentication and retries.

## Next step

To try both halves, connect the [MCP server](/mcp) in your assistant for campaign work, and create one API key for the trigger your backend will fire.