Last updated: August 2026

API vs MCP for posting: which should an agent use?

Use REST when the caller is your code: cron jobs, backend services, anything deterministic that needs retries, idempotency and logs. Use MCP when the caller is a model that should discover tools at runtime. MCP is a tool-discovery layer over HTTP, not a replacement for it, and most agents end up using both.

The question is not which protocol is better. It is which part of your system is holding the decision at the moment the post gets sent.

MCP sits on top of an API, it does not replace one

This is the misconception worth clearing first. MCP is a protocol for describing tools to a model and carrying calls to them. Behind almost every MCP server is an ordinary HTTP API. The OpenTweet MCP server calls the same endpoints documented in the REST reference. It is a different front door to the same building.

Two consequences follow. If a capability is missing from the underlying API, wrapping it in MCP will not create it. And choosing MCP does not remove the API from your architecture, it adds a layer with its own failure modes: a session that dropped, a tool list the client cached, a model that picked the wrong tool.

The one-line version

REST is an instruction. MCP is an offer. With REST you tell the service exactly what to do. With MCP you tell a model what it is allowed to do and let it choose. Pick based on whether the decision at that point in your system should be fixed or judged.

Nine axes that actually differ

REST API
MCP
Who decides what to send
Your code. The arguments are whatever you wrote.
The model. Arguments are generated per call.
Determinism
Same input, same request, every run.
Same prompt can produce different arguments.
Retries and idempotency
Send an Idempotency-Key and retry safely.
A retried tool call is a second model decision.
Tool discovery
You read the docs and hardcode the shape.
The client lists tools at runtime and adapts.
Adding a capability
You write the calling code for each new endpoint.
The server adds a tool, clients pick it up.
Latency per action
One HTTP round trip.
Handshake, tool listing, model choice, then the call.
Testing
curl, fixtures, assertions.
Needs a model in the loop or a stubbed client.
Observability
Your logs, your tracing, your error handling.
The client owns the transcript. Server-side logs only see the tool call.
Auth
Bearer header from your secret store.
Bearer header in the client config, which lives on the user machine.

The rows that decide most arguments are determinism, retries, and testing. Everything else is a preference. Those three are the ones that show up as an incident six months later.

Five real cases and the call

A cron that posts a daily changelog

REST

Nothing here needs judgement. You know the text, the time, and the account. Adding a model to the path adds cost, latency, and a way for the message to come out wrong.

A Claude or Cursor session where you draft and publish

MCP

The caller is a client you do not control and the whole value is that the model can compose, revise, and then act. Paste one URL and it has post, thread, and schedule tools.

An agent that reads support tickets and drafts posts for approval

Both

MCP for the drafting and the schedule call, REST for the queue your reviewers see and for the audit trail. The agent schedules, your backend reads and writes the same posts.

Bulk publishing 500 queued posts

REST

Per-call model overhead multiplied by 500 is real money and real time, and you want idempotency keys so a partial failure is safe to rerun. This is a batch job, not a conversation.

A product feature that posts on behalf of your users

REST

You need per-user error handling, retries, and webhooks that tell your system when a post went live. That is a backend integration with an HTTP API, whatever your agent stack looks like.

Retries are where the difference gets expensive

Over REST a retry is free of judgement. You send the same body with the same Idempotency-Key and the service either performs the action or returns the result of the one it already performed. Nothing new gets decided.

retry-safe.sh
# Same key, same body: retry as many times as you like.
curl -X POST https://opentweet.io/api/v1/posts \
  -H "Authorization: Bearer ot_your_key" \
  -H "Idempotency-Key: changelog-2026-08-31" \
  -H "Content-Type: application/json" \
  -d '{"text":"Changelog for 31 August is up.","publish_now":true}'

Over MCP a retry is a second model turn. The model may produce different text, call a different tool, or decide the previous attempt succeeded when it did not. That is not a flaw in MCP, it is what a non-deterministic caller means. It is also why the durable state, the queue and the audit trail, belongs on the API side even when the agent is doing the driving.

The same argument applies to notifications. If your system needs to know a post went live, subscribe a webhook on the API rather than asking the agent to report back, because a webhook fires whether or not a session is still open.

The both answer, concretely

The shape that holds up looks like this. Your agent surface, whether that is Claude, Cursor, or your own loop, talks MCP: it drafts, it revises, it schedules. Your backend talks REST: it enforces the queue, it retries, it logs, it reacts to webhooks. They operate on the same posts because they use the same key and the same account.

both.json
// Agent surface: one URL, tools discovered at runtime
{
  "mcpServers": {
    "opentweet": {
      "type": "streamable-http",
      "url": "https://mcp.opentweet.io/mcp",
      "headers": { "Authorization": "Bearer ot_your_key" }
    }
  }
}

// Backend: the same key, a fixed endpoint, your retry policy
// POST https://opentweet.io/api/v1/posts
// Authorization: Bearer ot_your_key

A post the agent scheduled shows up in a GET from your service, and a post your service queued is visible to the agent. That is the practical reason to pick a provider that ships both rather than an MCP-only or API-only tool. If you are also weighing workflow platforms, the comparison is in MCP vs Zapier vs n8n, and the agent-side setup is in give an MCP agent a Twitter tool.

One thing MCP does not decide for you

Whichever protocol you pick, the cost model is set by whose X connection you are using. Calling the X API directly bills per post and about $0.20 for a post with a link, over REST and over MCP alike. Flat-fee posting removes that variable from both paths. See the X API spend cap.

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Frequently asked questions

Should my agent post over a REST API or over MCP?

Use REST when your code decides what to post: cron jobs, backend services, anything that needs deterministic retries, idempotency keys, and a log line per call. Use MCP when a model decides at runtime and should discover the available tools itself. Most production systems use both, MCP in the chat surface and REST in the scheduled path.

Is MCP a replacement for a REST API?

No. MCP is a tool-discovery and transport protocol that sits in front of an API, it does not replace one. The OpenTweet MCP server calls the same endpoints the REST API exposes. If a capability is not in the underlying API, wrapping it in MCP does not create it.

Is MCP slower than calling a REST endpoint?

For the same action, yes, because MCP adds a session handshake and a tool listing before the call, and the model has to choose the tool and produce arguments. For an interactive agent that overhead is invisible. For a batch job publishing 500 posts, it is pure waste, and REST is the right call.

Which is easier to test, an API or an MCP tool?

REST, clearly. You can curl it, record fixtures, and assert on the response. MCP tool calls go through a model, so the same input can produce different arguments on different runs. Keep anything you need to assert on in the deterministic REST path and let MCP handle the parts where judgement is the point.

Can I use both MCP and the REST API with one key?

Yes. One ot_ key works as an Authorization header on https://opentweet.io/api/v1/posts and as the bearer header on the hosted MCP endpoint at https://mcp.opentweet.io/mcp. Same account, same posts, same schedule, so a post queued by an agent is visible to your backend and the other way around.

When is MCP clearly the better choice for posting?

When the caller is an AI client you do not control, such as Claude, Cursor, or ChatGPT in Developer Mode. Writing an integration for each of those is work you skip entirely if they all speak MCP. You paste one URL and every client gets the same tools.

Keep exploring

One key, a REST endpoint and a hosted MCP server. Use either, or both.

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You do not have to choose

The same ot_ key drives the REST endpoint and the hosted MCP server. Flat $11.99 a month, no X developer app.

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