MCP or REST? Ask who is doing the choosing.
This question gets debated as though it were about protocols. It is not. It is about where the decision lives. If a model decides which operation to run based on what somebody asked for, you want MCP. If your own code already knows the sequence, you want a POST request and nothing else.
Everything below follows from that one distinction.
Use MCP
when the model is the caller
- A user asks for something in natural language and the agent has to work out which tool fits.
- You want the same capability across several clients without writing an adapter for each.
- New tools should appear without you shipping a client update.
- You want the model to see JSON schemas rather than interpret prose documentation.
# one entry, then the agent takes over
{
"imagemcp": {
"url": "https://mcp.imagemcpserver.com/mcp",
"headers": { "x-api-key": "sk-img-gen-…" }
}
}Use REST
when your code is the caller
- A build step, a queue worker, a cron job — anywhere there is no model in the loop.
- You already know the sequence and there is nothing to discover.
- You want the fewest moving parts between your function and the work.
- You are calling from a language or runtime with no MCP client to hand.
await fetch(api + "/playground/generate", {
method: "POST",
headers: {
Authorization: `Bearer ${key}`,
"Content-Type": "application/json",
},
body: JSON.stringify({ prompt, aspectRatio }),
});Side by side
| MCP | REST | |
|---|---|---|
| Who decides what to call | The model, at runtime | Your code, at write time |
| How capabilities are found | Discovered from the server | Read from documentation by you |
| Argument correctness | Schema-guided; the model sees the enums | Your responsibility |
| Adding a new tool | Appears in the client automatically | Requires a code change |
| Setup cost | A config entry per client | None beyond an HTTP call |
| Works in a cron job | Awkward — needs a client | Naturally |
| Works inside a chat | Naturally | Not without a wrapper |
| Credits and logging | Identical | Identical |
The last row is the one worth noticing. Both routes hit the same handlers, deduct the same credits and write the same entry to your request log — so the choice is about ergonomics, not about capability or price.
In practice, most teams end up using both
Not as a hedge — because the two halves of the work genuinely have different shapes, and the same API key covers both.
Development
MCP in the editor
You are exploring. The agent should be able to try generate_transparent_image without you having read its parameters.
Production
REST in the service
The sequence is decided and tested. A protocol layer would add moving parts and buy nothing.
Automation
CLI in CI
A shell command that prints JSON, with the key in an environment variable. No bridge process to babysit.
MCP vs REST questions
Is MCP a replacement for a REST API?
No. They answer different questions. REST is how a program calls a service. MCP is how a model discovers and chooses a capability at runtime. A service that only offers MCP is hard to use from a cron job; one that only offers REST is hard for an agent to discover.
Does MCP cost more or run slower?
The billed work is identical — same operation, same credits, same log entry. MCP adds a thin protocol layer and, for stdio transports, a local process. That overhead is negligible next to the seconds an image model takes to respond.
Can I use the same API key for both?
Yes. One key authenticates the REST endpoints, the remote MCP endpoint and the CLI. That is deliberate — you should be able to prototype in your editor and ship to production without re-issuing credentials.
Which should I start with?
Whichever gets you to a real image fastest. If you have an agent already, connect MCP and ask it for a picture. If you are writing a script, POST to the endpoint. Neither choice locks you out of the other later.
What about the CLI — where does that fit?
It is a third door onto the same building, for agents that can run shell commands but do not speak MCP. It prints structured JSON, needs no dependencies, and works fine in CI where a bridge process would be awkward.