CLI vs MCP vs API for AI Agents: Which One Should You Use?

If you want an AI agent to humanize, paraphrase, grammar-check or detect text, SupWriter gives you three doors into the same engines: a Model Context Protocol server, a command-line tool, and a REST API. They cost the same, they draw on the same word credits, and they produce the same output.

They are not interchangeable, though. Picking the wrong one usually means writing a few hundred lines of glue that a different door would have handled for free — or worse, wiring an integration your agent cannot actually reach.

Here is how to choose, including what has changed in the agent landscape through 2026.

The 30-Second Answer

If you only remember one line: a chat agent wants MCP, an agent with a shell wants the CLI, and your own product wants the API.

What Each One Actually Is

MCP is an open protocol for describing tools to a language model. You add SupWriter as a connector once, and the client tells the model that four tools exist, what arguments they take, and when to use them. The model calls them on its own. There is nothing to install and no code to write.

The CLI is sw, a Node program installed with npm install -g supwriter. It reads standard input, writes standard output, handles authentication, splits long documents, retries rate limits, and exits with a distinct code per failure class. Anything that can run a shell command can use it.

The API is five HTTPS endpoints — /humanize, /paraphrase, /grammar, /detect, and /me for plan and credit information. You send JSON with a bearer token and get JSON back. Maximum control, maximum work.

All three are documented for machines as well as people: supwriter.com/skill.md is a single markdown file describing the whole API, written to be handed straight to a coding agent.

MCP: When Your Agent Lives in a Chat Window

MCP won the standardisation argument. By 2026 it has crossed 97 million monthly SDK downloads with over 10,000 public servers in production, with native support across Anthropic, OpenAI, Google and Microsoft models, and it is the default tool-calling pattern in LangChain, CrewAI, LangGraph and LlamaIndex.

For SupWriter specifically, MCP is the right pick when the human is in the loop and the interface is conversational. You say "clean this up so it doesn't read like ChatGPT wrote it" and the model decides to call humanize with the right tone. You never see an endpoint.

The practical advantages:

• Zero install. Paste a URL into your client's connector settings.
• Discovery is free. You do not tell the model the tools exist; the protocol does.
• Works on every plan. Base MCP over sign-in is available to all SupWriter accounts, not just paid ones — unlike the API and the CLI, which need Pro or Ultra.

The limits are real too. MCP is scoped to a client session, so it is awkward for anything unattended — no cron job, no CI check, no batch of 200 files. And the model decides when to call a tool, which is exactly what you want in a conversation and exactly what you do not want in a pipeline that must process every file every time.

We have per-client setup guides for Claude, ChatGPT and Cursor and VS Code, plus an overview on the MCP page.

CLI: When Your Agent Has a Shell

This is the category that grew fastest in 2026, and the one people underestimate.

A wave of autonomous agents now runs as a persistent process on hardware you control, with permission to execute shell commands. OpenClaw — the open-source assistant from PSPDFKit founder Peter Steinberger, past 68,000 GitHub stars and renamed from Clawdbot in January 2026 — runs as a long-running Node service that connects chat apps to an agent which can run shell commands, drive a browser and manage local files. Hermes Agent from Nous Research, released in February 2026, is a persistent daemon with code execution, sandboxed backends and the ability to write its own reusable skills. Grok Bot, which xAI launched in August 2026, gives each agent its own persistent cloud computer and is explicitly designed to operate tools the way a person would rather than depending on formal integrations.

Every one of those can run sw the moment it is installed. No connector, no client library, no schema.

That is not an accident of our design — it is where the ecosystem landed. Analyses of agent tooling in 2026 keep reaching the same conclusion: agents favour CLIs because a CLI handles auth once, exposes a surface that does not shift between API versions, and signals failure with an exit code rather than prose an agent has to interpret. Firecrawl's survey of agent CLI tools makes the same point about execution feedback — when a tool cannot tell an agent whether it succeeded, reliability collapses into guesswork about timeouts.

The CLI is also the only one of the three that solves problems you would otherwise solve yourself:

• Chunking. The API rejects anything over your per-request word cap outright. The CLI splits on paragraph boundaries and reassembles with the original spacing.
• Batching. sw humanize 'drafts/*.md' --out-dir out/ processes a folder in parallel.
• A build gate. sw detect 'content/**/*.md' --max-score 30 exits non-zero when content reads as machine-written, which makes AI detection a CI check rather than a post-mortem.
• Rate limits. Honours Retry-After on 429 and backs off on server errors.

The CLI post covers all of that properly, and the CLI page has the reference.

The catch: it needs Node 20.10 or newer, a shell, and a Pro or Ultra key.

API: When You Are Building the Product

If SupWriter is a feature inside something you ship, use the API directly. Not because the CLI cannot do it, but because you want control the CLI deliberately abstracts away — your own retry policy, your own error surfacing, your own progress UI, your own decisions about what to do when a user runs out of credits.

The API is also the right answer for a background worker with no terminal, a serverless function where spawning a process is awkward, or any runtime where you would rather not manage a globally installed binary that updates on its own schedule.

Practically: bearer token, JSON in, JSON out, a success and data envelope, and error.code values you can map to typed errors. GET /me reports plan, credit balance and the per-request word cap, so you can split long inputs before they are rejected instead of after.

Full reference lives in the developer docs. If you would rather not write the client yourself, hand your coding agent supwriter.com/skill.md and ask it to build one — that file exists for exactly this.

Side by Side

Rate limits and billing are identical across all three: one word credit per word, 60 requests a minute on Pro and 120 on Ultra, per key. AI detection is metered on all three — it is free only in the web app.

You Can Use More Than One

These are not exclusive, and most serious setups end up with two.

A realistic content team runs MCP in the editor so writers can clean up a paragraph mid-draft without leaving the document, and the CLI in CI so nothing reaches production above a detection threshold. A product team ships the API inside their app and still installs the CLI locally for one-off batch jobs.

There is a reasonable argument that this is now the expected shape for a developer tool. The 2026 consensus in agent tooling is that products shipping a CLI alongside a published agent skill or MCP server get adopted faster than products shipping only a dashboard and an SDK — because the agent doing the integration reaches for a shell binary before it reaches for your documentation.

Still Not Sure

Answer one question: can the thing calling SupWriter run a shell command?

If yes and it is unattended, use the CLI. If no and there is a human in a chat window, use MCP. If no and you are writing the code that calls it, use the API.

And if you are somewhere in between, start with the CLI. It is the fastest to try, it is the easiest to throw away, and if it turns out you need the API later, sw --help has already shown you the shape of every endpoint you will be calling.

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New to the tools themselves? Try the free AI humanizer or the AI detector in your browser first — 300 words free, no credit card required.