How do I authenticate?

Create a personal API key in Settings → API (Pro or Ultra plan) and send it as 'Authorization: Bearer sw_live_…' over HTTPS ('x-api-key: sw_live_…' also works). Keep the key server-side — never expose it in client code.

What are the API rate limits?

Pro: 60 requests/minute with up to 2,000 words per request. Ultra: 120 requests/minute with up to 5,000 words per request — per key. Over the limit returns a 429 with a Retry-After header, and every response carries X-RateLimit-Limit/Remaining/Reset. There's no separate API fee — calls draw on your plan's word credits.

How fast is the API response time?

Typically 8-15 seconds for texts under 1,000 words and 15-25 seconds for longer content up to 5,000 words, including full processing. Calls are synchronous — the result comes back in the same response.

How should I test the API during development?

Create a dedicated API key in Settings → API and test with short inputs to keep credit usage low; track requests in the 30-day usage chart there. Or hand the spec at /skill.md to your AI coding agent and let it build the integration for you.

Can I build the API into my own product?

Yes — API access is included on the Pro and Ultra plans, so you can integrate humanize, paraphrase, grammar, and detection into your CMS, content pipeline, or app. Calls consume your plan's word credits, and you can buy extra credits as you scale.

SupWriter

App Store

Now on iPhone & iPadApp Store ↗

Developer API

AI Humanizer API: Natural Writing Into Your Product

One REST API for humanizing, paraphrasing, grammar-checking, and one AI detector score with sentence-level highlights. Integrate natural writing into your CMS, content pipeline, or product with a few lines of code.

RESTful API

Simple JSON in, JSON out. Any language works.

Fast Processing

8-25 second response time depending on length

AI Detector Score

One AI detector score with sentence-level highlights

Simple Integration

Production-ready in under an hour

API overview

One REST API, four tools — humanize, paraphrase, grammar-check, and AI detection — the same engines that power the SupWriter app. Authenticate with a Bearer API key over HTTPS; every successful response is { "success": true, "data": { … } }.

https://supwriter.com/api/v1Get your API key

Full reference: API Developer Docs · Hand the spec to your agent: skill.md

Quickstart

Pick a tool and a language — copy, paste, ship. Swap in your own key from Settings → API.

Get your API key Pro/Ultra plans

Create an API key

Make a request Pick a feature and language

JavaScript · Humanize

const res = await fetch("https://supwriter.com/api/v1/humanize", {
  method: "POST",
  headers: {
    Authorization: `Bearer ${process.env.SUPWRITER_API_KEY}`,
    "Content-Type": "application/json",
  },
  body: JSON.stringify({ text: "Your text here." }),
});

const { success, data, error } = await res.json();
if (!success) throw new Error(error.code);
console.log(data.humanized_text);

What you can do

Four endpoints, one key. Each call consumes word credits from your plan.

1 credit / word of input

Humanize

Rewrite an AI-assisted draft so it reads naturally in the writer's own voice.

POST /api/v1/humanize

1 credit / word of output

Paraphrase

Rephrase while preserving meaning, then humanize — standard, formal, casual, academic, or creative.

POST /api/v1/paraphrase

1 credit / word of output

Grammar check

Fix grammar, spelling, and punctuation, then humanize — basic, thorough, or style.

POST /api/v1/grammar

1 credit / word analyzed

AI detection

Estimate how a draft may be read, with a sentence-level breakdown. Free in the SupWriter web app — metered (1 credit per word) on the API and MCP.

POST /api/v1/detect

Build it with your AI agent

Copy one prompt — your agent reads the full skill.md spec and builds all four tools.

Build with AIView skill.md

One prompt, every tool. Paste it into your AI coding agent — it reads the full spec at /skill.md, then either installs the sw CLI for scripts and CI, or builds a typed REST client for in-app features.

Your stack

Target stack: Next.js.

Integrate SupWriter into my project.

Read the full spec first and follow it exactly: https://supwriter.com/skill.md
It documents authentication (a server-side "Authorization: Bearer sw_live_..." key), the base URL https://supwriter.com/api/v1, five endpoints — POST /humanize, /paraphrase, /grammar and /detect, plus GET /me for plan, credits and limits — and the error codes, rate limits (Pro 60 / Ultra 120 requests per minute) and per-word credit billing.

There are two ways in. Pick the one that fits what I asked for.

1) Scripts, content pipelines and CI — use the official CLI instead of writing HTTP calls:

npm install -g supwriter

Authenticate with the SUPWRITER_API_KEY environment variable (do not run `sw login` in CI).
It reads stdin and writes stdout, so it composes:

cat draft.md | sw humanize > final.md
sw humanize 'drafts/*.md' --tone academic --out-dir humanized/
sw detect 'content/**/*.md' --max-score 30 # exits non-zero above the threshold

It already handles splitting documents over the per-request word limit, retrying 429s, and a distinct exit code per failure class. Run `sw --help` for the full reference — do not reimplement any of that by hand.

2) In-app features — build a typed client against the REST API that:
- reads the API key from a server-side env var, never the browser (HTTPS only),
- unwraps "data", maps "error.code" to typed errors, and retries 429s honoring Retry-After,
- reads limits.words_per_request from GET /me and splits longer inputs before sending,
- then adds a small UI to humanize, paraphrase, grammar-check and AI-detect text.

Use my existing stack and conventions, and keep the key server-side only.

Works with

ChatGPT

Claude

Claude Code

Codex

Cursor

GitHub Copilot

Antigravity

OpenCode+ any other AI agent

Plans & limits

API access is available on the Pro and Ultra plans.

Pro

$19.99/mo

REST API + static-token MCP access
60 requests / minute
2,000 words per request
15,000 word credits / month
All 50+ languages
All 4 tools: humanize, paraphrase, grammar, detect

Upgrade to Pro

Ultra

$39.99/mo

REST API + static-token MCP access
120 requests / minute
5,000 words per request
30,000 word credits / month
All 50+ languages
All 4 tools + priority support

Choose Ultra

API access requires Pro or Ultra. Every call — including AI detection — consumes word credits (in the web app, detection is free; the API meters it). Need more? Buy extra credits or compare plans.

Errors & rate limits

Errors use the envelope { "success": false, "error": { "code", "message" } } with a matching HTTP status.

Status	Code	Meaning
401	invalid_api_key	Missing or invalid API key.
403	plan_required	API access requires a Pro or Ultra plan.
403	insufficient_credits	Not enough word credits on your plan.
403	plan_limit	Text exceeds your plan's per-request word limit.
400	invalid_input	Missing or malformed 'text'.
429	rate_limited	Too many requests — slow down (honor Retry-After).
500	provider_error	Upstream processing failed; safe to retry.

Rate limit: Pro 60 / Ultra 120 requests per minute, per key — a 429 returns a Retry-After header.
Per-request word caps: Pro 2,000 words, Ultra 5,000 words.
Every response carries X-RateLimit-* headers.
Billing: 1 word credit per word. AI detection is metered on the API (it's free only in the web app).

Common integration patterns

CMS post-processing. Hook into your publish workflow — when a post is saved as a draft, call the API to humanize or grammar-check it before review. Works with WordPress (custom plugin), Contentful (webhook), and any CMS with an API.
Content pipeline automation. Add humanization as a step in your generation pipeline: AI writes the draft, your pipeline sends it to SupWriter, and the humanized version is stored or routed to editors.
In-product feature. Offer "humanize", "paraphrase", or "check for AI" inside your own writing tool or marketing app, powered by the API behind the scenes. Pro and Ultra include API access; calls draw on your plan's word credits.
Agentic workflows. Expose the same tools to your AI agents over MCP — Claude, ChatGPT, Cursor, Codex, and more — with the same key.

Related resources

API Developer Docs — the full endpoint reference, errors, and rate limits
MCP connector — use the tools inside Claude, ChatGPT, Cursor, and other AI clients
AI Humanizer for Content Agencies — scale production with API integration
AI Humanizer for SEO — rank without AI detection penalties
Pricing — compare API plan tiers and limits

Try the Humanizer Before You Integrate

Test the same engine that powers the API. Paste AI text below and see the output quality. Sign up free to try 300 words, no card needed.

Your Text

Style

Type

Lang

0 / 500 words

Humanize AI Writing in 3 Simple Steps

Turn an AI-assisted draft into clear, natural writing in your own voice, then check it before you submit.

Step 1

Your Text

Paste your AI-generated text here

Paste Your AI-Generated Text

Copy any content — assignments, blog posts, or AI-generated text.

Step 2

Result

Varies

Check before you submit

Check AI Score

See how much of your text is flagged as AI-written by detection systems.

Step 3

or select text & ask AI

Ask or type your prompt...

Edit or review selection

Improve writing

Fix grammar

Make shorter

Make longer

Use AI to do more

Continue writing

Humanize & Improve selection

Rewrite the draft so it sounds like you. Select any text to improve it, fix the grammar, shorten it, or lengthen it.

Check a draft before you submit

Detectors such as GPTZero, Turnitin, and ZeroGPT estimate whether a draft looks AI-written. They can also flag human writing.

SupWriter includes an AI detector so you can check a draft before you submit. It is a pre-check based on one scoring model, not the detector your school or client uses. Results vary by detector, text length and topic, and no tool can guarantee a result. How we test AI detectors

Detectors schools and clients use. SupWriter doesn't query them.

ZeroGPT

Copyleaks

Turnitin

QuillBot

GPTZero

Grammarly SaplingOpenAIClaudeGeminiGrok

Use the built-in AI detector (free on paid plans) to review a draft before you submit. A score is an estimate, not a guarantee.

Reviews

What our users say

What writers say about SupWriter

I've been using SupWriter for weeks, and it's become essential to my academic writing and research workflow. As a Ph.D. student in environmental law, I'm constantly drafting, refining, and translating complex texts, and SupWriter makes the whole process far more efficient and less stressful.

Henrry

Ph.D. Candidate

It's so great and I love this app. Everyone should use it. It helped me pass my essay!

Braxton H.

What impressed me most is how seamlessly SupWriter polishes my drafts. My clients noticed the difference in quality and tone immediately.

Brandon L.

Freelancer

Absolutely wonderful. The best AI humanizer I've used, especially for scientific writing.

Phoebe B.

Student

This tool lets me do so many things faster, and its features are genuinely useful.

Karima

Really good. I didn't expect it to be this great. It genuinely enhanced my writing. Give it a try.

Chris A.

Researcher

This product is so good. I'll choose it over and over again. It has my heart. ❤️

Rachel A.

Student

The Fix Selection feature is a game changer. I can select any text and improve it, fix the grammar, or make it shorter, and my content workflow has never been faster.

Aisha M.

Content Writer

Excellent service, really helpful for job applications, and it even has an AI detector. I'd highly recommend it to students, professors, and writers. An amazing writing tool.

Shania W.

It was amazing. I upgraded, and since then my work has consistently scored high marks.

Enzo H.

Student

The best humanizer ever.

Nikolay

It was great, especially in a foreign language. It helped the adjectives come out smoother than I expected!

Christina P.

Writer

I'm on the Ultra Monthly plan and bought $20 in extra credits. This platform is super good, and I'm planning to switch to the Yearly plan soon.

Ron

Ultra plan user

Precisely written and a very nice app. I recommend everyone give it a try.

Riya S.

Student

I was amazed to see the AI-flagged text identified so accurately. It really helps crack down on AI interventions.

Radhakrishnan

My professor commended the uniqueness of my essay after I used SupWriter to make it sound natural. Strongly recommended for students!

Marco D.

Student

The website is fantastic. It's clean, modern, and visually appealing. Navigation is intuitive, pages load fast, and everything works well on desktop and mobile. A smooth, enjoyable experience. I'm really impressed!

Korsmann

Student

It's been a great tool to use, and very helpful.

Adrianne

My experience with the AI detector was great. The interface is simple and easy to navigate, and results came back quickly with useful detail.

Kareem

Student

This experience has been amazing, and the service is extremely helpful.

Marshall

My experience was amazing. SupWriter is very easy to use, and it actually works!

Domenico F.

Student

Impressed by the quality, and cheaper than other platforms.

Steve

SupWriter is great. It helps me become a better writer and a better student overall.

Kal

Student

Developer Questions

API FAQ

Four, all under one key: humanize (rewrite AI-assisted drafts so they read naturally), paraphrase, grammar check, and one AI detector score with sentence-level highlights. They're the same engines that power the SupWriter editor.

Get Started Today

Ready to write with confidence?

Join thousands of students and writers who use SupWriter to make AI-assisted drafts sound like them.

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