AI humanizer API: who publishes one, at what price and limits

The table covers the twelve tools in the launch review set. "Published" means the vendor's own site states an API; "priced" means a price list this site could read.

Three of the twelve, Grammarly, QuillBot and Verva (formerly TwainGPT), state no API on the pages read on 8 November 2026. For the nine that do, the useful split is between the four with a public price list and the five without, because a pipeline cannot be budgeted on "contact us".

The four priced APIs sell in different units, so the table carries the vendor's unit and an approximate cost per 1,000 words where the vendor states one.

StealthGPT's API page lists Build at $19 a month for 100,000 Standard words, Grow at $49 for 275,000 and Scale at $99 for 600,000, states that humanization "counts input plus output words", and charges its Super model at 2.5 plan words per word, with extra usage at $0.20 per 1,000 Standard words and $0.50 per 1,000 Super words (StealthGPT, AI humanizer API, read 9 November 2026). Undetectable AI's developer page lists Starter at $29 a month for 72,500 words at $0.40 per 1,000 and Growth at $49 for 245,000 words at $0.20 per 1,000, and states that a new account's 250 free word credits can be used directly through the API (Undetectable AI, developer page, read 9 November 2026).

WriteHuman's API page lists Standard at $29 a month for 125,000 words and four Premium sizes from $69 for 400,000 words to $419 for 3 million, bills by input word count, sells top-up packs of 100,000 words for $25 that do not expire while a plan is active, and states that API access is paid only (WriteHuman, AI humanizer API, read 9 November 2026). Walter Writes AI states one credit pool for five APIs, humanizer, AI detector, image detector, grammar and plagiarism, from $49 a month for 300,000 credits up to $1,699 for 25 million, with most text endpoints costing 1 credit per word, read on 9 November 2026; its main limitation for a humanizer-only buyer is that the $49 entry price is the highest of the four and the pool is shared with four other products, so a plagiarism scan at 2 credits or an image check draws from the same balance.

GPTHuman states that its API is credit-based, that credits never expire and that free credits come with sign-up, but publishes no prices on the API page (GPTHuman, AI humanizer API, read 9 November 2026). HIX Bypass's pricing page lists Standard at $14.99 a month or $9.99 billed yearly for the web tool and links an API developer page without an API price (HIX Bypass, pricing, read 9 November 2026).

Price is only half the fit. A publishing pipeline that sends 3,000-word articles needs a per-call cap that accepts them, and a batch job needs a rate limit that will not stall it.

WriteHuman publishes the most complete limits: 2,000 words per call and 40 requests a minute on Standard, rising to 6,000 words and 160 requests a minute from its 800,000-word tier, read on 9 November 2026. Undetectable AI states no maximum usage limit but a default rate limit of 3 requests per second per IP address, read on 9 November 2026. StealthGPT and Walter Writes AI publish workflow features, async runs, batches and webhooks, rather than a per-call word cap, so a buyer has to test the cap or ask.

One more unit matters. StealthGPT counts output words as well as input, and its Super model multiplies both by 2.5, so 100,000 plan words on Build is roughly 20,000 words of Super humanization each way, by the vendor's own arithmetic, read on 9 November 2026. WriteHuman bills input only. Undetectable AI's web plans are "API compatible", meaning the same word balance serves both, read on 9 November 2026. Comparing the four on list price alone misreads them.

Not always, and the vendors say so. StealthGPT sells three API models, Super, Standard and Lite, and describes Standard as its "previous-generation humanizer", read on 9 November 2026. WriteHuman's API restricts its enhanced model and three-output scoring to Premium tiers; Standard gets one output from the base model, read on 9 November 2026. Undetectable AI lists three humanizer models on both API tiers. Walter Writes AI's API page does not state which of its web modes, Simple, Standard and Enhanced, the endpoint uses.

For a developer, that means the model named in the plan, not the brand, is what to test. The Undetectable AI review, StealthGPT review and Walter Writes AI review cover each vendor's web plans and modes on the same fields; this page covers only the API.

Two cautions close the guide. Several API pages advertise that output will pass named detectors; this site repeats none of those claims, since no vendor publishes a reproducible method behind them, and the only detector readings this site reports are its own, from the five public detectors named in the published method. And every price above is a list price read on one day; the guides hub carries the date on every page so a reader can tell when a figure is due a re-check.