From a development technique to a top management consideration – APIs have evolved into a business evolution catalyst. A provider’s core service can be shared, reused and deployed to create added value for the clientele.

API providers and users have built a huge ecosystem called Application programming interface (API) economy. API management market is expected to reach $5.1 billion in 2023.  Media streaming, financial services, travel industry and others have already embraced APIs.

Digital proofreading solutions aren’t the exception, market leaders are launching developer SDKs one by one. While Grammarly has joined (and left) the party, WebSpellChecker was a pioneer of grammar check API.

[Update]: Grammarly for developers will be discontinued on January 10, 2024.

Solutions vary by features and deployment options, and it’s challenging to say which of them is the best solution in 2023. But we’ll try. 

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WProofreader SDK for developers

Available as a cloud or on-premise solution. Get free trial.

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Spelling and grammar check API vs SDK

WebSpellChecker and Grammarly call their product SDK for devs, LanguageTool offers Proofreading APIs, Sapling AI sells separate API and SDK solutions. 

Are API and software development kit (SDK) the same? If not, what is the difference?

API (application programming interface) is a set of subroutine definitions, protocols, and tools for building software and applications. SDKs (software development kit) are collections of software development tools in one installable package.

SDKs commonly let users develop new applications using APIs. Pure APIs don’t include instructions for developers to create new systems.

There is also REST API (also known as RESTful API). A REST API is an application programming interface (API or web API) that conforms to the constraints of REST architectural style and allows for interaction with RESTful web services

REST API seem to go hand in hand with HTTP APIs, however, they don’t. 

An HTTP API is an API that uses Hypertext Transfer Protocol as the communication protocol between the two systems. HTTP APIs expose endpoints as API gateways for HTTP requests to have access to a server. REST API is not necessarily an HTTP API (since there are other transfer protocols available, yet they are less popular).

APIs have passed a long journey. Starting from the 1960s, the technology was gradually changing from ARPANET, ATTP, and TCP sessions to point-to-point interfaces, message-oriented middleware, and finally, integration as a service.

What’s the benefit of APIs and SDKs?

  • help businesses build better digital experiences and products;
  • accelerate innovation by exposing assets with partners and clients;
  • create an environment where systems can integrate with each other;
  • serve as a strategic asset to create business value;
  • can be leveraged by external customers. 

Among top API platform initiatives are: fast app development, internal app connection, ecosystems and B2B partner program development.

Top API grammar check solutions

We’ve investigated some of the existing proofreading solutions for developers and compared them by key criteria: check type, performance, limitations, features, pricing, etc.

We based these criteria on recommendations from experts who advise comparing API solutions by their usability, functionality, performance, flexibility, and documentation.

As a source of information, we took the providers’ product documentation and the content on the official websites.

Let’s start from a quick overview of each.


WProofreader SDK enables developers to build and extend proofreading functionality in their apps and websites. Main features include fast integration with WYSIWYG editors, grammar and spell check, multi-language support and AI-based engines, company-wide and user dictionaries. 

WProofreader SDK
WProofreader API

The solution can be deployed on WebSpellChecker Cloud AWS servers or on a company’s premises. Pricing depends on client proofreading volumes. You can test how WProofreader API works on the demos. Also available on the RapidAPI, the world’s largest API hub.


Grammarly for developers comes with text editor SDK. The API and UI integrates with your application and extends its proofreading capabilities.

Available features: spelling, grammar, fluency, conciseness, and tone. Importantly, Grammarly works only with texts written in English and dialects. The kit supports HTML elements and attributes and provides thin wrappers for React and Vue.js applications.

Effective January 10, 2024, Grammarly for Developers, including its SDK functionality and user experience, will no longer be available. Right now, Grammarly is not accepting new registrations for the product.


As a part of LanguageTool Enterprise, you can use their Proofreading API. The vendor says that the product is actively used by web agencies, websites and software companies. 

Engineers obtain spell check and grammar check options, multilingual support (12 languages), cloud-based or server-based deployment options. Other details are available per request.


Similarly to Grammarly, Sapling AI also has a developer SDK and separate API solution. Alongside basic spell and grammar capabilities, you also get the autocomplete feature.

You can get SDK access for testing purposes.

The vendor states that the main product benefits are: high accuracy, low latency, customization,  self-hosted/on-premise deployment. Developer guidelines can be found in the vendor documentation. 


Perfect Tense API offers both API and JavaScript integration methods to development firms and agencies.

The API gives users  complete control over how and when they send text in Perfect Tense and offers multiple response types and parameters that they can display.

The JavaScript library is for instant correction of spelling and grammar issues, and can be embedded without download or installation.


Ginger, one of the industry pioneers, also promotes its Grammar API.

The key benefits of Ginger solution are AI-powered text correction, full document proofreading, and custom correction explanations.

Synonyms & rephrase are additional options to basic grammar and spelling corrections. The API solution integrates with LMS/CRM/internal systems and can be locally installed.

There are no public technical details or sales information.


Probably, ProWritingAid has the largest array of dev SDKs. The product supports popular languages such as C#, JavaScript, Typescript, PHP, Python, Java.

What do you get in a bundle? AI grammar checking, contextual and entity spelling, terminology, writing style. Besides, per request, users can get extra features like readability check.

Pricing is scaled and depends on a number of calls.

Use cases of ProWritingAid API: essays marking, grammar checking inside text editors in apps and websites, providing feedback for English learners, translated text proofreading.


Outwrite, an AI writing assistant and a rephrasing tool, provides access to its services via Proofreading API. The vendor states that the solution has a low latency, detects complex mistakes, and is available in 99.9% uptime.

Engineers can use it for grammar checking, thesaurus and style suggestions, paraphrasing and custom dictionaries. Developers can test the Outwrite product with an API key.

Find more details about API providers in the comparison table below.

Comparing grammar correction API solutions

ProductWProofreader SDKGrammarly for developersProofreading APISapling API and SDKPerfect Tense APIGrammar API document spelling & grammar checkText and grammar checking APIProofreading API
Check typeSpelling, grammar, punctuation, and style, autocorrect & autodetect Spelling, grammar, punctuation, and styleSpelling, grammar, punctuation, and styleSpelling, grammar, punctuation, and styleSpelling and grammar Spelling and grammar Grammar, style, and terminology Spelling and grammar 
Performance (av. response time) ~160-350 ms / sentence (depends on the check type)real-time~ 300 ms / 1000 char~1 secondN/A150-250 ms server time / sentenceN/AN/A
Limitations50KB of text (50K char. incl. spaces) / API request (for Cloud users)

Server clients can increase this parameter
the number of API requests Only 20 POST requests / IP / min

75KB text / IP / min

20KB text / request
Recommended text size – 4000 char / requestdaily API limit20,000 char / call

2 documents or 120 sentences / min
calls / day

each text call is limited to 100,000 char ~ 22,000 words
Availability99.5% of the timeN/A, no public SLA providedN/AMonthly uptime – less than 95%N/A99% availabilityN/A99.9% uptime
SDKJavaScript components / integrations with RTEs/WYSIWYGwrappers for React and Vue.jsthe JSON API, Python LibraryJavaScript SDKNode.js and PHP SDKDeveloper SDKSDKs for C#, JavaScript, Typescript, PHP, Python, Javapre-built SDKs for Typescript and PHP
Extra features20+ languages, dictionaries (user and company-wide)customization; fluency, conciseness, and tonemultiple languagesautocomplete (beta), snippets, team management, dictionaries, custom mapping (rules)N/Afull document proofreadingerror explanation, definitions & recommendationscontextual spelling & thesaurus, terminology, writing style, poor readability style suggestions, contextual thesaurus, paraphrasing,
custom dictionary
DeploymentCloud and on-premiseCloud onlyCloud and on-premiseCloud and on-premiseCloudCloud and on-premiseCloud and on-premiseCloud
DocumentationWProofreader API Grammarly text editor SDKN/ASapling AI and SDKPerfect Tense APIGinger grammar correction APIProWritingAid API V2Outwrite API
Pricing plansper requestper request (participation in beta program is required)per requestper requestper requestper requestper request (scaled and depends on a number of API calls)per request

The bottom line

Digital proofreading vendors such as WebSpellChecker, Grammarly, LanguageTool, Ginger and others offer pre-built SDKs for developers and standalone APIs.

SDKs are ready-made components / plugins for HTML editors such as CKEditor, Froala or TinyMCE and typically come with UI. APIs are bare endpoints, a set ot commands which developers can use to access the vendor service core.

Solutions greatly vary. 

Almost all grammar check vendors have SDKs with wrappers for popular development frameworks and languages. 

As for tech details – average response time, limitations, accessibility – some vendors disclose this information, others provide it per request. Limitations are critical, as they define the number and size of requests you can send. 

For instance, WebSpellChecker, Ginger, and ProwritingAid allow sending large API requests while Sapling AI recommends not to exceed 4000 characters for single request.

Want to extend the proofreading capability of your website/app with multi language support, dictionaries, spelling autocorrect? Try WebSpellChecker. Sapling and LanguageTool are alternative options.   

Explore more

WProofreader SDK for developers

Available as a cloud or on-premise solution. Get free trial.

Explore more

Ginger and PerfectTense are OK for basic spell and grammar check. Grammarly and Outwrite offers extra options as paraphrasing. 

A key point in choosing an API vendor may be on-premise deployment. Grammarly offers only Cloud deployment, which may be inappropriate for security-conscious businesses. If you’re interested in on-premise API solutions, contact WebSpellChecker, LanguageTool, Sapling or Ginger.