Noplag Engine

Noplag Engine is an open-source matcher for verbatim and near-verbatim plagiarism checks against a corpus you control. It fits developers, publishers and institutions that can run software themselves or need an API-driven workflow. Choose it for traceable passage-level evidence and deployment control, not for paraphrase or AI-writing judgments.

Overview

Noplag Engine combines a locally operated matching engine with hosted service plans. The open-source edition searches your local corpus, while hosted tiers add broader sources, including live-web search on Premium and Enterprise. Apache 2.0 licensing and Docker Compose deployment let a technical team keep processing on its own hardware, but that flexibility brings responsibility for operations and security.

The maker dates the project to 2014 and is based in San Francisco. Its stated privacy position is that submitted documents are not used to train models; hosted retention can be configured, and EU data residency is available on request.

Key features

Winnowing fingerprints retrieve likely sources, then a seed-and-extend aligner identifies the character ranges that actually match. Reports provide similarity per source, matching passage offsets and coverage accounting. That combination is materially more useful than a single percentage when an editor or investigator must show exactly which text overlaps.

Ingestion accepts .txt, .md, .pdf and .docx files, with folder ingestion and HTTP upload. The /v1 API covers checks, reports, uploads, progress events and corpus management, so a team can place checking inside an existing service or editorial pipeline. It ships without authentication; exposing the API to users or a network therefore requires you to add an appropriate security layer.

Detection is deliberately narrow. The engine does not identify paraphrase, semantic similarity or AI-written text, so it cannot serve as a complete originality or authorship assessment. Hosted detection is calibrated for English, Spanish, Portuguese, Polish and Ukrainian at launch. Enterprise lists Canvas, Moodle and Blackboard integrations, while the features page also names Brightspace through LTI 1.3.

Pricing

The Free plan costs free, billed as three checks per month with 2,500 words per check, using the public web and Common Crawl. You can self-host for unlimited local checks, making this the sensible route for trying the service or running a small private corpus; hosted volume is the clear constraint. A 14-day trial is also offered.

Pro costs $9.99 a month, billed monthly. It raises usage to 50 checks per month and 25,000 words per check, and adds the Noplag Database, an academic index, REST API access and webhooks. This is the practical tier for makers and small teams that need automation rather than occasional manual checks.

Premium costs $19.99 a month, billed monthly, for 200 checks per month and 50,000 words per check. Live Web search, branded PDF reports and priority support make it a better fit for agencies and publishers handling client-facing output.

Enterprise uses custom pricing and custom check volume. It includes all corpora plus Live Web, on-premises or private-cloud deployment, SSO/SAML, LMS integrations, dedicated support and an SLA. Universities and regulated organizations are the intended audience, especially where deployment location and identity controls matter.

Platforms

Noplag Engine runs through an API, on Linux, macOS and Windows, as a self-hosted deployment, and on the web. Development requires Python 3.12 or newer; the Docker Compose setup uses PostgreSQL. The open-source package is best suited to teams comfortable maintaining that stack.

Who it's for

Free is for evaluating the hosted service or self-hosting local checks. Pro suits individual makers and small teams integrating checks into software. Premium is aimed at agencies and publishers that need live-web coverage and presentable reports. Enterprise is the choice for universities and regulated organizations requiring private deployment, identity integration and contractual support.

Pros and cons

Pros

Evidence at passage level: offsets and coverage accounting help reviewers verify and explain a match.

Deployment control: Apache 2.0 licensing, Docker Compose and local-corpus searching support self-hosted operation.

Useful automation surface: the API, uploads, progress events and webhooks can fit an existing workflow.

Broad file intake: common document formats, folders and HTTP uploads reduce preparation work.

Cons

No paraphrase or AI detection: substantial kinds of suspected misuse remain outside its scope.

Unauthenticated API: a production deployment needs additional access control and security planning.

Free hosted limits: three monthly checks and 2,500 words per check restrict regular web use.

Self-hosting demands technical ownership: Python, PostgreSQL and Docker Compose are not a turnkey setup for nontechnical users.

Alternatives

Choose Noplag for a similar freemium offering with three free monthly checks, 2,500 words per check and community Discord support. GPTZero is the better choice when AI detection is central, with a free allowance of 10,000 words per month and 50,000 characters. Pick Plagium for mobile access and five free daily searches, accepting its 4,000-character free-search limit and no API on the free account.

Winston AI suits users wanting AI and image detection alongside document and OCR scanning. Codequiry is for paid teams needing combined web and AI checks with team seats. Choose Copyscape for established URL scanning and monitoring options, Pangram for browser and Google Docs workflows, Plagius for a low-cost one-computer Windows license, or the plagiarism-detection software directory to compare more tools.

Verdict

Noplag Engine is a strong choice for technical teams that need controllable corpora, inspectable matching evidence and an API, with self-hosting as a genuine option. Look elsewhere if paraphrase analysis, AI detection or a ready-made consumer workflow is the primary requirement.