Overview

Noplag Engine is an open-source checker for finding copied passages in a corpus you control. It is best suited to technical teams and organizations that want local or self-hosted matching. Its passage-level reports and deployment flexibility are useful; its narrow matching scope is the key limitation.

Key features

Winnowing fingerprints identify candidate sources, which a seed-and-extend aligner then checks for matching character ranges. Reports break down similarity by source and include passage offsets and coverage accounting, giving reviewers more to investigate than a single score.

The local engine searches a corpus you supply; the hosted service adds live-web search. Ingestion supports .txt, .md, .pdf, and .docx files, including folders and HTTP uploads. The /v1 API provides checks, reports, uploads, progress events, and corpus management. That makes it a practical foundation for workflow integration, but it ships without authentication, so teams need to address access control before exposing it to users or a network.

Apache 2.0 licensing and Docker Compose support allow deployment on your own hardware, but self-hosting means taking responsibility for the environment. Development requires Python 3.12 or newer, and the Compose setup uses PostgreSQL. The maker says submitted documents are not used to train models; hosted-plan retention is configurable, and EU data residency is available on request.

Matching is limited to verbatim and near-verbatim text: it does not identify paraphrases, semantic similarity, or AI-written text. The hosted product says its detection is calibrated for English, Spanish, Portuguese, Polish, and Ukrainian at launch. These boundaries make it a poor fit when broader originality or AI-authorship screening is the priority.

Pricing

The Free plan costs 0.00 USD per free and includes 3 checks per month, up to 2,500 words per check, and public web plus Common Crawl. Self-hosting allows unlimited local checks, making Free a sensible route for trials or teams able to operate the engine themselves; hosted use is tightly capped.

Pro costs 9.99 USD per month, billed monthly, for 50 checks per month and up to 25,000 words per check, with the Noplag Database, academic index, REST API, and webhooks. It suits makers and small teams that need a regular hosted quota and workflow integration. Premium costs 19.99 USD per month, billed monthly, and raises the allowance to 200 checks per month and 50,000 words per check, adding Live Web search, branded PDF reports, and priority support. That is a better fit for agencies and publishers needing larger volumes or web coverage.

Enterprise has custom pricing and custom check volume. It includes all corpora and Live Web, on-premises or private-cloud deployment, SSO/SAML, LMS integrations, and dedicated support with an SLA, targeting universities and regulated organizations. The hosted plans add access and convenience, while Free's unlimited local checks require self-hosting; choose by weighing quota and corpus needs against operational responsibility.

Platforms

Noplag Engine supports API, Linux, macOS, self-hosted, web, and Windows. The listed API has no built-in authentication, an important constraint for deployments that need controlled access.

Who it's for

Developers, publishers, and institutions with a defined corpus and a need to inspect exact matching passages are the strongest fit. Teams that want a managed service can choose among hosted quotas, while organizations with infrastructure and privacy requirements can use self-hosting or Enterprise deployment options. Anyone needing paraphrase detection, semantic matching, or AI-writing detection should look elsewhere.

Pros and cons

Pros: Passage offsets, per-source similarity, and coverage accounting make matches easier to assess than a lone score.
Pros: Apache 2.0 licensing, local corpus search, and Docker Compose deployment give technical teams control over where the engine runs.
Pros: Hosted tiers add progressively larger quotas, broader corpora, and integrations such as LMS support at Enterprise level.
Cons: Matching does not cover paraphrase, semantic similarity, or AI-written text, limiting its use as a general originality screen.
Cons: Free hosted checking is limited to 3 checks per month, while unlimited local checking requires self-hosting.
Cons: The API ships without authentication, creating extra security work for deployments exposed beyond a trusted environment.

Alternatives

Noplag is the related choice for readers who want a freemium service with the same 3-check monthly Free allowance and 2,500-word cap, plus Community Discord support.

Dolos is worth considering for an open-source option with a free web app; its ZIP uploads are limited to 10 MB.

JPlag suits readers seeking free open-source local software who can meet its Java SE 25 requirement.

Plagium may fit readers who prefer web or mobile access and need up to five free daily searches, accepting a 4,000-character limit and no Deep Search or API access on the free account.

Copyscape is an alternative for website URL scans, with free scans limited per website and to 10 results per search.

GPTZero is the more relevant option when AI detection is the goal; its Free plan includes 10,000 words per month and 50,000 characters.

Originality.ai offers a free AI-detection-only plan with three scans per day and a 2,000-word scan limit.

Pangram is another alternative for readers seeking AI detection, with a free allowance of 2,000 words per day and three image-detection scans daily.

For a broader shortlist, browse Plagiarism Detection Software.

Verdict

Choose Noplag Engine if you need controlled-corpus matching, inspectable passage reports, and the option to self-host or integrate through an API. Its strongest case is technical control paired with clear evidence of where text overlaps. Look elsewhere if you need paraphrase or AI-writing detection, or if you cannot take on deployment security and administration.

	MOSS	Noplag Engine
Free plan	Yes	Yes
Free trial	No	Yes
Paid from	Free	$9.99/mo
Open source	No	No
Platforms	Linux, Web	api, Linux, macOS, self-hosted, Web, Windows
Free plan	Yes	Yes
AI detection	—	No
LMS integrations	—	Yes
API access	—	Yes
Maximum check size	—	25,000 words/check

MOSS vs Noplag Engine

MOSS

Noplag Engine