Overview

Dolos is a source-code similarity checker aimed chiefly at teachers reviewing programming work. It is the right kind of specialist tool when you need to compare submissions rather than scan essays or search the web for copied text.

Its strongest case is the combination of a free web app, useful pair-by-pair overlap reports, and options to analyze locally or host your own server. The main practical constraint is the web/API ZIP cap, alongside the need to choose a language when an archive contains several.

Key features

Code-focused reports

Dolos tokenizes source files, fingerprints token sequences, indexes matches, and reports similarity, total overlap, and the longest matching fragment for each pair of files. That gives an instructor concrete places to examine when reviewing possible overlap; it does not replace judgment about whether students were permitted to collaborate.

Language support and ZIP handling

Tree-sitter parsers cover common languages including Bash, C, C++, C#, Elm, JavaScript, Java, PHP, Python, and TypeScript. The web app detects languages from file extensions in a ZIP, but analyzes only the dominant language in a mixed-language archive by default. Selecting a language is important when the dominant-language assumption would miss submissions you need to compare.

API, local use, and privacy

A JSON API supports automated integration with exercise platforms such as Dodona. Public API submissions are capped at 10 MB, with the CLI or library recommended for larger work. The command-line tool can analyze files locally and show results in a browser; a Docker image is also provided, and users can run their own server.

The free web app runs on Ghent University servers in Belgium. Team Dodona says submitted files and reports are not shared or used commercially; users can delete their data, and older data is periodically deleted. Local and self-hosted options suit teams that prefer to keep analysis under their own control. Dolos is open source under the MIT license. Questions and support go through GitHub discussions, while bug, feature, and language requests can be filed as issues.

Pricing

The Dolos web app is free to use, with no free trial because the service is already free. Its ZIP uploads are limited to 10 MB. The free offer is a strong fit for individual teachers and modest batches of coursework, but larger submissions call for the CLI, library, or a self-hosted setup. No paid plan or seat-based tier is part of the offer.

Platforms

Dolos is available through the web, API, Linux, Windows, and self-hosted deployments. Its CSV report export is useful for carrying results into spreadsheet-based review, while local command-line analysis and Docker support offer alternatives to uploading files to the hosted app.

Who it's for

Dolos is best suited to teachers checking programming tests, exams, and exercises where collaboration is allowed within small groups. Its pairwise code reports can help focus a review, but the tool is not a fit for checking writing against web or academic sources: it offers neither web nor academic source matching.

Pros and cons

Pros: Free access and MIT-licensed open-source software make it practical for educators without a software budget.
Pros: Local analysis and self-hosting give schools an option beyond sending submissions to the hosted service.
Pros: Similarity, overlap, and longest-fragment reporting helps reviewers identify specific code regions worth examining.
Cons: The 10 MB ZIP ceiling on the public web app and API can rule out larger batches unless users switch to local or library-based analysis.
Cons: Mixed-language ZIPs are analyzed only in the dominant language unless a language is selected, so default handling can omit relevant code.
Cons: No web or academic source matching means it cannot serve as a general plagiarism checker for essays or published material.

Alternatives

For broader plagiarism checking, the Noplag free plan allows three monthly checks of up to 2,500 words each and includes public web and Common Crawl matching; choose it when prose and external-source comparison matter more than code submissions.

PlagAware is a better direction for readers who want a free allowance of 10 pages per month and monitoring for up to 30 URLs with a Content Protection Banner. PrePostSEO Plagiarism Checker has Android and iOS apps alongside desktop and web platforms, making it a more suitable option for readers who want to check from a phone.

Quetext AI Detector combines free AI detection and plagiarism checks, each up to 1,000 words, so choose it when AI detection is also part of the job. SmallSEOTools Plagiarism Checker offers free scans up to 1,000 words and is a straightforward alternative for short text submissions.

Copyscape suits readers looking for URL scans, with free scans limited per website and search results capped at 10. Originality.ai is the more targeted choice when AI detection is the priority: its free Basic plan is AI-detection-only, with three daily scans of up to 2,000 words. Plagiarism Checker X is a Windows-only alternative for readers who want a desktop application.

Browse more options in Plagiarism Checkers or Plagiarism Detection Software.

Verdict

Choose Dolos if you teach programming and need a free way to inspect similarities between code submissions, especially if local analysis or self-hosting matters. Its focused reports and flexible deployment make it a sensible specialist choice; look elsewhere for prose plagiarism, external-source matching, or mixed-language work that cannot be handled with a selected language.

	Dolos	Viper
Free plan	Yes	No
Free trial	No	No
Paid from	Free	—
Open source	No	No
Platforms	api, Linux, self-hosted, Web, Windows	Web
Free plan	Yes	No
Web source matching	No	—
Academic source matching	No	—
Report export formats	CSV	—
API access	Yes	Yes

Dolos vs Viper

Dolos

Viper