Dolos

Dolos is a specialist code-similarity checker for teachers reviewing programming work. It is most compelling for courses that need a free way to compare test, exam, or exercise submissions, including work where small-group collaboration is allowed.

Its file-pair reports and local or self-hosted options make it a focused choice for code, not a substitute for checking prose against the web or academic sources.

Overview

Dolos processes source files by tokenizing them, fingerprinting token sequences, and indexing matches. Its reports surface similarity, total overlap, and the longest matching fragment between file pairs, giving instructors specific passages to inspect rather than a simple claim of plagiarism. That evidence can guide review, but educators still need to interpret overlap in light of assignment rules and permitted collaboration.

The free web app accepts ZIP submissions and detects languages from file extensions. In a mixed-language ZIP, it analyzes only the dominant language unless a language is selected, so instructors should check that setting when submissions span languages.

Key features

Language support and reporting

Tree-sitter parsers cover common languages including Bash, C, C++, C#, Elm, JavaScript, Java, PHP, Python, and TypeScript. CSV export is useful when results need to be handled outside the report view. Dolos is open source under the MIT license, which makes it possible to run the software locally or host a server rather than rely on the public service.

Automation, privacy, and support

A JSON API supports automated integration with exercise platforms such as Dodona. The public API caps ZIP submissions at 10 MB; larger submissions are better suited to the CLI or library. The command-line tool can analyze locally and show results in a browser, and a Docker image is provided.

The 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. Questions and support go through GitHub discussions, while bugs, feature requests, and language requests can be filed as issues.

Pricing

Dolos is free to use. The Dolos web app plan costs nothing and includes ZIP uploads limited to 10 MB. There is no free trial because the plan is free. That makes it a practical option for educators who can work within the upload cap; larger jobs can use the CLI or library, or users can run their own server. The free web workflow does not match against web or academic sources.

Platforms

Dolos supports web, API, Linux, Windows, and self-hosted use. The local command-line option suits instructors who need offline analysis or larger submissions, while the hosted web app avoids running a server for ordinary ZIP uploads.

Who it's for

Dolos is best suited to teachers checking programming tests, exams, and exercises where collaboration may be allowed within small groups. Its code-specific comparisons help focus a review on overlapping fragments. It is not the right fit for checking essays or searching external sources for copied material.

Pros and cons

• Pros: Free web use and open-source licensing keep the basic workflow accessible and allow local or self-hosted deployment.
• Pros: Pair-level similarity, overlap totals, and longest matching fragments give instructors concrete material to review.
• Pros: JSON API integration and CLI support serve both automated exercise workflows and local analysis.
• Cons: Public web and API ZIP submissions are limited to 10 MB, which can constrain larger cohorts or archives.
• Cons: Mixed-language ZIPs are analyzed in only the dominant language by default, so language selection may be necessary.
• Cons: It focuses on source-code similarity and does not provide web or academic source matching.

Alternatives

For prose-oriented checks or broader plagiarism workflows, Plagiarism Checkers and Plagiarism Detection Software are relevant directories. Choose Noplag instead if you need a free monthly allowance of three checks, each up to 2,500 words, against public web and Common Crawl sources. Copyscape is another web-focused option, with free URL scans limited per website and search results limited to 10.

Originality.ai may suit readers who want AI detection in a free tier, with three scans a day and a 2,000-word limit per scan. PlagAware offers a free subscription tier with 10 pages per month and monitoring for up to 30 URLs. Plagiarism Checker X is a Windows alternative with a free basic plan that has word and daily search limits. Plagiarisma offers a free Windows desktop checker. Quetext AI Detector combines AI detection and plagiarism checking in its free tier, each up to 1,000 words. SmallSEOTools Plagiarism Checker offers free scans up to 1,000 words.

Verdict

Choose Dolos if you teach programming and want a free, code-focused way to inspect overlap between submissions, with the flexibility to automate, run locally, or self-host. Look elsewhere if you need matching against web or academic sources, or if your main task is checking prose.