Diffblue Cover is an AI-powered testing tool that automatically writes unit tests for Java and Kotlin code, using reinforcement learning to generate, run, and refine test candidates until they compile, pass, and add real coverage. It is built by Diffblue, a company that spun out of the University of Oxford's AI research group in 2016 and has focused exclusively on automated software testing ever since.
Who builds it
Diffblue was founded in 2016 by Daniel Kroening and Peter Schrammel, drawing on roughly a decade of Oxford research into program analysis; Mathew Lodge has been CEO since July 2019. The company raised a Series A round (reported as $22M / £17.3M) led by Goldman Sachs Principal Strategic Investments in 2017, with later backing from IP Group and AlbionVC. Diffblue counts large regulated enterprises among its customers, including Goldman Sachs, S&P Global, Citi, JP Morgan, and AWS.
How it works
Rather than suggesting code for a developer to accept or reject, Diffblue Cover works autonomously: it analyzes a Java or Kotlin codebase to map classes and methods, generates a test candidate for each testable method, runs the candidates, and uses reinforcement learning to iteratively adjust them until it converges on a suite that maximizes coverage. The whole process — plugin, CLI, and pipeline modes alike — runs inside the customer's own environment; Diffblue states that no code leaves the organization during test generation.
Core features
- Autonomous unit-test writing for Java and Kotlin, including mocks and assertions, with no prompting required.
- IntelliJ IDEA plugin for one-click test generation on classes and methods as you code.
- Command-line interface that configures itself from an existing Maven or Gradle build.
- CI/CD pipeline integration (GitHub, GitLab, Jenkins, Azure, AWS) to keep regression suites current automatically.
- Reporting and, on higher tiers, additional Optimize and Refactor tooling for existing test suites.
Pricing
Diffblue Cover is sold in four editions:
- Community — free, single user, up to 25 methods tested per month, IntelliJ plugin, community-forum support.
- Developer — around $30/user/month (or roughly $330/year), single user, up to 100 methods per month.
- Teams — roughly $30,000/year, 10+ user licenses, unlimited test generation for codebases up to 250,000 lines of code, plus CLI, pipeline, and reporting tools; a free trial is available.
- Enterprise — custom pricing for 20+ licenses and codebases beyond 250,000 lines, adding Optimize/Refactor tooling and a dedicated support team.
Third-party listings (Capterra) are the source for the Developer and Teams list prices; Diffblue's own pricing page did not expose these figures in a form we could independently confirm, so treat exact numbers as indicative and check with Diffblue for a current quote.
Who it's for
Diffblue Cover targets Java-heavy engineering organizations — particularly regulated industries like banking and financial services, where its existing customer base sits — that need to raise or maintain unit-test coverage on large, legacy-heavy codebases without pulling developers off feature work. Individual developers can try the Community and Developer editions for free or at low cost; Teams and Enterprise are built for organizations running Cover across many repositories in CI/CD, where automatically-maintained regression suites reduce the manual test-writing burden at scale.