License Compatibility Checker

License Compatibility Checker is a free web tool for assessing whether code under one software license can be used in a project under another. It treats compatibility as a directional question because bringing MIT code into a GPL project isn't the same as bringing GPL code into an MIT project. A user selects the license on the incoming code, the license of the destination project, and an intended usage scenario. Optional questions cover distribution, network access, modification, whether the finished product stays closed source, and how the code is combined. The result is a structured assessment of likely compatibility, conditions, conflicts, obligations, and points that need human review.

The checker uses a deterministic rules engine rather than a language model. Its license definitions contain structured facts about copyleft strength, source disclosure, attribution, patent terms, network use, and other dimensions. ESDecode says those facts are drawn from SPDX, the Open Source Initiative, the GNU Project, license stewards, and official texts. When a rule applies, the output exposes its identifier rather than presenting an unexplained verdict. Possible headline results include generally compatible, compatible with conditions, potential conflict, likely incompatible, manual review required, and unknown. This vocabulary is intentionally cautious and keeps uncertainty visible when the engine can't identify a license or safely generalize custom terms.

Context is important to the result. The engine distinguishes scenarios such as dependency use, source inclusion, modification, binary or source distribution, commercial and proprietary software, software as a service, internal use, resale, white labeling, and source code acquisition. It can also account for dynamic linking, static linking, direct source inclusion, or operation as a separate program. These details matter most with strong, weak, file level, and network copyleft licenses. The web form surfaces the core questions without making a visitor write code. Developers who need automation can install the underlying TypeScript and Node.js package and call the same compatibility logic from their own tooling.

The open-source package supports 21 defined license values, including common permissive families, GNU licenses, MPL, EPL, CDDL, AGPL, public domain equivalents, several source available licenses, proprietary terms, and unknown or custom cases. It normalizes common aliases and understands GNU version forms such as only or later. Beyond pairwise checks, the library includes analyzers for projects containing several licenses and for source code acquisition risk. It can parse license declarations from some Node.js and Composer manifest formats, report risk sections, and produce questions for a buyer to ask a code seller. Results can separate areas such as commercial use, modification, distribution, source disclosure, attribution, patents, copyleft, network use, and proprietary use. The package is framework independent, fully typed, published as an ES module, and has no runtime dependencies.

Transparency is its strongest advantage, but it also defines the limits. The engine reasons from declared license terms and the context a user supplies. It can't see how code is actually linked, confirm copyright ownership, interpret extra permissions hidden in a file header, or replace review of a proprietary agreement. License exceptions, dual license choices, and LicenseRef identifiers are treated as custom rather than confidently evaluated. Registry based dependency resolution isn't included, and some lockfile formats provide little or no license data. Content licenses are mostly outside scope. The project also notes that published foundation positions can still be treated differently by courts in different jurisdictions.

The hosted checker can be used without a displayed price, and the complete engine is available under the MIT license. That makes it useful for an early engineering or procurement screen, a repeatable CI adjacent check, or an educational comparison before a team reads the actual license text. It is not legal advice, and the maintainers explicitly recommend qualified review for ambiguous or high risk situations. Teams should therefore treat its output as a traceable starting point rather than a legal clearance certificate. Within that role, it offers more useful detail than a flat compatibility table because it combines direction, distribution model, integration method, obligations, source links, and stable rule identifiers in one result.