Not open source. Any use is allowed except competing with the publisher. Two years after release the version changes over to MIT.
On this page
What it does
The Functional Source License 1.0 is the first version of the license, replaced by 1.1 in February 2024. It lets you use, copy, change, build on and pass on the software for any purpose that is not a Competing Use. In this version a Competing Use is use in or for a commercial product or service that competes with the software, or with another product the publisher offers using it. The text names three examples: a substitute for one of the publisher's products, a use that exposes the interfaces of the software, and a product with the same or substantially similar functionality. Internal use, non-commercial education and non-commercial research stay permitted.
Details
The difference between 1.0 and 1.1 is worth knowing. In 1.0 the restricted terms end on the second anniversary of the day the version was made available, and the MIT terms take their place. In 1.1 the MIT grant is added next to the restricted terms instead. The practical result is the same two-year wait, and the older wording is the one that applies to a package published under 1.0. Version 1.0 also counts exposing the interfaces of the software as a competing use, which 1.1 no longer names.
What it allows and requires
Choose a category to see the full grant in one scannable list. Permissions show what the license allows, limits show what it withholds, and obligations show the conditions your release process must satisfy.
Permissions
Internal use
Using the software inside your own company, and access for your own people, is a permitted purpose.
Modify
You may change the software and build new works on it for any permitted purpose.
Distribute
You may pass copies on, as long as the use is not a competing one and the terms travel with the copy.
Patent use
The grant covers the publisher's patents, to the extent a permitted use would infringe them.
Limits
Competing use
You may not offer the software to others in a commercial product or service that substitutes for it, or for a product the publisher offers with it.
Use trademark
You may name the publisher as the origin of the software. The license grants no other right to its names or marks.
Patent claim
If you claim the software infringes a patent, your patent license under these terms ends immediately.
Hold liable
The authors disclaim warranty. Recipients cannot hold them liable for damages arising from the software, except where law forbids that disclaimer.
Obligations
Include license
Include a copy of these terms, or a link to them, with every copy, change or new work you pass on.
Include copyright
Leave every copyright notice that came with the software in place in anything you pass on.
Change license
Two years after a version was made available, these terms end for that version and MIT applies instead.
What FSL-1.0-MIT requires when you ship
FSL-1.0-MIT is readable source with use limits. The work is to match your use to the written grant, then keep the text on the record so procurement can read it. Do not treat a source-available identifier as OSI-style open source because the code is on GitHub.
You are not using it in or for a commercial product or service that competes with the software or with a product the publisher offers using it.
You are not exposing the interfaces of the software to others, which this version names as a competing use.
You use it for your own internal work, for non-commercial teaching or research, or in development and managed services for someone who already holds this license.
You include a copy of the terms, or a link to them, and leave every copyright notice in place, in any copy or change you pass on.
You have written down the version you depend on and the date the publisher made it available, because that date is when it changes over to MIT.
The duties named in FSL-1.0-MIT
The written grant is the source of the limits. These rows are the usual conditions teams have to match against their actual use, then keep on the record.
Include license
Include a copy of these terms, or a link to them, with every copy, change or new work you pass on.
Include copyright
Leave every copyright notice that came with the software in place in anything you pass on.
Change license
Two years after a version was made available, these terms end for that version and MIT applies instead.
Things to be aware of
- Teams read the MIT in the name as the license they have. MIT is what the version changes over to after two years, not what applies today.
- Teams apply the 1.1 wording to a 1.0 package. The two versions define a competing use differently, so read the text that shipped.
- Teams expect a project on 1.0 to stay there. A newer release can carry 1.1 instead, and the package you upgrade to is a fresh decision.
What the Functional Source License 1.0, MIT Change License does not do
Search results often flatten Functional Source License 1.0, MIT Change License into a slogan. These are the usual misreads. FSL-1.0-MIT is a grant with conditions, not a permission to skip the paperwork below.
- The Functional Source License is not an open source license. Readable code is not permission to build a product or service that competes with the publisher.
- It does not turn the whole project into MIT on one day. Each version changes over two years after that version was made available.
How FSL-1.0-MIT differs from nearby licenses
These licenses are often confused with FSL-1.0-MIT, but their release duties differ. Each row summarizes what the license requires when you ship. Open the linked page for the full checklist.
- FSL-1.0-MIT
- Not open source. Any use is allowed except competing with the publisher. Two years after release the version changes over to MIT.
- FSL-1.1-MIT
- Not open source. Any use is allowed except competing with the publisher. Two years after a version is released it is also under MIT.
- FSL-1.0-ALv2
- Not open source. Any use is allowed except competing with the publisher. Two years after release the version changes over to Apache 2.0.
- source-available
- You can read the code. That is not the same as permission to use it any way you like. Source available is not open source.
Common questions about Functional Source License 1.0, MIT Change License
Answers to common questions about what Functional Source License 1.0, MIT Change License requires, when its duties apply, and what evidence belongs with a release.
What is the Functional Source License 1.0, MIT Change License?
The Functional Source License 1.0 is the first version of the license, replaced by 1.1 in February 2024. It lets you use, copy, change, build on and pass on the software for any purpose that is not a Competing Use. In this version a Competing Use is use in or for a commercial product or service that competes with the software, or with another product the publisher offers using it. The text names three examples: a substitute for one of the publisher's products, a use that exposes the interfaces of the software, and a product with the same or substantially similar functionality. Internal use, non-commercial education and non-commercial research stay permitted.
What does FSL-1.0-MIT require when you ship a product?
You are not using it in or for a commercial product or service that competes with the software or with a product the publisher offers using it. You are not exposing the interfaces of the software to others, which this version names as a competing use. You use it for your own internal work, for non-commercial teaching or research, or in development and managed services for someone who already holds this license. You include a copy of the terms, or a link to them, and leave every copyright notice in place, in any copy or change you pass on. You have written down the version you depend on and the date the publisher made it available, because that date is when it changes over to MIT.
Is FSL-1.0-MIT open source?
Not in the OSI sense. FSL-1.0-MIT is source-available: you can read the code, but the grant restricts how you may use it. Read the actual text.
How is FSL-1.0-MIT different from Functional Source License 1.1, MIT Future License?
FSL-1.0-MIT asks this: Not open source. Any use is allowed except competing with the publisher. Two years after release the version changes over to MIT. Functional Source License 1.1, MIT Future License asks this: Not open source. Any use is allowed except competing with the publisher. Two years after a version is released it is also under MIT. Open the Functional Source License 1.1, MIT Future License page for what that license requires when you ship. Do not treat the SPDX ids as interchangeable because the short names look similar.
Where do I record FSL-1.0-MIT for a buyer?
The catalog records FSL-1.0-MIT as source available, and as a license a network deployment does not clear. A network distribution review item therefore appears on the checklist for SaaS and mixed projects, and publishing stays blocked until a person confirms it. The identifier is a templated one, so the fetch never lands it on confirmed, even when the text matches the publisher's copy word for word. A filled-in copy is labelled as the license with package-specific parameters instead of modified, and the text found in the package is put in front of you.
Where do I record FSL-1.0-MIT for a buyer?
The catalog records FSL-1.0-MIT as source available, and as a license a network deployment does not clear. A network distribution review item therefore appears on the checklist for SaaS and mixed projects, and publishing stays blocked until a person confirms it.
The identifier is a templated one, so the fetch never lands it on confirmed, even when the text matches the publisher's copy word for word. A filled-in copy is labelled as the license with package-specific parameters instead of modified, and the text found in the package is put in front of you.
Related licenses
See also
The hub is the pillar for this cluster. Sibling licenses are the other spokes. Product FAQ links explain how SourceTrust records the duty, not the license text itself.
Practical guidance for procurement review, not legal advice. Confirm high-stakes use with counsel.