What a CLA Actually Transfers
A Contributor Licence Agreement is a legal instrument under which contributors either assign copyright in their patches outright to the project steward, or grant the steward a broad enough licence to sublicence the work on any terms it chooses. The distinction between assignment and licence matters at the margins, but the practical effect is identical: the steward accumulates a consolidated copyright position that no individual contributor can veto.
Without that consolidation, changing a licence requires the agreement of every contributor whose code remains in the codebase. For a project with years of commits from hundreds of developers, that is effectively impossible. With it, the steward can relicence on its own initiative — quietly, quickly, and legally.
The Cases That Prove the Rule
HashiCorp's August 2023 move from the Mozilla Public Licence 2.0 to the Business Source Licence was announced in a single blog post and took effect immediately. It was possible because contributors to Terraform and the other HashiCorp tools had signed a CLA assigning copyright — or granting an equivalent perpetual, sublicensable licence — to HashiCorp, Inc. No contributor vote was held. None was required.
Elastic's 2021 relicensing of Elasticsearch and Kibana from Apache 2.0 to the Server Side Public Licence followed the same logic. Elastic N.V. held the consolidated copyright through its CLA, so the move required no external consent. When Elastic later returned both products to the GNU Affero General Public Licence in 2024, it exercised exactly the same lever in the opposite direction.
Redis Ltd made a comparable move in March 2024, shifting the Redis core from BSD to a dual RSALv2/SSPL licence. Again, Redis Ltd held copyright through contributor agreements; again, no contributor approval was sought or needed.
- CHRONOLOGY OF KEY RELICENSINGS ENABLED BY CLASAS RECORDED
- January 2021Elastic moves Elasticsearch and Kibana from Apache 2.0 to SSPL
- August 2023HashiCorp moves Terraform and related tools from MPL-2.0 to BUSL
- March 2024Redis Ltd moves Redis core from BSD to RSALv2/SSPL dual licence
- August 2024Elastic returns Elasticsearch and Kibana to AGPL
The contrast with the forks tells the story. When the community forked Terraform as OpenTofu under the Linux Foundation, it could not simply re-adopt the MPL-2.0 codebase and move forward under a different commercial arrangement — it had to audit which code was contributed under what terms, adopt a Developer Certificate of Origin rather than a CLA for future contributors, and work from the last MPL-licensed release. OpenSearch, Amazon Web Services' fork of Elasticsearch, faced the same structural problem: starting from the last Apache-licensed release and building forward without Elastic's consolidated copyright. A CLA does not just enable relicensing; its absence forces forks to begin again.
The Accountability Tension
The mechanism is legally clean but politically sharp. A contributor who signs a CLA is, in effect, trusting the steward not to use consolidated copyright against the spirit of the project. That trust is not enforceable. The Open Source Initiative and figures such as Bruce Perens and Simon Phipps have long noted that CLAs create a two-tier structure: the steward holds rights that contributors do not, which is precisely the asymmetry a relicensing requires.
Some stewards have tried to soften this with "inbound equals outbound" policies — promising that the project will never be relicensed to anything more restrictive than the current licence. The Apache Software Foundation's individual contributor licence agreement grants the Foundation a broad licence while the Foundation commits to distributing work under an OSI-approved licence. That commitment is a policy, not a contractual guarantee binding on successors, and it does not appear in the CLA itself.
MongoDB's SSPL episode illustrates the ceiling. MongoDB, Inc. held consolidated copyright and could relicence freely. What it could not do was force the OSI to approve the SSPL as an open-source licence — a recognition that copyright consolidation is a governance lever, not a reputational one.
The pattern across all these cases is consistent: the CLA is drafted quietly, signed by contributors who are focused on the code rather than the legal architecture around it, and then exercised years later when commercial circumstances change. The lag between signing and consequence is what makes contributor agreements so durable as a tool — and so disorienting when the tool is finally used.