Cosmo OSS Schema Registry Compatibility Mode for Apollo Router and Gateway is a feature that enables users to transition seamlessly from Apollo GraphOS to the open-source Cosmo schema registry without changing their existing Apollo Router or Gateway setup. This mode works by publishing Subgraphs to Cosmo, configuring WebHooks, using specific CLI commands to fetch Supergraph compositions, generating Supergraph Schema configuration files, and updating Apollo Router or Gateway configurations accordingly. The compatibility mode allows users to migrate from a closed-source solution (Apollo GraphOS) to an open-source one (Cosmo), enabling gradual migration without disrupting existing setups.