MV3

MV3 is short for Manifest V3, the third major version of the Chrome extension manifest specification. It defines how extensions declare their metadata, permissions, and capabilities to the browser, with a focus on security, privacy, and performance. MV3 represents a shift from MV2 in both architecture and API design, aiming to reduce resource usage and improve user control over extensions.

A key architectural change in MV3 is the replacement of persistent background pages with event-driven service workers. Service workers wake up to handle specific events and then shut down, reducing memory and processor overhead when extensions are idle. This change affects how extensions manage state, respond to events, and perform long-running tasks.

MV3 also reorganizes its API surface. The blocking webRequest API, which allowed extensions to intercept and modify network requests in real time, is being phased out in favor of the declarativeNetRequest API, which enforces filtering rules declaratively rather than through imperative code. In addition, the permission model was tightened and a single extension action API was introduced to unify the toolbar and popup UI concepts formerly known as browser_action and page_action.

Developers migrating from MV2 must adapt to the event-driven model, update background code to use service workers, adjust permission declarations, and rework network filtering around declarative rules. MV3 aims to enhance privacy by limiting continuous background activity and requiring explicit, rule-based network interactions.

Adoption has been gradual, with Chrome rolling out MV3 requirements for new extensions and migrating existing ones over time. Other major browsers have varied levels of MV3 support or compatibility, influencing cross-browser extension development and strategy.