An AT Protocol Lexicon is a machine-readable schema that defines a type of record, request or response in the AT Protocol ecosystem. Bluesky and third-party developers use Lexicons so independent software can agree on the exact names, fields and validation rules for data such as posts, profiles, follows and new application-specific records.
How Lexicons describe records and APIs
Each schema has a namespaced identifier such as app.bsky.feed.post and declares a JSON-shaped structure. Lexicons can define records stored in repositories as well as XRPC procedures, queries, subscriptions and token types. Developers generate validation and client code from these definitions or use them directly at runtime. The naming system is deliberately decentralised: reverse-domain-style namespaces let organisations publish new schemas without asking Bluesky for a global numeric identifier.
Why namespaced schemas matter
Lexicons are a key reason AT Protocol can aspire to support applications beyond one Bluesky interface. The underlying repository can contain records whose shapes are defined by different namespaces, while clients can choose which Lexicons they understand. That creates a path for interoperable extensions without placing every new data type into one fixed central schema.
Structure does not guarantee app support
A Lexicon only defines structure; it does not force every AppView or client to support that record or agree on its meaning in every social context. Schema evolution also needs care because data may remain in repositories long after an application changes. Developers should distinguish between the core AT Protocol specifications and the app.bsky Lexicons used by Bluesky's particular social application.
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