Do Codex and series features cost extra AI usage?

They use your own API keys, same as book generation, and the overhead is small by design. The story bible is compressed into a compact block with a strict size budget before it is sent, so chapters cost roughly the same as before. A few actions run extra AI requests only when you trigger them: "Extract from manuscript" (one request), the Consistency check (one request per finished chapter), character portraits (one image request per portrait), and in fiction the automatic chapter recap that powers series memory (one small request per written chapter). Nothing runs in the background without you.