Citations (API)

The Citations API is a feature of the Anthropic Messages API that enables Claude to automatically attach precise source references to the claims it makes when answering questions about user-provided documents. Instead of requiring developers to engineer complex prompts to extract verbatim quotes, the API handles document chunking internally and interleaves the generated response text with structured citation objects that point back to exact locations in the source material — page ranges for PDFs, character index ranges for plain text, or block index ranges for custom content. When citations are enabled, Claude processes each document by breaking it into sentence-level chunks (for PDFs and plain text) or using the developer-defined blocks (for custom content). The model then generates a response where each factual claim is accompanied by a citation object containing the quoted text, document index, document title, and precise location data. The quoted text itself (`cited_text`) is provided as a convenience field and does not count toward output token billing. Citations are designed for applications where verifiability and source attribution matter: legal research tools, financial document analysis, enterprise RAG pipelines, compliance workflows, and customer support systems. The feature is available across all current Claude models (with the exception of Claude Haiku 3), on the direct Anthropic API, Amazon Bedrock, and Google Cloud Vertex AI, at no additional cost beyond standard token pricing.