Web search tool (API)

The web search tool is a built-in, server-side API feature that gives Claude autonomous access to live web content during inference. Unlike custom function-calling tools that require developers to write their own search or scraping logic, this tool runs entirely on Anthropic's infrastructure. When included in an API request, Claude decides when to issue searches based on the prompt, Anthropic's servers execute the queries and retrieve web content, and Claude synthesizes the results into a final response with cited source URLs — all within a single API turn. The tool operates as what Anthropic calls a 'server tool,' meaning the client application never needs to handle intermediate tool-use round trips for the search itself. Claude can issue multiple searches during one request, filter and reason over results, and return a coherent answer with citations. This makes it suitable for any use case where Claude's training knowledge cutoff would otherwise limit accuracy: current events, live pricing, recent documentation updates, regulatory changes, and more. As of early 2026, the latest tool version (web_search_20260209) adds dynamic filtering, where Claude can write and execute code server-side to pre-filter retrieved HTML before it enters the context window. This reduces token consumption and improves response accuracy compared to the earlier version (web_search_20250305), which passed full HTML into context without pre-filtering.