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Anthropic added the ability to define tools in the system message midway through a conversation to Claude API

clearly official source

Anthropic has made it possible in the beta version of Claude API (header inline-tools-2026-09-15) to define and modify tools directly in the system message midway through a conversation, without having to edit the tools array or invalidate the prompt cache.

Anthropic has made a feature available in the beta version of Claude API that allows tools to be defined directly within the system message midway through an ongoing conversation. The feature runs under the beta header inline-tools-2026-09-15. The tool_addition block can carry a complete tool definition (tool: {"type": "tool_definition", "definition": {...}}), allowing a tool to be added, its schema to be changed, or a switch to a newer version of a server-side tool without having to modify the tools array or invalidate the prompt cache. The same header also covers adding and removing tools by reference.

In combination with the beta header mcp-client-2026-09-15 for the MCP connector, the definition can also be an entire MCP toolset. In this case, the API response records the list of tools loaded from each server in an mcp_tool_listing block, which fixes that list when it is sent back in the next request.

What changed

Why it matters

Developers building agents on Claude API can change tools and their schemas throughout a conversation without having to rewrite the entire tools array or lose the benefits of the prompt cache — this reduces both latency and costs for applications with long contexts and intensive tool use, including MCP servers.

Two audiences, two different impacts

What this means

01

For individuals

Developers working with Claude API and MCP tools can now change tool definitions and schemas during a conversation without having to rewrite the entire call and without losing the prompt cache.

What to do Try the beta header inline-tools-2026-09-15 (and optionally mcp-client-2026-09-15) in the Claude API documentation when working with dynamic tool definitions.
More practical updates →
02

For a business

Companies running agents on Claude API can reduce the costs and latency associated with prompt cache invalidation when updating tools, particularly in production deployments with long contexts or connections to MCP servers.

Development
What to decide Consider using the new beta feature in production Claude API integrations with tools or MCP servers where changes to tools have previously invalidated the prompt cache.
More business impacts →
Beta Claude API inline tools MCP system message tool definition

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Event sources

clearly official source · 1 publisher, 0 independent. We count feeds from the same owner only once.

1
Claude Platform Release Notes primary source · first detected Tools can now be defined inside a mid-conversation system message , in beta on the Claude API with the inline-tools-2026-09-15 beta header. A tool_addition block can carry the…