tools changedmanaged-agents/tools
Nearest release: v2.1.296, published under an hour after this site recorded the change. Shown because the two are within 24 hours of each other. Nothing here says the release caused the edit.
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14 Aug 2026
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501A custom tool's name can't start with `mcp__` or `ant__`, or match the name of a built-in agent tool. A request that breaks this rule returns a 400 error. An agent that already has a custom tool whose name starts with `ant__` accepts an update only if the update sends `tools` without that name, so rename or remove the tool in your next update. A new session is refused with a 400 error if such a tool is on its agent (after any [overrides](https://platform.claude.com/docs/en/managed-agents/sessions#override-agent-configuration-for-a-session)), on an agent in `subagents.predefined_agents`, or on an agent in `workflows.predefined_agents`. Sessions that already exist aren't affected.
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501503The agent calls its custom tools during a session. To receive the calls and return results, see [Session event stream](https://platform.claude.com/docs/en/managed-agents/events-and-streaming#handling-custom-tool-calls).
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503505For sessions that run in a self-hosted sandbox, the environment worker can [serve custom tools from the sandbox](https://platform.claude.com/docs/en/managed-agents/self-hosted-sandboxes-custom-tools). These can include tools that wrap an MCP server inside your network.
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