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Deferred tools are available to the agent but their complete schemas are withheld until the model discovers them. This reduces the initial tool list for agents with many optional capabilities.

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Mark a tool as deferred

Add hastekit.WithDeferred(true) to a function tool’s constructor. For MCP, pass mcpclient.WithDeferredTools("search_issues", "list_projects"); use the original server names, even when the connector has a prefix. Enable prompts.ResolveDeferredTools, or the standard resolver chain, on the system prompt. The agent adds its tool-search mechanism when deferred tools are present; the resolver explains that mechanism to the model.

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Execution behavior

The model discovers relevant tools, obtains their full schemas, and calls them normally. Deferral is not background execution, lazy authentication, or an access-control boundary. The application still configures the tools and connectors, and MCP discovery may still contact the server. Use it for large tool catalogs where most runs need only a subset. Keep a small, frequently used tool set visible directly. Do not defer a tool merely to hide a sensitive capability: filter it out or enforce authorization in middleware instead. A deferred tool can also require approval. Discovering its schema does not grant approval to execute it.