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Claude Code lazily loads tool schemas

·2026.04.28 22:34

Key point

Claude Code lazily loads tool schemas via ToolSearch instead of sending them all at once.

Details

Instead of sending the full schema of every tool at once, Claude Code guides the lazy-loading rule through a <system-reminder> inserted only into the first user message of the conversation. To use a needed tool, you must first call ToolSearch; calling a tool directly results in an InputValidationError.

  • Schemas are loaded using the format select:<name>[,<name>...].
  • This includes AskUserQuestion, CronCreate/Delete/List, Enter/ExitPlanMode, Enter/ExitWorktree, Monitor, NotebookEdit, PushNotification, RemoteTrigger, TaskOutput, TaskStop, TodoWrite, WebFetch, WebSearch, and more.
  • About 130 MCP tools such as Slack, Notion, and Gmail also follow the same structure.
  • Skills are also delivered via a separate <system-reminder> containing only the name and a one-line description.

In the author's hi test, about 38k tokens were consumed by the system instructions and reminders alone.

claude-codetool-searchlazy-loadingtool-schemafunction-calling
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