Claude/Code/Q/In what sense are MCP tools loaded on-demand

• In what sense are MCP tools "loaded on-demand" in Claude/Code?

  • Answer

    • AI/Response
      • "Loaded on-demand" refers to a specific Claude/Code feature called MCP Tool Search. Official docs: Scale with MCP Tool Search
      • How it works (from official docs):
        • Claude Code automatically enables Tool Search when MCP tool descriptions would consume more than 10% of the context window
        • When triggered: MCP tools are deferred rather than loaded into context upfront
        • Claude uses a built-in search tool (MCPSearch) to discover relevant MCP tools when it needs them
        • Only the tools Claude actually needs are then loaded into context
      • Why? Tool definitions from many MCP servers can be verbose and numerous — loading them all upfront could consume a significant slice of the context window even when most are never used in a session.
      • Default behaviour: Enabled by default (except when ANTHROPIC_BASE_URL points to a non-first-party host, since most proxies don't forward tool_reference blocks). Requires Sonnet 4+ or Opus 4+; Haiku models don't support it.
      • Control via env var ENABLE_TOOL_SEARCH:
        • (unset) — enabled by default
        • auto — activates when MCP tools exceed 10% of context
        • auto:<N> — custom threshold (e.g. auto:5 for 5%)
        • true — always on, including non-first-party base URLs
        • false — disabled; all MCP tools loaded upfront
      • You can also disable the search tool specifically: permissions.deny: ["MCPSearch"]

  • Related

    • MCP
    • Claude/Code