Feature Request: Support defer_loading and tool_search for dynamic tool discovery (parity with Anthropic & OpenAI)

[leeclouddragon]

Summary

Both Anthropic (Claude Sonnet 4.0+) and OpenAI (GPT-5.4+) now support deferred tool loading and tool search at the API level. The Gemini API currently lacks this capability.

Same feature request as googleapis/js-genai#1416 — filing here for visibility across both SDKs since this is an API-level feature.

Problem

When building AI agents with many tools (30+):

1. Context bloat: All tool schemas consume context tokens upfront. A typical multi-server MCP setup can consume 50k+ tokens in tool definitions alone.
2. Tool selection accuracy: Model accuracy degrades beyond 30-50 tools.
3. Client-side workarounds break caching: Pre-filtering tools before each API call changes the tools parameter, invalidating prefix cache.

What competitors have

Anthropic

• defer_loading: true on tools — schema sent but not rendered into model context
• tool_search_tool_regex/bm25 — model searches deferred tools, matched schemas expanded within same API call
• Zero extra round-trips, cache preserved
• Up to 10,000 tools

OpenAI

• Same defer_loading: true mechanism
• Namespaces to group tools — model only sees group name + description
• Discovered schemas injected at end of context window to preserve cache
• Hosted and client-executed search modes

Proposed for Gemini API

1. defer_loading: true on functionDeclarations
2. Built-in tool_search tool for on-demand discovery
3. Automatic schema expansion within generateContent call

Related issues

• Feature Request: Support defer_loading and tool_search for dynamic tool discovery (parity with Anthropic & OpenAI) js-genai#1416 (same request, JS SDK)
• Error when both search and function calls are in the tools[] config option js-genai#435 (search + function calling cannot be combined)
• Tool urlContext - Error 400 "Multiple tools are supported only when they are all search tools." - Is this wanted behavior? js-genai#1157 (urlContext + functionDeclarations cannot be combined)
• [Feature Request] Increase MCP tool limit from 100 to 500 google-gemini/gemini-cli#21823 (100 tool limit)