Anthropic Built a Fix That Proves the Problem 🔍
Right, so Anthropic dropped Tool Search on November 24th alongside Claude Opus 4.5, and I need you to sit with the implications for a second because they're brutal for MCP.
Tool Search lets you mark tools as defer_loading: true. When you do that, the tool's schema never enters your context window until Claude actually needs it. Claude gets a name and nothing else. When a task comes in that requires the tool, Claude calls the Tool Search tool, pulls the schema, and only then does it have the full definition loaded. Lazy loading for AI tools. Sounds boring. It's not.
Here's the number that should make you spit your tea out: accuracy went from 49% to 74% on Opus 4. On Opus 4.5, it climbed from 79.5% to 88.1%. Same model. Same tools. The only difference is not loading the tool definitions upfront.
I'll say it again. The mere presence of tool schemas in context was tanking accuracy by 25 percentage points. Twenty-five. Not because the model got confused by irrelevant tools (though it did). Because the context window was full of JSON nobody was using, and the model was drowning in it.
