Anthropic Just Admitted MCP Has a Context Problem
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. This is an admission dressed up as a feature I've been banging on about MCP's context cost since July. The Playwright MCP server alone burns 12.8k tokens just sitting there. Load four or five MCP servers and you've torched half your context window before asking the model to do anything useful. Tool Search is Anthropic's answer. And the answer is: don't load the tools. Think about that for a second. The company that created MCP built a feature whose entire purpose is avoiding the cost of loading MCP tool definitions. If that isn't an admission that the protocol has a context problem, I don't know what is. They didn't say "we've made MCP more efficient." They said "here's a way to not load MCP schemas until the last possible moment." The fix is the diagnosis. And the timing is proper interesting. Three weeks earlier, on November 5th, mcporter d