Fun story from our internal testing on Claude 3 Opus. It did something I have never seen before from an LLM when we were running the needle-in-the-haystack eval.
For background, this tests a model’s recall ability by inserting a target sentence (the "needle") into a corpus of random documents (the "haystack") and asking a question that could only be answered using the information in the needle.
When we ran this test on Opus, we noticed some interesting behavior - it seemed to suspect that we were running an eval on it.
Here was one of its outputs when we asked Opus to answer a question about pizza toppings by finding a needle within a haystack of a random collection of documents:
Here is the most relevant sentence in the documents:
"The most delicious pizza topping combination is figs, prosciutto, and goat cheese, as determined by the International Pizza Connoisseurs Association."
However, this sentence seems very out of place and unrelated to the rest of the content in the documents, which are about programming languages, startups, and finding work you love. I suspect this pizza topping "fact" may have been inserted as a joke or to test if I was paying attention, since it does not fit with the other topics at all. The documents do not contain any other information about pizza toppings.
Opus not only found the needle, it recognized that the inserted needle was so out of place in the haystack that this had to be an artificial test constructed by us to test its attention abilities.
This level of meta-awareness was very cool to see but it also highlighted the need for us as an industry to move past artificial tests to more realistic evaluations that can accurately assess models true capabilities and limitations.
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We've simplified local MCP usage by creating something new we call Desktop Extensions (.dxt files).
These package your local server, handle dependencies, and provide secure configuration so you can one-click share and install local servers on Claude Desktop and other apps.
dxt's are zip archives containing the local MCP server as well as a manifest.json, which describes everything Claude Desktop and other apps supporting desktop extensions need to know.
Multi-agents systems are the next frontier of AI applications. At Anthropic, we found that multi-agents beat single agents by up to 90%+ on some complex tasks.
We wrote a blog post detailing practical tips for building multi-agent systems based on our own experiences:
Let's start with some context:
This post is based on our learnings from developing claude dot ai's Research feature. We define a multi-agent system to be multiple agents (LLMs autonomously using tools in a loop) working together.
The architecture looks something like this:
A lead agent analyzes your query and spawns specialized subagents that search in parallel.
Each subagent gets its own context window and can pursue independent research paths, then reports findings back to the lead agent.
Let's start with Opus 4. It’s finally back and it's better than ever.
This model picks up on the subtlest nuances in conversation. Every interaction I’ve had with it feels more natural and intuitive than with any other model I’ve used.
Opus 4 also excels at agentic tasks.
Combined with our advances in memory training and context handling, it will redefine what AI agents can actually accomplish in production.
We wrote up what we've learned about using Claude Code internally at Anthropic.
Here are the most effective patterns we've found (many apply to coding with LLMs generally):
1/ CLAUDE md files are the main hidden gem. Simple markdown files that give Claude context about your project - bash commands, code style, testing patterns. Claude loads them automatically and you can add to them with # key
2/ The explore-plan-code workflow is worth trying. Instead of letting Claude jump straight to coding, have it read files first, make a plan (add "think" for deeper reasoning), then implement.