The latest Rex preview release includes a very important subsystem I've been working on for weeks to enable full forwards+backwards API compatibility with clients and servers. This was gating the Linux test program and massively opening up the Mac test program. We just invited another 1,000 (to Mac) and if it goes well this will step up considerably this week. The Linux test program will likely start this week (separate invite waves amongst existing testers). The API compatibility system is super cool. At a high level: Every API change is a rule on a JSON pointer (RFC 6901). For each message, we compile all the rules between your version and ours into a trie keyed by JSON path tokens. This happens lazily since we don't want a full quadratic explosion (a message with N revisions has N possible chains of length 1 to N). We don't want to build N^2/2 trie nodes when most WON'T be visited. Next, we wrote a custom recursive descent streaming JSON editor (forward-pass only). If a key doesn't match any subtree in the trie, we fast-path copy the full value onto the wire (nothing in it can possibly matter for compatibility). Equal chains of changes are interned to the same pointer, so when we broadcast an event to many old clients, we translate it once per distinct chain, not once per client. The result of all this plus some other work is that we simulated various shapes of 10,000 long revision chains (note: we're on revision 4 at the moment...) with pathological changes of various shapes and an upgrade costs about 35 _microseconds_ on a 20KB JSON body.

Opus 5.5 fast mode in the Claude Platform is no longer in research preview, and we’ve removed the waitlist! It’s the same Opus 5.5 but up to 2.5x faster, at $8 / $40 per million tokens. It's also available in Microsoft Foundry (Hosted on Azure), with Google Cloud coming soon. Fast mode in Claude Code remains unchanged, and uses extra usage credits. See our docs here: platform.claude.com/docs/en/… xitter.cf/ClaudeDevs/status/2054…

Roadster xitter.cf/i/broadcasts/1yKAPwNgg…

What's new in macOS 27.2 beta 4

The "whiteboard defense:" I should be able to pull you aside at any moment and ask you to explain any customer-facing system you've shipped. You should be able to clearly explain how it works and defend the decisions you made. This is my benchmark for responsible AI usage. I don't expect line-level familiarity with the code. I don't care if you remember the exact function name or implementation detail. You may not even know it. I don't care. But if I ask "why did you do X instead of Y?", "what happens if this actor behaves maliciously?", "what data structure did you use here and why?", or "where does this fail?" you should be able to answer confidently. For PoCs, demos, experiments, whatever: I don't care. Generate 100% of it and understand none of it. Speed over quality every time in those specific scenarios. But if you're shipping customer-facing work, you can't be shipping things you don't understand at a high level.

Jacob is correct here—we really do earnestly believe AI could kill all humans! I personally think it is >10% within the next decade. I believe Anthropic is trying its best, but we do not yet have a plan to solve alignment for superintelligence and are not clearly on track to.

iPhone Relay This could be a game changer. 👀 iOS 27 adds a new “iPhone Relay” feature that lets you use the same number on TWO iPhones and switch between them. • Main iPhone keeps the primary eSIM • Second iPhone gets a secondary eSIM • Same number on both iPhones • Switch between them anytime • Settings stay on the main iPhone Both iPhones need iOS 27, and carrier support appears to be required. For people who carry two iPhones, this could be seriously useful.