Member of Technical Staff at Anthropic. Previously at @github, @dependabot, @monzo, @gocardless.
Brooklyn, NY
Joined May 2009
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Harry Marr retweeted
Introducing Claude Opus 4.6. Our smartest model got an upgrade.
Opus 4.6 plans more carefully, sustains agentic tasks for longer, operates reliably in massive codebases, and catches its own mistakes.
It’s also our first Opus-class model with 1M token context in beta.
Harry Marr retweeted
Opus 4.5 is a very good model, in nearly every sense we know how to measure.
I’m also confident that it’s the model that we understand best as of its launch day: The system card includes 150 pages of research results, 50 of them on alignment.
It’s a good’un - really excited to see this ship. I’ve had lots of moments of joy watching it crush problems I thought it wouldn’t be able to handle. It’s not only smarter, but also more token-efficient *and* 1/3 the cost per token compared to the previous Opus.
Harry Marr retweeted
Big day - announcing @arcjet's Series A + our new local AI security model 🚀
An opt-in AI layer that runs expert security analysis for every request, entirely locally. 🏡
Accurate detection is the hardest part of security. 🎯
Legacy network-edge tools see packets, not users or logic. Real context lives in your code - where better decisions can actually be made. 🤖
That’s why we built Arcjet’s first AI security model. 🛡️
It runs inference locally in milliseconds, right inside your request handlers. Adds an extra layer to your defenses so you can ship faster and safer. 🍰
Arcjet now protects 500+ production apps used by 1,000+ developers - stopping bots, scrapers, spam, and fake accounts. 🕷️
So we’ve raised an $8.3M Series A led by @pluralplatform, bringing total funding to $12M. Also participating: @a16z, @seedcamp, @feross, and @jeffiel 💸
I'm excited to work alongside a small but exceptional team building the security platform that ships with your code. 🚀
ALT The Arcjet team, October 2025
Harry Marr retweeted
Introducing Claude Sonnet 4.5—the best coding model in the world.
It's the strongest model for building complex agents. It's the best model at using computers. And it shows substantial gains on tests of reasoning and math.
ALT A benchmark table for Claude Sonnet 4.5 showing leading performance across many domains, including agentic coding, computer use, mathematics, graduate-level reasoning, and financial analysis.
If you're interested in AI safety research, I can't think of a better way to spend a few months
We’re running another round of the Anthropic Fellows program.
If you're an engineer or researcher with a strong coding or technical background, you can apply to receive funding, compute, and mentorship from Anthropic, beginning this October. There'll be around 32 places.
ALT A drawing of two hands manipulating abstract shapes
Harry Marr retweeted
⚡️ We've just went live on ProductHunt with @lingodotdev!
From a @Cornell AI OSS hackathon to first customers, @ycombinator, @TechCrunch, $4.2M Seed round, and @ProductHunt today!
Come see what we've been building, and support us!
(link below)
Harry Marr retweeted
I’m excited to announce @arcjet has raised $3.6m seed funding to build the future of developer security, led by @zanelackey at @a16z! Also participating in the round are @seedcamp and a roster of great angels.
Although deploying code has become simpler, production security remains too difficult. Developers need a set of tools that solve real problems in the way they’re used to - with code.
Arcjet helps developers deal with:
- Spam & fraudulent signups
- Unwanted bots scraping content
- Enforcing dynamic, per user rate limits.
- Scanners and attacks like SQLi, XSS, etc
Embedding security rules into the application also means they can be tested. If you’ve ever turned on a new security tool then you know the pain of suddenly breaking production because you couldn’t test it locally or on staging!
For the last few years I’ve been playing with devtools all day every day for the @consoledotdev newsletter, so I’m excited to be working on bringing a focus on developer experience to security tooling!
More details on the Arcjet blog (link below)
@arupchak @charlesfitz @tweetsbycolin @duncanjennings @geoffbelknap @harrymarr @ianlivingstone @mxstbr @stopman @dessaigne @nitayj @synopsi @rishabhkaul @arcurn @swyx @t3dotgg @thomaspaulmann
Great to see Quotient launch — congrats @JuliaANeagu and team! Worth checking out if you're building LLM-powered products
I am excited to announce @QuotientAI!
Quotient enables developers to evaluate, improve and ship high quality AI products through fast, real-world, data-backed experimentation.
A =gemini(prompt, range) function in Google Sheets would be so damn useful. I'd gladly pay for the tokens.
It'd be trivial to build and could only help AI revenue. Seems like a no brainer. Curious why Google still hasn't shipped it
I went down a bit of a rabbit hole while trying to predict how many tokens an OpenAI prompt would use... hmarr.com/blog/counting-open…
Harry Marr retweeted
Today, Dependabot will cut false positives and reduce alert fatigue substantially -- to the tune of 15% of all npm alerts.
github.blog/2023-05-02-depen…
Dependabot's been taking some lessons on human social cues, so if you keep ignoring it, poor lil Dependabot will now take the hint and stop pestering you nitter.cf/github/status/16135967…
Welcome to a quiet(er) place. Dependabot now pauses pull request activity for inactive repos so you can focus on what matters most. github.blog/2023-01-12-a-sma…
Scrolling through the replies to this tweet brought a big smile to my face. Such a great reminder of who we’re building GitHub for.
I always enjoy watching these Descript product update videos. CEO-talking-at-the-camera videos usually feel over-produced, over-rehearsed, and full of marketing speak, but these manage to avoid that trap.
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Fun write up of a super cool feature—congrats on the launch.
I loved learning about how @jonhoo’s Noria project worked and it’s great to see more people (in addition to @readysetio) bringing this tech to life.
Today we're announcing PlanetScale Boost, an optimization layer that makes your existing SQL queries up to 1,000× faster.
It sounds too good to be true, but it's real and it's here. And there's so much I want to write about it! Here is my first write-up:
planetscale.com/blog/how-pla…
ICYMI, all open source projects on @GitHub can now choose to receive private vulnerability reports. No more 0-days showing up in your public issue tracker, no more setting up security mailboxes for your open source projects!
