Jarred Sumner
@jarredsumner

building @bunjavascript at @anthropicai. formerly: @stripe (twice) @thielfellowship. high school dropout. npm i -g bun

 San Francisco, CA
bun.com
Joined May 2014
3,974 Photos and videos
People frequently ask: > How is Bun sustainable? If I bet my company’s tech stack on Bun, will Bun still be around in a few years? We didn’t have a great answer to this question, until today
Bun
@bunjavascript
2 Dec 2025
Bun is joining Anthropic! bun.com/blog/bun-joins-anthr…
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JavaScript was never designed for the server. If that doesn't change soon, JavaScript on the server will be replaced by Rust. The fix needs to happen in the engine. Sound types. Ahead-of-time compilation. Threads with shared objects.
Arto Bendiken
@bendiken
Sep 23
"I love Rust! Rust is amazing ...if you never, ever, EVER have to look at it yourself." — @dhh
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JavaScript has been through this before. In 2011, Google set out to replace JavaScript with Dart, arguing JavaScript couldn't be fixed by evolving it. Then ES2015 shipped. It's time to do that again.
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JavaScript engines like JavaScriptCore and V8 have some of the best compiler tech in the world. An open question: with sound types, ahead-of-time compilation for statically typed code, and JITs for everything else, can JavaScript be competitive with Rust and C?
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Opus 5.5 is a very good model and I expect it will quickly become the daily driver for a lot of people. The writing style is so much better. It writes like Opus 4.6 and codes like Fable 5.1.
Claude
@claudeai
Sep 22
Introducing Claude Opus 5.5, the first model in our new Claude 5.5 family. It performs at the level of Claude Fable 5.1 for most tasks, and costs 40% less to run than Opus 5.
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Bun v1.4.3 ships on Monday
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In the next version of Bun `Bun.FetchSession` gives you a `fetch` function with its own keepalive connection pool
Dark slide with the Bun logo and a "new in Bun" badge. Large headline: "Bun.FetchSession". Subtitle: "fetch() with its own connection pool." Below it, a code sample: const api = new Bun.FetchSession({ proxy: "http://proxy:8080", tls: { ca }, keepAlive: { idleTimeout: 30 }, }); await api.fetch(url); new Octokit({ request: { fetch: api.fetch }, }); "api.fetch" is highlighted in both places. Three cards at the bottom read: "Own pool: isolated keep-alive", "Proxy + TLS: set once per session", "Drop-in: works in any SDK".

ALT Dark slide with the Bun logo and a "new in Bun" badge. Large headline: "Bun.FetchSession". Subtitle: "fetch() with its own connection pool." Below it, a code sample: const api = new Bun.FetchSession({ proxy: "http://proxy:8080", tls: { ca }, keepAlive: { idleTimeout: 30 }, }); await api.fetch(url); new Octokit({ request: { fetch: api.fetch }, }); "api.fetch" is highlighted in both places. Three cards at the bottom read: "Own pool: isolated keep-alive", "Proxy + TLS: set once per session", "Drop-in: works in any SDK".

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PHP got one thing right JavaScript never had In next week’s version of Bun
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Claude Code's idle memory dropped 14% on Linux with Bun v1.4.1
Line chart: "Claude Code — p90 idle memory usage (Linux)", headline "−14%, lower is better". Two lines plot 90th-percentile memory of idle Claude Code processes against process uptime, from about 1 hour to 2 days. The grey "Before" line rises from 490 MiB to 665 MiB; the orange "Bun v1.4.1" line stays below it the whole way, rising from 431 MiB to 572 MiB. Footnote: p90 RSS after 3+ min idle, Linux, Claude Code 2.1.260+ vs earlier versions, same days and session age, Sep 4–9, 2026; y-axis starts at 250 MiB.

ALT Line chart: "Claude Code — p90 idle memory usage (Linux)", headline "−14%, lower is better". Two lines plot 90th-percentile memory of idle Claude Code processes against process uptime, from about 1 hour to 2 days. The grey "Before" line rises from 490 MiB to 665 MiB; the orange "Bun v1.4.1" line stays below it the whole way, rising from 431 MiB to 572 MiB. Footnote: p90 RSS after 3+ min idle, Linux, Claude Code 2.1.260+ vs earlier versions, same days and session age, Sep 4–9, 2026; y-axis starts at 250 MiB.

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Excited to see everyone tomorrow! We’re at capacity with 130+ on the waitlist, so if you can’t make it please mark “can’t go” on Partiful so someone else can grab your spot
Jarred Sumner
@jarredsumner
Aug 28
We're throwing a party in SF for Bun v1.4 on September 9th. If you use Bun (or want to), come hang out partiful.com/e/GJtQ5hI0PaNDy…
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my favorite way to keep claude running overnight without /loop or /goal tell claude something like: “I’m going to sleep, and when I wake up I would love to see X result. Post frequent updates to <slack channel> as you make progress.”
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I feel like this started working around when claude started telling people “go to bed”
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Jarred Sumner retweeted
Bartek Igielski@Igloczek
Sep 4
moved a few projects from @bunjavascript 1.3.14 to 1.4.1, in each case memory usage dropped about 2/3, crazy
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Iza
@izadoesdev
Sep 5
bun 1.4 to bun 1.4.1, noticeable less memory on cold start, and seems to be less memory leaks, also lower CPU usage
Iza
@izadoesdev
Sep 4
switching from bun 1.3.4 to 1.4
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Iza
@izadoesdev
Sep 4
switching from bun 1.3.4 to 1.4
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“RuSt ReWrItE wILL kILl BuN”
Bun
@bunjavascript
Sep 4
Bun hit 32.8 million monthly downloads last month
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Jarred Sumner retweeted
Bun
@bunjavascript
Sep 4
Bun v1.4.1 - Fixes 202 issues, addressing 236 👍 - Reduced idle memory usage - HTTP/2 in Bun.serve() - Big upgrade for Bun's bundler - bun build --compile executables load faster & we shrunk bytecode - Bun starts 1ms faster bun.com/blog/bun-v1.4.1

Bun v1.4.1

Fixes 202 issues, addressing 236 👍. HTTP/2 in Bun.serve, Bun.write() streams Response bodies to disk, self-contained workspace node_modules, bun install --offline and --prefer-offline, WebSocket...

bun.com
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Jarred Sumner retweeted
Bun
@bunjavascript
Sep 4
Bun v1.4.1 reduces idle memory usage
Table: "Bun v1.4.1 reduces idle memory usage." Resident memory 3 minutes after 60 seconds of sustained load, lower is better. Columns: Bun 1.4.1, change vs 1.4.0, Bun 1.4.0, Bun 1.3.14, Node.js 26.7. Next.js SSR: 142 MB (−36%), 222, 1303, 195. Vite dev: 157 MB (−17%), 189, 303, 155. Express: 54 MB (−17%), 65, 77, 81. Fastify: 57 MB (−16%), 68, 80, 91. Elysia: 44 MB (no change), 46, 69, 96. Hono: 34 MB (no change), 35, 53, 92. Method: 60 s at a fixed request rate (40k req/s Elysia and Hono, 25k Express and Fastify, 5k Vite dev, 3k Next.js SSR), 64 connections, then 180 s idle; RSS sampled every 100 ms on Linux x64.

ALT Table: "Bun v1.4.1 reduces idle memory usage." Resident memory 3 minutes after 60 seconds of sustained load, lower is better. Columns: Bun 1.4.1, change vs 1.4.0, Bun 1.4.0, Bun 1.3.14, Node.js 26.7. Next.js SSR: 142 MB (−36%), 222, 1303, 195. Vite dev: 157 MB (−17%), 189, 303, 155. Express: 54 MB (−17%), 65, 77, 81. Fastify: 57 MB (−16%), 68, 80, 91. Elysia: 44 MB (no change), 46, 69, 96. Hono: 34 MB (no change), 35, 53, 92. Method: 60 s at a fixed request rate (40k req/s Elysia and Hono, 25k Express and Fastify, 5k Vite dev, 3k Next.js SSR), 64 connections, then 180 s idle; RSS sampled every 100 ms on Linux x64.

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Jarred Sumner retweeted
Bun
@bunjavascript
Sep 2
Bun v1.4.1 is a big upgrade to Bun’s bundler. On a 200-route React app over simulated 4G, navigating between routes is 38% faster than Bun v1.4.0, and 10% faster than Vite v8.2.2.
Slide titled “Route navigation on 4G”, subtitle “click → lazy route’s chunks fetched, parsed, and rendered by React · lower is better”. Three horizontal bars: Bun 1.4.0, 1104 ms (dark gray); Vite 8, 757 ms (gray); Bun 1.4.1, 681 ms (pink-to-orange gradient, labeled −38%). Table below with columns Bun 1.4.0 / Vite 8 / Bun 1.4.1: Requests per navigation 108 / 108 / 61; LCP, cold first load 736 ms / 560 ms / 576 ms; JS chunks emitted 918 / 908 / 750; Total JS shipped (gzip) 1.75 MB / 1.67 MB / 1.61 MB. Footnote: 200 lazy routes, 800 components, 30 npm deps, React Compiler on in both, headless Chrome, 170 ms RTT / 9 Mbps, cold cache, median of 7 runs × 12 routes; Bun built with bun build –splitting –min-chunk-size=16384 –react-compiler; Vite 8.2 with @vitejs/plugin-react 6 compiler: true.

ALT Slide titled “Route navigation on 4G”, subtitle “click → lazy route’s chunks fetched, parsed, and rendered by React · lower is better”. Three horizontal bars: Bun 1.4.0, 1104 ms (dark gray); Vite 8, 757 ms (gray); Bun 1.4.1, 681 ms (pink-to-orange gradient, labeled −38%). Table below with columns Bun 1.4.0 / Vite 8 / Bun 1.4.1: Requests per navigation 108 / 108 / 61; LCP, cold first load 736 ms / 560 ms / 576 ms; JS chunks emitted 918 / 908 / 750; Total JS shipped (gzip) 1.75 MB / 1.67 MB / 1.61 MB. Footnote: 200 lazy routes, 800 components, 30 npm deps, React Compiler on in both, headless Chrome, 170 ms RTT / 9 Mbps, cold cache, median of 7 runs × 12 routes; Bun built with bun build –splitting –min-chunk-size=16384 –react-compiler; Vite 8.2 with @vitejs/plugin-react 6 compiler: true.

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Merging 1 more memory usage reduction PR targeting long-running often idle servers & CLIs and then finishing release notes and then it will be Bun v1.4.1
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In the next version of Claude Code Claude Code starts 20% faster
Claude Code keynote-style slide on black. Headline: “Time to input box is now 20% faster.” Two horizontal bars: 2.1.250 at 397 ms (gray), 2.1.252 at 318 ms (orange). Footer: Linux x64, quiet box, median of 25 interleaved launches.

ALT Claude Code keynote-style slide on black. Headline: “Time to input box is now 20% faster.” Two horizontal bars: 2.1.250 at 397 ms (gray), 2.1.252 at 318 ms (orange). Footer: Linux x64, quiet box, median of 25 interleaved launches.

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This was a series of performance optimizations across Bun and JavaScriptCore that will benefit every app using bun build —compile, and not just Claude Code.
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