Hi Effecters!
Welcome back to This Week in Effect, your weekly update of the latest developments in the Effect community and ecosystem.
Effect is the TypeScript library for the AI era, helping developers build complex, synchronous, and asynchronous programs. One key feature that sets Effect apart is how it leverages structured concurrency to provide features such as async cancellation and safe resource management, making it easier to build robust, scalable, and efficient programs.
To get started, below you’ll find links to our documentation and our guide for installing Effect. Enjoy!
Recent major updates:
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New Module of the Week post by Maxwell Brown, featuring Cluster. Effect Cluster, Part 1 - Module of the Week #3 We’re kicking off a series on Effect Cluster with a focus on the actor model and how a cluster is laid out. We’ll dig deeper in Office Hours today -- bring your questions! Link in the thread.
effect.website/blog/module-of…
This week marks a major milestone for Effect and its community. Effect 4.0, our most ambitious release yet, has been declared stable. Read about the stable release notes in the announcement post and start migrating your projects from v3 to v4 following our migration guide.
Effect v4 is here. One ecosystem. Zero dependencies. The next chapter of Effect and the foundation for building reliable software and AI agents in TypeScript.
This was the final week of the Release Candidate cycle. The last stretch of RC hardening focused on concurrency correctness, Schema internals, and cluster reliability.
Concurrency correctness : FixedEffect.race losers being left running when the race ends during startup, fixed a lost failure during concurrent layer interruption, fixed theQueue batch taker wake loop on partial offers, and fixedAtomRegistry missing updates through stale nodes and leaked superseded builds. #
Schema : Stopped Schema unions from resolving candidate parsers on every decode, inlined checked sources of Schema JIT transformations, fixed Schema projection check preservation, fixed Schema brand identifier typing, and madeSchema.brand type-only. Also clarified schema branding guidance in the v3-to-v4 migration reference. #
Cluster & workflow : Guarded workflow replies against stale concurrent resets, retried failed cluster workflow run resets, and treated infinitePubSub capacity as unbounded. #
RPC : Failed in-flight RPC streams after a missed socket pong, fixed flat RPC client andAtomRpc inference for a union of tags, and fixed synchronous concurrentAtom.fn results. #
HttpApi : Added per-slotParseOptions annotations with aParseOptions fallback, and closed server WebSockets according to handler exit. #
Other fixes : FixedMutableHashMap key retention on equal-key cache refresh, fixed the encoded RPC interrupt fiber ID type for JSON null, and normalized partition result order across all modules.
And with that, the RC cycle wraps up. Thank you to everyone who tested the beta and RC builds and helped shape the release with bug reports, feedback, and contributions.
You can follow the full changelog in the effect repository. To catch up on more updates, check out Effect v4 Updates, starting from the recently published Effect v4 RC: August 2026 Updates.
Other technical updates from the past week.
- [Isolate RPC stream chunk decoding failures](https://github.com/Effect-TS/effect/pull/8627) (Bug Fix)
- [Keep shards acquired during an in-flight lock refresh](https://github.com/Effect-TS/effect/pull/8644) (Bug Fix)
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Michael and Sebastian merged the 4.0 stable release live at the Effect San Francisco meetup. Thanks to Andre Landgraf and Erik Thorelli for organizing, to CodeRabbit for hosting us on their rooftop, and to all the Effecters who joined in person
x@EffectTS_ live from SFlive at the @EffectTS_ meetup in sf with@esthor@MichaelArnaldi@thefubhy@RhysSullivan@kitlangton@andrelandgraf and the vibes are absolutely immaculateOne ecosystem. Zero dependencies. And amazing people. Effect v4 is here. One ecosystem. Zero dependencies. The next chapter of Effect and the foundation for building reliable software and AI agents in TypeScript. Effect feels insanely good to use with agents: - Typed errors/dependencies power up your type checking - DI makes designing good codebases easy - Built-ins for EVERYTHING, no agent creativity required An outrageous advantage for any non-frontend TS code Effect v4 is here. One ecosystem. Zero dependencies. The next chapter of Effect and the foundation for building reliable software and AI agents in TypeScript. Was thrilled to speak about @alchemy_run at the Effect 4.0 release last night. Great event and huge milestone for the@EffectTS_ team.Effect v4 is here. One ecosystem. Zero dependencies. The next chapter of Effect and the foundation for building reliable software and AI agents in TypeScript. #
Effect Paris 🇫🇷 meetup is scheduled for Tuesday, Oct 13! #
Only a few 2-day passes left for Effect Days 2026 ! The conference is scheduled for December 9–11 in Livorno, Italy.I’ll be speaking here about @warpdotco ’s effect journeyEffect Days is back 🇮🇹 Join us on December 9–11 for three days with the people building Effect and the people building with it. Workshops. Talks. Community. Early Bird tickets are now live ↓ effect.website/effect-days #
Effect Office Hours 🔥 every Wednesday at 10:00 AM ET / 16:00 CET on our YouTube channel . Visit our Effect Community Events calendar and subscribe for updates on upcoming Effect events.
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We’re thrilled to announce Ziverge as the first Effect Adoption Partner!
Here are 3 things we're thrilled to announce, as part of our partnership with @EffectfulTech, creators of @EffectTS_ 🔥 ➡️ Ziverge will be sponsoring Effect Days
➡️ We have produced an Effect SDK for [@GolemCloud](https://twitter.com/GolemCloud)
➡️ We've launched an [@EffectTS_](https://twitter.com/EffectTS_) Training Workshop
Sign up for
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- Most likely, or no way this is true.
Replying to @culer_1997 i actually think there's a chance i've shipped more effect to production than dillon has
@dillon_mulroy is this true
- 💯
Replying to @RhysSullivan I am exclusively making the agents write effect (also in what I am referring to in the OP) and it’s magical for agents
I've never felt more productive in my life than with Opus 5.5 and @EffectTS_
- The question…
what's the best full stack stack for using with Effect? anyone using the v4 rc with sveltekit?
- …and the answer🤍
Replying to @stolinski Basically anything. I use it with React Router. Any time you see an async backend function, you can fill it with Effect.
- Neon now has Effect bindings!
Effect 4.0 shipped yesterday night and today we're introducing @𝚗𝚎𝚘𝚗/𝚎𝚏𝚏𝚎𝚌𝚝: Effect v4 bindings for @𝚗𝚎𝚘𝚗/𝚜𝚍𝚔 Integrate Neon into your @EffectTS_ codebases: paginated lists as Streams, SDK errors handled with catchTag and Fiber interruption that cancels requests
- A new experimental
@xstate/effectpackage
Published @xstate/effect alpha: if you use @EffectTS_ and XState, try it out and let me know what you think! XState beta (and @xstate/effect beta) soon 🚢
stately.ai/docs/xstate/v6… 🏗️ Working on xstate/effect XState handles type-safe state-machine logic Effect makes the effectful boundary "type-safer" with typed failures, reqs, schemas, structured concurrency, and more Goal: run existing XState machines in Effect w/ minimal changes Follow along:
- An Effect-based auth library Introducing @yielded/auth, a new effect (v4)-based auth library. It's infinitely customizable, it can manage your db schemas, or you can bring your own, or you can use whatever sql adapter you want or even reimplement any func.
github.com/yielded-dev/au… #
stack-effect - a scaffolding CLI for full-stack TypeScript apps built on Effect, generating an Effect-first monorepo with community-aligned conventions and composable modules. By Lloyd Richards. #
effect-resonate - use Effect services, typed errors, and Layers withResonate durable execution. By Luke B. #
Bye - an open-source email and calendar platform inspired by HEY, built in TypeScript on Cloudflare primitives with Effect v4 and Alchemy. By chr33s. #
harbor-effect - an experimental framework that wrapsHarbor with Effect to run agent evaluations, collecting KPIs such as output tokens as structured data. By Hakim. #
imposters - Programmable mock servers for HTTP APIs and an in-memory S3. By Eliraz Kedmi. #
noite - A tiny, self-hostable PaaS for celld apps: git push deploys, with Durable Objects, SQL databases and object storage included. By Ryuz Corp.
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Don’t forget to listen to our Cause & Effect podcast hosted by Johannes Schickling and available on YouTube, X(Twitter), and audio platforms like Spotify and Apple Podcast.
John A. De Goes, the creator of ZIO, as well as CEO of Ziverge and Golem Cloud, joins Johannes Schickling and Michael Arnaldi to discuss how AI is reshaping software engineering, what developers should still learn, and why Effect’s primitives are especially valuable for coding agents.
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- Effect Office Hours 48 🔥
- Schema performance improvements in Effect v4
- PersistedQueue | Module of the Week #1 by Maxwell Brown.
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Looking for a role where you can work with Effect? We now have a dedicated Effect jobs page featuring open roles from companies that use Effect. We’ll keep it updated as new opportunities come in.
Explore Effect job opportunities → Disclaimer: Please note that these job postings are shared for informational purposes, and we encourage applicants to verify details directly with the hiring companies.
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The Effect Merch Store offers a selection of Effect-branded items designed for the community. All orders are processed and fully managed through Printful.
Effect Merch is here! We’re excited to announce the launch of our Effect merch page! Grab your favorite swag & show your love for Effect in style! 🧢
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That’s all for this week. Thank you for being a vital part of our community. Your feedback is highly valued as we fine-tune this format. Feel free to share your thoughts, and we’ll do our best to tailor it to the needs of our community.
Effect Community Team
Previous Effect v4 RC: September 2026 Updates