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pafin

<h3>Written in Rust, released under the MIT license. Built at pafin Hackathon 2026, our second in-house hackathon.</h3><p style="margin-bottom:1.2em;">pafin Inc., the company behind cryptact, a crypto tax platform with more than 200,000 users in Japan, has released fakestream as open source under the MIT license, which allows commercial use and modification. fakestream emulates Amazon Kinesis Data Streams on a developer's machine or in a CI job. It is the first project to come out of pafin Hackathon 2026, the in-house hackathon we held to accelerate AI adoption across the company.</p><h3><strong>Background</strong></h3><p style="margin-bottom:1.2em;">We have used Kinesis for about eight years. It moves transaction data between our services: the crawlers that fetch trades from exchanges and blockchains, the translation layer that normalizes them, and the ledger engine that computes gains and losses.</p><p style="margin-bottom:1.2em;">Reproducing that pipeline on a developer's machine and in CI, however, had been a problem for a long time. We ran LocalStack, with its JVM-based engine enabled to get the throughput we needed. Even with several gigabytes of heap it ran out of memory on large wallets, and every restart of the stack waited on the JVM to start. In CI, one instance per test job limited how many jobs fit on a runner. As DeFi grew and our data volume grew with it, that cost rose year after year.</p><p style="margin-bottom:1.2em;">In March 2026 LocalStack ended its Community edition. The remaining free tier requires an account and an auth token and is limited to non-commercial use. That change was what pushed us to solve a problem we had been working around.</p><p>&nbsp;</p><h3><strong>Why we built it</strong></h3><p style="margin-bottom:1.2em;">After looking at the existing alternatives, we measured what we actually call. It came to ten Kinesis operations: managing streams and shards, and reading and writing records. With a surface that small, we decided to use the hackathon week to write our own server in Rust, with no runtime like the JVM underneath it, and to see whether it could replace Kinesis in our development environment.</p><p style="margin-bottom:1.2em;">The replacement was finished during the hackathon, and fakestream now runs in our local development stacks every day.</p><p>&nbsp;</p><h3><strong>Results</strong></h3><p style="margin-bottom:1.2em;">Idle memory is about 2 MB, down from the multi-gigabyte heap the previous setup needed. Startup takes milliseconds, and the out-of-memory failures on large wallets are gone. In CI, each test job costs less, so a single runner fits more jobs in parallel.</p><p style="margin-bottom:1.2em;">On performance: on a developer laptop, replaying our own ingestion traffic at about twelve times its peak rate, fakestream sustained 171 MB/s of writes and 227 MB/s of reads. It had headroom left at that point; the load generator was the limit.</p><p>&nbsp;</p><h3><strong>Scope and caveats</strong></h3><p style="margin-bottom:1.2em;">fakestream runs as a single binary, and using it is a matter of pointing your AWS SDK's Kinesis endpoint at it. SQS, DynamoDB, S3 and the rest of your setup stay as they are.</p><p style="margin-bottom:1.2em;">It is a development and CI tool. It has no authentication, so it should run on trusted networks only: localhost, a CI job or a private container network. It is not a replacement for Kinesis in production.</p><p style="margin-bottom:1.2em;">Technical details — the operations it supports, the Kinesis limits it enforces, the persistence option and the full benchmarks — are published on the project site.</p><p>&nbsp;</p><h3><strong>Getting started</strong></h3><div style="border-radius:8px;box-shadow:0 4px 16px rgba(0,0,0,0.35);font-family:'SF Mono','Menlo','Consolas','Liberation Mono',monospace;font-size:0.85em;margin:1em 0 1.5em;overflow:hidden;"><div style="background-color:#313244;color:#a6adc8;font-size:0.75em;letter-spacing:0.08em;padding:7px 20px;text-transform:uppercase;">Shell</div><div style="background-color:#1e1e2e;color:#cdd6f4;line-height:1.7;overflow-x:auto;padding:16px 20px;"><div><span style="color:#89b4fa;">docker</span> run <span style="color:#eba0ac;">--rm</span> <span style="color:#eba0ac;">-p</span> 4567:4567 ghcr.io/pafin-inc/fakestream</div></div></div><p style="margin-bottom:0.6em;">For full setup instructions, see the repository or the project site.</p><ul><li><p style="margin-left:24px;">Repository: <a target="_blank" rel="noopener noreferrer" href="https://github.com/pafin-inc/fakestream">https://github.com/pafin-inc/fakestream</a></p></li><li><p style="margin-left:24px;">Project site: <a target="_blank" rel="noopener noreferrer" href="https://fakestream.pafin.com/">https://fakestream.pafin.com/</a></p></li></ul><p style="margin-top:0.8em;">fakestream is released under the MIT license. It is free to use, including commercially, and requires no account and no auth token. It can also be modified and redistributed, with no restrictions on building it into your own development environment or CI.</p><p>&nbsp;</p><h3><strong>About pafin Hackathon 2026</strong></h3><p>pafin Hackathon 2026 was our second in-house hackathon. We are accelerating AI adoption across the company, and the hackathon brought engineers and business teams together to build a wide range of ideas with AI, from internal tooling improvements and new cryptact features to entirely new services.</p><p><img style="aspect-ratio:4080/3072;" src="https://pafin-cortex-production.s3.ap-northeast-1.amazonaws.com/1_d725745900.jpg" alt="The opening of pafin Hackathon 2026" srcset="https://pafin-cortex-production.s3.ap-northeast-1.amazonaws.com/thumbnail_1_d725745900.jpg 207w,https://pafin-cortex-production.s3.ap-northeast-1.amazonaws.com/small_1_d725745900.jpg 500w,https://pafin-cortex-production.s3.ap-northeast-1.amazonaws.com/medium_1_d725745900.jpg 750w,https://pafin-cortex-production.s3.ap-northeast-1.amazonaws.com/large_1_d725745900.jpg 1000w," sizes="100vw" width="" height=""></p><p>&nbsp;</p><p><img style="aspect-ratio:3159/1778;" src="https://pafin-cortex-production.s3.ap-northeast-1.amazonaws.com/Kakao_Talk_20260629_100435147_28_2_83322eabac.jpg" alt="The fakestream team's presentation" srcset="https://pafin-cortex-production.s3.ap-northeast-1.amazonaws.com/thumbnail_Kakao_Talk_20260629_100435147_28_2_83322eabac.jpg 245w,https://pafin-cortex-production.s3.ap-northeast-1.amazonaws.com/small_Kakao_Talk_20260629_100435147_28_2_83322eabac.jpg 500w,https://pafin-cortex-production.s3.ap-northeast-1.amazonaws.com/medium_Kakao_Talk_20260629_100435147_28_2_83322eabac.jpg 750w,https://pafin-cortex-production.s3.ap-northeast-1.amazonaws.com/large_Kakao_Talk_20260629_100435147_28_2_83322eabac.jpg 1000w," sizes="100vw" width="" height=""></p><p>&nbsp;</p><p><img style="aspect-ratio:4080/3072;" src="https://pafin-cortex-production.s3.ap-northeast-1.amazonaws.com/2_f3cf3aba66.jpg" alt="A product demo during the hackathon" srcset="https://pafin-cortex-production.s3.ap-northeast-1.amazonaws.com/thumbnail_2_f3cf3aba66.jpg 207w,https://pafin-cortex-production.s3.ap-northeast-1.amazonaws.com/small_2_f3cf3aba66.jpg 500w,https://pafin-cortex-production.s3.ap-northeast-1.amazonaws.com/medium_2_f3cf3aba66.jpg 750w,https://pafin-cortex-production.s3.ap-northeast-1.amazonaws.com/large_2_f3cf3aba66.jpg 1000w," sizes="100vw" width="" height=""></p><p>&nbsp;</p><p><img style="aspect-ratio:4000/2252;" src="https://pafin-cortex-production.s3.ap-northeast-1.amazonaws.com/3_ebd45af3e1.jpg" alt="A team presenting its work" srcset="https://pafin-cortex-production.s3.ap-northeast-1.amazonaws.com/thumbnail_3_ebd45af3e1.jpg 245w,https://pafin-cortex-production.s3.ap-northeast-1.amazonaws.com/small_3_ebd45af3e1.jpg 500w,https://pafin-cortex-production.s3.ap-northeast-1.amazonaws.com/medium_3_ebd45af3e1.jpg 750w,https://pafin-cortex-production.s3.ap-northeast-1.amazonaws.com/large_3_ebd45af3e1.jpg 1000w," sizes="100vw" width="" height=""></p><p>&nbsp;</p><p><img style="aspect-ratio:4000/2252;" src="https://pafin-cortex-production.s3.ap-northeast-1.amazonaws.com/4_021085fc80.jpg" alt="A presentation during the final demos" srcset="https://pafin-cortex-production.s3.ap-northeast-1.amazonaws.com/thumbnail_4_021085fc80.jpg 245w,https://pafin-cortex-production.s3.ap-northeast-1.amazonaws.com/small_4_021085fc80.jpg 500w,https://pafin-cortex-production.s3.ap-northeast-1.amazonaws.com/medium_4_021085fc80.jpg 750w,https://pafin-cortex-production.s3.ap-northeast-1.amazonaws.com/large_4_021085fc80.jpg 1000w," sizes="100vw" width="" height=""></p><p>&nbsp;</p><div class="boilerplate" style="-webkit-text-stroke-width:0px;background-color:rgb(255, 255, 255);box-sizing:border-box;color:rgb(36, 53, 77);font-family:&quot;Plus Jakarta Sans&quot;, &quot;Plus Jakarta Sans Fallback&quot;, &quot;Plus Jakarta Sans&quot;, sans-serif;font-size:16px;font-style:normal;font-variant-caps:normal;font-variant-ligatures:normal;font-weight:400;letter-spacing:normal;orphans:2;text-align:start;text-decoration-color:initial;text-decoration-style:initial;text-decoration-thickness:initial;text-indent:0px;text-transform:none;white-space:normal;widows:2;word-spacing:0px;"><h3 style="box-sizing:border-box;color:rgb(36, 53, 77);font-size:1.75rem;line-height:1.33;margin-bottom:20px;margin-top:0px;scroll-margin-top:130px;"><strong>About pafin Inc.</strong></h3><p style="box-sizing:border-box;line-height:1.6em;margin-bottom:20px;margin-top:0px;"><strong>Services</strong><br><strong>cryptact - Crypto tax calculations: Simplified -</strong><br>Automatically calculate capital gains, access all the information you need for tax filing, and efficiently manage your portfolio -- all in one place. Trusted by over 200,000 users.<br>Website: <a style="box-sizing:border-box;color:rgb(42, 109, 176);text-decoration:none;transition:color 0.1s ease-out;" target="_blank" rel="noopener noreferrer" href="https://www.cryptact.com/en">https://www.cryptact.com/en</a><br>X: <a style="box-sizing:border-box;color:rgb(42, 109, 176);text-decoration:none;transition:color 0.1s ease-out;" target="_blank" rel="noopener noreferrer" href="https://x.com/CryptactGlobal">https://x.com/CryptactGlobal</a></p><p style="margin-bottom:20px;"><strong>Guardian — Crypto Tax Filing Support</strong><br>A full-service solution where crypto-specialized tax professionals and dedicated P&amp;L specialists handle everything from gain/loss calculations to filing your tax return.<br>Website: <a target="_blank" rel="noopener noreferrer" href="https://crypto-city.net/guardian">https://crypto-city.net/guardian</a></p><p style="margin-bottom:20px;"><strong>defitact - Your Web3 Sidekick -</strong><br>defitact allows you visualize, analyze and manage your DeFi assets, NFTs and transactions through a highly intuitive and interactive interface. All features are for free.<br>Website: <a target="_blank" rel="noopener noreferrer" href="https://www.defitact.com/">https://www.defitact.com/</a></p><p style="box-sizing:border-box;line-height:1.6em;margin-bottom:20px;margin-top:0px;"><strong>Company Name:</strong> pafin Inc.<br><strong>Date Founded:</strong> January 2018<br><strong>Representative Directors:</strong> Amin Azmoudeh, Co-CEO / Gaku Saito, Co-CEO<br><strong>Location:</strong> Frontier Kojimachi 5F, Kojimachi 3-2-4, Chiyoda City, Tokyo, Japan 102-0083</p></div><div class="contact" style="-webkit-text-stroke-width:0px;background-color:rgb(255, 255, 255);box-sizing:border-box;color:rgb(36, 53, 77);font-family:&quot;Plus Jakarta Sans&quot;, &quot;Plus Jakarta Sans Fallback&quot;, &quot;Plus Jakarta Sans&quot;, sans-serif;font-size:16px;font-style:normal;font-variant-caps:normal;font-variant-ligatures:normal;font-weight:400;letter-spacing:normal;orphans:2;text-align:start;text-decoration-color:initial;text-decoration-style:initial;text-decoration-thickness:initial;text-indent:0px;text-transform:none;white-space:normal;widows:2;word-spacing:0px;"><p style="box-sizing:border-box;line-height:1.6em;margin-bottom:20px;margin-top:0px;"><strong>Media Contact</strong><br>Email: <a style="box-sizing:border-box;color:rgb(42, 109, 176);text-decoration:none;transition:color 0.1s ease-out;" href="mailto:pr@pafin.com">pr@pafin.com</a></p></div><p style="border-top:1px solid #E5E7EB;color:#6B7280;font-size:0.8em;line-height:1.6;margin-top:32px;padding-top:16px;">fakestream is not affiliated with Amazon Web Services, Inc. Amazon Kinesis is a trademark of Amazon.com, Inc.</p>

pafin has open sourced fakestream, a small local emulator for Amazon Kinesis Data Streams. Written in Rust, MIT licensed, built at our second hackathon.

pafin open sources fakestream, a small local emulator for Amazon Kinesis Data Streams

ニュース

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Amazon Kinesis Data Streamsの軽量なローカルエミュレータ「fakestream」をOSS公開

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登壇のお知らせ 『OKJアカデミー OKJ×Metaplanet ～変化の激しい市場で挑戦を続ける理由～』9月15日

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登壇のお知らせ 『税金までしっかりわかる！JPYSCレンディングセミナー』8月19日

CoinPostに当社のAI連携機能とGtax統合に関する記事が掲載されました。

あたらしい経済に当社のGtax統合に関する記事が掲載されました。

<p>cryptact, Japan's most trusted crypto tax platform with more than 200,000 users since 2017, is now available in Australia. Founded by former Goldman Sachs executives, cryptact brings institutional-grade expertise to crypto tax, helping Australian investors calculate and lodge with accuracy and efficiency in line with ATO guidance. AI integration also lets users hand the tedious manual work of crypto tax preparation over to AI tools such as Claude or ChatGPT.<br>&nbsp;</p><p>For more details, please visit the <a target="_blank" rel="noopener noreferrer" href="https://www.cryptact.com/en/news/pr-australia-support-en">cryptact News Page</a>.</p>

As Japan's top crypto tax service, trusted by more than 200,000 users since 2017, cryptact is now available in Australia.

Japan's Leading Crypto Tax Service, cryptact, Now Available in Australia

プレスリリース

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クリプタクト、オーストラリアの確定申告および仮想通貨税制に対応

<figure class="table"><table><tbody><tr><td><span style="font-size:18px;"><strong>The deal unites automated capital-gains calculation with accounting journal-entry tools, serving individual investors, tax professionals, and corporate users on a single platform.</strong></span></td></tr></tbody></table></figure><p>cryptact, the crypto tax platform used by more than 200,000 investors and operated by pafin Inc., today announced it has completed the acquisition of Gtax, a competing crypto profit-and-loss calculation service operated by Gtax Inc. The two services will unite under the cryptact brand on October 5, 2026. Existing Gtax users will move to cryptact without re-registration, keeping their login credentials, transaction data, and profit-and-loss records intact.<br>&nbsp;</p><p>For more details, please visit the <a target="_blank" rel="noopener noreferrer" href="https://www.cryptact.com/en/news/gtax-integration-en">cryptact News Page.</a></p>

cryptact, the crypto tax platform used by more than 200,000 investors and operated by pafin Inc., today announced it has completed the acquisition of Gtax.

cryptact Acquires Gtax to Become Japan’s Standard for Crypto Tax Calculation 

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クリプタクトがGtaxを統合、日本の仮想通貨損益計算サービスの業界標準へ 

日本経済新聞 掲載のお知らせ

<p class="font-claude-response-body break-words whitespace-normal">On Saturday, June 27, 2026, Gaku Saito, Co-CEO of pafin (the company behind cryptact), will take the stage at "SBI DERIVATIVES DAY 2026," hosted by SBI SECURITIES Co., Ltd.</p><p class="font-claude-response-body break-words whitespace-normal">The session will explore the investment opportunities created by Japan's crypto-asset tax reform, along with a discussion of the value of Bitcoin. The session will be held in Japanese.</p><h3><strong>Event Overview</strong></h3><p class="font-claude-response-body break-words whitespace-pre-wrap">Event name: SBI DERIVATIVES DAY 2026<br>Host: SBI SECURITIES Co., Ltd.<br>Date and time: Saturday, June 27, 2026, 11:40 a.m. – 5:00 p.m. (planned)&nbsp;<br>Session title: "Crypto as an Asset Class: What Japan's Tax Reform Means for the Road Ahead"&nbsp;<br>Language: Japanese</p><p class="font-claude-response-body break-words whitespace-normal">Speakers:</p><ul><li>Gaku Saito — Co-CEO, pafin Inc.; Chairperson of the Taxation WG, Japan Cryptoasset Business Association (JCBA)</li><li>Tomohiko Kondo — President &amp; CEO, SBI VC Trade Co., Ltd.</li></ul><p class="font-claude-response-body break-words whitespace-normal">Moderator:</p><ul><li>Nanami Hiwa — Investor, Money Navigator, Financial Planner</li></ul><p class="font-claude-response-body break-words whitespace-pre-wrap">Session time: 12:45 p.m. – 1:15 p.m.<br>Admission: Free<br>Venue: TODA Hall &amp; Conference, Hall A — TODA BUILDING 4F, 1-7-1 Kyobashi, Chuo-ku, Tokyo 104-0031, Japan</p><p class="font-claude-response-body break-words whitespace-normal">For event details and registration, please see the following (links to the SBI SECURITIES website): <a class="underline underline-offset-2 decoration-1 decoration-current/40 hover:decoration-current focus:decoration-current" target="_blank" rel="noopener noreferrer" href="https://www.sbisec.co.jp/contents/lp/2026/sbiderivativesday_260627.html">https://www.sbisec.co.jp/contents/lp/2026/sbiderivativesday_260627.html</a></p>

pafin Co-CEO Gaku Saito will take the stage at "SBI DERIVATIVES DAY 2026," hosted by SBI SECURITIES, on June 27.

pafinからの最新ニュース

Yahoo!ファイナンスに当社執筆の暗号資産税制に関する記事が掲載されました。

Amazon Kinesis Data Streamsの軽量なローカルエミュレータ「fakestream」をOSS公開

登壇のお知らせ『OKJアカデミー OKJ×Metaplanet ～変化の激しい市場で挑戦を続ける理由～』9月15日

登壇のお知らせ『税金までしっかりわかる！JPYSCレンディングセミナー』8月19日

CoinPostに当社のAI連携機能とGtax統合に関する記事が掲載されました。

あたらしい経済に当社のGtax統合に関する記事が掲載されました。

クリプタクト、オーストラリアの確定申告および仮想通貨税制に対応

クリプタクトがGtaxを統合、日本の仮想通貨損益計算サービスの業界標準へ

日本経済新聞掲載のお知らせ

登壇のお知らせ『SBI DERIVATIVES DAY 2026』6月27日

pafinからの最新ニュース

Yahoo!ファイナンスに当社執筆の暗号資産税制に関する記事が掲載されました。

Amazon Kinesis Data Streamsの軽量なローカルエミュレータ「fakestream」をOSS公開

登壇のお知らせ 『OKJアカデミー OKJ×Metaplanet ～変化の激しい市場で挑戦を続ける理由～』9月15日

登壇のお知らせ 『税金までしっかりわかる！JPYSCレンディングセミナー』8月19日

CoinPostに当社のAI連携機能とGtax統合に関する記事が掲載されました。

あたらしい経済に当社のGtax統合に関する記事が掲載されました。

クリプタクト、オーストラリアの確定申告および仮想通貨税制に対応

クリプタクトがGtaxを統合、日本の仮想通貨損益計算サービスの業界標準へ

日本経済新聞 掲載のお知らせ

登壇のお知らせ 『SBI DERIVATIVES DAY 2026』6月27日

登壇のお知らせ『OKJアカデミー OKJ×Metaplanet ～変化の激しい市場で挑戦を続ける理由～』9月15日

登壇のお知らせ『税金までしっかりわかる！JPYSCレンディングセミナー』8月19日

日本経済新聞掲載のお知らせ

登壇のお知らせ『SBI DERIVATIVES DAY 2026』6月27日