When I left Sixt, I sent a farewell letter to the people I worked with. This is that letter, with the names of people and internal systems left out. Those it thanks know who they are.
Dear friends, colleagues, and everyone I have shared a project, a debate, a lunch, a late evening, a weekend, or an experience with:
I am writing to wrap up my thesis at Sixt. As with every thesis, the part that lasts may not be the chapters but the acknowledgments, so let me start there.
My journey with Sixt began four years ago, and I still remember the first day clearly. I arrived in Pullach hours early and sat at the station finishing a paper for a research conference. It was due that day, and I had worked on it for months, including the night before. When time was up, I knew I had to let it go rather than polish it toward perfection. Then I walked into the building, waited to be picked up, and set out to see what would come next. Today I leave as a completely different person, and most of that difference came from all of you. Thank you.
Thank you for giving me the opportunity to begin this journey. Thank you for walking me into this place, for connecting me with the people around it, and for having my back ever since. Thank you for guiding me through how the old pricing systems worked. Thank you for building their replacement with me and taking it live. Thank you for the interesting challenges, and for helping me understand the business motivation behind them. For the fun and laughter, the calm and the hard work, the fights and struggles, the bug reports and fixes, and the on-call nights: thank you all. When I joined, the whole area ran on a legacy stack. Today it is fully modernized.
Thank you for the fantastic collaboration on pricing automation. Thank you for all the fun, sometimes heated, but always constructive discussions. Thank you for the jokes, and for our debates about idiomatic Go. Thank you for the data analysis, and for the investigations we did together. Together we greatly simplified our systems and laid the groundwork for data-driven pricing automation.
Thank you for the inspiring talk that started a new pricing system, and for the trust and help in moving things forward. I still clearly remember the afternoon when we quickly sketched its architecture diagrams. Thank you for one of the most exciting weeks and weekends, when we brought its very first version live within days, and that was before AI. Thank you for scaling it into a truly production-grade system.
Thank you for the opportunities, for building the environment, and for shaping how we apply AI. My trajectory would have been entirely different if we had not debated an architecture on the blackboard a year and a half ago. Thank you for teaming up with such motivation, entrepreneurial pragmatism, and hard work to bring up our AI platform and the ecosystem around it. Thank you for being inspiring sparring partners on the LLM gateway, agent memory, and agent sandboxes. Thank you for volunteering your spare time to polish the platform. Thank you for adopting early and experimenting first, which set things up for everyone who came after. Thank you for making our systems talk to each other. Thank you for debating all sorts of security concerns around AI with me, and for setting up and evaluating the guardrails. Thank you for the great discussions around Go, and for contributing to the wider Go community. Thank you for the journey of transforming customer service. Thank you for the legendary infrastructure support through all these years, and for the long nights we spent together. Thanks to all of this effort, our AI stack is where it is today.
Thank you for the support, the brainstorming, the discussions, and the building that made Python a first-class citizen and grew a Python community. Thank you for polishing, contributing to, and helping scale the automated code review. Without you, these community-driven things would not have been adopted so widely.
And thank you to everyone not mentioned here, as the space is too narrow to contain.
Thank you all, for the agency, the energy, the understanding, and the listening. We fought hard and then settled completely. Together we reshaped our surroundings for the better, and together we created value and meaning.
There is one spirit I want to share. I watched many areas that looked stable get rebuilt within short periods. The people who acted first were not the most certain ones; they were the ones willing to be wrong in public. So try to be creative before being correct, and stay open-minded when a prior no longer holds. Most of what we build here will be replaced, and some of it already has been, by us. Before Sixt, I spent years with existentialism and nihilism and reached a simple belief, and the years since have confirmed it: there may be no meaning waiting for us, but we still choose to create, therefore meaning exists. So push things forward where you stand.
The beauty of being human is that we can adapt. Identity keeps shifting as long as we keep looking for new things to do. Hence the one idea I keep returning to: if we don’t do it now, we may never do it at all. So pursue what we truly want, tune out the noise, work hard, and look after ourselves and the people we love along the way.
I loved my time here. It shaped me, and I will carry it with me.
I will close with Bruce Lee’s rendering of a much older Chinese idea about water, 上善若水:
Empty your mind.
Be formless, shapeless, like water.
You put water into a cup, it becomes the cup.
You put water into a bottle, it becomes the bottle.
You put it into a teapot, it becomes the teapot.
Now, water can flow, or it can crash.
Be water, my friend.
We will see each other down the road,
Changkun
Disclaimer: This text is purely generated by Changkun.
It has been re-processed from the original to avoid disclosure issues.
离开 Sixt 时，我给共事过的人写了一封告别信。下面就是这封信，略去了人名和内部系统的名字。信里感谢的每一位，自己会知道。
亲爱的朋友们、同事们，以及每一位和我一起做过项目、争论过问题、吃过午饭、熬过夜、过过周末、有过共同经历的人：
我写这封信，是为了给我在 Sixt 的这篇“论文”收尾。和每篇论文一样，最后留下来的也许不是正文的章节，而是致谢，所以我就从致谢写起。
我和 Sixt 的旅程开始于四年前，第一天的情形我至今记得很清楚。那天我提前几个小时到了 Pullach，坐在车站里赶一篇投给学术会议的论文。当天就是截稿日，而我已经为它忙了好几个月，前一晚也还在写。时间到了，我知道必须放手，而不是继续把它打磨到完美。然后我走进办公楼，等人来接我，去看接下来会发生什么。今天我离开时，已经完全是另一个人，而这些变化大多来自你们。谢谢。
谢谢你们给了我开始这段旅程的机会。谢谢你带我走进这个地方，把我介绍给周围的人，并且从那以后一直在背后撑着我。谢谢你们带我弄懂旧的定价系统是怎么运转的。谢谢你们和我一起把它的替代者做出来，送上线。谢谢你们带来那些有意思的难题，也让我明白了它们背后的业务动机。为了那些欢乐和笑声、平静和辛苦、争吵和挣扎、bug 报告和修复，还有值班的夜晚：谢谢大家。我加入时，整个领域都跑在旧的技术栈上；今天，它已经完全现代化了。
谢谢你们在定价自动化上的出色合作。谢谢那些有趣、有时激烈、但始终有建设性的讨论。谢谢那些玩笑，还有我们关于怎样写出地道的 Go 的争论。谢谢你们做的数据分析，还有我们一起做过的那些排查。我们一起大大简化了系统，也为数据驱动的定价自动化打下了基础。
谢谢那场给了我启发的分享，一个新的定价系统由此开始；也谢谢你的信任，和推动事情向前的帮助。我至今清楚地记得那个下午，我们很快画出了它的架构图。谢谢你们陪我度过了最激动人心的几周和几个周末：我们在几天之内就让它的第一个版本上线了，那还是在有 AI 之前。谢谢你们把它扩展成了一个真正生产级的系统。
谢谢你给我的机会，谢谢你营造的环境，也谢谢你塑造了我们应用 AI 的方式。如果一年半以前我们没有在黑板前争论过那个架构，我的轨迹会完全不同。谢谢你和我搭档，带着那样的干劲、创业者式的务实和辛勤，一起把我们的 AI 平台和它周边的生态建了起来。谢谢你们在 LLM 网关、智能体记忆和智能体沙盒上做我的陪练，给了我很多启发。谢谢你们拿出业余时间来打磨这个平台。谢谢你们最早采用、最先尝试，为后来的所有人铺好了路。谢谢你们让我们的各个系统能够彼此对话。谢谢你们和我争论 AI 带来的各种安全问题，并且搭建、评估了护栏。谢谢那些关于 Go 的精彩讨论，以及你们对更广泛的 Go 社区的贡献。谢谢你们和我一起走过改造客户服务的这段路。谢谢这些年来传奇般的基础设施支持，以及我们一起熬过的那些长夜。正是因为这一切努力，我们的 AI 技术栈才走到了今天。
谢谢你们的支持、头脑风暴、讨论和动手，让 Python 成了一等公民，也建起了一个 Python 社区。谢谢你们打磨自动化代码审查，为它做贡献，并帮它推广开来。没有你们，这些由社区推动的东西不会被这样广泛地采用。
也谢谢所有没有在这里提到的人，这里空白太小，写不下。
谢谢大家，谢谢你们的主动、活力、理解和倾听。我们争得很凶，然后彻底达成一致。我们一起把周围变得更好，也一起创造了价值和意义。
有一种精神我想分享。我看到过许多看似稳定的领域，在很短的时间里被重建。最先行动的人并不是最有把握的人，而是愿意当众犯错的人。所以，先求有创造力，再求正确；当先验不再成立时，保持开放。我们在这里建的东西，大部分都会被替换掉；其中一些已经被替换了，而且是我们自己动的手。来 Sixt 之前，我在存在主义和虚无主义里泡了好些年，得出一个简单的信念，此后这些年的经历也印证了它：也许并没有什么意义在等着我们，但我们仍然选择去创造，所以意义存在。所以，在你所站的地方，把事情往前推。
人之为人的美好，在于我们能够适应。只要我们还在寻找新的事情去做，身份就会不断变化。所以我总是回到同一个想法：如果现在不做，也许就永远不会做了。所以去追求我们真正想要的，屏蔽噪音，努力工作，一路上照顾好自己和所爱的人。
我爱在这里的时光。它塑造了我，我会带着它走下去。
最后，我想用李小龙对一个古老得多的中国思想的演绎来收尾，那就是关于水的“上善若水”：
清空你的思绪。
无形，无式，像水一样。
把水倒进杯子，它就成了杯子。
把水倒进瓶子，它就成了瓶子。
把水倒进茶壶，它就成了茶壶。
水可以流动，也可以冲击。
像水一样吧，我的朋友。
后会有期，
长坤
声明：本文完全由长坤亲自撰写。
为避免披露问题，本文在原文基础上做了重新处理。