Senior Fullstack SDK Engineer

Hi, I’m Josh, Engineering Manager at Customer.io. We’re looking for a curious, AI-forward Fullstack SDK Engineer to join our SDK team and help us build the tools that developers rely on to integrate with Customer.io.

Over 9,000 companies - from scrappy startups to global brands - use our platform to send billions of emails, push notifications, in-app messages, and SMS every day. Customer.io powers automated communication that people actually want to receive.

We help teams send smarter, more relevant messages using real-time behavioral data. Under the hood: Go, React, Ember and AI help us ship fast and scale with confidence.

You own problems end to end. You move fast, act like an owner, and thrive in ambiguity. You've led complex projects before, whether officially or not, and you're ready to do it again.

You think like a developer using your tools, not just an engineer building them. You care about API design, developer experience, and everything in between.

You bring rigor and creativity. Best practices matter - but never more than forward motion.

• →Build and evolve Customer.io's SDKs across platforms - including Android, iOS, Node, and Go - and the developer tools that support them
• →Lead SDK initiatives end to end: set technical direction, write the code, manage the release, and own the outcome
• →Improve SDK delivery infrastructure - CI, automated testing, versioning, publishing, and release processes - so shipping is reliable and low-friction
• →Partner closely with product and design to turn loosely defined goals into a great developer experience
• →Use AI agents to make multi-file changes across unfamiliar codebases and languages by scoping the work, writing the prompt, and verifying the output
• →Bring strong customer empathy to the role - understanding what the engineers integrating with our SDKs actually need, and occasionally engaging directly with them

• Expert-level competency in at least one established programming language, with strong software design and API design fundamentals
• Experience building and evolving mature, production-quality software, with a clear understanding of how systems fit together and how other developers consume them
• Exceptional fluency with current LLMs and agentic coding tools (e.g., Claude, Cursor, Codex) - using AI to understand unfamiliar codebases, work across languages, and run things in parallel or autonomously
• High autonomy in ambiguous environments - you clarify the problem, validate its value, and drive implementation without close direction
• Strong written and verbal communication, with the ability to explain technical decisions and tradeoffs clearly
• Ability to work effectively across multiple languages and stacks; prior experience in every language we support (Swift, Kotlin, Node, Go) is not required

Nice to have: experience building external-facing SDKs, APIs, or developer tools; experience owning CI, testing, versioning, or release infrastructure; or forward-deployed engineering experience.

No gotchas, no trick questions - just a clear, human process designed to help both of us make an informed decision.

• Application - We review everyone with care. Tell us why you're interested.
• Recruiter Call (30 mins) - Let’s chat about what you’re looking for and how we work.
• Hiring Manager & AI Skills Interview (60 mins) - We'll explore ownership, autonomy, and collaboration, then dig into how you actually use AI in your day-to-day workflow.
• Practical AI-Enabled Coding Round (60 mins) - A live, collaborative session where you'll extend a sample SDK using AI tools. Wwe want to see how you think and work with AI as part of your process.

All final candidates will be asked to complete a background check and employment verifications as part of our pre-employment process.

Customer.io recognizes the stifling impact of systemic injustice on diverse communities. We commit to using our influence to increase inclusion and equity within the tech industry. We strive to build an inclusive team culture, implement bias-free hiring practices, and develop community partnerships to expand our global impact.

Zoom is the only video conference platform that we use, virtual interviews will be conducted using the video capability (i.e., not via the chat), and offers will be extended in writing on official Customer.io letterhead. Please be vigilant in all of your job search activity, and if you have any questions please contact jobs@customer.io.

We believe in empathy, transparency, responsibility, and, yes, a little awkwardness. If you’re excited by what you read and want to build software that makes communication better for everyone—apply now.