Went through the full loop at Twilio for a senior SWE role in early 2026. Got to final round, didn't take the offer for comp reasons, but the process was genuinely interesting so figured I'd post.
The system design round at the senior / L5 level is 45 minutes. They open with a pretty open-ended prompt, something like "design a notification delivery system that handles SMS, email, and push at scale." Which, you know, is literally Twilio's product, so be ready for that. It's not a trick, they just want to see if you know your own domain.
What they actually care about: How you handle rate limiting across channels. This came up almost immediately. Think about per-user limits, carrier throttles, campaign-level caps. Durability and retry semantics. If a message fails at the carrier level, what happens? Exponential backoff, dead-letter queues, that kind of thing. Idempotency. Twilio's whole API is idempotent by design (idempotency keys), so they expect you to think about this in your designs. Multi-tenancy isolation. When your platform serves thousands of B2B customers, how do you make sure one noisy tenant doesn't affect others?
The interviewer was pretty collaborative, not adversarial. They'd redirect with "what if volume spikes 10x during a Flash Sale?" type questions. Classic breadth probe.
I didn't get a whiteboard but we worked in a shared doc. No code required in the system design round.
Prep that actually helped: Reading their engineering blog on how they handle SMS at scale, and brushing up on webhook delivery reliability patterns. The Twilio developer docs are basically a cheat sheet for what their internal systems look like.
One thing I wasn't ready for: they asked me how I'd monitor delivery success rates across different carriers and geographies. Very ops-aware question for a design interview. Come with something on observability.
Happy to answer questions if you're prepping for the Twilio senior SWE loop.