How Varsha Parthasarathy became engineer #1 at Flex — solving a problem she'd experienced

When Varsha Parthasarathy was pregnant, she needed a letter from her doctor's office just to use her own HSA money on pregnancy-related expenses. Everything was already in her chart. She still had to send her husband or her sibling to the admin office to chase down the paperwork. Four months after her first kid was born, running on very little sleep and definitely not planning a career move, she sat down with her former Lyft colleague Miguel — one of the cofounders of Flex (S23) — to work together for a week in San Francisco. By the end of that week, she was in.

Flex helps businesses accept HSA and FSA payments, so people can use their pre-tax health dollars on products and services they're already buying. It plugs into a merchant's existing checkout and shows up as another way to pay, like Klarna or Google Pay.

From self-driving cars to healthcare payments

Before Flex, Varsha worked at Lyft's self-driving division on the autonomy side — data annotation pipelines, supervised metrics, sensor data stores, offline ground-truth perception. She also spent a couple of years as an engineering manager there, which taught her that the people matter the most.

She already knew Miguel as a coworker. Then Sam, Flex's other cofounder, reached out as a reference call that turned into something much longer. "I came away really liking Sam," she says. But she also knew that working with someone at a big company is different from building something from scratch with a handful of people. So she tested it: a week side by side, watching how Miguel thought through problems and made decisions. "By the end of that week, I just knew these were people I wanted to build with."

What being engineer #1 actually looks like

Varsha was Flex's first engineer. She doesn't have a formal title. If pressed, she describes herself as "the engineer who also does support, analyzes queue failures and metrics, and every now and then jumps on a sales call or helps onboard a merchant."

She still remembers the company's first Thanksgiving. She was on call for both engineering and support — answering questions from customers and merchants having trouble checking out while simultaneously chasing down a decimal computation bug. "In that one weekend, I touched more parts of the product than I would have in months at a big company, and I loved every bit of it."

Sales calls, merchant onboarding, content pipelines, marketing compliance — none of it was in her job description. None of it existed before she did it.

A Slack bot that became Flex's first line of triage

On-call comes around often on a small engineering team, and Varsha wanted to spend less time on support and more time building. So she started writing small bots. One was a Slack bot that helped her figure out what was going on when a payment failed. Over time, it grew into the Flex Investigator. Give it an order number or a customer email, and it traces what happened across Flex's systems in about fifteen seconds — something that used to take around twenty minutes. Today it's the team's first line of on-call triage.

Nobody had to sign off on it first. "Sam and Miguel trust us to go build what the team needs, and we deliver."

She also built a personal agentic bot that lives behind a Slack emoji: an agent that listens for events, finds answers to questions people ask her, and keeps working around the clock, even pushing PRs. On the tools side, she reaches for Claude Code for implementation, Codex for brainstorming and planning, and a personal finance bot she built and named Paisa Bot — paisa means money in her language — to track markets and think through investing strategies.

Founders who are "very human"

Varsha says anyone on the team can go to Sam and Miguel with uncomfortable or awkward questions, directly, and they actually want that. "It goes both ways. They've given the team a lot of trust, and we trust them right back. Whenever those questions come up, there's always some action that comes out of it. It never just ends with a conversation."

What surprised her most about startup life: she never felt like she had to choose between work and her kid. "And the founders — I didn't expect them to be this fun and chill. They're very human, and that makes them so easy to work with."

Who would thrive here

Varsha's advice is specific:

• If you enjoy solving problems across different domains and don't have much ego about which problems you work on, a startup is a great space for you.
• Pick a company where you truly believe in the mission. "There aren't many companies working where payments and healthcare meet, and building something that keeps getting better for both customers and merchants is far from easy. There are still so many hard problems left to solve, and that's exactly the kind of environment I enjoy."