# Payabli builds best-in-class docs for complex payment APIs with Fern

**Date:** 2026-05-12  
**Source:** https://buildwithfern.com/customers/payabli

> How Payabli migrated from Mintlify in three weeks, switched OpenAPI to Fern Definition, and built interactive developer tools their team had always wanted.

---
<Eyebrow>The problem</Eyebrow>

## Mintlify couldn't keep up with Payabli's growing API complexity

Payabli's payment processing APIs include endpoints with dozens of conditional fields based on payment method, merchant service type, and other factors. Their docs were originally built on Mintlify, but platform updates kept introducing changes that broke or overrode Payabli's custom styling, and the team had no way to prevent it. That became a deal-breaker.

On the API definition side, managing their specs with OpenAPI Specification created constant friction. The team had to develop creative workarounds just to get basic documentation published. Between the two, the documentation team was spending roughly 30% of its effort on spec and platform management, leaving little room to invest in the developer experience.

Payabli started using Fern in April 2025. The migration from Mintlify to Fern Docs took just three weeks. The team also moved from OpenAPI to Fern Definition to better handle the complexity of their payment APIs, and uses Fern to generate SDKs in eight languages.

<Quote author="Casey Smith" role="Head of Technical Writing at Payabli">
  Using the OpenAPI specification to describe our APIs always felt like trying to force a square peg
  into a round hole. Fern Definition is intuitive enough that new team members can contribute
  quickly and our complex data models don't require workarounds to describe accurately. It feels
  like it was made to describe our APIs.
</Quote>

<Eyebrow>The solution</Eyebrow>

## Fern Definition, custom components, and single-source SDKs

Key capabilities Payabli unlocked with Fern:

- **Fern Definition for complex payment APIs.** Payabli switched from OpenAPI to Fern Definition, eliminating cumbersome oneOf/anyOf patterns and gaining clean type aliasing and union support that actually fit their payment data models. Spec management went from the team's biggest time sink to a small fraction of their workload, freeing them to focus on developer experience.
- **Interactive tools built directly into the docs.** With that time back, the team built custom React components including a transaction lifecycle explorer, an ACH timeline calculator, embedded component demos, and step-by-step code walkthroughs, the kinds of developer tools Payabli had always wanted to build but couldn't prioritize when maintenance ate most of their bandwidth.
- **Single source, multiple outputs.** From their Fern Definition files, Payabli generates a complete API reference, SDKs in 8 languages, and code examples across their developer guides, all staying in sync automatically without manual cross-updates.
- **Full control over styling and navigation.** Fern's stable CSS selectors let Payabli fully brand their docs without worrying about upstream changes breaking their styling, the exact problem they'd hit with Mintlify. Navigation configuration gives the team complete control over information architecture across 212 documented endpoints.

<Eyebrow>Looking ahead</Eyebrow>

## Better docs, better payment integrations

With tooling friction removed, Payabli's documentation team is focused on building the developer enablement resources that were previously impossible to prioritize: more interactive tools, comprehensive guides, and quickstarts.

For Payabli, the move to Fern is part of a larger investment in making their platform the simplest choice for SaaS companies that need embedded payments, starting with documentation that actually matches the quality of their APIs.