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

The problem

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

Payabli's docs were originally built on Mintlify, but platform updates kept breaking or overriding their custom styling, with no way to prevent it. For a company whose docs are a core part of the product, losing control of the developer experience was a deal-breaker.

The harder problem was the APIs themselves: sophisticated, with lots of union types and large payloads. Realistic examples aren't a nicety for these APIs. They're how developers read them at all, so Payabli treats named examples as critical infrastructure. But on APIs that change daily, keeping hundreds of examples accurate by hand was untenable. Payabli also had no generated SDKs, so developers hand-rolled their own clients against complex specs.

Payabli started using Fern in April 2025, and migrated off Mintlify to Fern Docs in three weeks. Their spec became the single source for everything developer-facing: the API reference, the SDKs, and the examples referenced throughout their guides.

Our APIs are sophisticated, and a field typed as string doesn't tell a developer whether to send a transaction ID, a dollar amount, or a status code. A realistic example value helps them make a leap they couldn't make otherwise, and it keeps them out of our support queue.

The solution

One source of truth for examples, SDKs, and docs

Payabli drives everything developer-facing from a single spec. A change there flows out to every surface that references it, so a three-person team keeps docs, examples, and eight SDKs in sync as their APIs change daily.

Key capabilities Payabli unlocked with Fern:

• 300+ reusable examples, maintained once. Endpoint request and response snippets ship with Fern Docs out of the box, so keeping examples in sync went from daily manual upkeep on every page to a component the team just drops in. Payabli defines each example once in the spec, and those snippets now appear more than 300 times across their docs. Their APIs change daily, so without that reuse every change would mean tracking down each affected example and spending valuable minutes editing and render-testing it by hand. Instead, one edit to the example in the spec propagates everywhere it is referenced: the API reference, the developer guides, and the SDK code samples.
• Generating SDKs surfaced issues in the API itself. Payabli shipped no SDKs before Fern; now they generate eight from a single spec. Turning that spec into code for the first time surfaced inconsistencies the team cleaned up at the source. A single change now regenerates all eight through CI, and forward-compatible patterns keep older integrations working as their APIs change daily. Each SDK reads as native to its language rather than a generated translation of another. The SDKs also carry the field descriptions from Payabli's API definition as doc comments. A developer hovering over a method in their editor sees what each field expects without opening the docs, which counts most for payment fields whose meaning isn't obvious from the type alone.
• Interactive tools built into the docs. Payabli's writers build custom React components directly in Fern Docs: a transaction lifecycle explorer, an ACH timeline calculator, embedded demos of their payment components, and step-by-step code walkthroughs that guide developers through real integration flows. These are the kinds of developer tools Payabli had always wanted to build but couldn't prioritize when maintenance ate most of their bandwidth.
• Built for agents and humans. Payabli wants its docs useful to both without overcorrecting toward either, and <llms-only> and <llms-ignore> tags let them tailor each page to its reader. Payabli uses detailed SVG diagrams across their docs. Those help a human reader but are noise to an agent that can't render them, so they keep the SVG out of the Markdown and write the diagram's logic out directly: fewer steps and less room for misinterpretation. Their docs run to roughly two million tokens, so Fern's section-level llms.txt and llms-full.txt point an agent at just the slice it needs.

Looking ahead

Docs that match the APIs

Freed from that upkeep, Payabli's three-person team ships far more than its size suggests, and pours the time into the developer enablement it couldn't prioritize before: more interactive tools, comprehensive guides, and quickstarts.

The move to Fern is part of a larger investment in making Payabli the simplest choice for software companies that need embedded payments, starting with documentation and SDKs that match the quality of the APIs underneath them.