Most API documentation tools demo well. The buyer regret usually shows up six months later, when the spec has drifted, the SDK examples are stale, the docs URL plan needs redoing, or the renewal price is double what the sales call quoted. This is the criteria checklist I use when buyers ask which tool to pick.
Five questions that decide the migration risk
1. How cleanly does the tool ingest your OpenAPI 3.0 / 3.1 spec?
Anything that requires bespoke MDX or a custom DSL is a switching cost waiting to happen. The good signal is: drop in the spec, get a working reference page in one step. The bad signal is: there's a "config file" that mirrors the spec and you have to keep them in sync.
Test it with your real spec, not a sample. Schemas with oneOf / discriminator, multipart endpoints, and streaming responses are where rendering quality varies most.
2. Do SDK examples stay tied to the spec?
Hand-written examples drift. Spec-driven examples don't. Look for tools that generate request/response examples from your operation examples: or, better, from the schema itself.
If the SDK examples come from a separate generator, ask how the two stay in sync at release time.
3. Does the docs site emit llms.txt?
The agent-readable index matters more every quarter. GPTs, Claude, and Perplexity-style answer engines treat llms.txt as the canonical map of a docs site. Tools that don't emit it leave you behind on the "AI-readable" axis even if the human-readable site is gorgeous.
4. What does the migration off this tool look like in two years?
The reversibility question. If you adopt the tool and decide to switch in two years, what do you take with you? The spec is yours. The MDX is yours. The redirects map is yours. But:
- Are the generated SDK package names dependent on the tool?
- Does the docs URL structure carry custom routes that don't map cleanly to OpenAPI?
- Are the examples written in the tool's component library?
Tools that lock you into proprietary components have a higher exit cost. Tools that let you take the spec, the markdown, and the redirects map are reversible.
5. Is the pricing public, and does it scale by something predictable?
Pricing pages that hide live SDK counts, seat counts, or "API project" definitions behind sales calls are a yellow flag. Tools that publish plans and clearly state what counts as a seat / project / SDK are easier to budget against.
The right question is not "is it cheap" but "is the bill at year three predictable from what I can see today."
Where each category of tool tends to land
The full multi-column comparison lives on /best-api-documentation-tools/. The short version:
- Mintlify — visual polish, AI search, fast to ship a docs site. Lighter on OpenAPI-native SDK generation in the same workflow.
- ReadMe — strong hosted docs hub with API logs and recipes. Pricing scales with usage.
- Fern — docs + SDKs together. Native OpenAPI workflow. Less migration-report tooling.
- GitBook — broad knowledge-base platform. Good when API reference is one section among many.
- Redocly — strongest on OpenAPI governance (lint, bundling, multi-output). Less product polish on the docs-site side.
- Bloom — OpenAPI workflow with docs + SDKs + migration reports + llms.txt in one tool. Newer, fewer languages today.
What to do next
If you're partway through an evaluation, /best-api-documentation-tools/ has the side-by-side matrix with honest losses called out. If you're early in the process, the inventory questions above are more important than the tool comparison — answer those first, then run two short bake-offs with your real spec.
If you already use Stainless, /migrate/stainless-to-bloom/ has the specific path. If you use Mintlify, ReadMe, Fern, or GitBook, the migration hub has the per-provider versions of the same path.