The two real open-source Stainless alternatives are OpenAPI Generator and Kiota — both free, both self-hosted, both community- or Microsoft-maintained with no vendor pricing tiers. Neither generates hosted docs, publishes packages for you, or diffs SDK compatibility across versions; that's the work you take on in exchange for zero licensing cost and zero vendor lock-in. Sourced isn't in this category — it's the hosted, low-effort path with a free tier, positioned below as the tradeoff on the other side.
The vendor-dependency argument for OSS just got a real-world example. Stainless — a hosted vendor, not an OSS project — announced in May 2026 that it's joining Anthropic; per that post, new signups, projects, and SDKs stopped immediately, and Stainless said it's winding down its hosted SDK generator. WorkOS's coverage frames the takeaway directly: the hosted SDK generator is going away. Neither OpenAPI Generator nor Kiota can disappear this way — there's no company to acquire. If you have a live Stainless project rather than choosing your first tool, the migration guide covers what to do with the spec and config you already own.
OpenAPI Generator
OpenAPI Generator is community-maintained, hosted on GitHub, with no licensing cost and no usage tiers. Per its own site (fetched September 2026), it ships 50+ client generators across 40+ languages and technologies, including Java, Kotlin, Go, PHP, Ruby, and JavaScript.
Setup, per the same source:
- Install via npm, Homebrew, or Docker.
- Have an OpenAPI spec (YAML or JSON).
- Run
openapi-generator-cli generate -i <spec-file> -g <language> -o <output-directory>.
That's it for a first client. What it doesn't do: host docs, publish to a registry, check whether a new generation broke your public SDK surface, or track drift between your spec and a hand-maintained config. Customization is template-based — you can override generation behavior, but you're maintaining those templates yourself, indefinitely, across every language you target.
Kiota
Kiota is Microsoft's open-source OpenAPI-to-client generator, free with no tiers. Per Microsoft's own docs (fetched September 2026), its CLI commands are search, download, show, generate, update, info, login, and logout. The core workflow is:
kiota generate --openapi <path-or-url> --language <language> --output <path>
Supported languages, with maturity levels as documented in the project README (fetched September 2026): CSharp, Go, Java, PHP, and Python are Stable; Dart, Ruby, and TypeScript are Preview. Kiota also writes a kiota-lock.json file per output directory, capturing the generation parameters and a hash of the source spec — kiota update re-runs generation only if the spec or parameters changed, which is a useful drift-detection primitive that OpenAPI Generator doesn't have built in.
Kiota grew out of Microsoft Graph tooling, but it works against any OpenAPI 3.x description, not just Microsoft's own APIs.
What neither tool gives you
- Hosted docs. Neither produces a docs site. You'd pair either with a separate docs tool (Redocly, GitBook, or a hand-rolled static site) or none at all.
- Registry publishing. Both stop at generated source code. Publishing to npm or PyPI, including trusted-publisher setup, is a CI step you build yourself.
- Compatibility diffing. Neither tool tells you whether today's generation broke a method signature customers depend on. That check, if you want it, is custom tooling or a manual review.
- llms.txt or MCP output. Neither generates these formats natively.
- A free-vs-paid decision to make at all. There isn't one — no seats, no endpoint caps, no account.
When OSS wins
- You already have CI infrastructure and a team member who owns generator templates.
- You need a language neither Sourced nor most hosted platforms cover (OpenAPI Generator's 40+ languages is the widest net available anywhere).
- Vendor lock-in or a recurring bill is the dealbreaker, not the engineering time.
- You want the generated code to live entirely inside your own monorepo and pipeline with no external service in the loop.
- You're generating clients for internal tooling, not customer-facing SDKs, where docs, publishing, and compatibility reports matter less.
When hosted wins
- You need docs live today, not after a separate docs-tool evaluation.
- You're migrating off an existing generator (like Stainless) and want a compatibility report against your live SDK before you touch anything customer-facing — that's not something either OSS tool produces.
- You want
llms.txtand MCP server output without building a separate pipeline for each. - Nobody on the team wants to own template maintenance indefinitely across two or three languages.
- You want to evaluate the output before committing engineering time, which is what a free hosted preview is for.
Comparison
| Dimension | OpenAPI Generator | Kiota | Sourced |
|---|---|---|---|
| Cost | Free, no tiers | Free, no tiers | Free preview tier, paid to publish |
| Languages | 50+ generators, 40+ languages | 9 languages, maturity varies | 2 (TypeScript, Python) |
| Hosted docs | No | No | Yes, from the same spec |
| Publishing automation | No — build your own CI | No — build your own CI | Registry readiness checks, approval-gated |
| Compatibility diff vs. existing SDK | No | No | Yes |
| llms.txt / MCP output | No | No | Yes |
| Maintenance owner | You (templates, CI, drift) | You (CI, drift; lock file helps) | Sourced (generation), you (approval) |
Honest scope
If your team already runs a generator pipeline and the only pain point is that Stainless costs money or gates docs behind a paid tier, OpenAPI Generator or Kiota replaces the SDK-generation half of Stainless for $0, forever, with no account. It does not replace the docs half, the compatibility-checking half, or the publishing-automation half — those stay unsolved, or become separate tools you assemble yourself. Sourced exists for teams who'd rather not assemble that pipeline: same starting point (an OpenAPI spec), free preview, but hosted docs, SDK previews, and a compatibility report come out of the same run.
Nothing stops you from using both — generate a quick client locally with OpenAPI Generator to sanity-check your spec, then run the same spec through Sourced when you're ready for docs and a release-readiness check. One scope note: Sourced's generated SDKs focus on TypeScript and Python today — the two ecosystems where teams publish first — narrower than OpenAPI Generator's 40+ languages or Kiota's 9, but with hosted docs and a compatibility report included in the same run.
FAQ
Is OpenAPI Generator actively maintained?
Yes — it's a long-running community project on GitHub with frequent releases and 50+ generator targets, per its own site as of September 2026.
Is Kiota tied to Microsoft Graph, or does it work with any API?
It works with any OpenAPI 3.x description. Its CLI includes a registry search feature that surfaces public API descriptions including Microsoft Graph, but generation itself only needs a spec.
Do either of these tools generate hosted docs?
No. Both are client-code generators only. Pairing either with a docs tool (or Sourced) is a separate decision.
Which has broader language support, OpenAPI Generator or Kiota?
OpenAPI Generator — 50+ generators across 40+ languages, versus Kiota's 9 languages with varying maturity levels.
Is Sourced open source?
No. Sourced is a hosted product with a free preview tier — unlimited SDK/docs previews and up to two hosted noindex docs review sites and one hosted MCP server, no credit card, with paid plans starting at publish.
Can I switch from OpenAPI Generator to Sourced later without losing anything?
Your OpenAPI spec is the source of truth either way, so nothing is lost — you'd just point the same spec at Sourced and compare the output, including a docs preview OpenAPI Generator never produced.
Try the hosted path
Create hosted docs from your repo or start free with a spec upload — the shortest path from OpenAPI spec to live docs and SDK previews if you'd rather not assemble the OSS pipeline yourself. Run your spec through the OpenAPI validator first regardless of which generator you pick — a spec that fails validation produces bad output in every tool on this page.