Document base-url-env / x-fern-base-url-env for SDK base URL from an environment variable

[devin-ai-integration]

Summary

Generated SDKs can now read their base URL from an environment variable, the same way API keys already default from env vars. The API definition names the variable (base-url-env in api.yml, x-fern-base-url-env in OpenAPI, or base-url-env under api.environments in generators.yml), and the generated client falls back to it when the caller passes neither baseUrl nor environment. This lets customers point one SDK build at staging, production, or a self-hosted deployment via configuration instead of hard-coding or passing a base URL in code. Only the TypeScript SDK generator implements the fallback today, and the docs say so explicitly.

Implements docs for: feat(typescript): read SDK base URL from configured environment variable (fern-api/fern#17736) — fern-api/fern#17736
Implements docs for: feat(cli): add base-url-env for SDK base URL environment variable (fern-api/fern#17705) — fern-api/fern#17705

Pages changed

• fern/products/api-def/ferndef/api-yml/environments.mdx — new "Base URL from an environment variable" section (canonical home): api.yml example, resolution order (baseUrl > environment > env var > default environment), fern check valid-base-url-env warning, TypeScript-only note.
• fern/products/api-def/openapi/extensions/server-names.mdx — new ## x-fern-base-url-env section with an openapi.yml example and the precedence rule (generators.yml base-url-env wins over the spec).
• fern/products/api-def/openapi/extensions/overview.md — table row for x-fern-base-url-env linking to that section.
• fern/products/sdks/reference/generators-yml-reference.mdx — environments ParamField documents base-url-env, its precedence over x-fern-base-url-env, and that overriding environments alone keeps the spec's value.
• fern/products/sdks/generators/typescript/configuration.mdx — guardProcessEnvAccess now covers the base URL env read; requireBaseUrl gains a sentence on the client's base URL resolution order (including standalone sub-clients).

Auto-generated changelogs (typescript/changelog/2026-09-16.mdx, cli-changelog/2026-09-15.mdx) were not edited. No Docs-product changelog entry (SDK/API definition feature).

Checks: pre-commit run --files <changed> (Vale passes; remaining findings are pre-existing on untouched lines) and npx fern-api@5.59.0 check --warnings (0 errors).

Link to Devin session: https://app.devin.ai/sessions/dcb35859297d4ee3bddc62db8cf40148
Open in Devin Desktop: https://app.devin.ai/desktop/session/dcb35859297d4ee3bddc62db8cf40148?variant=devin

[devin-ai-integration]

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[github-actions]

🌿 Preview your docs: https://fern-preview-devin-1789751303-base-url-env-docs.docs.buildwithfern.com/learn

Here are the markdown pages you've updated:

• learn/api-definitions/ferndef/api-yml/environments
• learn/api-definitions/openapi/extensions/server-names-and-url-templating
• learn/sdks/generators/typescript/configuration
• learn/sdks/reference/generators-yml