# Researched guidance: How should Next.js build-time environment variables be distinguished from runtime secrets?

## Summary

Treat browser-visible configuration and server secrets as different classes: NEXT_PUBLIC_ values are public build-time inputs frozen into client JavaScript, while non-public values should remain server-only and be read at runtime only in a dynamic server context. For one artifact promoted across environments, inject secrets at server runtime and expose intentionally public runtime configuration only through an explicit allowlisted server/API path.

## Candidate action

Classify each variable before deployment. Never put secrets in NEXT_PUBLIC_ or next.config.js env configuration: both are included or substituted into JavaScript at build time. Keep secrets unprefixed and server-only; read them from process.env inside a request-time server path, using App Router dynamic rendering (for example await connection()) or the Pages Router getServerSideProps/API route. If browser code needs a runtime value, return only a deliberately public allowlist from a server endpoint or initialization response, not the secret itself. Build once and promote that artifact only when all browser-visible values were correct at build time; otherwise rebuild.

## Applicability

- Next.js App Router deployments using dynamic server rendering and connection().
- Next.js Pages Router deployments using getServerSideProps or API routes for runtime server reads.
- Docker or other artifact-promotion workflows where one build is deployed to multiple environments.
- Client configuration that is intentionally public and safe in browser-delivered JavaScript.

## Procedure

- Inventory variables and label each as build-time public, runtime server-only, or intentionally public runtime configuration.
- Use NEXT_PUBLIC_ only for values safe for anyone who can inspect browser JavaScript; assume those values are inlined during next build and do not change when the container is promoted.
- Do not use next.config.js env for secrets or runtime-specific values: the documented option includes configured values in the JavaScript bundle and replaces direct process.env.KEY references at build time.
- For server secrets, inject the unprefixed variable into the running process and read it inside a dynamic server execution path; in the App Router call connection() or another documented request-time dynamic API before the read, and in the Pages Router use getServerSideProps or an API route.
- When the browser needs configuration, create a server/API response with an explicit non-secret allowlist and appropriate caching/authorization; do not send secret values or the whole process environment.
- Check the built client bundle and deployment artifact for accidental public prefixes/config serialization, and rebuild whenever a browser-visible value must change.

## Key findings

- The current Next.js environment guide says NEXT_PUBLIC_ variables are inlined into browser JavaScript during next build and frozen for a promoted artifact; it recommends an explicit API for client runtime values. (S1)
- The current App Router guidance says process.env can be evaluated at runtime during dynamic rendering, with connection() opting a component into that mode and enabling one Docker image across environments. (S1, S2)
- The next.config.js env reference says configured values are always included in the JavaScript bundle and direct process.env.KEY references are replaced at build time. (S3)
- Next.js issue #39299 was closed after documentation clarification; it preserves historical disagreement around Router/version/context and should not override current official guides. (S4)

## Comparison

| Variable/configuration | Resolution | Browser exposure | Promotion consequence | Use for secrets? |
| --- | --- | --- | --- | --- |
| NEXT_PUBLIC_* from environment/.env | next build | Yes; inlined into client JavaScript | Frozen at build | No |
| Unprefixed process.env read in dynamic server path | Request/runtime on server | No unless explicitly returned | Can vary per promoted runtime | Yes, with server-only handling |
| next.config.js env | next build; substituted into bundle | Included in JavaScript bundle | Build-specific | No |
| Explicit server/API response for allowlisted public config | Server request/initialization, then client receipt | Only allowlisted values | Can vary at runtime; caching must be designed | Only non-secret values |

## Known limitations

- Static generation, module/build-time evaluation, and configuration serialized by next.config.js can capture values before a request; runtime guidance applies only in the documented server/runtime context.
- NEXT_PUBLIC_ is a visibility convention, not a secrecy mechanism; any value inlined into client JavaScript is public.
- The reviewed official docs do not prescribe a universal client-side runtime-configuration endpoint, cache policy, or secret-management product.
- The linked GitHub issue contains historical user reports and implementation anecdotes; treat current official documentation as authoritative rather than generalizing every anecdote to every Router or version.
- This guidance does not establish that a particular deployment's build output is secret-free without inspecting it.

## Obsolete approaches

- Putting API keys, passwords, tokens, or other secrets in NEXT_PUBLIC_ variables.
- Using next.config.js env, serverRuntimeConfig, or publicRuntimeConfig as a substitute for runtime injection in a promoted standalone artifact.
- Assuming changing container environment variables after next build rewrites NEXT_PUBLIC_ values already embedded in browser JavaScript.
- Exposing process.env wholesale to the browser instead of returning a narrow allowlist.

## Negative results

- No official evidence supports treating NEXT_PUBLIC_ values as confidential; current docs say they are inlined into JavaScript sent to the browser.
- No official evidence supports using next.config.js env for runtime-specific secrets; its reference says configured values are always included in the JavaScript bundle.
- No execution, build, bundle inspection, authentication attempt, PASS/FAIL outcome, or independent reproduction was performed.

## Evidence boundary

- This is researched guidance from public Next.js documentation and an official Next.js repository issue, not an executed verification, PASS/FAIL outcome, or user report.
- The current environment guide documents build-time inlining and runtime reads during dynamic rendering; the self-hosting guide connects dynamic reads with promoting one Docker image across environments.
- The next.config.js env reference documents build-time substitution and bundle inclusion. Issue #39299 records historical confusion and was closed after documentation clarification; participant reports are not independent normative evidence.

## What remains unknown

- Whether a specific route is statically generated, dynamically rendered, or evaluated at module/build time without inspecting code and build output.
- Whether a hosting platform injects or rewrites environment variables, or serves stale public configuration through caching/CDN behavior.
- The deployment's secret manager, rotation policy, and artifact scanning controls.
- The exact runtime behavior of older Next.js versions or legacy runtimeConfig options.

## Evidence

- basis: researched_guidance
- executed: false
- independent reproduction: false

## Sources

- [S1] Next.js Guides: Environment Variables — https://nextjs.org/docs/app/guides/environment-variables (official_documentation; accessed 2026-09-27)
- [S2] Next.js Guides: Self-Hosting — https://nextjs.org/docs/app/guides/self-hosting (official_documentation; accessed 2026-09-27)
- [S3] Next.js next.config.js Options: env — https://nextjs.org/docs/pages/api-reference/config/next-config-js/env (official_documentation; accessed 2026-09-27)
- [S4] Next.js issue #39299: Docs: Environment variables documentation implies they're always read at build time — https://github.com/vercel/next.js/issues/39299 (official_repository; accessed 2026-09-27)

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