Sandbox variables and secrets

Configure environment variables and secrets in Blaxel sandboxes to securely manage API keys, credentials, and runtime configuration values.

Environment variables

There are multiple ways to configure environment variables in Blaxel sandboxes.

Environment variables cannot be added or changed after a sandbox is created. Set all required variables in your Dockerfile, or at sandbox creation time.

When building new sandbox images

Environment variables defined in the sandbox image with the ENV Dockerfile directive are available in every sandbox created from it.

FROM blaxel/base-image:latest

ENV NODE_ENV=production
ENV PORT=3000

Never add secrets in a Dockerfile.

This also applies when creating sandbox images with the declarative image builder:

TypeScript

import { ImageInstance } from "@blaxel/core";

const sandbox = await ImageInstance.fromRegistry("node:20-alpine")
  .workdir("/app")
  .env({
    NODE_ENV: "production",
    PORT: "3000",
  })
  .build({ name: "my-sandbox", memory: 4096 });

When instantiating sandboxes from existing images

When instantiating sandboxes from existing images, any environment variables defined in the base image are automatically available. Additional environment variables can be set using the following methods.

Set variables at sandbox creation time

Pass envs as an array of name/value objects when creating a sandbox with the Blaxel SDKs. These are set as environment variables and are available to every process running inside the deployed sandbox by default (although they can be overridden by process-level variables, discussed in the next section).

TypeScript

import { SandboxInstance } from "@blaxel/core";

const sandbox = await SandboxInstance.createIfNotExists({
  name: "my-sandbox",
  image: "blaxel/base-image:latest",
  region: "us-pdx-1",
  envs: [
    { name: "NODE_ENV", value: "production" },
    { name: "PORT", value: "3000" },
  ],
});

When executing sandbox processes

Environment variables can also be set at process execution, applicable only to that specific process.

TypeScript

import { SandboxInstance } from "@blaxel/core";

const sandbox = await SandboxInstance.get("my-sandbox");

const process = await sandbox.process.exec({
  command: "node server.js",
  env: {
    PORT: "8080",
    LOG_LEVEL: "debug",
  },
});

Secrets

Use proxy injection (recommended)

The recommended way to inject secrets into a sandbox is with the Blaxel proxy. This intercepts outbound HTTPS requests from the sandbox and injects secrets server-side using {{SECRET:name}} placeholders. The sandbox code never sees raw API keys or credentials.

See the proxy routing with secrets injection documentation for examples.

Use a .env.build file

Build variables let you pass secrets and configuration values into the Docker build phase without exposing them at runtime. This is useful when your build process needs credentials that should never appear inside the deployed sandbox.

Create a .env.build file in the root of your project for build secrets. A common example of this is installing private npm packages, which require an NPM_TOKEN during npm install. Variables defined here are injected during the build phase only and are never persisted in the runtime environment.

.env.build

MY_SECRET_BUILD_VAR=I_AM_A_SECRET

Use the --build-env-file argument to bl deploy to specify a custom file name or path instead of the default .env.build.

Ensure that .env.build is ignored during commits to avoid accidentally making secrets public.