> ## Documentation Index
> Fetch the complete documentation index at: https://microsanbox-staging-appcypher-sdk-runtime-bootstrap.mintlify.site/llms.txt
> Use this file to discover all available pages before exploring further.

# Secrets

> TypeScript secret substitution and placeholder passthrough

Secrets expose a placeholder to the guest and substitute the real value only for an allowed host and enabled request location. A placeholder in any other location is blocked unless the destination is explicitly allowed to receive it unchanged.

```typescript theme={null}
await using sandbox = await Sandbox.builder("worker")
  .secret((s) =>
    s.env("GH_TOKEN")
      .value(token)
      .allow("github.com")
      .allow("api.github.com")
      .substituteInHeaders(false)
      .substituteInQuery(false)
      .substituteInBody(true)
      .allowPassthroughFor("api.anthropic.com")
      .requireTlsIdentity(true)
      .violationAction("block-and-terminate"),
  )
  .secretViolationAction("block-and-log")
  .create();
```

`allow()` accepts exact hosts and wildcard patterns such as `*.example.com`. Use `allowAnyHostDangerous(true)` only when every destination may receive the real secret.

Substitution defaults to headers enabled, query disabled, and body disabled. Basic authentication follows the header setting. Disabling substitution does not make a placeholder inert: the placeholder is still blocked unless the request host matches `allowPassthroughFor()`.

Violation actions are `block`, `block-and-log`, and `block-and-terminate`. The per-secret action overrides the sandbox-wide action. Passthrough is a host policy, not a violation action.

See [Secrets](/sandboxes/secrets) for CLI and YAML syntax.