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Document x-localstack header #634

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1 change: 1 addition & 0 deletions src/content/docs/aws/capabilities/config/configuration.md
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Expand Up @@ -40,6 +40,7 @@ Options that affect the core LocalStack system.
| `SERVICES`| `s3,sqs` | A comma-delimited string of services. Check the [internal health endpoint](/aws/capabilities/networking/internal-endpoints#localstack-endpoints) `/_localstack/health` for valid service names. If `SERVICES` is set LocalStack will only load the listed services. All other services will be disabled and cannot be used. |
| `ALLOW_NONSTANDARD_REGIONS` | `0` (default) | Allows the use of non-standard AWS regions. By default, LocalStack only accepts [standard AWS regions](https://docs.aws.amazon.com/general/latest/gr/rande.html). |
| `PARITY_AWS_ACCESS_KEY_ID` | `0` (default) | Enables the use production-like access key IDs. By default, LocalStack issues keys with `LSIA...` and `LKIA...` prefix, and will reject keys that start with `ASIA...` or `AKIA...`. |
| `LOCALSTACK_RESPONSE_HEADER_ENABLED` | `1` (default) \| `0` | Whether LocalStack adds the [`x-localstack` response header](/aws/capabilities/networking/internal-endpoints#x-localstack-response-header) to every AWS API response. The header value is the LocalStack version and lets client tools detect LocalStack and its version. |

## CLI

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18 changes: 18 additions & 0 deletions src/content/docs/aws/capabilities/networking/internal-endpoints.md
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Expand Up @@ -37,3 +37,21 @@ curl -v --request POST --header "Content-Type: application/json" --data '{"acti
The API path for the AWS internal resources is `/_aws`.
These endpoints offer LocalStack-specific features in addition to the ones offered by the AWS services.
For instance, `/aws/services/sqs/messages` conveniently access all messages within a SQS queue, without deleting them.

### `x-localstack` response header

LocalStack adds an `x-localstack` HTTP header to every response served by its AWS gateway.
The header value is the LocalStack version string (for example, `2026.3.1.dev65`), so client tools can detect both that they are talking to LocalStack and which version is running in a single round-trip.

```bash
curl -s -i http://localhost.localstack.cloud:4566/_localstack/health | grep -i x-localstack
# x-localstack: 2026.3.1.dev65
```

:::note
Before LocalStack `v2026.04`, the header value was the static string `true`.
Starting with `v2026.04`, it returns the LocalStack version instead.
Clients that only check for the *presence* of the header remain compatible.
:::

The header is enabled by default and can be disabled by setting [`LOCALSTACK_RESPONSE_HEADER_ENABLED`](/aws/capabilities/config/configuration#core) to `0`.