CORS And Responses

Use the gateway CORS block to centralize browser access policy instead of reimplementing it in every upstream. The CORS config defines which origins, methods,

Use the gateway CORS block to centralize browser access policy instead of reimplementing it in every upstream. The CORS config defines which origins, methods, and headers are allowed for cross-origin requests. This page explains every CORS field and how the gateway applies them to responses.

Last reviewed: 2026-03-06

When to use this

Use the CORS and response reference when you need to control how the gateway handles browser cross-origin requests or what shape responses take for static, error, and operational endpoints. CORS is configured globally and applies to all routes unless overridden by an explicit OPTIONS handler.

Key concepts

• The CORS block is global -- it applies the same origin, method, and header policy to all routes. Per-route CORS overrides require explicit OPTIONS path entries.

• The gateway returns a 204 No Content response for OPTIONS preflight requests automatically. You only need an explicit OPTIONS route if a specific path requires different CORS headers.

• Static response routes can return JSON objects, strings, booleans, or null. The gateway serializes the response field as-is and sets Content-Type: application/json.

• Error responses follow a consistent JSON shape with error and message fields. 401 errors include details about which JWT check failed (expiry, issuer, audience).

• The gateway adds a powered-by response header by default. This can be useful for debugging which gateway version handled a request.

Repo-grounded example

{
  "cors": {
    "allow_origins": ["https://app.example.com"],
    "allow_methods": ["GET", "POST", "OPTIONS"],
    "allow_headers": ["Content-Type", "Authorization", "X-Refresh-Token"],
    "expose_headers": ["X-Request-Id"],
    "allow_credentials": true,
    "max_age": 300
  }
}

This snippet defines a complete CORS block with allow_origins, allow_methods, allow_headers, expose_headers, allow_credentials, and max_age. Each field maps to a specific CORS response header that the gateway attaches to every response.

Troubleshooting

• If the browser shows a CORS error, check that allow_origins includes the exact origin (scheme + domain + port) your frontend uses -- wildcards are not supported when allow_credentials is true.

• If preflight requests return 404 instead of 204, verify that the global CORS block is present in your config and that no explicit OPTIONS route is shadowing the default behavior.

• If a static response returns null as a string instead of JSON null, make sure the response field is set to null (no quotes) in the config JSON.

• If error responses do not include the expected message field, confirm you are testing against a current gateway version -- older versions used a different error shape.

Related docs

• cors

• introduction

• gateway troubleshooting matrix