How can I send multiple Set-Cookie headers from API Gateway using a proxied Lambda

As of November 2018 this is possible using the multiValueHeaders field in the response instead of headers (see announcement).

As an example instead of:

{
  "statusCode": 200,
  "body": "testing multiple set-cookie headers",
  "headers": {
    "X-Test-Header": "baking experiment",
    "Set-Cookie": "cookie1=chocolate-chip",
    "Set-Cookie": "cookie2=oatmeal",
    "Content-Type": "text/plain"
  }
}

You can respond with:

{
  "statusCode": 200,
  "body": "testing multiple set-cookie headers",
  "multiValueHeaders": {
    "X-Test-Header": ["baking experiment"],
    "Set-Cookie": ["cookie1=chocolate-chip", "cookie2=oatmeal"],
    "Content-Type": ["text/plain"]
  }
}

Note that you can use a mix of headers and multiValueHeaders:

{
  "statusCode": 200,
  "body": "testing multiple set-cookie headers",
  "headers": {
    "X-Test-Header": "baking experiment",
    "Content-Type": "text/plain"
  },
  "multiValueHeaders": {
    "Set-Cookie": ["cookie1=chocolate-chip", "cookie2=oatmeal"]
  }
}

However using the same header in both will mean that the value under headers is dropped.

See the documentation for more details.

When using only the header field (as available prior to Nov 2018) I tried sending the following manually curated JSON as a response:

{
  "statusCode": 200,
  "body": "testing multiple set-cookie headers",
  "headers": {
    "X-Test-Header": "baking experiment",
    "Set-Cookie": "cookie1=chocolate-chip",
    "Set-Cookie": "cookie2=oatmeal",
    "Content-Type": "text/plain"
  }
}

The cookies that API gateway returns in response to a CURL request are:

< Content-Type: text/plain
< Content-Length: 35
< Connection: keep-alive
< Date: Thu, 29 Sep 2016 11:22:09 GMT
< Set-Cookie: cookie2=oatmeal
< X-Test-Header: baking experiment
< X-Cache: Miss from cloudfront

As you can see the first Set-Cookie is dropped on the floor.

As Mark B pointed out, you can/should achieve this by setting multiple cookie name/value pairs in a single Set-Cookie header. The browser should interpret this correctly.

Cookie: a=1; b=2

Edit: as pointed out by OP, there are use cases that require multiple instances of the header. We've added it our backlog along with supporting multiple header names on incoming requests.

Use multiValueHeaders:

response.multiValueHeaders = {
  "Set-Cookie": [
    'cookie1=value1',
    'cookie1=value1'
  ]
}

or:

{
    "statusCode": httpStatusCode,
    "headers": { "headerName": "headerValue", ... },
    "multiValueHeaders": { "headerName": ["headerValue", "headerValue2",...], ... },
    "body": "..."
}

https://docs.aws.amazon.com/apigateway/latest/developerguide/set-up-lambda-proxy-integrations.html#api-gateway-simple-proxy-for-lambda-output-format