AWS lambda api gateway error "Malformed Lambda proxy response"

Usually, when you see Malformed Lambda proxy response, it means your response from your Lambda function doesn't match the format API Gateway is expecting, like this

{
    "isBase64Encoded": true|false,
    "statusCode": httpStatusCode,
    "headers": { "headerName": "headerValue", ... },
    "body": "..."
}

If you are not using Lambda proxy integration, you can login to API Gateway console and uncheck the Lambda proxy integration checkbox.

Also, if you are seeing intermittent Malformed Lambda proxy response, it might mean the request to your Lambda function has been throttled by Lambda, and you need to request a concurrent execution limit increase on the Lambda function.

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If lambda is used as a proxy then the response format should be

{
"isBase64Encoded": true|false,
"statusCode": httpStatusCode,
"headers": { "headerName": "headerValue", ... },
"body": "..."
}

Note : The body should be stringified

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Yeah so I think this is because you're not actually returning a proper http response there which is why you're getting the error.

personally I use a set of functions like so:

    module.exports = {
        success: (result) => {
            return {
                statusCode: 200,
                headers: {
                    "Access-Control-Allow-Origin" : "*", // Required for CORS support to work
                    "Access-Control-Allow-Credentials" : true // Required for cookies, authorization headers with HTTPS
                },
                body: JSON.stringify(result),
            }
        },
        internalServerError: (msg) => {
            return {
                statusCode: 500,
                headers: {
                    "Access-Control-Allow-Origin" : "*", // Required for CORS support to work
                    "Access-Control-Allow-Credentials" : true // Required for cookies, authorization headers with HTTPS
                },
                body: JSON.stringify({
                    statusCode: 500,
                    error: 'Internal Server Error',
                    internalError: JSON.stringify(msg),
                }),
            }
        }
} // add more responses here.

Then you simply do:

var responder = require('responder')

// some code

callback(null, responder.success({ message: 'hello world'}))

---

For Python3:

import json

def lambda_handler(event, context):
    return {
        'statusCode': 200,
        'headers': {
            'Content-Type': 'application/json',
            'Access-Control-Allow-Origin': '*'
        },
        'body': json.dumps({
            'success': True
        }),
        "isBase64Encoded": False
    }

Note the body isn't required to be set, it can just be empty:

        'body': ''

---

I had this issue, which originated from an invalid handler code which looks completely fine:

exports.handler = (event, context) => {
    return {
       isBase64Encoded: false,
       body: JSON.stringify({ foo: "bar" }),
       headers: {
          'Access-Control-Allow-Origin': '*',
       },
       statusCode: 200,
    };
}

I got the hint from examining the somewhat confusing API Gateway response logs:

> Endpoint response body before transformations: null

The way to fix it would be to either

Add the async keyword (async function implicitly returns a Promise):

exports.handler = async (event, context) => {
    return {
        isBase64Encoded: false,
        body: JSON.stringify({ foo: "bar" }),
        headers: {
            'Access-Control-Allow-Origin': '*',
        },
        statusCode: 200,
    };
}

Return a Promise:

exports.handler = (event, context) => {
    return new Promise((resolve) => resolve({
        isBase64Encoded: false,
        body: JSON.stringify({ foo: "bar" }),
        headers: {
            'Access-Control-Allow-Origin': '*',
        },
        statusCode: 200,
    }));
}

Use the callback:

exports.handler = (event, context, callback) => {
    callback({
        isBase64Encoded: false,
        body: JSON.stringify({ foo: "bar" }),
        headers: {
            'Access-Control-Allow-Origin': '*',
        },
        statusCode: 200,
    });
}

My handler was previously declared async without ever using await, so I removed the async keyword to reduce complexity of the code, without realizing that Lambda expects either using async/await/Promise or callback return method.

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From the AWS docs

In a Lambda function in Node.js, To return a successful response, call callback(null, {"statusCode": 200, "body": "results"}). To throw an exception, call callback(new Error('internal server error')). For a client-side error, e.g., a required parameter is missing, you can call callback(null, {"statusCode": 400, "body": "Missing parameters of ..."}) to return the error without throwing an exception.

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I've tried all of above suggestion but it doesn't work while body value is not String

return {
    statusCode: 200,
    headers: {
        "Content-Type": "application/json",
        "Access-Control-Allow-Origin": "*"
    },
    body: JSON.stringify({
        success: true
    }),
    isBase64Encoded: false
};

---

Just a piece of code for .net core and C# :

using Amazon.Lambda.APIGatewayEvents;
...
var response = new APIGatewayProxyResponse
{
   StatusCode = (int)HttpStatusCode.OK,
   Body = JsonConvert.SerializeObject(new { msg = "Welcome to Belarus! :)" }),
   Headers = new Dictionary<string, string> { { "Content-Type", "application/json" } }
};
return response;

Response from lambda will be :

{"statusCode":200,"headers":{"Content-Type":"application/json"},"multiValueHeaders":null,"body":"{\"msg\":\"Welcome to Belarus! :)\"}","isBase64Encoded":false}

Response from api gateway will be :

{"msg":"Welcome to Belarus! :)"}

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A very very special case, if you pass the headers directly there is a chance you have this header:

"set-cookie": [ "........" ]

But Amazon needs this:

"set-cookie": "[ \\"........\\" ]"

---

For anyone else who struggles when the response appears valid. This does not work:

callback(null,JSON.stringify( {
  isBase64Encoded: false,
  statusCode: 200,
  headers: { 'headerName': 'headerValue' },
  body: 'hello world'
})

but this does:

callback(null,JSON.stringify( {
  'isBase64Encoded': false,
  'statusCode': 200,
  'headers': { 'headerName': 'headerValue' },
  'body': 'hello world'
})

Also, it appears that no extra keys are allowed to be present on the response object.

---

If you're using Go with https://github.com/aws/aws-lambda-go, you have to use events.APIGatewayProxyResponse.

func hello(ctx context.Context, event ImageEditorEvent) (events.APIGatewayProxyResponse, error) {
    return events.APIGatewayProxyResponse{
        IsBase64Encoded: false,
        StatusCode:      200,
        Headers:         headers,
        Body:            body,
    }, nil
}

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I had this error because I accidentally removed the variable ServerlessExpressLambdaFunctionName from the CloudFormation AWS::Serverless::Api resource. The context here is https://github.com/awslabs/aws-serverless-express "Run serverless applications and REST APIs using your existing Node.js application framework, on top of AWS Lambda and Amazon API Gateway"

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In case the above doesn't work for anyone, I ran into this error despite setting the response variable correctly.

I was making a call to an RDS database in my function. It turned out that what was causing the problem was the security group rules (inbound) on that database.

You'll probably want to restrict the IP addresses that can access the API, but if you want to get it working quick / dirty to test out if that change fixes it you can set it to accept all like so (you can also set the range on the ports to accept all ports too, but I didn't do that in this example):

https://i.stack.imgur.com/e5lr8.png

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A common cause of the "Malformed Lambda proxy response" error is headers that are not {String: String, ...} key/values pairs.

Since set-cookie headers can and do appear in multiples, they are represented in http.request.callback.response as the set-cookie key having an Array of Strings value instead of a single String. While this works for developers, AWS API Gateway doesn't understand it and throws a "Malformed Lambda proxy response" error.

My solution is to do something like this:

function createHeaders(headers) {
  const singleValueHeaders = {}
  const multiValueHeaders = {}
  Object.entries(headers).forEach(([key, value]) => {
    const targetHeaders = Array.isArray(value) ? multiValueHeaders : singleValueHeaders
    Object.assign(targetHeaders, { [key]: value })
  })

  return {
    headers: singleValueHeaders,
    multiValueHeaders,
  }
}

var output = {
  ...{
    "statusCode": response.statusCode,
    "body": responseString
  },
  ...createHeaders(response.headers)
}

Note that the ... above does not mean Yada Yada Yada. It's the ES6 spread operator.

---

Here's another approach. Configure the mapping template in your API gateway integration request and response. Go to IntegrationRequest -> MappingTemplate -> select "When there are no templates defined" -> type application/json for content-type. Then you don't have to explicitly send a json. Even the response you get at your client can be a plain string.

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The format of your function response is the source of this error. For API Gateway to handle a Lambda function's response, the response must be JSON in this format:

{ "isBase64Encoded": true|false, "statusCode": httpStatusCode, "headers": { "headerName": "headerValue", ... }, "body": "..." }

Here's an example function in Node.js with the response correctly formatted:

exports.handler = (event, context, callback) => {

var responseBody = {
    "key3": "value3",
    "key2": "value2",
    "key1": "value1"
};

var response = {
    "statusCode": 200,
    "headers": {
        "my_header": "my_value"
    },
    "body": JSON.stringify(responseBody),
    "isBase64Encoded": false
};
callback(null, response);

};

Ref: https://aws.amazon.com/premiumsupport/knowledge-center/malformed-502-api-gateway/

---

Python 3.7

Before

{
    "isBase64Encoded": False,
    "statusCode": response.status_code,
    "headers": {
                  "Content-Type": "application/json",
               },
     "body": response.json()
}

After

{
    "isBase64Encoded": False,
    "statusCode": response.status_code,
    "headers": {
                  "Content-Type": "application/json",
               },
     "body": str(response.json()) //body must be of string type
}

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If you're just new to AWS and just want your URL working,

If you haven't created a trigger for your Lambda Function, navigate to the function in Lambda Functions app and create trigger choosing API Gateway.

Navigate to API Gateway App -> Choose your Particular Lambda's API Gateway (Method execution) -> Click on INTEGRATION Request -> Uncheck "Use Lambda Proxy integration" (check box).

Then click on "<-Method Execution" & click on Test Client section. Provide the options and click test button. You should see a success response.

If you are still unable to get a success response, create an alias for the correct version (if you have multiple versions in the Lambda Function)

Pick the URL from the logs and use your POST/GET Tool (Postman) and choose authentication as AWS Signature - provide your authentication keys(AccessKey & SecretKey) in the postman request with AWS Region & Service Name as lambda.

P.S : This may only help beginners and may be irrelevant to others.

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Most likely your returning body is in JSON format, but only STRING format is allowed for Lambda proxy integration with API Gateway.

So wrap your old response body with JSON.stringify().

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