diff --git a/smithy-aws-protocol-tests/model/awsJson1_0/errors.smithy b/smithy-aws-protocol-tests/model/awsJson1_0/errors.smithy
index 7da6a4b55a2..123b82b6467 100644
--- a/smithy-aws-protocol-tests/model/awsJson1_0/errors.smithy
+++ b/smithy-aws-protocol-tests/model/awsJson1_0/errors.smithy
@@ -58,6 +58,23 @@ apply InvalidGreeting @httpResponseTests([
               }""",
         bodyMediaType: "application/json",
     },
+    {
+        id: "AwsJson10FooErrorWithMultipleAmznErrorTypes",
+        documentation: """
+            API Gateway always adds its own x-amzn-errortype header on gateway responses.
+            However, this header does not match the error configured in customers model and \
+            results in two X-Amzn-Errortype header values. \
+            The first value is provided by the customer and the second value is from API Gateway \
+
+            Clients need to split the header value on ',' and take only the first element. \
+            For example, 'InvalidGreeting,InvalidParameterException' is to be interpreted as 'InvalidGreeting'.""",
+        protocol: awsJson1_0,
+        code: 400,
+        headers: {
+            "X-Amzn-Errortype": "InvalidGreeting,InvalidParameterException",
+        },
+        appliesTo: "client",
+    },
 ])
 
 /// This error is thrown when a request is invalid.
diff --git a/smithy-aws-protocol-tests/model/awsJson1_1/errors.smithy b/smithy-aws-protocol-tests/model/awsJson1_1/errors.smithy
index c57b6fed010..f344ccb8a5f 100644
--- a/smithy-aws-protocol-tests/model/awsJson1_1/errors.smithy
+++ b/smithy-aws-protocol-tests/model/awsJson1_1/errors.smithy
@@ -51,6 +51,23 @@ apply InvalidGreeting @httpResponseTests([
               }""",
         bodyMediaType: "application/json",
     },
+    {
+        id: "AwsJson11FooErrorWithMultipleAmznErrorTypes",
+        documentation: """
+            API-Gateway always adds its own x-amzn-errortype header on gateway responses.
+            However, this header does not match the error configured in customers model and \
+            results in two X-Amzn-Errortype header values. \
+            The first value is provided by the customer and the second value is from API Gateway \
+
+            Clients need to split the header value on ',' and take only the first element. \
+            For example, 'InvalidGreeting,InvalidParameterException' is to be interpreted as 'InvalidGreeting'.""",
+        protocol: awsJson1_1,
+        code: 400,
+        headers: {
+            "X-Amzn-Errortype": "InvalidGreeting,InvalidParameterException",
+        },
+        appliesTo: "client",
+    },
 ])
 
 /// This error is thrown when a request is invalid.
diff --git a/smithy-aws-protocol-tests/model/restJson1/errors.smithy b/smithy-aws-protocol-tests/model/restJson1/errors.smithy
index 7e387fe0363..b5a6f7a710b 100644
--- a/smithy-aws-protocol-tests/model/restJson1/errors.smithy
+++ b/smithy-aws-protocol-tests/model/restJson1/errors.smithy
@@ -95,7 +95,23 @@ apply InvalidGreeting @httpResponseTests([
               }""",
         bodyMediaType: "application/json",
     },
+    {
+        id: "RestJsonWithMultipleAmznErrorTypes",
+        documentation: """
+            API-Gateway always adds its own x-amzn-errortype header on gateway responses.
+            However, this header does not match the error configured in customers model and \
+            results in two X-Amzn-Errortype header values. \
+            The first value is provided by the customer and the second value is from API Gateway \
 
+            Clients need to split the header value on ',' and take only the first element. \
+            For example, 'InvalidGreeting,InvalidParameterException' is to be interpreted as 'InvalidGreeting'.""",
+        protocol: restJson1,
+        code: 400,
+        headers: {
+            "X-Amzn-Errortype": "InvalidGreeting,InvalidParameterException",
+        },
+        appliesTo: "client",
+    }
 ])
 
 /// This error is thrown when a request is invalid.