[AWS] Custom Runtime error response format inconsistent with Lambda Runtime

[mrjv]

Description

When a Spring Cloud Function throws an exception, the error response JSON returned by the Custom Runtime uses a different stackTrace format than the standard Lambda Runtime. This makes it difficult to parse error responses consistently across runtime types:

• Lambda Runtime returns stackTrace as a JSON array of strings (one entry per frame).
• Custom Runtime returns stackTrace as a single string with newline-separated frames (including Caused by sections inlined).

Reproducing Code

@SpringBootApplication
public class Application {

    public static void main(String[] args) {
        SpringApplication.run(Application.class, args);
    }

    @Bean
    public Function<Map<String, String>, String> hello() {
        return event -> {
            throw new RuntimeException("Something went wrong");
        };
    }

    @Bean
    public Function<Map<String, String>, String> helloNested() {
        return event -> {
            Exception cause = new IllegalStateException("Root cause error");
            throw new RuntimeException("Wrapper exception", cause);
        };
    }
}

Lambda Runtime Response — Simple Exception

{
  "errorMessage": "Something went wrong",
  "errorType": "java.lang.RuntimeException",
  "stackTrace": [
    "com.example.lambda.Application.lambda$hello$0(Application.java:24)",
    "org.springframework.cloud.function.context.catalog.SimpleFunctionRegistry$FunctionInvocationWrapper.invokeFunctionAndEnrichResultIfNecessary(SimpleFunctionRegistry.java:1027)",
    "org.springframework.cloud.function.context.catalog.SimpleFunctionRegistry$FunctionInvocationWrapper.invokeFunction(SimpleFunctionRegistry.java:973)",
    "org.springframework.cloud.function.context.catalog.SimpleFunctionRegistry$FunctionInvocationWrapper.doApply(SimpleFunctionRegistry.java:811)",
    "org.springframework.cloud.function.context.catalog.SimpleFunctionRegistry$FunctionInvocationWrapper.apply(SimpleFunctionRegistry.java:635)",
    "org.springframework.cloud.function.adapter.aws.FunctionInvoker.handleRequest(FunctionInvoker.java:94)",
    "java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(Unknown Source)",
    "java.base/java.lang.reflect.Method.invoke(Unknown Source)"
  ]
}

Custom Runtime Response — Simple Exception

{
  "errorType": "RuntimeException",
  "errorMessage": "Something went wrong",
  "stackTrace": "java.lang.RuntimeException: Something went wrong\n\tat com.example.lambda.Application.lambda$hello$0(Application.java:24)\n\tat org.springframework.cloud.function.context.catalog.SimpleFunctionRegistry$FunctionInvocationWrapper.invokeFunctionAndEnrichResultIfNecessary(SimpleFunctionRegistry.java:1027)\n\tat org.springframework.cloud.function.context.catalog.SimpleFunctionRegistry$FunctionInvocationWrapper.invokeFunction(SimpleFunctionRegistry.java:973)\n\tat org.springframework.cloud.function.context.catalog.SimpleFunctionRegistry$FunctionInvocationWrapper.doApply(SimpleFunctionRegistry.java:811)\n\tat org.springframework.cloud.function.context.catalog.SimpleFunctionRegistry$FunctionInvocationWrapper.apply(SimpleFunctionRegistry.java:635)\n\tat org.springframework.cloud.function.adapter.aws.CustomRuntimeEventLoop.eventLoop(CustomRuntimeEventLoop.java:153)\n\tat org.springframework.cloud.function.adapter.aws.CustomRuntimeEventLoop.lambda$run$0(CustomRuntimeEventLoop.java:95)\n\tat java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1090)\n\tat java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:614)\n\tat java.base/java.lang.Thread.run(Thread.java:1474)\n"
}

Lambda Runtime Response — Nested Exception

{
  "errorMessage": "Wrapper exception",
  "errorType": "java.lang.RuntimeException",
  "stackTrace": [
    "com.example.lambda.Application.lambda$helloNested$0(Application.java:36)",
    "org.springframework.cloud.function.context.catalog.SimpleFunctionRegistry$FunctionInvocationWrapper.invokeFunctionAndEnrichResultIfNecessary(SimpleFunctionRegistry.java:1027)",
    "org.springframework.cloud.function.context.catalog.SimpleFunctionRegistry$FunctionInvocationWrapper.invokeFunction(SimpleFunctionRegistry.java:973)",
    "org.springframework.cloud.function.context.catalog.SimpleFunctionRegistry$FunctionInvocationWrapper.doApply(SimpleFunctionRegistry.java:811)",
    "org.springframework.cloud.function.context.catalog.SimpleFunctionRegistry$FunctionInvocationWrapper.apply(SimpleFunctionRegistry.java:635)",
    "org.springframework.cloud.function.adapter.aws.FunctionInvoker.handleRequest(FunctionInvoker.java:94)",
    "java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(Unknown Source)",
    "java.base/java.lang.reflect.Method.invoke(Unknown Source)"
  ]
}

Custom Runtime Response — Nested Exception

{
  "errorType": "RuntimeException",
  "errorMessage": "Wrapper exception",
  "stackTrace": "java.lang.RuntimeException: Wrapper exception\n\tat com.example.lambda.Application.lambda$helloNested$0(Application.java:36)\n\tat org.springframework.cloud.function.context.catalog.SimpleFunctionRegistry$FunctionInvocationWrapper.invokeFunctionAndEnrichResultIfNecessary(SimpleFunctionRegistry.java:1027)\n\tat org.springframework.cloud.function.context.catalog.SimpleFunctionRegistry$FunctionInvocationWrapper.invokeFunction(SimpleFunctionRegistry.java:973)\n\tat org.springframework.cloud.function.context.catalog.SimpleFunctionRegistry$FunctionInvocationWrapper.doApply(SimpleFunctionRegistry.java:811)\n\tat org.springframework.cloud.function.context.catalog.SimpleFunctionRegistry$FunctionInvocationWrapper.apply(SimpleFunctionRegistry.java:635)\n\tat org.springframework.cloud.function.adapter.aws.CustomRuntimeEventLoop.eventLoop(CustomRuntimeEventLoop.java:153)\n\tat org.springframework.cloud.function.adapter.aws.CustomRuntimeEventLoop.lambda$run$0(CustomRuntimeEventLoop.java:95)\n\tat java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1090)\n\tat java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:614)\n\tat java.base/java.lang.Thread.run(Thread.java:1474)\nCaused by: java.lang.IllegalStateException: Root cause error\n\tat com.example.lambda.Application.lambda$helloNested$0(Application.java:35)\n\t... 9 more\n"
}

Expected Behavior

The Custom Runtime should return stackTrace in the same format as the Lambda Runtime — a JSON array of strings with one stack frame per element — to allow consistent error parsing regardless of deployment target.

[mrjv]

Actually the errorType field is also inconsistent as shown in the examples above, so ideally that would be updated too.

The Custom Runtime Response returns the simple class name while the AWS Runtime returns the full class name. The expected behavior would be to use the full class name in both cases (e.g. java.lang.RuntimeException instead of RuntimeException).