Is there a solid step by step guide on how to solve this build error?

Runtime JAR files in the classpath should have the same version.

For the life of me I can't find what's pulling in the wrong version of stdlib.

try running the gradle "dependencies" task on your project and find where the two versions come from

I am using Maven and yeah running

dependency:analyze

did not really help me. It just says that the dependency is undeclared but somehow it's there.

running

dependency:tree

?

no, running

dependency:analyze

but I will try

tree

I believe stdlib-jre8 is compiled with bytecode targeting Java 8 rather than Java 6.

kotlin-stdlib-jre8

is an additional artifact for

kotlin-stdlib

with java 8 specific API. You need to have both in runtime if you want to use

kotlin-stdlib-jre8

. Usually

kotlin-stdlib

and

kotlin-stdlib-jre7

are pulled in as transitive dependencies of

kotlin-stdlib-jre8

Thanks for clarifying 🙂