Cosmo Router is an Open Source GraphQL API Gateway that implements the GraphQL Federation specification, allowing multiple GraphQL APIs to be combined into a unified Graph using GraphQL Federation. The Cosmo Router integrates Subgraphs, which are managed by separate teams and can support requests over HTTP and subscriptions over WebSockets via SSE. To ensure seamless end-to-end functionality, correctness, and superior system performance, the team developed an integration testing framework that abstracts away complexity and ensures deterministic and reproducible tests for distributed systems like Cosmo Router. The framework uses gqlgen to implement Subgraphs and run them in the same process as tests, allowing for debugging into Subgraphs if necessary, NATS for Event-Driven Federated Subscriptions, and the testserver provided by NATS to define test cases. The team also implemented features such as parallel testing, counter-based assertions, and deterministic tracing to ensure fast, easy-to-write, and easy-to-debug tests that meet the requirements of a complex system like Cosmo Router.