In accordance with an embodiment, described herein is a system and method for providing runtime tracing for a web-based client that accesses a transactional middleware platform using an extension interface. The extension interface provides an extension server and a class library to make application services in the transactional middleware platform available to web-based clients. A trace generator can be provided to compose a trace filter specifying categories of trace points, and to generate trace records based on the composed trace filter and trace points encountered at runtime. Code snippets can be inserted into each class in the class library for use in invoking the trace generator. At runtime, one or more objects instantiated from the class library can invoke the trace generator to generate trace records that track a particular web-based client end to end, from the time it starts a service call to the time it gets a response.