Python: Kernel JSON Schema Builder support for Forward Refs

[moonbox3]

The current Kernel JSON Schema Builder is missing support to properly handle Forward References when building a schema. This is marked as a TODO in code and is untested.

https://github.com/microsoft/semantic-kernel/blob/fbb26b5cf6a71d0cbfaa9c9e9a53997d2e34cbfb/python/semantic_kernel/schema/kernel_json_schema_builder.py#L55C9-L55C27

[github-actions]

This issue is stale because it has been open for 90 days with no activity.

[grvnmttl]

Picking this back up — it was auto-closed as stale but the gap is still present on main today.

I dug into the exact failure mode: plain (non-cyclic) forward references already resolve correctly in KernelJsonSchemaBuilder.build_model_schema today. The real bug is that a self-referential or mutually-recursive KernelBaseModel (e.g. a tree node, linked list, or two models that reference each other) causes KernelJsonSchemaBuilder.build() to recurse infinitely and crash with RecursionError: maximum recursion depth exceeded, rather than a NameError on the unresolved name.

I have a fix ready that detects the cycle and emits a standard JSON Schema $defs/$ref pair instead of recursing — the same mechanism Pydantic's own model_json_schema() uses for recursive models. It only activates when a cycle is actually present, so today's fully-inlined schema output for non-recursive models is unchanged. Added test coverage for the simple forward-ref case, self-reference, a mutual two-model cycle, and the structured_output=True path — none of which were covered before. Opening a PR shortly.