The selected metaclass is assigned to the generated type, but its construction hooks are never invoked. StructMeta_new_inner calls PyType_Type.tp_new(metatype, ...) directly, bypassing the selected metaclass's __new__ and __init__. That breaks the documented StructMeta subclass pattern for supplying default struct configuration.

At exact head b11a57d52c3329fa460a1a1d60e7059106796119, using the documented pattern with a metaclass that does config.setdefault("kw_only", True) produces these divergent results:

defstruct: calls=[]; signature=(child: int, *, base: int); Dynamic(2, base=1) succeeds
class syntax: calls=[("new", "Static"), ("init", "Static")]; signature=(*, base: int, child: int); Static(2, base=1) raises TypeError

The added custom-metaclass test only delegates from __new__ and checks the resulting metaclass, so it cannot detect that the hook was skipped. Please preserve the selected metaclass's hooks exactly once (including their configuration/validation behavior) and add a regression covering a hook with an observable effect.

Verification: exact base f51f378335b01dc0026dc6553a0b9e1915a8edae reproduces the reported Struct types cannot define __slots__ failure; the two added tests pass on the head; the complete unit suite passes (6382 passed, 143 skipped). The hook/configuration probe above remains red on the head.

Reviewed with OpenAI Codex assistance

The follow-up still does not preserve the selected metaclass's construction semantics, and the direct tp_init call introduces a regression for ordinary class syntax. At exact head 995ca0b6ffc0ae4cdfeff1de0babce5b3347bc72, PyType_Type.tp_new(type, ...) still bypasses CustomMeta.__new__; type->tp_init is then invoked inside StructMeta_new_inner, before the struct fields/configuration are installed.

For defstruct I observe only [("init", "Dynamic")], with __struct_fields__ still missing during that hook, signature (child: int, *, base: int), and a positional child accepted. Equivalent class syntax observes [("new", "Static"), ("init", "Static"), ("init", "Static")], signature (*, base: int, child: int), and rejects the positional child. A metaclass whose __new__ raises for the generated name is likewise bypassed by defstruct but honored by class syntax.

The added regression test is also red as written because it retains the hooks recorded while creating Base; the complete local unit run is 1 failed, 6382 passed, 143 skipped on CPython 3.13.5. Clearing those setup calls would still expose the missing __new__ and unapplied kw_only policy. The public Build and Test, docs-pr, and CodSpeed workflows for this head are currently action_required with zero jobs.

There is also a deterministic failure-path reference leak in the new code. Py_CLEAR(args) was moved below both the tp_new and tp_init failure branches, while cleanup: never decrefs args. Repeating 25 custom-__init__ validation failures increases the refcount of a namespace sentinel by 25 on this head; exact base increases it by 0.

Please dispatch construction through the selected metaclass on the original namespace/configuration (or otherwise preserve the same lifecycle), so __new__ and __init__ each run exactly once at the class-syntax-equivalent point, validation/config hooks are honored, and all failure-owned references are released. The compatible-MRO and metaclass-conflict behavior did match class syntax in my probes.

Reviewed with OpenAI Codex assistance.