diff --git a/src/agent_env/artifact/universe.py b/src/agent_env/artifact/universe.py
index e9cef1bb..a8fc89ee 100644
--- a/src/agent_env/artifact/universe.py
+++ b/src/agent_env/artifact/universe.py
@@ -2,7 +2,7 @@
 
 from __future__ import annotations
 
-from typing import ClassVar
+from typing import Any, ClassVar
 
 from agent_env.artifact.artifact import Artifact
 
@@ -21,7 +21,7 @@ class Universe(Artifact):
     # ``type`` literal default (e.g. ``type: Literal["environment_universe"] = "environment_universe"``).
     __is_abstract__: ClassVar[bool] = True
 
-    def __init_subclass__(cls, **kwargs):
+    def __init_subclass__(cls, **kwargs: Any) -> None:
         super().__init_subclass__(**kwargs)
         # A subclass is concrete if its ``type`` field has a non-empty default.
         type_field = cls.model_fields.get("type")
diff --git a/src/agent_env/providers/sandbox_providers/sandbox_provider.py b/src/agent_env/providers/sandbox_providers/sandbox_provider.py
index 39854700..32a71655 100644
--- a/src/agent_env/providers/sandbox_providers/sandbox_provider.py
+++ b/src/agent_env/providers/sandbox_providers/sandbox_provider.py
@@ -187,7 +187,7 @@ def from_config(cls, **config: Any) -> Self:
     # Whether create_container(private_network=True) puts containers on a network they share.
     PRIVATE_NETWORK: ClassVar[bool] = False
 
-    def __init_subclass__(cls, **kwargs):
+    def __init_subclass__(cls, **kwargs: Any) -> None:
         super().__init_subclass__(**kwargs)
         cls.SUPPORTED_SANDBOX_MODES = frozenset(
             mode for mode, method in ((SANDBOX_MODE_CONTAINER, "create_sandbox"), (SANDBOX_MODE_VM, "create_vm"))
diff --git a/src/agent_env/task_step/task_step.py b/src/agent_env/task_step/task_step.py
index 5dae12e8..eb544bbf 100644
--- a/src/agent_env/task_step/task_step.py
+++ b/src/agent_env/task_step/task_step.py
@@ -90,7 +90,7 @@ class TaskStep(ABC):
     # Fields holding env, A2A agent or artifact ids; None means undeclared.
     entity_refs: ClassVar[tuple[EntityRef, ...] | None] = None
 
-    def __init_subclass__(cls, **kwargs) -> None:
+    def __init_subclass__(cls, **kwargs: Any) -> None:
         super().__init_subclass__(**kwargs)
         # Never inherited: a subclass may add id fields its parent's declaration misses.
         if "entity_refs" not in vars(cls):
diff --git a/tst/unit/test_subclass_hooks_are_annotated.py b/tst/unit/test_subclass_hooks_are_annotated.py
new file mode 100644
index 00000000..2e521f51
--- /dev/null
+++ b/tst/unit/test_subclass_hooks_are_annotated.py
@@ -0,0 +1,25 @@
+"""agent_env ships py.typed, so a consumer's type checker checks its subclasses of agent_env's classes, and defining one calls
+the base's ``__init_subclass__``, which a strict checker refuses to call when it's untyped (``no-untyped-call``). Every such
+hook is annotated."""
+
+import ast
+from pathlib import Path
+
+SRC = Path(__file__).resolve().parents[2] / "src" / "agent_env"
+
+
+def _unannotated(hook: ast.FunctionDef) -> list[str]:
+    args = hook.args
+    after_cls = [*args.posonlyargs, *args.args][1:]
+    params = [*after_cls, *args.kwonlyargs, *filter(None, (args.vararg, args.kwarg))]
+    return [param.arg for param in params if param.annotation is None] + (["return"] if hook.returns is None else [])
+
+
+def test_every_init_subclass_hook_is_annotated():
+    unannotated = {
+        f"{path.relative_to(SRC)}:{node.lineno}": missing
+        for path in sorted(SRC.rglob("*.py"))
+        for node in ast.walk(ast.parse(path.read_text()))
+        if isinstance(node, ast.FunctionDef) and node.name == "__init_subclass__" and (missing := _unannotated(node))
+    }
+    assert unannotated == {}