diff --git a/tests/test_ata_constraint_map_validation.py b/tests/test_ata_constraint_map_validation.py
new file mode 100644
index 000000000..5f398a904
--- /dev/null
+++ b/tests/test_ata_constraint_map_validation.py
@@ -0,0 +1,140 @@
+"""Fail-first public-boundary contracts for ATA semantic controls."""
+
+from __future__ import annotations
+
+import numpy as np
+import pytest
+
+import fast_mlsirm.ata as ata
+from fast_mlsirm.types import MLSIRMParams
+
+
+class _HostileString:
+    """Object whose representation callbacks must never run during validation."""
+
+    def __init__(self) -> None:
+        self.str_calls = 0
+        self.repr_calls = 0
+
+    def __str__(self) -> str:
+        self.str_calls += 1
+        raise RuntimeError("ATA_CONSTRAINT_STR_SENTINEL")
+
+    def __repr__(self) -> str:
+        self.repr_calls += 1
+        raise RuntimeError("ATA_CONSTRAINT_REPR_SENTINEL")
+
+
+class _HostileInteger:
+    """Object whose integer-conversion hooks must never run during validation."""
+
+    def __init__(self) -> None:
+        self.int_calls = 0
+        self.index_calls = 0
+
+    def __int__(self) -> int:
+        self.int_calls += 1
+        raise RuntimeError("ATA_CONSTRAINT_INT_SENTINEL")
+
+    def __index__(self) -> int:
+        self.index_calls += 1
+        raise RuntimeError("ATA_CONSTRAINT_INDEX_SENTINEL")
+
+
+def _bank() -> tuple[MLSIRMParams, np.ndarray]:
+    """Return a small valid calibrated bank for ATA boundary tests."""
+    bank = MLSIRMParams(
+        theta=np.array([[0.0]], dtype=np.float64),
+        alpha=np.log(np.array([0.8, 1.0, 1.2, 1.4], dtype=np.float64)),
+        b=np.array([-1.0, -0.25, 0.5, 1.0], dtype=np.float64),
+        xi=np.zeros((1, 1), dtype=np.float64),
+        zeta=np.zeros((4, 1), dtype=np.float64),
+        tau=-30.0,
+    )
+    return bank, np.zeros(4, dtype=np.int64)
+
+
+def _base_call(monkeypatch: pytest.MonkeyPatch, **overrides: object) -> tuple[int, Exception | None]:
+    """Call ATA with a counted information boundary and return observed failure."""
+    bank, factor_id = _bank()
+    information_calls = 0
+
+    def counted_information(*_args: object, **_kwargs: object) -> np.ndarray:
+        nonlocal information_calls
+        information_calls += 1
+        return np.ones((1, 4), dtype=np.float64)
+
+    monkeypatch.setattr(ata, "item_information_matrix", counted_information)
+    kwargs: dict[str, object] = {
+        "content": np.array(["A", "A", "B", "B"], dtype=object),
+        "seed": 0,
+    }
+    kwargs.update(overrides)
+
+    failure: Exception | None = None
+    try:
+        ata.assemble_to_target(
+            bank,
+            factor_id,
+            np.array([0.0], dtype=np.float64),
+            np.array([100.0], dtype=np.float64),
+            length=2,
+            model="MIRT",
+            **kwargs,
+        )
+    except Exception as exc:  # noqa: BLE001 - the test records the public boundary.
+        failure = exc
+    return information_calls, failure
+
+
+def test_content_constraint_key_rejects_hostile_string_before_scoring(monkeypatch: pytest.MonkeyPatch) -> None:
+    """Content-map keys must be validated, not coerced through ``str`` callbacks."""
+    hostile = _HostileString()
+
+    calls, failure = _base_call(monkeypatch, min_per_content={hostile: 1})
+
+    assert calls == 0
+    assert isinstance(failure, ValueError)
+    assert str(failure) == "content constraint keys must be strings"
+    assert hostile.str_calls == 0
+    assert hostile.repr_calls == 0
+
+
+def test_content_constraint_count_rejects_hostile_integer_before_scoring(monkeypatch: pytest.MonkeyPatch) -> None:
+    """Content-map counts must be exact integers without invoking conversion hooks."""
+    hostile = _HostileInteger()
+
+    calls, failure = _base_call(monkeypatch, max_per_content={"A": hostile})
+
+    assert calls == 0
+    assert isinstance(failure, ValueError)
+    assert str(failure) == "content constraint counts must be integers"
+    assert hostile.int_calls == 0
+    assert hostile.index_calls == 0
+
+
+def test_exposure_count_key_rejects_hostile_integer_before_scoring(monkeypatch: pytest.MonkeyPatch) -> None:
+    """Exposure-map identities must not invoke arbitrary integer conversion callbacks."""
+    hostile = _HostileInteger()
+
+    calls, failure = _base_call(monkeypatch, exposure_counts={hostile: 1})
+
+    assert calls == 0
+    assert isinstance(failure, ValueError)
+    assert str(failure) == "exposure_counts keys and values must be integers"
+    assert hostile.int_calls == 0
+    assert hostile.index_calls == 0
+
+
+@pytest.mark.parametrize("field,value", [("seed", True), ("seed", 1.5), ("exposure_max", True), ("exposure_max", 1.5)])
+def test_scalar_controls_require_exact_integers_before_scoring(
+    monkeypatch: pytest.MonkeyPatch,
+    field: str,
+    value: object,
+) -> None:
+    """Boolean/fractional scalar controls must not be silently coerced to integers."""
+    calls, failure = _base_call(monkeypatch, **{field: value})
+
+    assert calls == 0
+    assert isinstance(failure, ValueError)
+    assert str(failure) == f"{field} must be an integer"