diff --git a/core/physics/biography_wiring.py b/core/physics/biography_wiring.py
index c9ca97f4..91abf3e8 100644
--- a/core/physics/biography_wiring.py
+++ b/core/physics/biography_wiring.py
@@ -122,8 +122,10 @@ class BiographyProvenanceRecord:
 
 
 def _content_id(payload: Mapping[str, Any]) -> str:
-    raw = json.dumps(payload, sort_keys=True, separators=(",", ":"), default=str)
-    return hashlib.sha256(raw.encode("utf-8")).hexdigest()[:24]
+    # Full 256-bit digest (64 hex); no ``default=str`` — non-serializable
+    # payload elements fail closed with a typed ``TypeError`` (ADR-0244 §2.7).
+    raw = json.dumps(payload, sort_keys=True, separators=(",", ":"))
+    return hashlib.sha256(raw.encode("utf-8")).hexdigest()
 
 
 def biography_provenance_record(
diff --git a/core/physics/cognitive_lifecycle.py b/core/physics/cognitive_lifecycle.py
index e18253d7..c16daee5 100644
--- a/core/physics/cognitive_lifecycle.py
+++ b/core/physics/cognitive_lifecycle.py
@@ -124,13 +124,33 @@ class EgressValidationError(CognitiveLifecycleError):
 # --- Content addressing ----------------------------------------------------------
 
 
+def _le_f64_bytes(arr: np.ndarray) -> bytes:
+    """Canonical little-endian float64 bytes for cross-platform-stable digests.
+
+    ADR-0244 §2.7 byte-order guard: coerce to little-endian float64 before
+    hashing so the digest is identical on every little-endian platform and
+    deterministic on big-endian ones. Byte-wise a no-op on the M1/x86 targets,
+    but an explicit contract rather than an implicit platform assumption — and a
+    coercion, not an ``assert`` (the assert form is stripped under ``-O``).
+    """
+    contiguous = np.ascontiguousarray(arr, dtype=np.float64)
+    return contiguous.astype(np.dtype("<f8"), copy=False).tobytes()
+
+
 def _content_id(payload: Mapping[str, Any]) -> str:
-    raw = json.dumps(payload, sort_keys=True, separators=(",", ":"), default=str)
-    return hashlib.sha256(raw.encode("utf-8")).hexdigest()[:24]
+    # Full 256-bit digest (64 hex). No ``default=str``: a non-serializable
+    # payload element fails closed with a typed ``TypeError`` at the
+    # serialization boundary rather than silently collapsing distinct objects
+    # onto identical string forms (ADR-0244 §2.7).
+    raw = json.dumps(payload, sort_keys=True, separators=(",", ":"))
+    return hashlib.sha256(raw.encode("utf-8")).hexdigest()
 
 
 def _psi_digest(psi: np.ndarray) -> str:
-    return hashlib.sha256(np.ascontiguousarray(psi, dtype=np.float64).tobytes()).hexdigest()[:24]
+    # Full 256-bit digest over canonical little-endian float64 bytes — no
+    # 96-bit truncation (birthday-collision floor at 2^48), no platform
+    # byte-order ambiguity (ADR-0244 §2.7).
+    return hashlib.sha256(_le_f64_bytes(psi)).hexdigest()
 
 
 def _as_psi(x: np.ndarray, name: str, *, error: type[CognitiveLifecycleError]) -> np.ndarray:
@@ -271,7 +291,7 @@ class ProblemHamiltonian:
             _content_id(
                 {
                     "domain": str(self.domain),
-                    "matrix_sha": hashlib.sha256(arr.tobytes()).hexdigest(),
+                    "matrix_sha": hashlib.sha256(_le_f64_bytes(arr)).hexdigest(),
                     "metadata": {k: str(v) for k, v in sorted(meta.items())},
                 }
             ),
diff --git a/core/physics/self_authorship.py b/core/physics/self_authorship.py
index 55ef6f88..8fafd782 100644
--- a/core/physics/self_authorship.py
+++ b/core/physics/self_authorship.py
@@ -39,8 +39,10 @@ class AuthorshipProposal:
 
 
 def _content_id(payload: Mapping[str, Any]) -> str:
-    raw = json.dumps(payload, sort_keys=True, separators=(",", ":"), default=str)
-    return hashlib.sha256(raw.encode("utf-8")).hexdigest()[:24]
+    # Full 256-bit digest (64 hex); no ``default=str`` — non-serializable
+    # payload elements fail closed with a typed ``TypeError`` (ADR-0244 §2.7).
+    raw = json.dumps(payload, sort_keys=True, separators=(",", ":"))
+    return hashlib.sha256(raw.encode("utf-8")).hexdigest()
 
 
 class SelfAuthorshipMiner:
diff --git a/tests/test_adr_0243_cognitive_lifecycle.py b/tests/test_adr_0243_cognitive_lifecycle.py
index 497286d1..6f249f23 100644
--- a/tests/test_adr_0243_cognitive_lifecycle.py
+++ b/tests/test_adr_0243_cognitive_lifecycle.py
@@ -569,8 +569,8 @@ def test_egress_refuses_certificate_not_bound_to_state():
     cert = outcome.relaxation.certificate
     # Independent gold for the binding: byte digest of the certified state.
     gold_digest = hashlib.sha256(
-        np.ascontiguousarray(psi, dtype=np.float64).tobytes()
-    ).hexdigest()[:24]
+        np.ascontiguousarray(psi, dtype=np.float64).astype(np.dtype("<f8"), copy=False).tobytes()
+    ).hexdigest()
     assert cert.psi_digest == gold_digest
     assert cert.as_dict()["psi_digest"] == gold_digest
 
diff --git a/tests/test_adr_0244_semantic_rigor.py b/tests/test_adr_0244_semantic_rigor.py
new file mode 100644
index 00000000..3bbbe6fe
--- /dev/null
+++ b/tests/test_adr_0244_semantic_rigor.py
@@ -0,0 +1,99 @@
+"""ADR-0244 §2.7 semantic-rigor pins (cohesion directive Mandates 4 + 5).
+
+Content-address keys must (1) retain the full 256-bit SHA-256 digest — no
+96-bit (24-hex) truncation, which floors a birthday collision at 2^48 and can
+corrupt content-addressed merge keys; (2) fail closed on non-serializable
+payload elements rather than silently coercing them via ``default=str``; and
+(3) hash a canonical little-endian float64 byte layout so digests are identical
+across little-endian platforms and deterministic on big-endian ones.
+"""
+
+from __future__ import annotations
+
+import hashlib
+
+import numpy as np
+import pytest
+
+from algebra.rotor import make_rotor_from_angle
+from core.physics.cognitive_lifecycle import (
+    _content_id as lifecycle_content_id,
+    _le_f64_bytes,
+    _psi_digest,
+)
+from core.physics.biography_wiring import _content_id as biography_content_id
+from core.physics.self_authorship import _content_id as authorship_content_id
+
+_CONTENT_ID_FUNCS = (
+    lifecycle_content_id,
+    biography_content_id,
+    authorship_content_id,
+)
+
+
+def _sample_psi() -> np.ndarray:
+    return np.ascontiguousarray(make_rotor_from_angle(0.7, 6), dtype=np.float64)
+
+
+# --- Mandate 4: full 256-bit digests (no 96-bit truncation) -----------------------
+
+
+@pytest.mark.parametrize("content_id", _CONTENT_ID_FUNCS)
+def test_content_id_is_full_256_bit_hex(content_id) -> None:
+    digest = content_id({"a": 1, "b": ["x", 2.0], "c": True})
+    assert len(digest) == 64
+    int(digest, 16)  # valid lowercase hex
+
+
+def test_psi_digest_is_full_256_bit_hex() -> None:
+    digest = _psi_digest(_sample_psi())
+    assert len(digest) == 64
+    int(digest, 16)
+
+
+def test_full_digest_extends_the_old_truncated_prefix() -> None:
+    """The widening is an un-truncation, not a different hash: the old 24-hex
+    form is exactly the 64-hex digest's prefix (same underlying bytes)."""
+    psi = _sample_psi()
+    full = _psi_digest(psi)
+    legacy_prefix = hashlib.sha256(_le_f64_bytes(psi)).hexdigest()[:24]
+    assert full[:24] == legacy_prefix
+
+
+# --- Mandate 4: fail closed on silent coercion ------------------------------------
+
+
+@pytest.mark.parametrize("content_id", _CONTENT_ID_FUNCS)
+def test_content_id_fails_closed_on_non_serializable(content_id) -> None:
+    class _Opaque:
+        pass
+
+    with pytest.raises(TypeError):
+        content_id({"bad": _Opaque()})
+
+
+@pytest.mark.parametrize("content_id", _CONTENT_ID_FUNCS)
+def test_content_id_is_deterministic(content_id) -> None:
+    payload = {"k": "v", "n": 3, "xs": [1, 2, 3]}
+    assert content_id(payload) == content_id(dict(payload))
+
+
+# --- Mandate 5: canonical little-endian byte-order contract -----------------------
+
+
+def test_le_bytes_are_byte_order_canonical() -> None:
+    """A native-endian array and an explicit big-endian-dtype array carrying the
+    same values must hash identically — the coercion, not the platform, decides
+    the bytes."""
+    psi = _sample_psi()
+    native = np.ascontiguousarray(psi, dtype=np.float64)
+    big_endian = native.astype(np.dtype(">f8"))
+    assert _le_f64_bytes(native) == _le_f64_bytes(big_endian)
+    assert _psi_digest(native) == _psi_digest(big_endian)
+
+
+def test_le_bytes_match_explicit_little_endian_layout() -> None:
+    psi = _sample_psi()
+    expected = np.ascontiguousarray(psi, dtype=np.dtype("<f8")).tobytes()
+    assert _le_f64_bytes(psi) == expected
+    assert len(_le_f64_bytes(psi)) == 32 * 8  # 32 float64 components