diff --git a/tests/explorer/test_decision_route_timestamp.py b/tests/explorer/test_decision_route_timestamp.py
new file mode 100644
index 00000000..4b568619
--- /dev/null
+++ b/tests/explorer/test_decision_route_timestamp.py
@@ -0,0 +1,237 @@
+"""Regression tests for the /api/decisions 422 bug.
+
+``ContextGraph.record_decision()`` stores ``timestamp`` as a POSIX float
+(``datetime.now().timestamp()``).  ``DecisionResponse.timestamp`` is typed
+``Optional[str]``.  Without the ``_normalize_timestamp`` field-validator on
+``DecisionResponse`` the raw float fails Pydantic validation and every decision
+endpoint returns 422.
+
+The validator lives on ``DecisionResponse`` in ``semantica/explorer/schemas.py``
+and converts float/int epochs to ISO-8601 strings via
+``datetime.fromtimestamp(value, tz=timezone.utc).isoformat()``.
+
+``_node_to_decision()`` must pass the raw stored value through unchanged so the
+validator can do its job.  A route-level ``str()`` cast would pre-empt the
+validator and produce raw numeric strings instead of ISO-8601, breaking the API
+contract and all callers that call ``datetime.fromisoformat()`` on the result.
+
+Each test below is written so that it *fails* when the route-level ``str()``
+cast is present (i.e. it would have caught the regression introduced by the
+incorrect fix).
+"""
+
+import math
+from datetime import datetime
+
+import pytest
+from fastapi.testclient import TestClient
+from pydantic import ValidationError
+
+from semantica.context.context_graph import ContextGraph
+from semantica.explorer.app import create_app
+from semantica.explorer.routes.decisions import _node_to_decision
+from semantica.explorer.schemas import DecisionResponse
+from semantica.explorer.session import GraphSession
+
+
+# ---------------------------------------------------------------------------
+# Helpers
+# ---------------------------------------------------------------------------
+
+def _decision_node(timestamp):
+    """Minimal node dict as returned by the graph session layer."""
+    return {
+        "id": "d-test",
+        "type": "decision",
+        "properties": {
+            "category": "loan_underwriting",
+            "scenario": "A-7291 review",
+            "reasoning": "DTI within policy",
+            "outcome": "approved",
+            "confidence": 0.94,
+            "timestamp": timestamp,
+        },
+    }
+
+
+def _recorded_client():
+    """TestClient backed by a graph built with real record_decision() calls.
+
+    This is the path that was broken in production: record_decision() stores
+    timestamp as a float epoch, which must come out the other side as an
+    ISO-8601 string, not a raw numeric string.
+    """
+    graph = ContextGraph(advanced_analytics=False)
+    graph.record_decision(
+        category="credit_application",
+        scenario="Personal loan, $85k income, 31% DTI",
+        reasoning="Income meets threshold; employment stable",
+        outcome="proceed_to_underwriting",
+        confidence=0.88,
+        entities=["applicant_A7291"],
+    )
+    return TestClient(create_app(session=GraphSession(graph)))
+
+
+# ---------------------------------------------------------------------------
+# Unit tests: _node_to_decision() → DecisionResponse
+#
+# Each assertion must fail when the route contains the incorrect str() cast:
+#   timestamp=None if ... is None else str(properties.get("timestamp"))
+# because that cast turns floats into numeric strings such as "1786513069.69"
+# rather than ISO-8601 strings such as "2026-08-12T05:37:49+00:00".
+# ---------------------------------------------------------------------------
+
+def test_float_timestamp_becomes_iso8601():
+    """A POSIX-float epoch must be normalised to an ISO-8601 string.
+
+    Fails with the str() cast because str(1786513069.694965) ==
+    '1786513069.694965', which is not a valid isoformat string.
+    """
+    decision = _node_to_decision(_decision_node(timestamp=1786513069.694965))
+
+    assert isinstance(decision.timestamp, str)
+    # Must parse as a valid ISO-8601 datetime — this is the key assertion that
+    # the incorrect str() cast breaks.
+    parsed = datetime.fromisoformat(decision.timestamp)
+    # Round-trip: parsed timestamp must be within 1 s of the original epoch.
+    assert abs(parsed.timestamp() - 1786513069.694965) < 1.0
+
+
+def test_int_timestamp_becomes_iso8601():
+    """An integer epoch (no sub-second component) must also become ISO-8601.
+
+    Fails with the str() cast because str(1786513069) == '1786513069'.
+    """
+    decision = _node_to_decision(_decision_node(timestamp=1786513069))
+
+    assert isinstance(decision.timestamp, str)
+    parsed = datetime.fromisoformat(decision.timestamp)
+    assert abs(parsed.timestamp() - 1786513069) < 1.0
+
+
+def test_none_timestamp_stays_none():
+    """A stored None must remain None, not become the string 'None'."""
+    decision = _node_to_decision(_decision_node(timestamp=None))
+
+    assert decision.timestamp is None
+
+
+def test_missing_timestamp_key_stays_none():
+    """A node without a timestamp key at all must not raise and must be None."""
+    node = {
+        "id": "d-no-ts",
+        "type": "decision",
+        "properties": {"category": "x", "outcome": "y"},
+    }
+
+    decision = _node_to_decision(node)
+
+    assert decision.timestamp is None
+
+
+def test_iso_string_passes_through_unchanged():
+    """An already-ISO-8601 string must be returned verbatim."""
+    iso = "2026-08-12T10:04:20+00:00"
+    decision = _node_to_decision(_decision_node(timestamp=iso))
+
+    assert decision.timestamp == iso
+
+
+def test_nan_timestamp_raises_validation_error():
+    """NaN must be rejected by the validator, not silently accepted.
+
+    With the str() cast, str(nan) == 'nan' bypasses the validator's finiteness
+    check and is silently accepted — this test would pass the incorrect version
+    of the code if it expected 'nan', but it correctly expects a ValidationError.
+    """
+    with pytest.raises(ValidationError):
+        _node_to_decision(_decision_node(timestamp=float("nan")))
+
+
+def test_inf_timestamp_raises_validation_error():
+    """Positive infinity must be rejected, not silently accepted as 'inf'."""
+    with pytest.raises(ValidationError):
+        _node_to_decision(_decision_node(timestamp=float("inf")))
+
+
+def test_negative_inf_timestamp_raises_validation_error():
+    """Negative infinity must be rejected, not silently accepted as '-inf'."""
+    with pytest.raises(ValidationError):
+        _node_to_decision(_decision_node(timestamp=float("-inf")))
+
+
+def test_out_of_range_epoch_raises_validation_error():
+    """A millisecond epoch accidentally passed as seconds must be rejected.
+
+    With the str() cast, str(1723600000000) is silently accepted as a string.
+    The validator correctly raises ValidationError for out-of-range epochs.
+    """
+    with pytest.raises(ValidationError):
+        _node_to_decision(_decision_node(timestamp=1723600000000))
+
+
+# ---------------------------------------------------------------------------
+# Integration tests: full HTTP path through TestClient
+#
+# These exercise the complete production path:
+#   record_decision() → float stored in graph → HTTP GET → JSON response
+#
+# They are the definitive check: if the route emits numeric strings instead of
+# ISO-8601 the fromisoformat() assertion below fails immediately.
+# ---------------------------------------------------------------------------
+
+def test_list_decisions_float_timestamp_serialised_as_iso8601():
+    """GET /api/decisions must return ISO-8601 timestamps for all decisions.
+
+    This is the exact production failure path.  record_decision() stores
+    timestamp as a float; the endpoint must return an ISO-8601 string, not a
+    raw numeric string like '1786513069.69'.
+    """
+    with _recorded_client() as client:
+        response = client.get("/api/decisions")
+
+    assert response.status_code == 200
+    payload = response.json()
+    assert len(payload) >= 1
+
+    for item in payload:
+        ts = item["timestamp"]
+        assert isinstance(ts, str), f"timestamp must be str, got {type(ts)}"
+        # This is the line that fails when the str() cast is present:
+        datetime.fromisoformat(ts)
+
+
+def test_get_decision_float_timestamp_serialised_as_iso8601():
+    """GET /api/decisions/{id} must return an ISO-8601 timestamp."""
+    with _recorded_client() as client:
+        decision_id = client.get("/api/decisions").json()[0]["decision_id"]
+        response = client.get(f"/api/decisions/{decision_id}")
+
+    assert response.status_code == 200
+    ts = response.json()["timestamp"]
+    assert isinstance(ts, str)
+    datetime.fromisoformat(ts)
+
+
+def test_get_precedents_float_timestamp_serialised_as_iso8601():
+    """GET /api/decisions/{id}/precedents must return ISO-8601 timestamps."""
+    graph = ContextGraph(advanced_analytics=False)
+    for i in range(3):
+        graph.record_decision(
+            category="risk",
+            scenario=f"loan assessment scenario {i}",
+            reasoning="standard criteria",
+            outcome="approved",
+            confidence=0.9,
+        )
+
+    with TestClient(create_app(session=GraphSession(graph))) as client:
+        decision_id = client.get("/api/decisions").json()[0]["decision_id"]
+        response = client.get(f"/api/decisions/{decision_id}/precedents")
+
+    assert response.status_code == 200
+    for item in response.json():
+        ts = item["timestamp"]
+        assert isinstance(ts, str)
+        datetime.fromisoformat(ts)