diff --git a/development/TTP_TAGGING.md b/development/TTP_TAGGING.md
index 5046cc11..278ff66c 100644
--- a/development/TTP_TAGGING.md
+++ b/development/TTP_TAGGING.md
@@ -2519,6 +2519,8 @@ lands; PII rule §6 type assertion is GREEN today).
 
 **E.2.2 — Idempotency + replay-safety property tests** (`tests/ttp/test_idempotency.py`)
 
+**Status:** ✅ done.
+
 - Hypothesis property: for any valid input tuple, `compute_tag_uuid`
   returns the same string twice in a row. Determinism.
 - Distinct input tuples produce distinct UUIDs (collision resistance
diff --git a/tests/ttp/test_idempotency.py b/tests/ttp/test_idempotency.py
new file mode 100644
index 00000000..82f29b02
--- /dev/null
+++ b/tests/ttp/test_idempotency.py
@@ -0,0 +1,111 @@
+"""Idempotency and replay-safety tests for ``compute_tag_uuid`` (E.2.2).
+
+The deterministic UUIDv5 derivation is the load-bearing primitive
+behind ``INSERT OR IGNORE`` replay safety: the worker must be able to
+re-process the same source events any number of times — crash recovery,
+backfill, manual re-run — and converge to the same tag set.
+
+The replay-safety lock asserts the *exact* parameter set so a future
+contributor adding ``created_at``, ``os.getpid()``, ``random.random()``
+or any other non-deterministic input cannot silently break replay
+safety; the test breaks first.
+"""
+from __future__ import annotations
+
+import inspect
+from typing import Optional
+
+from hypothesis import given, settings
+from hypothesis import strategies as st
+
+from decnet.web.db.models.ttp import _TTP_TAG_NS, compute_tag_uuid
+
+
+# ── Determinism ─────────────────────────────────────────────────────
+
+
+_input_tuple = st.tuples(
+    st.text(min_size=1, max_size=32),
+    st.text(min_size=1, max_size=64),
+    st.text(min_size=1, max_size=32),
+    st.integers(min_value=0, max_value=10_000),
+    st.text(min_size=1, max_size=16),
+    st.one_of(st.none(), st.text(min_size=1, max_size=16)),
+)
+
+
+@given(t=_input_tuple)
+@settings(max_examples=100, deadline=None)
+def test_compute_tag_uuid_is_deterministic(
+    t: tuple[str, str, str, int, str, Optional[str]],
+) -> None:
+    a = compute_tag_uuid(*t)
+    b = compute_tag_uuid(*t)
+    assert a == b
+
+
+# ── Collision resistance ────────────────────────────────────────────
+
+
+@given(
+    tuples=st.lists(_input_tuple, min_size=2, max_size=200, unique=True),
+)
+@settings(max_examples=50, deadline=None)
+def test_distinct_inputs_yield_distinct_uuids(
+    tuples: list[tuple[str, str, str, int, str, Optional[str]]],
+) -> None:
+    uuids = {compute_tag_uuid(*t) for t in tuples}
+    # Distinct input tuples → distinct UUIDs in the practical input
+    # space. UUIDv5 is 122-bit; collisions in N≤200 are
+    # vanishingly unlikely.
+    assert len(uuids) == len(tuples)
+
+
+# ── Golden-value lock ───────────────────────────────────────────────
+
+
+def test_compute_tag_uuid_golden_value() -> None:
+    """Pinned input → pinned UUID. Drift = breaking change.
+
+    Worked example from the design doc (``find_recursive_root`` / R0014
+    on ``cmd_42``). If this assertion ever needs to flip, every
+    existing tag UUID in production has been silently invalidated —
+    treat as a migration event.
+    """
+    assert (
+        compute_tag_uuid("command", "cmd_42", "R0014", 2, "T1083", None)
+        == "9aa491e5-f03b-5d8f-9eb5-161becedcdd6"
+    )
+    assert (
+        compute_tag_uuid("command", "cmd_42", "R0015", 1, "T1548", "T1548.001")
+        == "4fd57b14-c135-544c-97d9-4fabc1051584"
+    )
+
+
+def test_namespace_constant_is_pinned() -> None:
+    """The namespace UUID itself is part of the contract — regenerating
+    it would invalidate every tag UUID."""
+    assert str(_TTP_TAG_NS) == "1ca31f08-5522-5aae-8371-fe81f0e39de3"
+
+
+# ── Replay-safety lock: parameter set ───────────────────────────────
+
+
+def test_compute_tag_uuid_parameter_set_is_locked() -> None:
+    """The accepted inputs MUST be exactly the six identity fields.
+
+    Adding ``created_at``, a process PID, a random salt, or any other
+    non-deterministic input silently breaks replay safety. This test
+    is the trip-wire: a contributor must update it deliberately to
+    change the input set.
+    """
+    sig = inspect.signature(compute_tag_uuid)
+    names = tuple(sig.parameters)
+    assert names == (
+        "source_kind",
+        "source_id",
+        "rule_id",
+        "rule_version",
+        "technique_id",
+        "sub_technique_id",
+    )