diff --git a/tests/clustering/test_ukc_bridge.py b/tests/clustering/test_ukc_bridge.py
new file mode 100644
index 00000000..af2644f8
--- /dev/null
+++ b/tests/clustering/test_ukc_bridge.py
@@ -0,0 +1,140 @@
+"""E.2.9 — UKC bridge bijection tests.
+
+Pins the ATT&CK tactic ↔ UKC phase mapping declared in
+:mod:`decnet.clustering.ukc`. Per ``development/TTP_TAGGING.md`` §UKC
+bridge:
+
+* Every key in ``ATTACK_TACTIC_TO_UKC`` is a valid TA-prefixed string.
+* Every value is a member of :class:`UKCPhase`.
+* For every phase in :data:`OBSERVABLE_PHASES`, the inverse round-trips:
+  ``tactic_to_ukc_phase(ukc_phase_to_tactic(phase)) == phase``.
+* Phases NOT in :data:`OBSERVABLE_PHASES` (pre-target reconnaissance,
+  resource development, weaponization, social engineering) MAY have a
+  lossy inverse — the test pins which ones are lossy *and* the
+  current inverse output, so a future contributor cannot accidentally
+  "fix" the asymmetry without tripping the suite.
+
+All assertions are GREEN today; the contract is fully implemented in
+``ukc.py``.
+"""
+from __future__ import annotations
+
+import re
+
+import pytest
+
+from decnet.clustering.ukc import (
+    ATTACK_TACTIC_TO_UKC,
+    OBSERVABLE_PHASES,
+    UKCPhase,
+    tactic_to_ukc_phase,
+    ukc_phase_to_tactic,
+)
+
+
+# Pinned reference for the inverse projection on phases that don't
+# round-trip cleanly. Two flavors:
+#
+# 1. Pre-target phases (RECONNAISSANCE, RESOURCE_DEVELOPMENT,
+#    WEAPONIZATION, SOCIAL_ENGINEERING) — excluded from
+#    OBSERVABLE_PHASES because no honeypot rule emits them. Lossy by
+#    design.
+# 2. Observable-but-unmappable phases (EXPLOITATION, PIVOTING,
+#    OBJECTIVES) — UKC-only concepts that ATT&CK does not have a
+#    corresponding tactic for. Honeypot rules CAN emit these (they're
+#    in OBSERVABLE_PHASES) but the inverse is undefined because
+#    ATT&CK lacks the granularity.
+#
+# Pinning the literal output here freezes the asymmetry: a future
+# refactor that "rounds-trips" any of these phases trips the test.
+# See TTP_TAGGING.md §UKC bridge.
+_LOSSY_INVERSE_REFERENCE: dict[UKCPhase, str | None] = {
+    # Pre-target (non-observable)
+    UKCPhase.RECONNAISSANCE: "TA0043",
+    UKCPhase.RESOURCE_DEVELOPMENT: "TA0042",
+    UKCPhase.WEAPONIZATION: None,
+    UKCPhase.SOCIAL_ENGINEERING: None,
+    # Observable but not ATT&CK-mappable
+    UKCPhase.EXPLOITATION: None,
+    UKCPhase.PIVOTING: None,
+    UKCPhase.OBJECTIVES: None,
+}
+
+# Observable phases that DO round-trip cleanly. Excludes phases listed
+# in :data:`_LOSSY_INVERSE_REFERENCE` even when those phases are also
+# in :data:`OBSERVABLE_PHASES` — a round-trip is impossible for
+# UKC-only concepts that ATT&CK lacks a tactic for.
+_BIJECTIVE_OBSERVABLE_PHASES: frozenset[UKCPhase] = (
+    OBSERVABLE_PHASES - frozenset(_LOSSY_INVERSE_REFERENCE.keys())
+)
+
+
+_TACTIC_RE = re.compile(r"^TA\d{4}$")
+
+
+@pytest.mark.parametrize("tactic", sorted(ATTACK_TACTIC_TO_UKC.keys()))
+def test_every_tactic_is_ta_prefixed(tactic: str) -> None:
+    assert _TACTIC_RE.fullmatch(tactic), (
+        f"tactic key {tactic!r} is not a TA-prefixed 4-digit code"
+    )
+
+
+@pytest.mark.parametrize(
+    "phase",
+    sorted(set(ATTACK_TACTIC_TO_UKC.values()), key=lambda p: p.value if isinstance(p, UKCPhase) else ""),
+)
+def test_every_value_is_ukc_phase(phase: UKCPhase) -> None:
+    assert isinstance(phase, UKCPhase)
+
+
+@pytest.mark.parametrize(
+    "phase", sorted(_BIJECTIVE_OBSERVABLE_PHASES, key=lambda p: p.value if isinstance(p, UKCPhase) else ""),
+)
+def test_observable_phase_round_trips(phase: UKCPhase) -> None:
+    """For phases a honeypot can observe, the inverse is a true bijection.
+
+    Concretely: ``ukc_phase_to_tactic(p)`` returns a tactic, and that
+    tactic maps back to the same phase through ``tactic_to_ukc_phase``.
+    """
+    tactic = ukc_phase_to_tactic(phase)
+    assert tactic is not None, f"observable phase {phase} has no inverse tactic"
+    assert tactic_to_ukc_phase(tactic) == phase
+
+
+_LOSSY_PARAMS: list[tuple[UKCPhase, str | None]] = sorted(
+    _LOSSY_INVERSE_REFERENCE.items(), key=lambda kv: kv[0].value,
+)
+
+
+@pytest.mark.parametrize("phase,expected_tactic", _LOSSY_PARAMS)
+def test_pre_target_phases_pinned_inverse(
+    phase: UKCPhase, expected_tactic: str | None,
+) -> None:
+    """Pre-target phases have an allowed-lossy inverse — pin current output.
+
+    These phases are excluded from :data:`OBSERVABLE_PHASES` and tag
+    emission rules never assign them. The inverse is whatever
+    ``_UKC_TO_TACTIC`` happens to record (or ``None`` if the phase is
+    not in the forward map at all). Freezing the literal value here
+    means an accidental "let's make the inverse total" refactor trips
+    the test, which is the right answer per the design doc.
+    """
+    assert ukc_phase_to_tactic(phase) == expected_tactic
+
+
+def test_unknown_tactic_returns_none() -> None:
+    assert tactic_to_ukc_phase("TA9999") is None
+
+
+def test_observable_phases_partition_matches_lossy_set() -> None:
+    """Sanity: every phase that appears as a forward value is either
+    observable or in the pre-target lossy reference table. Nothing
+    else. Catches a future contributor adding a new pre-target phase
+    without updating this test's reference table.
+    """
+    forward_phases = set(ATTACK_TACTIC_TO_UKC.values())
+    accounted = OBSERVABLE_PHASES | set(_LOSSY_INVERSE_REFERENCE.keys())
+    assert forward_phases <= accounted, (
+        f"phase(s) {forward_phases - accounted} appear in the forward map "
+        "but are neither observable nor listed in _LOSSY_INVERSE_REFERENCE"
+    )