diff --git a/tests/test_weights.py b/tests/test_weights.py
new file mode 100644
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--- /dev/null
+++ b/tests/test_weights.py
@@ -0,0 +1,180 @@
+"""Tests for ``arborist.substrate.weights`` — WeightSet defaults +
+``from_dict`` adapter (#000012 Phase 1a).
+
+Pure-stdlib dataclass + classmethod-style adapter; small surface.
+This pins:
+
+  - documented defaults (lifted from the module docstring)
+  - greek-letter and Python-safe key aliases ("lambda" ↔ "lambda_")
+  - missing-keys fall-through to defaults
+  - extra-keys silently ignored
+  - string values coerce to float
+  - frozen-dataclass invariant (mutation raises)
+"""
+
+from __future__ import annotations
+
+import dataclasses
+
+import pytest
+
+from arborist.substrate.weights import (
+    DEFAULT_WEIGHTS,
+    WeightSet,
+    from_dict,
+)
+
+
+# --- defaults pinned to the docstring ------------------------------
+
+
+def test_default_weights_match_documented_values():
+    """Each weight in the dataclass docstring is pinned. If a future
+    PR changes a default, this test fires and forces an explicit
+    update of both the docstring AND the test."""
+    assert DEFAULT_WEIGHTS.alpha == 1.0
+    assert DEFAULT_WEIGHTS.beta == 1.0
+    assert DEFAULT_WEIGHTS.gamma == 1.0
+    assert DEFAULT_WEIGHTS.delta == 0.5
+    assert DEFAULT_WEIGHTS.epsilon == 0.3
+    assert DEFAULT_WEIGHTS.zeta == 0.0   # validator-diversity off in single-validator
+    assert DEFAULT_WEIGHTS.eta == 2.0    # regression > improvement weight
+    assert DEFAULT_WEIGHTS.theta == 0.5
+    assert DEFAULT_WEIGHTS.iota == 0.0   # reserved
+    assert DEFAULT_WEIGHTS.kappa == 0.0  # reserved
+    assert DEFAULT_WEIGHTS.lambda_ == 0.5
+
+
+def test_eta_is_heavier_than_alpha():
+    """Documented invariant from the docstring:
+    'η (RegressionPenalty) heavier than improvement weights so the
+    scorer is biased toward refusing regressions.'"""
+    assert DEFAULT_WEIGHTS.eta > DEFAULT_WEIGHTS.alpha
+    assert DEFAULT_WEIGHTS.eta > DEFAULT_WEIGHTS.beta
+    assert DEFAULT_WEIGHTS.eta > DEFAULT_WEIGHTS.gamma
+
+
+def test_three_batteries_equal_by_default():
+    """Documented invariant: 'α = β = γ (3 batteries weighted equally
+    by default).'"""
+    assert DEFAULT_WEIGHTS.alpha == DEFAULT_WEIGHTS.beta == DEFAULT_WEIGHTS.gamma
+
+
+def test_reserved_weights_are_zero_phase_1a():
+    """Phase 1a discipline: validator-diversity / security / complexity
+    are zero by default. v8 multi-validator deployments must opt in."""
+    assert DEFAULT_WEIGHTS.zeta == 0.0
+    assert DEFAULT_WEIGHTS.iota == 0.0
+    assert DEFAULT_WEIGHTS.kappa == 0.0
+
+
+# --- as_dict --------------------------------------------------------
+
+
+def test_as_dict_returns_all_eleven_fields():
+    d = DEFAULT_WEIGHTS.as_dict()
+    expected_keys = {
+        "alpha", "beta", "gamma", "delta", "epsilon", "zeta",
+        "eta", "theta", "iota", "kappa", "lambda_",
+    }
+    assert set(d.keys()) == expected_keys
+
+
+def test_as_dict_uses_lambda_underscore_key():
+    """``lambda`` is a Python keyword → dataclass uses ``lambda_``;
+    as_dict() carries it forward unchanged. Callers serializing to
+    JSON / YAML rename to bare ``lambda`` if needed."""
+    d = DEFAULT_WEIGHTS.as_dict()
+    assert "lambda_" in d
+    assert "lambda" not in d
+
+
+# --- from_dict adapter ---------------------------------------------
+
+
+def test_from_dict_uses_greek_letter_keys():
+    ws = from_dict({"alpha": 0.7, "beta": 0.8, "gamma": 0.9})
+    assert ws.alpha == 0.7
+    assert ws.beta == 0.8
+    assert ws.gamma == 0.9
+    # Unspecified keys fall through to defaults.
+    assert ws.delta == DEFAULT_WEIGHTS.delta
+
+
+def test_from_dict_translates_lambda_to_lambda_underscore():
+    """JSON/YAML configs use bare ``lambda`` (the Greek letter, not
+    the keyword). from_dict translates to the Python-safe field
+    name ``lambda_``."""
+    ws = from_dict({"lambda": 0.99})
+    assert ws.lambda_ == 0.99
+    # Bare 'lambda' did not survive into a hidden field.
+    assert "lambda" not in ws.as_dict()
+
+
+def test_from_dict_lambda_underscore_key_also_accepted():
+    """For Python callers writing config dicts directly, ``lambda_``
+    is the natural key name; from_dict accepts it too."""
+    ws = from_dict({"lambda_": 0.42})
+    assert ws.lambda_ == 0.42
+
+
+def test_from_dict_missing_keys_fall_through_to_defaults():
+    """Empty dict → identical to DEFAULT_WEIGHTS."""
+    ws = from_dict({})
+    assert ws == DEFAULT_WEIGHTS
+
+
+def test_from_dict_partial_override_preserves_other_defaults():
+    ws = from_dict({"alpha": 0.5})
+    assert ws.alpha == 0.5
+    # Every other field is unchanged.
+    assert ws.beta == DEFAULT_WEIGHTS.beta
+    assert ws.eta == DEFAULT_WEIGHTS.eta
+    assert ws.lambda_ == DEFAULT_WEIGHTS.lambda_
+
+
+def test_from_dict_ignores_unknown_keys():
+    """Defensive: extra keys (e.g. "mu" — never defined) are
+    silently dropped rather than raising. Forward-compat with
+    config files that include vendor-specific tags."""
+    ws = from_dict({"alpha": 0.6, "mu": 1.234, "extra_metadata": "ignore"})
+    assert ws.alpha == 0.6
+    # mu / extra_metadata silently dropped.
+    assert not hasattr(ws, "mu")
+
+
+def test_from_dict_coerces_string_to_float():
+    """JSON/YAML may pass numeric weights as strings. from_dict
+    coerces via float()."""
+    ws = from_dict({"alpha": "0.7", "lambda": "0.3"})
+    assert ws.alpha == 0.7
+    assert ws.lambda_ == 0.3
+
+
+def test_from_dict_int_input_becomes_float():
+    ws = from_dict({"alpha": 1})
+    assert ws.alpha == 1.0
+    assert isinstance(ws.alpha, float)
+
+
+# --- frozen invariant ----------------------------------------------
+
+
+def test_weight_set_is_frozen():
+    """WeightSet is dataclass(frozen=True) — mutation raises
+    FrozenInstanceError. Pinned so a future @dataclass change
+    forces this test to be updated explicitly."""
+    with pytest.raises(dataclasses.FrozenInstanceError):
+        DEFAULT_WEIGHTS.alpha = 99.0
+
+
+def test_weight_sets_equal_by_value():
+    """Two WeightSet instances with identical fields compare equal —
+    standard dataclass equality. Used by tests + audit-trail
+    serialization."""
+    a = WeightSet(alpha=0.5, beta=0.6)
+    b = WeightSet(alpha=0.5, beta=0.6)
+    assert a == b
+    # Different values → not equal.
+    c = WeightSet(alpha=0.5, beta=0.7)
+    assert a != c