diff --git a/tests/test_pi_star_code.py b/tests/test_pi_star_code.py
new file mode 100644
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+++ b/tests/test_pi_star_code.py
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+"""code-py-ast@v1 π* tests.
+
+Python source → deterministic AST S-expression. KATs mirror the
+docstring's equivalence-class examples: whitespace / comment /
+quote-style / numeric-formatting invariance vs identifier and
+operator distinctness. Plus invalid-input cone (non-Python,
+non-bytes) and determinism.
+
+Note: this projection is projective, not invertible — the canonical
+S-expression is NOT valid Python, so re-canonicalizing the output
+raises (covered by an explicit test).
+
+Backfills the test-coverage gap for arborist/pi_star/code.py
+identified during the 2026-05-10 zero-coverage sweep.
+"""
+
+from __future__ import annotations
+
+import pytest
+
+from arborist.pi_star import get
+from arborist.pi_star.protocol import PiStarError
+
+
+@pytest.fixture
+def ps():
+    return get("code-py-ast@v1")
+
+
+def _canon(ps, src: str) -> bytes:
+    return ps.canonicalize(src.encode("utf-8"))
+
+
+# --- registry presence + metadata ------------------------------------
+
+
+def test_registry_contains_code_py_ast():
+    ps = get("code-py-ast@v1")
+    assert ps.name == "code-py-ast"
+    assert ps.version == "v1"
+    assert ps.domain == "code"
+
+
+# --- positive shape --------------------------------------------------
+
+
+def test_empty_module(ps):
+    out = _canon(ps, "")
+    # Module(body=[] type_ignores=[]) — minimal AST shape.
+    assert out.startswith(b"(Module ")
+    assert b"body=[]" in out
+
+
+def test_single_expression(ps):
+    out = _canon(ps, "1 + 2")
+    assert b"BinOp" in out
+    assert b"Add" in out
+    assert b"value=1" in out
+    assert b"value=2" in out
+
+
+def test_assignment(ps):
+    out = _canon(ps, "x = 1")
+    assert b"Assign" in out
+    assert b"'x'" in out
+    assert b"value=1" in out
+
+
+def test_function_def(ps):
+    out = _canon(ps, "def foo(a, b):\n    return a + b\n")
+    assert b"FunctionDef" in out
+    assert b"'foo'" in out
+    assert b"Return" in out
+
+
+# --- equivalence classes preserved -----------------------------------
+
+
+@pytest.mark.parametrize(
+    "a,b",
+    [
+        # Whitespace / indentation
+        ("x=1", "x = 1"),
+        ("x = 1", "x  =  1"),
+        # Comments
+        ("x = 1", "x = 1  # comment"),
+        ("# leading\nx = 1", "x = 1"),
+        # Quote style
+        ("x = 'foo'", 'x = "foo"'),
+        # Trailing semicolons
+        ("x = 1", "x = 1;"),
+        # Numeric formatting (underscores in int literals)
+        ("x = 1000", "x = 1_000"),
+    ],
+)
+def test_equivalent_sources_collapse(ps, a, b):
+    assert _canon(ps, a) == _canon(ps, b)
+
+
+# --- equivalence classes kept distinct -------------------------------
+
+
+@pytest.mark.parametrize(
+    "a,b",
+    [
+        # Identifier names
+        ("x = 1", "y = 1"),
+        # Operator types
+        ("a + b", "a - b"),
+        ("a * b", "a / b"),
+        # Argument order in calls
+        ("f(a, b)", "f(b, a)"),
+        # Literal vs variable
+        ("x = 1", "x = 2"),
+        # String vs bytes
+        ("x = 'a'", "x = b'a'"),
+    ],
+)
+def test_distinct_sources_stay_distinct(ps, a, b):
+    assert _canon(ps, a) != _canon(ps, b)
+
+
+# --- determinism + ordering ------------------------------------------
+
+
+def test_repeated_canonicalize_is_stable(ps):
+    src = "class X:\n    def f(self): return 42\n    y = [1, 2, 3]\n"
+    a = _canon(ps, src)
+    b = _canon(ps, src)
+    c = _canon(ps, src)
+    assert a == b == c
+
+
+def test_field_order_is_lexical(ps):
+    """_canonical_sexp sorts node._fields for cross-version stability."""
+    out = _canon(ps, "x = 1")
+    # In an Assign node, lexical sort puts 'targets' before 'value'.
+    body_idx = out.index(b"Assign")
+    targets_idx = out.index(b"targets=", body_idx)
+    value_idx = out.index(b"value=", body_idx)
+    assert targets_idx < value_idx
+
+
+# --- source-position fields excluded ---------------------------------
+
+
+def test_line_position_does_not_change_canonical(ps):
+    """lineno / col_offset are not in `_fields` so they're excluded;
+    the same statement at different line numbers canonicalizes the same."""
+    a = _canon(ps, "x = 1")
+    b = _canon(ps, "\n\nx = 1")
+    assert a == b
+
+
+def test_position_fields_excluded(ps):
+    """Source-position metadata MUST NOT appear in canonical output."""
+    out = _canon(ps, "x = 1\ny = 2\n")
+    # Confirm none of the excluded fields leaked into the encoding.
+    for excluded in (b"lineno=", b"col_offset=", b"end_lineno=", b"end_col_offset="):
+        assert excluded not in out, f"position field {excluded!r} leaked"
+
+
+# --- invalid-input cone ----------------------------------------------
+
+
+def test_non_python_raises(ps):
+    with pytest.raises(PiStarError, match="not valid Python"):
+        ps.canonicalize(b"def (")
+
+
+def test_unbalanced_paren_raises(ps):
+    with pytest.raises(PiStarError, match="not valid Python"):
+        ps.canonicalize(b"x = (1 + 2")
+
+
+def test_non_bytes_raises(ps):
+    with pytest.raises(PiStarError, match="expects bytes"):
+        ps.canonicalize("x = 1")  # type: ignore[arg-type]
+
+
+def test_bytearray_accepted(ps):
+    """bytearray is an accepted subtype per the bytes/bytearray guard."""
+    out = ps.canonicalize(bytearray(b"x = 1"))
+    assert b"Assign" in out
+    assert b"value=1" in out
+
+
+# --- projective (not invertible) -------------------------------------
+
+
+def test_canonical_output_is_not_python_source(ps):
+    """Re-canonicalizing the canonical S-expression raises — by design."""
+    canonical = _canon(ps, "x = 1")
+    # The S-expression starts with `(Module ...)` which isn't a Python
+    # expression. ast.parse rejects it.
+    with pytest.raises(PiStarError, match="not valid Python"):
+        ps.canonicalize(canonical)
+
+
+# --- value types ------------------------------------------------------
+
+
+def test_bool_serialized_as_bool_not_int(ps):
+    """bool subclasses int — handler must dispatch on bool first."""
+    out = _canon(ps, "x = True")
+    # If bool fell through to int, we'd see `value=1`. Must see True.
+    assert b"value=True" in out
+    assert b"value=1" not in out
+
+
+def test_none_handled(ps):
+    out = _canon(ps, "x = None")
+    assert b"value=None" in out
+
+
+def test_float_literal(ps):
+    out = _canon(ps, "x = 3.14")
+    assert b"value=3.14" in out
+
+
+def test_complex_literal(ps):
+    out = _canon(ps, "x = 1j")
+    assert b"value=1j" in out
+
+
+def test_bytes_literal(ps):
+    out = _canon(ps, "x = b'hi'")
+    assert b"value=b'hi'" in out