diff --git a/baml_language/sdk_tests/crates/python_pydantic2/type_shapes/customizable/roundtrip_tests/test_interfaces.py b/baml_language/sdk_tests/crates/python_pydantic2/type_shapes/customizable/roundtrip_tests/test_interfaces.py
new file mode 100644
index 0000000000..138340184e
--- /dev/null
+++ b/baml_language/sdk_tests/crates/python_pydantic2/type_shapes/customizable/roundtrip_tests/test_interfaces.py
@@ -0,0 +1,156 @@
+"""Roundtrip coverage for `baml_sdk.interfaces` — interface types (BEP-044).
+
+Interfaces are BAML-side contracts, not serializable host SDK models:
+codegen surfaces interface-typed boundary positions as `typing.Any`
+(`client_codegen.rs`: `TirTy::Interface(..) => cg::Ty::BuiltinUnknown`).
+The *values* crossing the boundary are always concrete class instances.
+
+These tests pin:
+
+  - return position: the host receives the concrete implementing class
+    (for both the in-body `implements` form — `Square`/`Rect` — and the
+    out-of-body `implements Shape for Circle` form)
+  - parameter position: the host passes a concrete instance and the
+    engine dispatches interface methods on it
+  - round trips preserve the concrete class in bare, list, optional,
+    and class-field positions
+  - KNOWN GAPS (pinned so a fix flips these tests and forces an
+    intentional update): no encode-time conformance check for interface
+    params, and impl-method host bindings point at unregistered names
+
+The python bridge encodes host instances by their runtime class, so all
+of the above works here. The nodejs SDK currently diverges — its bridge
+encodes by the declared codegen type, losing class identity in interface
+positions; see roundtrip_interfaces.test.ts for the pinned node behavior.
+"""
+
+import pytest
+
+import baml_sdk  # noqa: F401  — initializes the BAML runtime
+from baml_bridge.errors import BamlPanic
+from baml_sdk.interfaces import (
+    Circle,
+    Rect,
+    ShapeBox,
+    Square,
+    box_area,
+    return_circle_as_shape,
+    return_rect_as_shape,
+    return_square_as_shape,
+    round_trip_optional_shape,
+    round_trip_shape,
+    round_trip_shape_box,
+    round_trip_shape_list,
+    shape_area,
+    shape_area_async,
+    sum_areas,
+)
+
+
+# ── return position: host receives the concrete implementing class ──────
+
+
+def test_return_square_as_shape():
+    s = return_square_as_shape()
+    assert isinstance(s, Square)
+    assert s.side == 5
+
+
+def test_return_rect_as_shape():
+    s = return_rect_as_shape()
+    assert isinstance(s, Rect)
+    assert s == Rect(width=3, height=4)
+
+
+def test_return_circle_as_shape():
+    # Circle implements Shape via the out-of-body `implements ... for` form.
+    s = return_circle_as_shape()
+    assert isinstance(s, Circle)
+    assert s.radius == 2
+
+
+# ── parameter position: engine dispatches interface methods ─────────────
+
+
+def test_shape_area_dispatches_on_square():
+    assert shape_area(s=Square(side=5)) == 25
+
+
+def test_shape_area_dispatches_on_rect():
+    assert shape_area(s=Rect(width=3, height=4)) == 12
+
+
+def test_shape_area_dispatches_on_out_of_body_impl():
+    assert shape_area(s=Circle(radius=2)) == 12
+
+
+async def test_shape_area_async_dispatches():
+    assert await shape_area_async(s=Square(side=6)) == 36
+
+
+def test_sum_areas_mixes_implementors():
+    assert sum_areas(a=Square(side=2), b=Rect(width=3, height=4)) == 16
+
+
+# ── round trips preserve the concrete class ─────────────────────────────
+
+
+def test_round_trip_shape_preserves_concrete_class():
+    r = round_trip_shape(s=Rect(width=2, height=3))
+    assert isinstance(r, Rect)
+    assert r == Rect(width=2, height=3)
+
+
+def test_round_trip_shape_list_preserves_each_element():
+    shapes = [Square(side=1), Rect(width=2, height=3), Circle(radius=4)]
+    assert round_trip_shape_list(shapes=shapes) == shapes
+
+
+def test_round_trip_optional_shape_none():
+    assert round_trip_optional_shape(s=None) is None
+
+
+def test_round_trip_optional_shape_value():
+    r = round_trip_optional_shape(s=Circle(radius=1))
+    assert isinstance(r, Circle)
+    assert r.radius == 1
+
+
+def test_round_trip_shape_box_field_position():
+    r = round_trip_shape_box(b=ShapeBox(shape=Square(side=2)))
+    assert isinstance(r, ShapeBox)
+    assert isinstance(r.shape, Square)
+    assert r.shape.side == 2
+
+
+def test_box_area_dispatches_through_field():
+    assert box_area(b=ShapeBox(shape=Rect(width=2, height=5))) == 10
+
+
+# ── KNOWN GAPS — these pin *current* behavior, not desired behavior ─────
+
+
+def test_non_implementor_panics_at_dispatch_not_encode():
+    # There is no encode-time conformance check for interface-typed
+    # params (the codegen type is `typing.Any`): a value whose class
+    # does not implement Shape encodes fine and only fails inside the
+    # VM when the virtual call cannot resolve. A future encode- or
+    # call-time check would surface a TypeError instead — update this
+    # pin when that lands. NB: BamlPanic derives BaseException.
+    with pytest.raises(BamlPanic, match="could not resolve interface method"):
+        shape_area(s=ShapeBox(shape=Square(side=1)))
+
+
+def test_primitive_into_interface_param_panics_at_dispatch():
+    with pytest.raises(BamlPanic, match="could not resolve interface method"):
+        shape_area(s=42)
+
+
+def test_impl_method_host_binding_is_not_callable():
+    # sdkgen emits `area`/`area_async` bindings on implementing classes
+    # (named `user.interfaces.Square.area`), but the engine registers
+    # interface-impl methods under a synthetic `Shape$for$Square` name,
+    # so calling the binding from the host panics with "Function not
+    # found". Update this pin when the binding and registration agree.
+    with pytest.raises(BamlPanic, match="Function not found"):
+        Square(side=3).area()
diff --git a/baml_language/sdk_tests/crates/typescript_node/type_shapes/customizable/roundtrip_interfaces.test.ts b/baml_language/sdk_tests/crates/typescript_node/type_shapes/customizable/roundtrip_interfaces.test.ts
new file mode 100644
index 0000000000..72aa9ef69e
--- /dev/null
+++ b/baml_language/sdk_tests/crates/typescript_node/type_shapes/customizable/roundtrip_interfaces.test.ts
@@ -0,0 +1,152 @@
+// Roundtrip coverage for baml_sdk/interfaces — interface types (BEP-044).
+// Counterpart of python_pydantic2/.../roundtrip_tests/test_interfaces.py.
+//
+// Interfaces are BAML-side contracts, not serializable host SDK models:
+// codegen surfaces interface-typed boundary positions as `unknown`
+// (client_codegen.rs: `TirTy::Interface(..) => cg::Ty::BuiltinUnknown`).
+//
+// KNOWN GAP (nodejs bridge) — unlike the python SDK, where all of these
+// scenarios work: outbound host→engine encoding here is driven by the
+// declared codegen type, so a class instance passed in an interface
+// position (`unknown`) loses its class identity and encodes as a plain
+// object. The engine then cannot dispatch interface methods on it ("VM
+// internal error: virtual call could not resolve interface method"), and
+// round-tripped interface positions come back as plain objects rather
+// than class instances. Interface *returns* are unaffected: the value
+// leaves the engine carrying its concrete class, so the host receives a
+// real class instance. The tests below pin this *current* behavior, not
+// desired behavior — update them when the nodejs bridge encodes by the
+// value's runtime class (as python does) or codegen grows a structural
+// interface representation.
+import "./baml_sdk/index.js"; // initializes the BAML runtime
+import { describe, it, expect } from "vitest";
+import {
+  Circle,
+  Rect,
+  ShapeBox,
+  Square,
+  box_area,
+  return_circle_as_shape,
+  return_rect_as_shape,
+  return_square_as_shape,
+  round_trip_optional_shape,
+  round_trip_shape,
+  round_trip_shape_box,
+  round_trip_shape_list,
+  shape_area,
+  shape_area_async,
+  sum_areas,
+} from "./baml_sdk/interfaces/index.js";
+
+describe("roundtrip interfaces", () => {
+  // ── return position: host receives the concrete implementing class ──
+
+  it("return_square_as_shape", () => {
+    const s = return_square_as_shape();
+    expect(s).toBeInstanceOf(Square);
+    expect((s as Square).side).toBeCloseTo(5);
+  });
+
+  it("return_rect_as_shape", () => {
+    const s = return_rect_as_shape();
+    expect(s).toBeInstanceOf(Rect);
+    expect((s as Rect).width).toBeCloseTo(3);
+    expect((s as Rect).height).toBeCloseTo(4);
+  });
+
+  it("return_circle_as_shape (out-of-body impl)", () => {
+    const s = return_circle_as_shape();
+    expect(s).toBeInstanceOf(Circle);
+    expect((s as Circle).radius).toBeCloseTo(2);
+  });
+
+  // ── parameter position — KNOWN GAP: class identity lost on encode, so
+  // the engine cannot dispatch interface methods on host instances ─────
+
+  it("shape_area panics on Square (class identity lost)", () =>
+    expect(() => shape_area(new Square({ side: 5 }))).toThrow(
+      /could not resolve interface method/,
+    ));
+
+  it("shape_area panics on Rect", () =>
+    expect(() => shape_area(new Rect({ width: 3, height: 4 }))).toThrow(
+      /could not resolve interface method/,
+    ));
+
+  it("shape_area panics on out-of-body impl", () =>
+    expect(() => shape_area(new Circle({ radius: 2 }))).toThrow(
+      /could not resolve interface method/,
+    ));
+
+  it("shape_area_async rejects", async () =>
+    await expect(shape_area_async(new Square({ side: 6 }))).rejects.toThrow(
+      /could not resolve interface method/,
+    ));
+
+  it("sum_areas panics", () =>
+    expect(() =>
+      sum_areas(new Square({ side: 2 }), new Rect({ width: 3, height: 4 })),
+    ).toThrow(/could not resolve interface method/));
+
+  // ── round trips — KNOWN GAP: interface positions come back as plain
+  // objects (field values survive, class identity does not) ────────────
+
+  it("round_trip_shape returns a plain object", () => {
+    const r = round_trip_shape(new Rect({ width: 2, height: 3 }));
+    expect(r).not.toBeInstanceOf(Rect);
+    expect(r).toEqual({ width: 2, height: 3 });
+  });
+
+  it("round_trip_shape_list returns plain objects", () => {
+    const r = round_trip_shape_list([
+      new Square({ side: 1 }),
+      new Rect({ width: 2, height: 3 }),
+      new Circle({ radius: 4 }),
+    ]);
+    expect(r[0]).not.toBeInstanceOf(Square);
+    expect(r).toEqual([{ side: 1 }, { width: 2, height: 3 }, { radius: 4 }]);
+  });
+
+  it("round_trip_optional_shape null", () =>
+    expect(round_trip_optional_shape(null)).toBeNull());
+
+  it("round_trip_optional_shape value returns a plain object", () => {
+    const r = round_trip_optional_shape(new Circle({ radius: 1 }));
+    expect(r).not.toBeInstanceOf(Circle);
+    expect(r).toEqual({ radius: 1 });
+  });
+
+  it("round_trip_shape_box keeps the box, loses the field's class", () => {
+    // The box itself is a declared class type, so its identity survives;
+    // its interface-typed `shape` field decays to a plain object.
+    const r = round_trip_shape_box(new ShapeBox({ shape: new Square({ side: 2 }) }));
+    expect(r).toBeInstanceOf(ShapeBox);
+    expect(r.shape).not.toBeInstanceOf(Square);
+    expect(r.shape).toEqual({ side: 2 });
+  });
+
+  it("box_area panics (field dispatch)", () =>
+    expect(() =>
+      box_area(new ShapeBox({ shape: new Rect({ width: 2, height: 5 }) })),
+    ).toThrow(/could not resolve interface method/));
+
+  // ── KNOWN GAPS shared with the python SDK ────────────────────────────
+
+  it("non-implementor panics at dispatch, not encode", () => {
+    // No encode-time conformance check for interface-typed params: a
+    // non-implementing value encodes fine and only fails inside the VM.
+    expect(() =>
+      shape_area(new ShapeBox({ shape: new Square({ side: 1 }) })),
+    ).toThrow(/could not resolve interface method/);
+    expect(() => shape_area(42)).toThrow(/could not resolve interface method/);
+  });
+
+  it("impl-method host binding is not callable", () => {
+    // sdkgen emits `area`/`area_async` bindings on implementing classes
+    // (named `user.interfaces.Square.area`), but the engine registers
+    // interface-impl methods under a synthetic `Shape$for$Square` name,
+    // so calling the binding from the host panics with "Function not
+    // found". Update this pin when the binding and registration agree.
+    expect(() => new Square({ side: 3 }).area()).toThrow(/Function not found/);
+  });
+});
diff --git a/baml_language/sdk_tests/fixtures/type_shapes/baml_src/ns_interfaces/types.baml b/baml_language/sdk_tests/fixtures/type_shapes/baml_src/ns_interfaces/types.baml
new file mode 100644
index 0000000000..0a680ffef7
--- /dev/null
+++ b/baml_language/sdk_tests/fixtures/type_shapes/baml_src/ns_interfaces/types.baml
@@ -0,0 +1,106 @@
+// Coverage for interface types (BEP-044) at the host SDK boundary.
+//
+// Interfaces are BAML-side contracts, not serializable host SDK models:
+// codegen surfaces interface-typed boundary positions as opaque values
+// (`typing.Any` / `unknown` — see `client_codegen.rs`
+// `TirTy::Interface(..) => cg::Ty::BuiltinUnknown`). The *values* crossing
+// the boundary are always concrete class instances, so what these fixtures
+// pin is:
+//
+//   - return position: the host receives the concrete implementing class
+//   - parameter position: the host passes a concrete instance and the
+//     engine dispatches interface methods on it
+//   - encode → FFI → decode round trips preserve the concrete class, in
+//     bare, list, optional, and class-field positions
+//
+// The two SDKs currently diverge on the inbound direction: the python
+// bridge encodes host instances by their runtime class (all of the above
+// works), while the nodejs bridge encodes by the declared codegen type
+// (`unknown`), losing class identity in interface positions. Each SDK's
+// test file pins its own current behavior.
+
+interface Shape {
+  function area(self) -> int
+}
+
+// In-body `implements` form.
+class Square {
+  side int
+  implements Shape {
+    function area(self) -> int {
+      self.side * self.side
+    }
+  }
+}
+
+class Rect {
+  width int
+  height int
+  implements Shape {
+    function area(self) -> int {
+      self.width * self.height
+    }
+  }
+}
+
+// Out-of-body `implements ... for ...` form.
+class Circle {
+  radius int
+}
+
+implements Shape for Circle {
+  function area(self) -> int {
+    3 * self.radius * self.radius
+  }
+}
+
+// Interface in a class-field position.
+class ShapeBox {
+  shape Shape
+}
+
+// ── interface in return position ────────────────────────────────────────
+
+function return_square_as_shape() -> Shape {
+  Square { side: 5 }
+}
+
+function return_rect_as_shape() -> Shape {
+  Rect { width: 3, height: 4 }
+}
+
+function return_circle_as_shape() -> Shape {
+  Circle { radius: 2 }
+}
+
+// ── interface in parameter position (engine-side method dispatch) ───────
+
+function shape_area(s: Shape) -> int {
+  s.area()
+}
+
+function sum_areas(a: Shape, b: Shape) -> int {
+  a.area() + b.area()
+}
+
+// ── encode → FFI → decode round trips ───────────────────────────────────
+
+function round_trip_shape(s: Shape) -> Shape {
+  s
+}
+
+function round_trip_shape_list(shapes: Shape[]) -> Shape[] {
+  shapes
+}
+
+function round_trip_optional_shape(s: Shape?) -> Shape? {
+  s
+}
+
+function round_trip_shape_box(b: ShapeBox) -> ShapeBox {
+  b
+}
+
+function box_area(b: ShapeBox) -> int {
+  b.shape.area()
+}