diff --git a/scripts/generators/prism.py b/scripts/generators/prism.py
new file mode 100644
index 0000000..566d824
--- /dev/null
+++ b/scripts/generators/prism.py
@@ -0,0 +1,393 @@
+"""Planar prism solids written as STEP (AP214) or IGES 5.3 (186 manifold solid B-reps), with
+assemblies: STEP next-assembly-usage occurrences placed by item-defined transformations, IGES
+subfigure definitions (308) placed by singular subfigure instances (408).
+
+Standard library only. Every model here is generated from code, so the files are reproducible:
+run the model's script and compare the SHA-256 with its README.
+
+A part is a polygon (counter-clockwise, in the XY plane, millimetres) extruded along +Z by a
+height. Faces: bottom, top and one side per polygon edge, all planar, normals outward.
+"""
+
+import math
+
+
+# ---------- geometry ----------
+
+def regular(n, r, phase=0.0):
+    return [(r * math.cos(phase + 2 * math.pi * i / n), r * math.sin(phase + 2 * math.pi * i / n)) for i in range(n)]
+
+
+def rect(w, d):
+    return [(-w / 2, -d / 2), (w / 2, -d / 2), (w / 2, d / 2), (-w / 2, d / 2)]
+
+
+def l_section(w, d, t):
+    """An L: a w x d bounding box with legs t thick (not convex)."""
+    return [(0, 0), (w, 0), (w, t), (t, t), (t, d), (0, d)]
+
+
+def area2(poly):
+    return sum(poly[i][0] * poly[(i + 1) % len(poly)][1] - poly[(i + 1) % len(poly)][0] * poly[i][1] for i in range(len(poly)))
+
+
+class Part:
+    def __init__(self, name, poly, height):
+        if area2(poly) <= 0:
+            raise ValueError(f"{name}: polygon must be counter-clockwise")
+        self.name, self.poly, self.h = name, poly, height
+
+    @property
+    def faces(self):
+        return len(self.poly) + 2
+
+    @property
+    def edges(self):
+        return 3 * len(self.poly)
+
+    @property
+    def vertices(self):
+        return 2 * len(self.poly)
+
+
+# A placement is (rotation 3x3 as rows, translation). Rotations are built from axis angles.
+
+def rot(axis, deg):
+    a = math.radians(deg)
+    c, s = math.cos(a), math.sin(a)
+    x, y, z = axis
+    n = math.sqrt(x * x + y * y + z * z)
+    x, y, z = x / n, y / n, z / n
+    return [[c + x * x * (1 - c), x * y * (1 - c) - z * s, x * z * (1 - c) + y * s],
+            [y * x * (1 - c) + z * s, c + y * y * (1 - c), y * z * (1 - c) - x * s],
+            [z * x * (1 - c) - y * s, z * y * (1 - c) + x * s, c + z * z * (1 - c)]]
+
+
+IDENTITY = [[1, 0, 0], [0, 1, 0], [0, 0, 1]]
+
+
+def matmul(a, b):
+    return [[sum(a[i][k] * b[k][j] for k in range(3)) for j in range(3)] for i in range(3)]
+
+
+class Node:
+    """An assembly: a name and children (Part or Node), each with a rotation and translation."""
+
+    def __init__(self, name, children=None):
+        self.name = name
+        self.children = children or []  # [(Part | Node, R, t)]
+
+    def add(self, child, R=IDENTITY, t=(0, 0, 0)):
+        self.children.append((child, R, tuple(t)))
+        return self
+
+
+def count(node):
+    """(solid instances, face instances, distinct parts, assembly nodes, occurrences)."""
+    parts, nodes = set(), set()
+    def walk(n):
+        nodes.add(id(n))
+        s = f = o = 0
+        for c, _, _ in n.children:
+            o += 1
+            if isinstance(c, Part):
+                parts.add(id(c)); s += 1; f += c.faces
+            else:
+                a, b, d = walk(c); s += a; f += b; o += d
+        return s, f, o
+    s, f, o = walk(node)
+    return dict(solids=s, faces=f, distinct_parts=len(parts), assemblies=len(nodes), occurrences=o)
+
+
+def f6(x):
+    """A real in the shortest form that round-trips at 1e-9 mm, never '-0.'."""
+    v = round(x, 9)
+    if v == 0:
+        return "0."
+    s = repr(v)
+    if "e" in s or "E" in s:
+        s = f"{v:.12E}"
+        return s
+    return s if "." in s else s + "."
+
+
+# ---------- STEP AP214 ----------
+
+def step_text(root, file_name, description):
+    out = []
+
+    def add(s):
+        out.append(s)
+        return len(out)
+
+    def pt(p):
+        return add(f"CARTESIAN_POINT('',({f6(p[0])},{f6(p[1])},{f6(p[2])}))")
+
+    def dr(d):
+        return add(f"DIRECTION('',({f6(d[0])},{f6(d[1])},{f6(d[2])}))")
+
+    def ax(o, z, x):
+        return add(f"AXIS2_PLACEMENT_3D('',#{pt(o)},#{dr(z)},#{dr(x)})")
+
+    # context
+    mm = add("(LENGTH_UNIT()NAMED_UNIT(*)SI_UNIT(.MILLI.,.METRE.))")
+    rad = add("(NAMED_UNIT(*)PLANE_ANGLE_UNIT()SI_UNIT($,.RADIAN.))")
+    sr = add("(NAMED_UNIT(*)SI_UNIT($,.STERADIAN.)SOLID_ANGLE_UNIT())")
+    unc = add(f"UNCERTAINTY_MEASURE_WITH_UNIT(LENGTH_MEASURE(1.E-06),#{mm},'distance_accuracy_value','')")
+    ctx = add(f"(GEOMETRIC_REPRESENTATION_CONTEXT(3)GLOBAL_UNCERTAINTY_ASSIGNED_CONTEXT((#{unc}))"
+              f"GLOBAL_UNIT_ASSIGNED_CONTEXT((#{mm},#{rad},#{sr}))REPRESENTATION_CONTEXT('',''))")
+    appctx = add("APPLICATION_CONTEXT('core data for automotive mechanical design processes')")
+    add(f"APPLICATION_PROTOCOL_DEFINITION('international standard','automotive_design',2000,#{appctx})")
+    pctx = add(f"PRODUCT_CONTEXT('',#{appctx},'mechanical')")
+    pdctx = add(f"PRODUCT_DEFINITION_CONTEXT('part definition',#{appctx},'design')")
+
+    def product(name):
+        p = add(f"PRODUCT('{name}','{name}','',(#{pctx}))")
+        add(f"PRODUCT_RELATED_PRODUCT_CATEGORY('part',$,(#{p}))")
+        f = add(f"PRODUCT_DEFINITION_FORMATION('','',#{p})")
+        pd = add(f"PRODUCT_DEFINITION('design','',#{f},#{pdctx})")
+        pds = add(f"PRODUCT_DEFINITION_SHAPE('','',#{pd})")
+        return pd, pds
+
+    origin = None
+
+    def brep(part):
+        n, h = len(part.poly), part.h
+        P = [(x, y, 0.0) for x, y in part.poly] + [(x, y, h) for x, y in part.poly]
+        V = [add(f"VERTEX_POINT('',#{pt(p)})") for p in P]
+
+        def edge(a, b):
+            pa, pb = P[a], P[b]
+            d = [pb[k] - pa[k] for k in range(3)]
+            L = math.sqrt(sum(x * x for x in d))
+            u = [x / L for x in d]
+            vec = add(f"VECTOR('',#{dr(u)},{f6(L)})")
+            line = add(f"LINE('',#{pt(pa)},#{vec})")
+            return add(f"EDGE_CURVE('',#{V[a]},#{V[b]},#{line},.T.)")
+
+        bot = [edge(i, (i + 1) % n) for i in range(n)]
+        top = [edge(n + i, n + (i + 1) % n) for i in range(n)]
+        ver = [edge(i, n + i) for i in range(n)]
+
+        def oe(e, fwd):
+            return f"#{add(f'ORIENTED_EDGE({chr(39)}{chr(39)},*,*,#{e},{'.T.' if fwd else '.F.'})')}"
+
+        def face(oes, o, z, x):
+            loop = add(f"EDGE_LOOP('',({','.join(oes)}))")
+            bound = add(f"FACE_OUTER_BOUND('',#{loop},.T.)")
+            plane = add(f"PLANE('',#{ax(o, z, x)})")
+            return add(f"ADVANCED_FACE('',(#{bound}),#{plane},.T.)")
+
+        faces = [face([oe(bot[i], False) for i in reversed(range(n))], P[0], (0, 0, -1), (1, 0, 0)),
+                 face([oe(top[i], True) for i in range(n)], P[n], (0, 0, 1), (1, 0, 0))]
+        for i in range(n):
+            j = (i + 1) % n
+            dx, dy = P[j][0] - P[i][0], P[j][1] - P[i][1]
+            L = math.hypot(dx, dy)
+            faces.append(face([oe(bot[i], True), oe(ver[j], True), oe(top[i], False), oe(ver[i], False)],
+                              P[i], (dy / L, -dx / L, 0), (dx / L, dy / L, 0)))
+        shell = add(f"CLOSED_SHELL('',({','.join(f'#{f}' for f in faces)}))")
+        return add(f"MANIFOLD_SOLID_BREP('{part.name}',#{shell})")
+
+    reps, pds_of = {}, {}
+
+    def define(node):
+        """Product + shape representation for a part or an assembly (once each)."""
+        nonlocal origin
+        if id(node) in reps:
+            return reps[id(node)]
+        pd, pds = product(node.name)
+        o = ax((0, 0, 0), (0, 0, 1), (1, 0, 0))
+        if isinstance(node, Part):
+            rep = add(f"ADVANCED_BREP_SHAPE_REPRESENTATION('{node.name}',(#{o},#{brep(node)}),#{ctx})")
+        else:
+            rep = add(f"SHAPE_REPRESENTATION('{node.name}',(#{o}),#{ctx})")
+        add(f"SHAPE_DEFINITION_REPRESENTATION(#{pds},#{rep})")
+        reps[id(node)] = (pd, rep, o)
+        if not isinstance(node, Part):
+            k = 0
+            for child, R, t in node.children:
+                cpd, crep, co = define(child)
+                k += 1
+                nm = f"{child.name}:{k}"
+                nauo = add(f"NEXT_ASSEMBLY_USAGE_OCCURRENCE('{nm}','{nm}','',#{pd},#{cpd},$)")
+                npds = add(f"PRODUCT_DEFINITION_SHAPE('','',#{nauo})")
+                z = (R[0][2], R[1][2], R[2][2])
+                x = (R[0][0], R[1][0], R[2][0])
+                tgt = ax(t, z, x)
+                # the child's origin (in the child's representation) maps to the placement in the parent's
+                idt = add(f"ITEM_DEFINED_TRANSFORMATION('','',#{co},#{tgt})")
+                rr = add(f"(REPRESENTATION_RELATIONSHIP('','',#{crep},#{rep})REPRESENTATION_RELATIONSHIP_WITH_TRANSFORMATION(#{idt})SHAPE_REPRESENTATION_RELATIONSHIP())")
+                add(f"CONTEXT_DEPENDENT_SHAPE_REPRESENTATION(#{rr},#{npds})")
+        return reps[id(node)]
+
+    define(root)
+    head = ("ISO-10303-21;\nHEADER;\n"
+            f"FILE_DESCRIPTION(('{description}'),'2;1');\n"
+            f"FILE_NAME('{file_name}','2026-10-20T00:00:00',('raykholo'),(''),'tangency-labs/models generator','python3 scripts/generators','');\n"
+            "FILE_SCHEMA(('AUTOMOTIVE_DESIGN { 1 0 10303 214 1 1 1 1 }'));\nENDSEC;\nDATA;\n")
+    body = "".join(f"#{i + 1}={s};\n" for i, s in enumerate(out))
+    return head + body + "ENDSEC;\nEND-ISO-10303-21;\n"
+
+
+# ---------- IGES 5.3 ----------
+
+class Iges:
+    """Directory entries and parameter data, written in fixed 80-column records."""
+
+    def __init__(self):
+        self.ents = []  # (type, form, params list, status, matrix_de, label)
+
+    def add(self, typ, params, form=0, status="00000000", matrix=0, label=""):
+        self.ents.append([typ, form, params, status, matrix, label])
+        return 2 * len(self.ents) - 1  # the entity's DE sequence number
+
+    def set_status(self, de, status):
+        self.ents[(de - 1) // 2][3] = status
+
+    def text(self, file_name, max_coord):
+        def h(s):
+            return f"{len(s)}H{s}"
+        g = [h(","), h(";"), h("tangency-labs/models generator"), h(file_name), h("python3 scripts/generators"),
+             h("prism.py IGES 5.3"), "32", "38", "6", "308", "15", h(file_name), "1.", "2", h("MM"), "1", "1.",
+             h("20261020.000000"), "1.E-06", f6(max_coord), h("raykholo"), h(""), "11", "0", h("20261020.000000")]
+        s_lines = ["Generated by scripts/generators (github.com/tangency-labs/models), CC0"]
+
+        def wrap(items, width, end):
+            """Join parameters with ',' and end with ';', split into lines of at most width chars."""
+            lines, cur = [], ""
+            for i, it in enumerate(items):
+                tok = it + (end if i == len(items) - 1 else ",")
+                if len(cur) + len(tok) > width:
+                    lines.append(cur); cur = ""
+                cur += tok
+            lines.append(cur)
+            return lines
+
+        g_lines = wrap(g, 72, ";")
+        d_lines, p_lines = [], []
+        for idx, (typ, form, params, status, matrix, label) in enumerate(self.ents):
+            de = 2 * idx + 1
+            pl = wrap([str(typ)] + [str(p) for p in params], 64, ";")
+            start = len(p_lines) + 1
+            for line in pl:
+                p_lines.append(f"{line:<64}{de:>8}")
+            d_lines.append(f"{typ:>8}{start:>8}{0:>8}{0:>8}{0:>8}{0:>8}{matrix:>8}{0:>8}{status:>8}")
+            d_lines.append(f"{typ:>8}{0:>8}{0:>8}{len(pl):>8}{form:>8}{'':>8}{'':>8}{label[:8]:>8}{0:>8}")
+        rec = []
+        rec += [f"{l:<72}S{i + 1:>7}" for i, l in enumerate(s_lines)]
+        rec += [f"{l:<72}G{i + 1:>7}" for i, l in enumerate(g_lines)]
+        rec += [f"{l:<72}D{i + 1:>7}" for i, l in enumerate(d_lines)]
+        rec += [f"{l:<72}P{i + 1:>7}" for i, l in enumerate(p_lines)]
+        rec.append(f"S{len(s_lines):>7}G{len(g_lines):>7}D{len(d_lines):>7}P{len(p_lines):>7}{'':<40}T{1:>7}")
+        return "\n".join(rec) + "\n"
+
+
+DEP = "00010000"     # physically dependent
+DEF = "00000200"     # independent, used as a definition (subfigure definitions)
+
+
+def iges_solid(ig, part):
+    """A 186 manifold solid B-rep for the prism; every entity under it is physically dependent."""
+    n, hgt = len(part.poly), part.h
+    P = [(x, y, 0.0) for x, y in part.poly] + [(x, y, hgt) for x, y in part.poly]
+    vl = ig.add(502, [len(P)] + [f6(c) for p in P for c in p], form=1, status=DEP)
+    edges = []  # (a, b) vertex indices, 1-based in the vertex list
+
+    def line(a, b):
+        pa, pb = P[a], P[b]
+        return ig.add(110, [f6(c) for c in pa + pb], status=DEP)
+
+    E = [(i, (i + 1) % n) for i in range(n)] + [(n + i, n + (i + 1) % n) for i in range(n)] + [(i, n + i) for i in range(n)]
+    curves = [line(a, b) for a, b in E]
+    params = [len(E)]
+    for (a, b), c in zip(E, curves):
+        params += [c, vl, a + 1, vl, b + 1]
+    el = ig.add(504, params, form=1, status=DEP)
+    bot = list(range(n)); top = [n + i for i in range(n)]; ver = [2 * n + i for i in range(n)]
+
+    def loop(es):  # [(edge index 0-based, forward)]
+        p = [len(es)]
+        for e, fwd in es:
+            p += [0, el, e + 1, 1 if fwd else 0, 0]
+        return ig.add(508, p, form=1, status=DEP)
+
+    def plane(o, z):
+        pt = ig.add(116, [f6(c) for c in o], status=DEP)
+        dn = ig.add(123, [f6(c) for c in z], status=DEP)
+        return ig.add(190, [pt, dn], status=DEP)
+
+    def face(es, o, z):
+        return ig.add(510, [plane(o, z), 1, 1, loop(es)], form=1, status=DEP)
+
+    faces = [face([(bot[i], False) for i in reversed(range(n))], P[0], (0, 0, -1)),
+             face([(top[i], True) for i in range(n)], P[n], (0, 0, 1))]
+    for i in range(n):
+        j = (i + 1) % n
+        dx, dy = P[j][0] - P[i][0], P[j][1] - P[i][1]
+        L = math.hypot(dx, dy)
+        faces.append(face([(bot[i], True), (ver[j], True), (top[i], False), (ver[i], False)], P[i], (dy / L, -dx / L, 0)))
+    shell = ig.add(514, [len(faces)] + [x for f in faces for x in (f, 1)], form=1, status=DEP)
+    return ig.add(186, [shell, 1, 0], status=DEP, label=part.name[:8].upper())
+
+
+def iges_text(root, file_name):
+    """root: a Node. Every Node becomes a 308 definition (deepest first); every child placement a 408.
+    The top Node's children are written as independent 408 instances (the file's assembly)."""
+    ig = Iges()
+    solid_de, sub_de, depth = {}, {}, {}
+
+    def matrix(R, t):
+        return ig.add(124, [f6(R[i][j]) if j < 3 else f6(t[i]) for i in range(3) for j in range(4)], status=DEP)
+
+    def part_sub(part):
+        # a part becomes a one-member subfigure, so every placement is a 408
+        if id(part) not in sub_de:
+            s = iges_solid(ig, part)
+            sub_de[id(part)] = ig.add(308, [0, f"{len(part.name)}H{part.name}", 1, s], status=DEF)
+            depth[id(part)] = 0
+        return sub_de[id(part)]
+
+    def instance(child, R, t, status):
+        d = part_sub(child) if isinstance(child, Part) else node_sub(child)
+        # an unrotated placement uses the 408's own X, Y, Z; a rotated one puts rotation and translation in
+        # the 408's transformation matrix (124), so both placement mechanisms are exercised (X' = M (S x + T)).
+        if R == IDENTITY:
+            return ig.add(408, [d, f6(t[0]), f6(t[1]), f6(t[2]), "1."], status=status)
+        m = matrix(R, t)
+        return ig.add(408, [d, "0.", "0.", "0.", "1."], status=status, matrix=m)
+
+    def node_sub(node):
+        if id(node) not in sub_de:
+            members = [instance(c, R, t, DEP) for c, R, t in node.children]
+            dmax = 1 + max(depth[id(c)] for c, _, _ in node.children)
+            sub_de[id(node)] = ig.add(308, [dmax, f"{len(node.name)}H{node.name}", len(members)] + members, status=DEF)
+            depth[id(node)] = dmax
+        return sub_de[id(node)]
+
+    for c, R, t in root.children:
+        instance(c, R, t, "00000000")
+    return ig.text(file_name, 1000.0)
+
+
+def expected(root):
+    """Analytic totals for checking a reader: solid count, total volume (mm3), world bounding box."""
+    vol, lo, hi, n = 0.0, [math.inf] * 3, [-math.inf] * 3, 0
+
+    def walk(node, R, t):
+        nonlocal vol, n
+        for c, Rc, tc in node.children:
+            R2 = matmul(R, Rc)
+            t2 = [sum(R[i][k] * tc[k] for k in range(3)) + t[i] for i in range(3)]
+            if isinstance(c, Part):
+                n += 1
+                vol += abs(area2(c.poly)) / 2 * c.h
+                for x, y in c.poly:
+                    for z in (0.0, c.h):
+                        p = [sum(R2[i][k] * (x, y, z)[k] for k in range(3)) + t2[i] for i in range(3)]
+                        for i in range(3):
+                            lo[i], hi[i] = min(lo[i], p[i]), max(hi[i], p[i])
+            else:
+                walk(c, R2, t2)
+    walk(root, IDENTITY, [0, 0, 0])
+    return dict(solids=n, volume=round(vol, 3), bbox=[round(v, 3) for v in lo + hi])
diff --git a/scripts/generators/untrimmed_surface_body.py b/scripts/generators/untrimmed_surface_body.py
new file mode 100644
index 0000000..507abb1
--- /dev/null
+++ b/scripts/generators/untrimmed_surface_body.py
@@ -0,0 +1,105 @@
+"""step/untrimmed-surface-body: an open surface body (an open box, 100 x 80 x 50 mm, no lid) whose
+floor lies on a B-spline surface 4,000 mm square, trimmed to the 100 x 80 mm floor. The body's
+bounding box is the trimmed faces' box; a reader that measures or frames the underlying surface
+instead reports a box 40 times too wide.
+
+    python3 scripts/generators/untrimmed_surface_body.py step/untrimmed-surface-body
+"""
+
+import os
+import sys
+
+sys.path.insert(0, os.path.dirname(os.path.abspath(__file__)))
+from prism import f6  # noqa: E402
+
+W, D, H = 100.0, 80.0, 50.0
+BIG = 2000.0  # the floor's surface spans -BIG..BIG in X and Y
+
+
+def step():
+    out = []
+
+    def add(s):
+        out.append(s)
+        return len(out)
+
+    def pt(p):
+        return add(f"CARTESIAN_POINT('',({f6(p[0])},{f6(p[1])},{f6(p[2])}))")
+
+    def dr(d):
+        return add(f"DIRECTION('',({f6(d[0])},{f6(d[1])},{f6(d[2])}))")
+
+    mm = add("(LENGTH_UNIT()NAMED_UNIT(*)SI_UNIT(.MILLI.,.METRE.))")
+    rad = add("(NAMED_UNIT(*)PLANE_ANGLE_UNIT()SI_UNIT($,.RADIAN.))")
+    sr = add("(NAMED_UNIT(*)SI_UNIT($,.STERADIAN.)SOLID_ANGLE_UNIT())")
+    unc = add(f"UNCERTAINTY_MEASURE_WITH_UNIT(LENGTH_MEASURE(1.E-06),#{mm},'distance_accuracy_value','')")
+    ctx = add(f"(GEOMETRIC_REPRESENTATION_CONTEXT(3)GLOBAL_UNCERTAINTY_ASSIGNED_CONTEXT((#{unc}))"
+              f"GLOBAL_UNIT_ASSIGNED_CONTEXT((#{mm},#{rad},#{sr}))REPRESENTATION_CONTEXT('',''))")
+    app = add("APPLICATION_CONTEXT('core data for automotive mechanical design processes')")
+    add(f"APPLICATION_PROTOCOL_DEFINITION('international standard','automotive_design',2000,#{app})")
+    pc = add(f"PRODUCT_CONTEXT('',#{app},'mechanical')")
+    pdc = add(f"PRODUCT_DEFINITION_CONTEXT('part definition',#{app},'design')")
+    prod = add(f"PRODUCT('untrimmed-surface-body','untrimmed-surface-body','',(#{pc}))")
+    add(f"PRODUCT_RELATED_PRODUCT_CATEGORY('part',$,(#{prod}))")
+    pdf = add(f"PRODUCT_DEFINITION_FORMATION('','',#{prod})")
+    pd = add(f"PRODUCT_DEFINITION('design','',#{pdf},#{pdc})")
+    pds = add(f"PRODUCT_DEFINITION_SHAPE('','',#{pd})")
+
+    # the open box: floor at z=0, four walls up to z=H; vertices bottom 0-3, top 4-7
+    P = [(0, 0, 0), (W, 0, 0), (W, D, 0), (0, D, 0), (0, 0, H), (W, 0, H), (W, D, H), (0, D, H)]
+    V = [add(f"VERTEX_POINT('',#{pt(p)})") for p in P]
+    E = {}
+
+    def edge(a, b):
+        if (a, b) in E:
+            return E[(a, b)], True
+        if (b, a) in E:
+            return E[(b, a)], False
+        pa, pb = P[a], P[b]
+        d = [pb[k] - pa[k] for k in range(3)]
+        L = sum(x * x for x in d) ** 0.5
+        vec = add(f"VECTOR('',#{dr([x / L for x in d])},{f6(L)})")
+        line = add(f"LINE('',#{pt(pa)},#{vec})")
+        E[(a, b)] = add(f"EDGE_CURVE('',#{V[a]},#{V[b]},#{line},.T.)")
+        return E[(a, b)], True
+
+    def loop(vs):
+        oes = []
+        for i in range(len(vs)):
+            e, fwd = edge(vs[i], vs[(i + 1) % len(vs)])
+            oes.append(f"#{add(f'ORIENTED_EDGE({chr(39)}{chr(39)},*,*,#{e},{chr(46)}{chr(84) if fwd else chr(70)}{chr(46)})')}")
+        lp = add(f"EDGE_LOOP('',({','.join(oes)}))")
+        return add(f"FACE_OUTER_BOUND('',#{lp},.T.)")
+
+    def plane(o, z, x):
+        a = add(f"AXIS2_PLACEMENT_3D('',#{pt(o)},#{dr(z)},#{dr(x)})")
+        return add(f"PLANE('',#{a})")
+
+    # the floor's surface: a flat bilinear B-spline patch, 4,000 mm square, normal -Z (outward, down)
+    cp = [[pt((-BIG, -BIG, 0)), pt((BIG, -BIG, 0))], [pt((-BIG, BIG, 0)), pt((BIG, BIG, 0))]]
+    rows = ",".join("(" + ",".join(f"#{c}" for c in r) + ")" for r in cp)
+    # u along +Y, v along +X: du x dv = Y x X = -Z, so the surface normal points out of the box
+    bss = add(f"B_SPLINE_SURFACE_WITH_KNOTS('',1,1,({rows}),.PLANE_SURF.,.F.,.F.,.F.,(2,2),(2,2),(0.,1.),(0.,1.),.UNSPECIFIED.)")
+    faces = [add(f"ADVANCED_FACE('floor',(#{loop([0, 3, 2, 1])}),#{bss},.T.)")]
+    walls = [([0, 1, 5, 4], (0, -1, 0), (1, 0, 0)), ([1, 2, 6, 5], (1, 0, 0), (0, 1, 0)),
+             ([2, 3, 7, 6], (0, 1, 0), (-1, 0, 0)), ([3, 0, 4, 7], (-1, 0, 0), (0, -1, 0))]
+    for vs, n, x in walls:
+        faces.append(add(f"ADVANCED_FACE('',(#{loop(vs)}),#{plane(P[vs[0]], n, x)},.T.)"))
+    shell = add(f"OPEN_SHELL('',({','.join(f'#{f}' for f in faces)}))")
+    model = add(f"SHELL_BASED_SURFACE_MODEL('untrimmed-surface-body',(#{shell}))")
+    o = add(f"AXIS2_PLACEMENT_3D('',#{pt((0, 0, 0))},#{dr((0, 0, 1))},#{dr((1, 0, 0))})")
+    rep = add(f"MANIFOLD_SURFACE_SHAPE_REPRESENTATION('untrimmed-surface-body',(#{o},#{model}),#{ctx})")
+    add(f"SHAPE_DEFINITION_REPRESENTATION(#{pds},#{rep})")
+    head = ("ISO-10303-21;\nHEADER;\nFILE_DESCRIPTION(('open surface body on a much larger surface'),'2;1');\n"
+            "FILE_NAME('untrimmed-surface-body.step','2026-10-20T00:00:00',('raykholo'),(''),'tangency-labs/models generator','python3 scripts/generators','');\n"
+            "FILE_SCHEMA(('AUTOMOTIVE_DESIGN { 1 0 10303 214 1 1 1 1 }'));\nENDSEC;\nDATA;\n")
+    return head + "".join(f"#{i + 1}={s};\n" for i, s in enumerate(out)) + "ENDSEC;\nEND-ISO-10303-21;\n"
+
+
+if __name__ == "__main__":
+    out = sys.argv[1] if len(sys.argv) > 1 else "."
+    os.makedirs(out, exist_ok=True)
+    path = os.path.join(out, "untrimmed-surface-body.step")
+    with open(path, "w", newline="\n") as f:
+        f.write(step())
+    print(path, dict(faces=5, area=W * D + 2 * (W + D) * H, bbox=[0, 0, 0, W, D, H], surface_extent=[-BIG, -BIG, BIG, BIG]))
diff --git a/step/untrimmed-surface-body/README.md b/step/untrimmed-surface-body/README.md
new file mode 100644
index 0000000..b9219cb
--- /dev/null
+++ b/step/untrimmed-surface-body/README.md
@@ -0,0 +1,34 @@
+# untrimmed-surface-body
+
+An open surface body (a box without a lid, 100 × 80 × 50 mm) whose floor lies on a B-spline surface
+4,000 mm square, trimmed to the 100 × 80 mm floor. The body's bounding box is 100 × 80 × 50 mm;
+its floor's underlying surface is 40 times wider.
+
+- **File:** `untrimmed-surface-body.step`
+- **Author:** raykholo
+- **Made with:** `scripts/generators/untrimmed_surface_body.py` (Python 3, standard library only).
+  It writes STEP AP214 (`AUTOMOTIVE_DESIGN`) directly, in millimetres, with a distance accuracy of
+  1e-6 mm. Run `python3 scripts/generators/untrimmed_surface_body.py step/untrimmed-surface-body` to
+  reproduce the file byte for byte.
+- **Application protocol:** AP214, as a manifold surface shape representation (one shell-based
+  surface model, one open shell).
+- **Size:** 1 surface body; 5 faces (4 planes and 1 bilinear B-spline surface), 12 edges, 8
+  vertices. 6.3 KB.
+- **SHA-256:** `2decd6561114628a67d4e3ee396f56ee4ae1dfbdacc2ef4acfe1b5841ca4552b`
+
+## Why it is interesting
+
+Surface models exported from CAD often keep a face's whole underlying surface, much larger than the
+trimmed face. A reader that measures, frames or bounds the surface instead of the trimmed face
+reports a box of about 4,000 × 4,000 × 50 mm, zooms the view out to it, or sizes meshing and
+tolerances from it. The faces themselves are simple, so any difference comes from the bounding.
+
+## Check values
+
+| | |
+|---|---|
+| Bodies | 1 open surface body (no solid) |
+| Faces | 5 |
+| Area | 26,000 mm² |
+| Bounding box | x 0 … 100, y 0 … 80, z 0 … 50 mm |
+| Floor's underlying surface | x −2,000 … 2,000, y −2,000 … 2,000 mm at z 0 |
diff --git a/step/untrimmed-surface-body/untrimmed-surface-body.step b/step/untrimmed-surface-body/untrimmed-surface-body.step
new file mode 100644
index 0000000..76debca
--- /dev/null
+++ b/step/untrimmed-surface-body/untrimmed-surface-body.step
@@ -0,0 +1,167 @@
+ISO-10303-21;
+HEADER;
+FILE_DESCRIPTION(('open surface body on a much larger surface'),'2;1');
+FILE_NAME('untrimmed-surface-body.step','2026-10-20T00:00:00',('raykholo'),(''),'tangency-labs/models generator','python3 scripts/generators','');
+FILE_SCHEMA(('AUTOMOTIVE_DESIGN { 1 0 10303 214 1 1 1 1 }'));
+ENDSEC;
+DATA;
+#1=(LENGTH_UNIT()NAMED_UNIT(*)SI_UNIT(.MILLI.,.METRE.));
+#2=(NAMED_UNIT(*)PLANE_ANGLE_UNIT()SI_UNIT($,.RADIAN.));
+#3=(NAMED_UNIT(*)SI_UNIT($,.STERADIAN.)SOLID_ANGLE_UNIT());
+#4=UNCERTAINTY_MEASURE_WITH_UNIT(LENGTH_MEASURE(1.E-06),#1,'distance_accuracy_value','');
+#5=(GEOMETRIC_REPRESENTATION_CONTEXT(3)GLOBAL_UNCERTAINTY_ASSIGNED_CONTEXT((#4))GLOBAL_UNIT_ASSIGNED_CONTEXT((#1,#2,#3))REPRESENTATION_CONTEXT('',''));
+#6=APPLICATION_CONTEXT('core data for automotive mechanical design processes');
+#7=APPLICATION_PROTOCOL_DEFINITION('international standard','automotive_design',2000,#6);
+#8=PRODUCT_CONTEXT('',#6,'mechanical');
+#9=PRODUCT_DEFINITION_CONTEXT('part definition',#6,'design');
+#10=PRODUCT('untrimmed-surface-body','untrimmed-surface-body','',(#8));
+#11=PRODUCT_RELATED_PRODUCT_CATEGORY('part',$,(#10));
+#12=PRODUCT_DEFINITION_FORMATION('','',#10);
+#13=PRODUCT_DEFINITION('design','',#12,#9);
+#14=PRODUCT_DEFINITION_SHAPE('','',#13);
+#15=CARTESIAN_POINT('',(0.,0.,0.));
+#16=VERTEX_POINT('',#15);
+#17=CARTESIAN_POINT('',(100.0,0.,0.));
+#18=VERTEX_POINT('',#17);
+#19=CARTESIAN_POINT('',(100.0,80.0,0.));
+#20=VERTEX_POINT('',#19);
+#21=CARTESIAN_POINT('',(0.,80.0,0.));
+#22=VERTEX_POINT('',#21);
+#23=CARTESIAN_POINT('',(0.,0.,50.0));
+#24=VERTEX_POINT('',#23);
+#25=CARTESIAN_POINT('',(100.0,0.,50.0));
+#26=VERTEX_POINT('',#25);
+#27=CARTESIAN_POINT('',(100.0,80.0,50.0));
+#28=VERTEX_POINT('',#27);
+#29=CARTESIAN_POINT('',(0.,80.0,50.0));
+#30=VERTEX_POINT('',#29);
+#31=CARTESIAN_POINT('',(-2000.0,-2000.0,0.));
+#32=CARTESIAN_POINT('',(2000.0,-2000.0,0.));
+#33=CARTESIAN_POINT('',(-2000.0,2000.0,0.));
+#34=CARTESIAN_POINT('',(2000.0,2000.0,0.));
+#35=B_SPLINE_SURFACE_WITH_KNOTS('',1,1,((#31,#32),(#33,#34)),.PLANE_SURF.,.F.,.F.,.F.,(2,2),(2,2),(0.,1.),(0.,1.),.UNSPECIFIED.);
+#36=DIRECTION('',(0.,1.0,0.));
+#37=VECTOR('',#36,80.0);
+#38=CARTESIAN_POINT('',(0.,0.,0.));
+#39=LINE('',#38,#37);
+#40=EDGE_CURVE('',#16,#22,#39,.T.);
+#41=ORIENTED_EDGE('',*,*,#40,.T.);
+#42=DIRECTION('',(1.0,0.,0.));
+#43=VECTOR('',#42,100.0);
+#44=CARTESIAN_POINT('',(0.,80.0,0.));
+#45=LINE('',#44,#43);
+#46=EDGE_CURVE('',#22,#20,#45,.T.);
+#47=ORIENTED_EDGE('',*,*,#46,.T.);
+#48=DIRECTION('',(0.,-1.0,0.));
+#49=VECTOR('',#48,80.0);
+#50=CARTESIAN_POINT('',(100.0,80.0,0.));
+#51=LINE('',#50,#49);
+#52=EDGE_CURVE('',#20,#18,#51,.T.);
+#53=ORIENTED_EDGE('',*,*,#52,.T.);
+#54=DIRECTION('',(-1.0,0.,0.));
+#55=VECTOR('',#54,100.0);
+#56=CARTESIAN_POINT('',(100.0,0.,0.));
+#57=LINE('',#56,#55);
+#58=EDGE_CURVE('',#18,#16,#57,.T.);
+#59=ORIENTED_EDGE('',*,*,#58,.T.);
+#60=EDGE_LOOP('',(#41,#47,#53,#59));
+#61=FACE_OUTER_BOUND('',#60,.T.);
+#62=ADVANCED_FACE('floor',(#61),#35,.T.);
+#63=ORIENTED_EDGE('',*,*,#58,.F.);
+#64=DIRECTION('',(0.,0.,1.0));
+#65=VECTOR('',#64,50.0);
+#66=CARTESIAN_POINT('',(100.0,0.,0.));
+#67=LINE('',#66,#65);
+#68=EDGE_CURVE('',#18,#26,#67,.T.);
+#69=ORIENTED_EDGE('',*,*,#68,.T.);
+#70=DIRECTION('',(-1.0,0.,0.));
+#71=VECTOR('',#70,100.0);
+#72=CARTESIAN_POINT('',(100.0,0.,50.0));
+#73=LINE('',#72,#71);
+#74=EDGE_CURVE('',#26,#24,#73,.T.);
+#75=ORIENTED_EDGE('',*,*,#74,.T.);
+#76=DIRECTION('',(0.,0.,-1.0));
+#77=VECTOR('',#76,50.0);
+#78=CARTESIAN_POINT('',(0.,0.,50.0));
+#79=LINE('',#78,#77);
+#80=EDGE_CURVE('',#24,#16,#79,.T.);
+#81=ORIENTED_EDGE('',*,*,#80,.T.);
+#82=EDGE_LOOP('',(#63,#69,#75,#81));
+#83=FACE_OUTER_BOUND('',#82,.T.);
+#84=CARTESIAN_POINT('',(0.,0.,0.));
+#85=DIRECTION('',(0.,-1.,0.));
+#86=DIRECTION('',(1.,0.,0.));
+#87=AXIS2_PLACEMENT_3D('',#84,#85,#86);
+#88=PLANE('',#87);
+#89=ADVANCED_FACE('',(#83),#88,.T.);
+#90=ORIENTED_EDGE('',*,*,#52,.F.);
+#91=DIRECTION('',(0.,0.,1.0));
+#92=VECTOR('',#91,50.0);
+#93=CARTESIAN_POINT('',(100.0,80.0,0.));
+#94=LINE('',#93,#92);
+#95=EDGE_CURVE('',#20,#28,#94,.T.);
+#96=ORIENTED_EDGE('',*,*,#95,.T.);
+#97=DIRECTION('',(0.,-1.0,0.));
+#98=VECTOR('',#97,80.0);
+#99=CARTESIAN_POINT('',(100.0,80.0,50.0));
+#100=LINE('',#99,#98);
+#101=EDGE_CURVE('',#28,#26,#100,.T.);
+#102=ORIENTED_EDGE('',*,*,#101,.T.);
+#103=ORIENTED_EDGE('',*,*,#68,.F.);
+#104=EDGE_LOOP('',(#90,#96,#102,#103));
+#105=FACE_OUTER_BOUND('',#104,.T.);
+#106=CARTESIAN_POINT('',(100.0,0.,0.));
+#107=DIRECTION('',(1.,0.,0.));
+#108=DIRECTION('',(0.,1.,0.));
+#109=AXIS2_PLACEMENT_3D('',#106,#107,#108);
+#110=PLANE('',#109);
+#111=ADVANCED_FACE('',(#105),#110,.T.);
+#112=ORIENTED_EDGE('',*,*,#46,.F.);
+#113=DIRECTION('',(0.,0.,1.0));
+#114=VECTOR('',#113,50.0);
+#115=CARTESIAN_POINT('',(0.,80.0,0.));
+#116=LINE('',#115,#114);
+#117=EDGE_CURVE('',#22,#30,#116,.T.);
+#118=ORIENTED_EDGE('',*,*,#117,.T.);
+#119=DIRECTION('',(1.0,0.,0.));
+#120=VECTOR('',#119,100.0);
+#121=CARTESIAN_POINT('',(0.,80.0,50.0));
+#122=LINE('',#121,#120);
+#123=EDGE_CURVE('',#30,#28,#122,.T.);
+#124=ORIENTED_EDGE('',*,*,#123,.T.);
+#125=ORIENTED_EDGE('',*,*,#95,.F.);
+#126=EDGE_LOOP('',(#112,#118,#124,#125));
+#127=FACE_OUTER_BOUND('',#126,.T.);
+#128=CARTESIAN_POINT('',(100.0,80.0,0.));
+#129=DIRECTION('',(0.,1.,0.));
+#130=DIRECTION('',(-1.,0.,0.));
+#131=AXIS2_PLACEMENT_3D('',#128,#129,#130);
+#132=PLANE('',#131);
+#133=ADVANCED_FACE('',(#127),#132,.T.);
+#134=ORIENTED_EDGE('',*,*,#40,.F.);
+#135=ORIENTED_EDGE('',*,*,#80,.F.);
+#136=DIRECTION('',(0.,1.0,0.));
+#137=VECTOR('',#136,80.0);
+#138=CARTESIAN_POINT('',(0.,0.,50.0));
+#139=LINE('',#138,#137);
+#140=EDGE_CURVE('',#24,#30,#139,.T.);
+#141=ORIENTED_EDGE('',*,*,#140,.T.);
+#142=ORIENTED_EDGE('',*,*,#117,.F.);
+#143=EDGE_LOOP('',(#134,#135,#141,#142));
+#144=FACE_OUTER_BOUND('',#143,.T.);
+#145=CARTESIAN_POINT('',(0.,80.0,0.));
+#146=DIRECTION('',(-1.,0.,0.));
+#147=DIRECTION('',(0.,-1.,0.));
+#148=AXIS2_PLACEMENT_3D('',#145,#146,#147);
+#149=PLANE('',#148);
+#150=ADVANCED_FACE('',(#144),#149,.T.);
+#151=OPEN_SHELL('',(#62,#89,#111,#133,#150));
+#152=SHELL_BASED_SURFACE_MODEL('untrimmed-surface-body',(#151));
+#153=CARTESIAN_POINT('',(0.,0.,0.));
+#154=DIRECTION('',(0.,0.,1.));
+#155=DIRECTION('',(1.,0.,0.));
+#156=AXIS2_PLACEMENT_3D('',#153,#154,#155);
+#157=MANIFOLD_SURFACE_SHAPE_REPRESENTATION('untrimmed-surface-body',(#156,#152),#5);
+#158=SHAPE_DEFINITION_REPRESENTATION(#14,#157);
+ENDSEC;
+END-ISO-10303-21;