A T depends on every connection between its copper and its group

Anchoring a T only on the connection whose copper it touched missed the
connections that tie that copper to the T's own group. When one of those
was ripped, the T stayed routed but cut off: on charge-sense freeroute
counted 15 unrouted while KiCad found 27 opens. Routed connections of a net
form a forest, so the path back is unique; the T now depends on all of it.
KiCad's opens and freeroute's count agree again (18 against 17, the extra
one a GND pour island left to the fill).

src/freeroute/cli.py

@@ -30,8 +30,8 @@ from freeroute.ses import write_ses
 #: which optional flags each engine actually implements. Anything not listed for
 #: an engine is rejected by :func:`_validate` rather than quietly ignored.
 ENGINE_OPTIONS: dict[str, frozenset[str]] = {
-    "grid": frozenset({"rip_up", "max_passes", "seed", "orders", "promote", "access", "negotiate", "rollback"}),
-    "exact": frozenset({"shove", "diagonal", "rip_up", "max_passes", "seed", "orders", "promote", "access", "negotiate", "rollback"}),
+    "grid": frozenset({"rip_up", "max_passes", "seed", "orders", "promote", "access", "negotiate", "rollback", "tree"}),
+    "exact": frozenset({"shove", "diagonal", "rip_up", "max_passes", "seed", "orders", "promote", "access", "negotiate", "rollback", "tree"}),
     "room": frozenset({"shove", "pack"}),
 }
 
@@ -48,6 +48,7 @@ _FLAG_NAMES = {
     "access": "--access-cost",
     "negotiate": "--negotiate",
     "rollback": "--no-rollback",
+    "tree": "--no-tree",
 }
 
 
@@ -184,6 +185,15 @@ def build_parser() -> argparse.ArgumentParser:
         "connections a rip-up displaces are rerouted at once and the rip-up is undone "
         "if fewer connections end up routed",
     )
+    parser.add_argument(
+        "--no-tree",
+        dest="tree_nets",
+        action="store_false",
+        help="grid and exact engines: join a multi-pin net's pins as a chain in pin "
+        "order, each connection ending on one chosen pin, as before. By default they are "
+        "paired by a minimum spanning tree and a connection may end on copper its net "
+        "has already routed",
+    )
     parser.add_argument(
         "--hole-to-hole",
         type=float,
@@ -223,6 +233,7 @@ def main(argv: list[str] | None = None) -> int:
             ("access", args.access_cost is not None),
             ("negotiate", args.negotiate is not None),
             ("rollback", not args.rollback),
+            ("tree", not args.tree_nets),
         )
         if asked
     ]
@@ -260,6 +271,7 @@ def main(argv: list[str] | None = None) -> int:
             "negotiate": args.negotiate or 0,
             "hole_to_hole_mm": args.hole_to_hole,
             "rollback": args.rollback,
+            "tree_nets": args.tree_nets,
         }
     elif args.engine == "grid":
         options = {
@@ -272,6 +284,7 @@ def main(argv: list[str] | None = None) -> int:
             "negotiate": args.negotiate or 0,
             "hole_to_hole_mm": args.hole_to_hole,
             "rollback": args.rollback,
+            "tree_nets": args.tree_nets,
         }
     if args.engine == "room":
         result = build_rooms_routing_result(

src/freeroute/route/exact_router.py

@@ -102,6 +102,7 @@ def route_board_exact(
     access_cost: float = 0.0,
     negotiate: int = 0,
     rollback: bool = True,
+    tree_nets: bool = True,
     net_widths: dict[int, int] | None = None,
     via_drill: int | None = None,
     hole_to_hole: int = 0,
@@ -138,6 +139,7 @@ def route_board_exact(
         access_cost=access_cost,
         negotiate=negotiate,
         rollback=rollback,
+        tree_nets=tree_nets,
         net_widths=net_widths,
         via_drill=via_drill,
         hole_to_hole=hole_to_hole,
@@ -194,12 +196,22 @@ def route_board_exact(
     # 25-pin supply net costs that one connection, not the whole net.
     rejected: list[tuple[int, str, str]] = []
     ends_list = result.connection_ends or [("?", "?", "")] * len(result.connections)
+    anchors_list = getattr(result, "connection_anchors", None) or [()] * len(result.connections)
     # each net is checked at the width it will be written at: its class's
     # when all of its connections went in at that, else the default
     net_half = dict(result.net_half)
-    for (net_no, wires, vias), ends in sorted(
-        zip(result.connections, ends_list, strict=False), key=lambda c: c[0][0]
+    # a connection that ended on another's copper (a T into its net's tree)
+    # dangles if that one is dropped, so it is dropped too; connections come
+    # in commit order, anchors first
+    dropped_at: set[int] = set()
+    for pos, ((net_no, wires, vias), ends, anchors) in sorted(
+        enumerate(zip(result.connections, ends_list, anchors_list, strict=False)),
+        key=lambda c: c[1][0][0],
     ):
+        if any(a in dropped_at for a in anchors):
+            dropped_at.add(pos)
+            rejected.append((net_no, *ends))
+            continue
         wire_half = net_half.get(net_no, half_width)
         pieces = _copper_pieces(wires, vias, wire_half, via_layers, via_half)
         boxes = [(box, box_layers) for box, box_layers, _ in pieces]
@@ -220,6 +232,7 @@ def route_board_exact(
             )
         ):
             rejected.append((net_no, *ends))
+            dropped_at.add(pos)
             continue  # exact-clearance conflict -> drop (may be recovered by shove)
         acc = accepted.get(net_no)
         if acc is None:

src/freeroute/route/grid_router.py

@@ -65,7 +65,7 @@ class RouteResult:
 
     __slots__ = (
         "wires", "vias", "half_width", "unrouted", "connections", "connection_ends", "failed",
-        "net_half", "narrowed",
+        "net_half", "narrowed", "connection_anchors",
     )
 
     def __init__(self, half_width: int) -> None:
@@ -79,6 +79,9 @@ class RouteResult:
         #: what each routed connection joins, and the zone both ends sit in
         #: (or ""), parallel to ``connections``
         self.connection_ends: list[tuple[str, str, str]] = []
+        #: for each routed connection, the earlier ones (positions in
+        #: ``connections``) whose copper it ended on, parallel to ``connections``
+        self.connection_anchors: list[tuple[int, ...]] = []
         #: connections left unrouted: (net_no, one end, the other end)
         self.failed: list[tuple[int, str, str]] = []
         #: half width of each net routed wider than the default (its class)
@@ -127,6 +130,9 @@ class _Connection:
         "half",
         "stubs",
         "stub_near",
+        "anchors",
+        "copper",
+        "seq",
     )
 
     def __init__(self, index: int, net_no: int, a: Pin, b: Pin) -> None:
@@ -158,6 +164,14 @@ class _Connection:
         # routed at the default width instead; the half width it went in at
         self.narrow = False
         self.half = 0
+        # connections whose copper this one ended on (a T into its net's
+        # tree): ripping one of them leaves this one dangling
+        self.anchors: frozenset[int] = frozenset()
+        # the nodes of its committed copper (path cells, via cells on every
+        # layer), where another connection of its net may end
+        self.copper: frozenset[Node] = frozenset()
+        # commit order: a connection is always committed after its anchors
+        self.seq = 0
 
 
 class GridRouter:
@@ -188,8 +202,17 @@ class GridRouter:
         net_widths: dict[int, int] | None = None,
         via_drill: int | None = None,
         hole_to_hole: int = 0,
+        tree_nets: bool = True,
     ) -> None:
         self.board = board
+        # Multi-pin nets: pair a net's pin groups by a minimum spanning tree,
+        # and let a connection start or end on copper its net already routed
+        # (a T into the tree), not only on one chosen pin.
+        self.tree = bool(tree_nets)
+        self._dependents: dict[int, set[int]] = {}
+        self._commits = 0
+        self._plane_nets: set[int] = set()
+        self._net_conns: dict[int, list[_Connection]] = {}
         # Pin access: while a pin still has connections to route, the cells
         # its route would leave by (its end cells, the via it may need there,
         # and the clearance around both) cost other nets this much extra, so
@@ -699,6 +722,13 @@ class GridRouter:
             goal_cell = self._group_target(conn.a_group)
         starts = self._group_ends(conn.a_group, conn.net_no, conns, allow_ripup)
         goals = self._group_ends(conn.b_group, conn.net_no, conns, allow_ripup)
+        # either side may also start or end on copper its net already routed
+        a_tree = self._tree_ends(conn, conn.a_group)
+        b_tree = self._tree_ends(conn, conn.b_group)
+        for node in a_tree:
+            starts.setdefault(node, None)
+        for node in b_tree:
+            goals.setdefault(node, None)
         if not starts or not goals:
             return None
         # A start that is already an end joins at once, except that ending on
@@ -711,6 +741,7 @@ class GridRouter:
         ]
         if common:
             conn.a_end, conn.b_end = starts[common[0]], goals[common[0]]
+            conn.anchors = _anchors([common[0]], starts, goals, a_tree, b_tree)
             return [common[0]]
         net_no = conn.net_no
 
@@ -776,6 +807,7 @@ class GridRouter:
                 if ok:
                     path = _reconstruct(came_from, current)
                     conn.a_end, conn.b_end = starts.get(path[0]), end
+                    conn.anchors = _anchors(path, starts, goals, a_tree, b_tree)
                     return path
             cx, cy, cl = current
             base = g_score[current]
@@ -822,6 +854,55 @@ class GridRouter:
                         )
         return None
 
+    def _tree_ends(self, conn: _Connection, group) -> dict[Node, frozenset[int]]:
+        """Copper its net has already routed on ``group``'s side of ``conn``: a
+        route may start or end anywhere on it. Each node maps to what a route
+        ending there relies on: the connection that laid the copper and every
+        one between it and ``group`` (routed connections of a net form a
+        forest, so that path is unique). Ripping any of them would cut the
+        route off. Empty for plane nets, which keep their drops to the plane,
+        and when tree routing is off."""
+        if not self.tree or conn.net_no in self._plane_nets:
+            return {}
+        adjacent: dict[int, list[tuple[int, _Connection]]] = {}
+        for c in self._net_conns.get(conn.net_no, ()):
+            if c is conn or not c.routed:
+                continue
+            a, b = id(c.a_group), id(c.b_group)
+            adjacent.setdefault(a, []).append((b, c))
+            adjacent.setdefault(b, []).append((a, c))
+        owners: dict[Node, frozenset[int]] = {}
+        path_to: dict[int, frozenset[int]] = {id(group): frozenset()}
+        stack = [id(group)]
+        while stack:
+            here = stack.pop()
+            for other, c in adjacent.get(here, ()):
+                if other in path_to:
+                    continue
+                path = path_to[here] | {c.index}
+                path_to[other] = path
+                stack.append(other)
+                for node in c.copper:
+                    # a walled cell's via moved into the pad: no copper there
+                    if node not in self._keepout and node not in self._via_only:
+                        owners.setdefault(node, path)
+        return owners
+
+    def _with_dependents(self, victims: list[_Connection], conns: list[_Connection]) -> list[_Connection]:
+        """``victims`` and every routed connection that ended on their copper,
+        directly or through another: ripping a route rips the routes teed onto
+        it, which would otherwise dangle. Anchors come before what hangs on
+        them, as they were committed."""
+        found = {v.index: v for v in victims}
+        stack = list(victims)
+        while stack:
+            for i in self._dependents.get(stack.pop().index, ()):
+                c = conns[i]
+                if c.routed and i not in found:
+                    found[i] = c
+                    stack.append(c)
+        return sorted(found.values(), key=lambda c: (c.seq, c.index))
+
     # --- pin stubs ----------------------------------------------------------
     # A pin rarely sits on a cell centre, so every route starts and ends with a
     # short stub from the pin to its first cell. The grid has to see that
@@ -1431,6 +1512,13 @@ class GridRouter:
             # via k joins wire segments k and k + 1
             conn.vias[0 if idx == 0 else idx - 1] = location
         conn.routed = True
+        self._commits += 1
+        conn.seq = self._commits
+        conn.copper = frozenset(path) | frozenset(
+            (*cell, layer) for cell in conn.via_cells for layer in self.layers
+        )
+        for anchor in conn.anchors:
+            self._dependents.setdefault(anchor, set()).add(conn.index)
         self._settle(conn, -1)
 
     def _forget_stubs(self, conn: _Connection) -> None:
@@ -1473,6 +1561,12 @@ class GridRouter:
         conn.wires = []
         conn.vias = []
         conn.routed = False
+        for anchor in conn.anchors:
+            hanging = self._dependents.get(anchor)
+            if hanging is not None:
+                hanging.discard(conn.index)
+        conn.anchors = frozenset()
+        conn.copper = frozenset()
         conn.rip_count += 1
         self._settle(conn, +1)
 
@@ -1688,7 +1782,7 @@ class GridRouter:
                     crossed |= self._stub_owners(
                         end, node[:2], node[2], conn.net_no, conns, self._conn_half(conn)
                     )
-            victims = [conns[i] for i in sorted(crossed)]
+            victims = self._with_dependents([conns[i] for i in sorted(crossed)], conns)
             if self.rollback:
                 return self._ripup_transaction(conn, path, victims, conns)
             for victim in victims:
@@ -1699,7 +1793,7 @@ class GridRouter:
     def _ripup_transaction(self, conn, path, victims, conns) -> bool:
         """Route ``conn`` through ``victims``, reroute them at once in free
         space, and undo it all if fewer connections end up routed."""
-        saved = [(v, v.path, v.a_end, v.b_end, v.rip_count) for v in victims]
+        saved = [(v, v.path, v.a_end, v.b_end, v.rip_count, v.anchors) for v in victims]
         own_rips = conn.rip_count
         for victim in victims:
             self._rip(victim)
@@ -1717,8 +1811,8 @@ class GridRouter:
                 self._rip(victim)
         self._rip(conn)
         conn.rip_count = own_rips
-        for victim, old_path, a_end, b_end, rips in saved:
-            victim.a_end, victim.b_end = a_end, b_end
+        for victim, old_path, a_end, b_end, rips, anchors in saved:
+            victim.a_end, victim.b_end, victim.anchors = a_end, b_end, anchors
             self._commit(victim, old_path)
             victim.rip_count = rips
         return False
@@ -1740,6 +1834,7 @@ class GridRouter:
                 planes_by_net.setdefault(zone.net_nos[0], []).append(zone)
         plane_ids = frozenset(id(z) for zs in planes_by_net.values() for z in zs)
         conns: list[_Connection] = []
+        self._plane_nets: set[int] = set()
         nets = set(pins_by_net) | set(zones_by_net) | set(planes_by_net)
         # deterministic: fewer-pin nets first, then by net number
         for net_no in sorted(nets, key=lambda n: (len(pins_by_net.get(n, [])), n)):
@@ -1752,12 +1847,18 @@ class GridRouter:
                 # a plane net: every other group goes straight to the plane
                 # (usually a stub and a via), not pad to pad across the board
                 pairs = [(g, hubs[0]) for g in groups if g is not hubs[0]]
+                self._plane_nets.add(net_no)
+            elif self.tree:
+                pairs = _spanning_pairs(groups)
             else:
                 pairs = list(zip(groups, groups[1:], strict=False))
             for ga, gb in pairs:
                 conn = _Connection(len(conns), net_no, _first(ga[0]), _first(gb[0]))
                 conn.a_group, conn.b_group = ga, gb
                 conns.append(conn)
+        self._net_conns: dict[int, list[_Connection]] = {}
+        for conn in conns:
+            self._net_conns.setdefault(conn.net_no, []).append(conn)
         return conns
 
     def _fixed_pieces(self, net_no: int) -> list[_FixedPiece]:
@@ -1807,18 +1908,22 @@ class GridRouter:
         return wide, narrowed
 
     def _snapshot(self, conns: list[_Connection]) -> list[tuple]:
+        # in commit order, so a connection teed onto another comes after it;
+        # its anchors as positions in this list
+        routed = sorted((c for c in conns if c.routed), key=lambda c: (c.seq, c.index))
+        position = {c.index: i for i, c in enumerate(routed)}
         return [
             (c.net_no, [(layer, list(pts)) for layer, pts in c.wires], list(c.vias),
-             (*_ends_of(c), self._left_to_zone(c)))
-            for c in conns
-            if c.routed
+             (*_ends_of(c), self._left_to_zone(c)),
+             tuple(sorted(position[a] for a in c.anchors if a in position)))
+            for c in routed
         ]
 
     def _result_from(self, snapshot, unrouted: int) -> RouteResult:
         result = RouteResult(self.half_width)
         written: set[tuple[int, IntPoint]] = set()
         result.unrouted = unrouted
-        for net_no, wires, vias, ends in snapshot:
+        for net_no, wires, vias, ends, anchors in snapshot:
             for layer, points in wires:
                 result.add_wire(net_no, layer, points)
             for location in vias:
@@ -1829,7 +1934,8 @@ class GridRouter:
                 result.add_via(net_no, location)
             result.connections.append((net_no, wires, vias))
             result.connection_ends.append(ends)
-        routed = {ends[:2] + (net_no,) for net_no, _w, _v, ends in snapshot}
+            result.connection_anchors.append(anchors)
+        routed = {ends[:2] + (net_no,) for net_no, _w, _v, ends, _a in snapshot}
         result.failed = [
             (c.net_no, *_ends_of(c), self._left_to_zone(c))
             for c in self._conns
@@ -1842,6 +1948,53 @@ def _first(items):
     return items[0] if items else None
 
 
+def _anchors(path, starts, goals, a_tree, b_tree) -> frozenset[int]:
+    """What a found path relies on: the connections between the copper it
+    starts or ends on and its own groups."""
+    found: set[int] = set()
+    if starts.get(path[0]) is None and path[0] in a_tree:
+        found |= a_tree[path[0]]
+    if goals.get(path[-1]) is None and path[-1] in b_tree:
+        found |= b_tree[path[-1]]
+    return frozenset(found)
+
+
+def _group_points(group) -> list[tuple[int, int]]:
+    pins, zones = group
+    if pins:
+        return [(p.location.x, p.location.y) for p in pins]
+    points = []
+    for zone in zones:
+        box = zone.bounding_box()
+        points.append(((box.ll.x + box.ur.x) // 2, (box.ll.y + box.ur.y) // 2))
+    return points or [(0, 0)]
+
+
+def _spanning_pairs(groups) -> list[tuple]:
+    """A minimum spanning tree over a net's groups (Prim, Manhattan distance
+    between their nearest pins), as (in the tree, joining it) pairs: each
+    connection joins the nearest group not yet joined, not the next in pin
+    order."""
+    if len(groups) < 2:
+        return []
+    points = [_group_points(g) for g in groups]
+
+    def gap(i: int, j: int) -> int:
+        return min(abs(ax - bx) + abs(ay - by) for ax, ay in points[i] for bx, by in points[j])
+
+    best = {j: (gap(0, j), 0) for j in range(1, len(groups))}
+    pairs = []
+    while best:
+        j = min(best, key=lambda k: (best[k][0], k))
+        _, i = best.pop(j)
+        pairs.append((groups[i], groups[j]))
+        for k in best:
+            d = gap(j, k)
+            if d < best[k][0]:
+                best[k] = (d, j)
+    return pairs
+
+
 def _group_label(group) -> str:
     pins, zones = group
     parts = [f"{p.component_ref}-{p.name}" for p in pins[:3]]
@@ -2039,6 +2192,7 @@ def route_board(
     access_cost: float = 0.0,
     negotiate: int = 0,
     rollback: bool = True,
+    tree_nets: bool = True,
     net_widths: dict[int, int] | None = None,
     via_drill: int | None = None,
     hole_to_hole: int = 0,
@@ -2062,6 +2216,7 @@ def route_board(
         access_cost=access_cost,
         negotiate=negotiate,
         rollback=rollback,
+        tree_nets=tree_nets,
         net_widths=net_widths,
         via_drill=via_drill,
         hole_to_hole=hole_to_hole,

src/freeroute/route/pipeline.py

@@ -121,6 +121,7 @@ def route_dsn_board(
     negotiate: int = 0,
     hole_to_hole_mm: float = 0.25,
     rollback: bool = True,
+    tree_nets: bool = True,
 ) -> tuple[RouteResult, int, list[str]]:
     """Route a parsed :class:`DsnBoard`; return the board-unit result, the scale,
     and the layer names. ``layers`` restricts routing to those signal-layer
@@ -146,6 +147,7 @@ def route_dsn_board(
         access_cost=access_cost,
         negotiate=negotiate,
         rollback=rollback,
+        tree_nets=tree_nets,
         net_widths=_net_widths_board(dsn, scale),
         via_drill=_via_drill_board(dsn, scale),
         hole_to_hole=_mm_to_board(dsn, scale, hole_to_hole_mm) or 0,
@@ -201,6 +203,7 @@ def build_routing_result(
     negotiate: int = 0,
     hole_to_hole_mm: float = 0.25,
     rollback: bool = True,
+    tree_nets: bool = True,
 ) -> RoutingResult:
     """Route ``dsn`` (grid track) and convert to a DSN-unit RoutingResult."""
     route, scale, layer_names = route_dsn_board(
@@ -211,6 +214,7 @@ def build_routing_result(
         negotiate=negotiate,
         hole_to_hole_mm=hole_to_hole_mm,
         rollback=rollback,
+        tree_nets=tree_nets,
     )
     return _to_routing_result(route, dsn, scale, layer_names)
 
@@ -232,6 +236,7 @@ def route_dsn_board_exact(
     negotiate: int = 0,
     hole_to_hole_mm: float = 0.25,
     rollback: bool = True,
+    tree_nets: bool = True,
 ) -> tuple[ExactRouteResult, int, list[str]]:
     """Route a parsed :class:`DsnBoard` with the exact-geometry (orthogonal,
     DRC-verified) track; return the exact result, the scale, and layer names.
@@ -261,6 +266,7 @@ def route_dsn_board_exact(
         access_cost=access_cost,
         negotiate=negotiate,
         rollback=rollback,
+        tree_nets=tree_nets,
         net_widths=_net_widths_board(dsn, scale),
         via_drill=_via_drill_board(dsn, scale),
         hole_to_hole=_mm_to_board(dsn, scale, hole_to_hole_mm) or 0,
@@ -285,6 +291,7 @@ def build_exact_routing_result(
     negotiate: int = 0,
     hole_to_hole_mm: float = 0.25,
     rollback: bool = True,
+    tree_nets: bool = True,
 ) -> RoutingResult:
     """Route ``dsn`` (exact track) and convert to a DSN-unit RoutingResult."""
     exact, scale, layer_names = route_dsn_board_exact(
@@ -303,6 +310,7 @@ def build_exact_routing_result(
         negotiate=negotiate,
         hole_to_hole_mm=hole_to_hole_mm,
         rollback=rollback,
+        tree_nets=tree_nets,
     )
     return _to_routing_result(exact.result, dsn, scale, layer_names)
 

tests/route/test_tree_nets.py

@@ -0,0 +1,170 @@
+"""Multi-pin nets: spanning-tree pairs, and connections that end on copper
+their net has already routed (a T into the tree)."""
+
+from __future__ import annotations
+
+import pytest
+
+from freeroute.board import build_board
+from freeroute.dsn import parse_dsn
+from freeroute.route import exact_router
+from freeroute.route.grid_router import GridRouter, RouteResult
+from freeroute.route.pipeline import _rule_clearance_dsn, _rule_width_dsn
+
+# P1 and P2 are nearest each other, so the tree pairs them first; P3 sits
+# under the middle of that leg, 50 mm below it and 90 mm from either pin.
+TEE = """(pcb "tee.dsn"
+  (parser (string_quote ") (space_in_quoted_tokens on) (host_cad "t") (host_version "1"))
+  (resolution um 10)
+  (unit um)
+  (structure
+    (layer F.Cu (type signal) (property (index 0)))
+    (boundary (path pcb 0  0 0  200000 0  200000 -100000  0 -100000  0 0))
+    (rule (width 2000) (clearance 2000))
+  )
+  (library
+    (padstack Rect_Pad (shape (rect F.Cu -1000 -1000 1000 1000)) (attach off))
+    (image PAD (pin Rect_Pad 1 0 0))
+  )
+  (placement
+    (component PAD
+      (place P1 60000 -20000 front 0)
+      (place P2 140000 -20000 front 0)
+      (place P3 100000 -70000 front 0)
+    )
+  )
+  (network
+    (net N (pins P1-1 P2-1 P3-1))
+    (class default (rule (width 2000) (clearance 2000)))
+  )
+  (wiring)
+)
+"""
+
+
+def _router(tree_nets=True, text=TEE):
+    dsn = parse_dsn(text)
+    s = dsn.resolution.value
+    return GridRouter(
+        build_board(dsn),
+        trace_width=round(_rule_width_dsn(dsn) * s),
+        clearance=round(_rule_clearance_dsn(dsn) * s),
+        orthogonal=True,
+        tree_nets=tree_nets,
+    )
+
+
+def test_pairs_follow_a_spanning_tree_not_the_pin_order():
+    router = _router()
+    conns = router._build_connections()
+    pairs = [(c.a.component_ref, c.b.component_ref) for c in conns]
+    assert pairs == [("P1", "P2"), ("P1", "P3")]
+    chain = _router(tree_nets=False)._build_connections()
+    assert [(c.a.component_ref, c.b.component_ref) for c in chain] == [("P1", "P2"), ("P2", "P3")]
+
+
+def test_the_third_pin_tees_into_the_first_leg():
+    router = _router()
+    result = router.route()
+    first, second = router._conns
+    assert result.unrouted == 0
+    # the second connection starts on the first one's copper, mid-leg
+    start = second.path[0]
+    assert start in first.copper and second.a_end is None
+    assert second.anchors == {first.index}
+    assert router._dependents[first.index] == {second.index}
+    assert abs(router._cell_center(start[:2]).x - 1_000_000) <= router.step
+    # and so its copper is the 50 mm drop, not 90 mm back to a pin
+    assert len(second.path) < len(router._conns[0].path)
+
+
+def test_without_trees_each_connection_ends_on_a_pin():
+    router = _router(tree_nets=False)
+    router.route()
+    assert all(c.anchors == frozenset() for c in router._conns)
+    assert all(c.a_end is not None and c.b_end is not None for c in router._conns)
+
+
+def test_ripping_an_anchor_takes_what_hangs_on_it():
+    router = _router()
+    router.route()
+    first, second = router._conns
+    victims = router._with_dependents([first], router._conns)
+    assert victims == [first, second]  # anchors first, as committed
+    for victim in victims:
+        router._rip(victim)
+    assert not first.routed and not second.routed
+    assert not router._dependents.get(first.index)
+    assert not router._cell_owner
+
+
+def test_the_snapshot_lists_anchors_before_what_hangs_on_them():
+    router = _router()
+    result = router.route()
+    assert result.connection_anchors == [(), (0,)]
+
+
+def test_the_exact_engine_drops_a_tee_whose_anchor_it_rejects(monkeypatch):
+    """A connection teed onto copper the exact check refuses would dangle."""
+    dsn = parse_dsn(TEE)
+    board = build_board(dsn)
+    real = exact_router.route_board
+
+    def grid_then_spoil(*args, **kwargs):
+        result: RouteResult = real(*args, **kwargs)
+        assert result.connection_anchors == [(), (0,)]
+        # move the first leg off the board edge so the exact check rejects it
+        net, wires, vias = result.connections[0]
+        result.connections[0] = (net, [(layer, [type(p)(p.x, p.y + 2_000_000) for p in pts])
+                                       for layer, pts in wires], vias)
+        return result
+
+    monkeypatch.setattr(exact_router, "route_board", grid_then_spoil)
+    s = dsn.resolution.value
+    exact = exact_router.route_board_exact(
+        board,
+        trace_width=round(_rule_width_dsn(dsn) * s),
+        clearance=round(_rule_clearance_dsn(dsn) * s),
+    )
+    assert len(exact.result.failed) == 2  # the leg and the T hanging on it
+    assert not exact.result.wires
+
+
+@pytest.mark.parametrize("tree_nets", [True, False])
+def test_occupancy_stays_honest(tree_nets):
+    router = _router(tree_nets)
+    router.route()
+    conns = router._conns
+    assert all(conns[i].routed for i in router._cell_owner.values())
+    for anchor, hanging in router._dependents.items():
+        for i in hanging:
+            assert conns[i].routed and conns[anchor].routed
+
+
+# P1-P2 along the top, P3 far off to the lower right, P4 just under the
+# middle of the P1-P2 leg.
+CHAIN_TEE = TEE.replace(
+    """      (place P3 100000 -70000 front 0)
+""",
+    """      (place P3 190000 -90000 front 0)
+      (place P4 100000 -40000 front 0)
+""",
+).replace("(pins P1-1 P2-1 P3-1)", "(pins P1-1 P2-1 P3-1 P4-1)")
+
+
+def test_a_tee_depends_on_every_connection_back_to_its_group():
+    """Joined as P1-P2, P2-P3, P3-P4, P4's connection tees onto P1-P2 copper,
+    which reaches P3's group only through P2-P3. Ripping P2-P3 must take the T
+    too, or it would be counted routed while cut off from P3."""
+    router = _router(text=CHAIN_TEE)
+    conns = router._conns = router._build_connections()
+    groups = {c.a.component_ref: c.a_group for c in conns}
+    groups.update({c.b.component_ref: c.b_group for c in conns})
+    for conn, (a, b) in zip(conns, (("P1", "P2"), ("P2", "P3"), ("P3", "P4")), strict=True):
+        conn.a_group, conn.b_group = groups[a], groups[b]
+    for conn in conns:
+        assert router._try_route(conn, conns)
+    first, second, third = conns
+    assert third.path[0] in first.copper  # the T lands on the far leg
+    assert third.anchors == {first.index, second.index}
+    assert router._with_dependents([second], conns) == [second, third]

tests/test_cli.py

@@ -160,6 +160,7 @@ def test_cli_supported_combinations_are_accepted(tmp_path):
         "access": ["--access-cost", "5"],
         "negotiate": ["--negotiate", "2"],
         "rollback": ["--no-rollback"],
+        "tree": ["--no-tree"],
     }
     for engine, options in ENGINE_OPTIONS.items():
         for option in options: