diff --git a/zig/src/core/lanes/fill_test.zig b/zig/src/core/lanes/fill_test.zig
new file mode 100644
index 0000000000..57768e3d53
--- /dev/null
+++ b/zig/src/core/lanes/fill_test.zig
@@ -0,0 +1,117 @@
+//! Suffix-match grafts (lanes/fill.zig) on the fake target: a shared round that trims a grafted window stays exact.
+const std = @import("std");
+const Config = @import("config.zig").Config;
+const Cost = @import("config.zig").Cost;
+const Engine = @import("engine.zig").Engine;
+const sm = @import("stream.zig");
+const fake = @import("fake.zig");
+const win = @import("windows.zig");
+const SuffixLookup = @import("proposer.zig").SuffixLookup;
+
+const gpa = std.testing.allocator;
+
+/// The drafts the grafts follow: the head's, held by the target (no proposer, as the Metal CLI's streams), or copies.
+const Trunk = enum { held, copied };
+
+const Run = struct {
+    replies: [][]u32,
+    reasons: []sm.Reason,
+    drafted: u64, // draft rows verified, all streams
+    branch_rows: u64, // of those, grafted rows
+    branch_kept: u64, // of those, rows on the accepted path
+
+    fn deinit(r: Run) void {
+        for (r.replies) |x| gpa.free(x);
+        gpa.free(r.replies);
+        gpa.free(r.reasons);
+    }
+};
+
+fn run(trunk: Trunk, drafts: bool, n: usize, prompt: []const u32) !Run {
+    // the cost of each row, not the 32-row cap, is what makes the allocator cut the held set's windows
+    var costs: [16]Cost = undefined;
+    for (&costs, 1..) |*c, w| c.* = .{ .width = @intCast(w), .ms = 5.0 + 0.8 * @as(f64, @floatFromInt(w)) };
+    const head = drafts and trunk == .held;
+    // a copied trunk shares 4 rows a round, so the allocator cuts into it
+    const rows: u32 = if (trunk == .held) 32 else 4;
+    var cfg = try Config.init(gpa, .{ .exact_width = 16, .gpu_tokens = true, .mtp = head, .speculate = head, .speculate_early = false, .drafts = 4, .window_costs = &costs, .mtp_step_ms = 0.5, .hidden_rows = true, .batch_rows = rows, .max_streams = 8, .draft_streams = true }, 16, 15);
+    defer cfg.deinit(gpa);
+    cfg.fill_lanes = 9; // the Metal CLI's --lanes 8
+    // the reply repeats from its first token, so suffix matches find branches; the head ranks its held chain
+    var target: fake.Fake = .{ .gpa = gpa, .cycle_after = 0, .probe_prompt = prompt.len, .ranked = true };
+    defer target.deinit();
+    var clock: fake.FixedClock = .{};
+    var engine = Engine.init(gpa, &cfg, target.backend(), clock.clock());
+    defer engine.deinit();
+    const streams = try gpa.alloc(sm.Stream, n);
+    defer gpa.free(streams);
+    const proposers = try gpa.alloc(SuffixLookup, n);
+    defer gpa.free(proposers);
+    for (streams, proposers) |*s, *p| {
+        p.* = try SuffixLookup.init(gpa, .{ .min_match = 4 });
+        const proposer = if (trunk == .copied) p.proposer() else null;
+        s.* = try sm.Stream.init(gpa, .{ .id = "s", .prompt = prompt, .max_new = 120, .eos = &.{96}, .drafts = drafts, .proposer = proposer });
+    }
+    defer for (streams, proposers) |*s, *p| {
+        s.deinit(gpa);
+        p.deinit();
+    };
+    for (streams) |*s| try engine.addStream(s);
+    while (engine.activeCount() > 0) try engine.step();
+    var out: Run = .{ .replies = try gpa.alloc([]u32, n), .reasons = try gpa.alloc(sm.Reason, n), .drafted = 0, .branch_rows = 0, .branch_kept = 0 };
+    for (out.replies, out.reasons, streams) |*r, *why, *s| {
+        r.* = try gpa.dupe(u32, s.emitted());
+        why.* = s.reason;
+        out.drafted += s.drafted;
+        out.branch_rows += s.branch_rows;
+        out.branch_kept += s.branch_accepted;
+    }
+    return out;
+}
+
+test "a shared round that trims a window with suffix-match grafts replies as the one-token decode" {
+    const prompt = [_]u32{ 3, 1, 4, 1, 5, 9, 2, 6, 5, 3, 5, 8, 9, 7, 9, 3, 2, 3, 8, 4 };
+    for ([_]Trunk{ .held, .copied }) |trunk| {
+        const want = try run(trunk, false, 1, &prompt);
+        defer want.deinit();
+        const got = try run(trunk, true, 2, &prompt);
+        defer got.deinit();
+        try std.testing.expect(got.branch_kept > 0 and got.branch_kept <= got.branch_rows and got.branch_rows <= got.drafted);
+        for (got.replies, got.reasons) |r, why| {
+            try std.testing.expectEqualSlices(u32, want.replies[0], r);
+            try std.testing.expectEqual(want.reasons[0], why);
+        }
+    }
+}
+
+test "allocate keeps a grafted window's first drafts at every count, the trunk's before the grafts" {
+    // flat costs, so a round fills its batch rows and the window's count is batch_rows - 1
+    var costs: [16]Cost = undefined;
+    for (&costs, 1..) |*c, w| c.* = .{ .width = @intCast(w), .ms = 5.0 };
+    const Window = struct { kind: win.Kind, held: u32, tokens: []const u32, parents: []const i32 };
+    const windows = [_]Window{
+        // three held drafts, then grafts: one after the last held draft, one off the first held draft and its child
+        .{ .kind = .head, .held = 3, .tokens = &.{ 7, 8, 9 }, .parents = &.{ -1, 0, 1, 2, 0, 4 } },
+        // three copied drafts, then grafts past their end: one after the last, its child and that child's sibling
+        .{ .kind = .copy, .held = 0, .tokens = &.{ 4, 1, 5, 7, 8, 9 }, .parents = &.{ -1, 0, 1, 2, 3, 3 } },
+    };
+    for (windows) |t| for (1..1 + t.parents.len) |c| {
+        var cfg = try Config.init(gpa, .{ .exact_width = 16, .gpu_tokens = true, .mtp = true, .speculate = true, .speculate_early = false, .drafts = 4, .window_costs = &costs, .mtp_step_ms = 0.5, .hidden_rows = true, .batch_rows = @intCast(c + 1), .max_streams = 8, .draft_streams = true }, 16, 15);
+        defer cfg.deinit(gpa);
+        var target: fake.Fake = .{ .gpa = gpa };
+        defer target.deinit();
+        var clock: fake.FixedClock = .{};
+        var engine = Engine.init(gpa, &cfg, target.backend(), clock.clock());
+        defer engine.deinit();
+        var s = try sm.Stream.init(gpa, .{ .id = "s", .prompt = &.{ 3, 1, 4 }, .max_new = 64, .drafts = true });
+        defer s.deinit(gpa);
+        var plans = [_]win.Plan{.{ .stream = &s, .position = 3, .kind = t.kind, .held = t.held, .tokens = t.tokens, .parents = t.parents, .branch_from = 4 }};
+        try win.allocate(&engine, &plans);
+        const p = plans[0];
+        try std.testing.expectEqual(c, p.count());
+        try std.testing.expectEqual(@min(t.held, c), p.held);
+        try std.testing.expectEqualSlices(u32, t.tokens[0 .. c - @min(t.held, c)], p.tokens);
+        try std.testing.expectEqualSlices(i32, t.parents[0..c], p.parents.?);
+        try std.testing.expectEqual(@as(u32, if (c > 3) 4 else 0), p.branch_from);
+    };
+}
diff --git a/zig/src/core/lanes/lanes.zig b/zig/src/core/lanes/lanes.zig
index e943bf9d46..48c1c61242 100644
--- a/zig/src/core/lanes/lanes.zig
+++ b/zig/src/core/lanes/lanes.zig
@@ -45,6 +45,7 @@ test {
     _ = @import("gpu_full_test.zig");
     _ = @import("drafted_test.zig");
     _ = @import("pricing_test.zig");
+    _ = @import("fill_test.zig");
 }
 
 pub const learned_disk = @import("learned_disk.zig");
diff --git a/zig/src/core/lanes/windows.zig b/zig/src/core/lanes/windows.zig
index 36200960d2..11a029d171 100644
--- a/zig/src/core/lanes/windows.zig
+++ b/zig/src/core/lanes/windows.zig
@@ -230,9 +230,16 @@ pub fn allocate(e: *Engine, plans: []Plan) !void {
             p.tokens = &.{};
             p.parents = null;
         } else if (p.parents) |parents| {
-            const tree = try accept.sanitizeTree(a, p.tokens, parents, c);
-            p.tokens = tree.tokens;
+            // the held drafts lead the window (placeholders here), so the first kept rows stay held
+            const drafts = try a.alloc(u32, p.count());
+            @memset(drafts[0..p.held], 0);
+            @memcpy(drafts[p.held..], p.tokens);
+            const tree = try accept.sanitizeTree(a, drafts, parents, c);
+            const held = @min(p.held, c);
+            p.held = held;
+            p.tokens = tree.tokens[held..];
             p.parents = tree.parents;
+            if (c < p.branch_from) p.branch_from = 0; // the grafts follow the trunk's drafts: this count keeps none
         } else if (p.held > 0) {
             p.held = c;
         } else p.tokens = p.tokens[0..c];