diff --git a/zigux/tests/phase1_rbtree_cached_bench_replay.zig b/zigux/tests/phase1_rbtree_cached_bench_replay.zig
new file mode 100644
index 00000000000000..9c1532cb789bd4
--- /dev/null
+++ b/zigux/tests/phase1_rbtree_cached_bench_replay.zig
@@ -0,0 +1,119 @@
+const std = @import("std");
+const rbtree = @import("rbtree");
+
+const iterations_rbtree = 4_000;
+
+const RbEntry = struct {
+    key: i32,
+    node: rbtree.Node = .{},
+};
+
+fn less(a: *const rbtree.Node, b: *const rbtree.Node) bool {
+    const lhs: *const RbEntry = @fieldParentPtr("node", a);
+    const rhs: *const RbEntry = @fieldParentPtr("node", b);
+    return lhs.key < rhs.key;
+}
+
+const CachedReplay = struct {
+    checksum: u64,
+    initial_leftmost_key: i32,
+    erase_non_leftmost_returned_leftmost: bool,
+    promoted_leftmost_key: i32,
+    replacement_leftmost_key: i32,
+    inserted_leftmost_key: i32,
+    add_cached_leftmost_keys: [3]i32,
+};
+
+fn runCachedReplay() CachedReplay {
+    var cached_entries = [_]RbEntry{
+        .{ .key = 10 },
+        .{ .key = 5 },
+        .{ .key = 20 },
+        .{ .key = 15 },
+    };
+    var cached_replacement = RbEntry{ .key = 10 };
+    var new_leftmost = RbEntry{ .key = 3 };
+    var cached_root = rbtree.RootCached.init();
+    var add_cached_leftmost_keys = [_]i32{ -1, -1, -1 };
+    var add_cached_leftmost_count: usize = 0;
+
+    for (&cached_entries) |*entry| {
+        if (rbtree.addCached(&entry.node, &cached_root, less)) |leftmost| {
+            const leftmost_entry: *const RbEntry = @fieldParentPtr("node", leftmost);
+            add_cached_leftmost_keys[add_cached_leftmost_count] = leftmost_entry.key;
+            add_cached_leftmost_count += 1;
+        }
+    }
+
+    const initial_leftmost_entry: *const RbEntry = @fieldParentPtr(
+        "node",
+        rbtree.firstCached(&cached_root).?,
+    );
+    const erase_non_leftmost_returned_leftmost =
+        rbtree.eraseCached(&cached_entries[2].node, &cached_root) == null;
+    const still_leftmost_entry: *const RbEntry = @fieldParentPtr(
+        "node",
+        rbtree.firstCached(&cached_root).?,
+    );
+    std.debug.assert(still_leftmost_entry.key == initial_leftmost_entry.key);
+
+    const promoted_leftmost = rbtree.eraseCached(&cached_entries[1].node, &cached_root) orelse unreachable;
+    const promoted_leftmost_entry: *const RbEntry = @fieldParentPtr("node", promoted_leftmost);
+
+    rbtree.replaceNodeCached(&cached_entries[0].node, &cached_replacement.node, &cached_root);
+    const replacement_leftmost_entry: *const RbEntry = @fieldParentPtr(
+        "node",
+        rbtree.firstCached(&cached_root).?,
+    );
+
+    if (rbtree.addCached(&new_leftmost.node, &cached_root, less)) |leftmost| {
+        const leftmost_entry: *const RbEntry = @fieldParentPtr("node", leftmost);
+        add_cached_leftmost_keys[add_cached_leftmost_count] = leftmost_entry.key;
+        add_cached_leftmost_count += 1;
+    }
+    const inserted_leftmost_entry: *const RbEntry = @fieldParentPtr(
+        "node",
+        rbtree.firstCached(&cached_root).?,
+    );
+
+    var checksum: u64 = 0;
+    checksum +%= @as(u64, @intFromBool(rbtree.first(&cached_root.root) == rbtree.firstCached(&cached_root)));
+    checksum +%= @intCast(initial_leftmost_entry.key);
+    checksum +%= @as(u64, @intFromBool(erase_non_leftmost_returned_leftmost));
+    checksum +%= @intCast(still_leftmost_entry.key);
+    checksum +%= @intCast(promoted_leftmost_entry.key);
+    checksum +%= @as(u64, @intFromBool(rbtree.first(&cached_root.root) == rbtree.firstCached(&cached_root)));
+    checksum +%= @intCast(replacement_leftmost_entry.key);
+    checksum +%= @intCast(inserted_leftmost_entry.key);
+    checksum +%= @as(u64, @intFromBool(rbtree.first(&cached_root.root) == rbtree.firstCached(&cached_root)));
+
+    return .{
+        .checksum = checksum,
+        .initial_leftmost_key = initial_leftmost_entry.key,
+        .erase_non_leftmost_returned_leftmost = erase_non_leftmost_returned_leftmost,
+        .promoted_leftmost_key = promoted_leftmost_entry.key,
+        .replacement_leftmost_key = replacement_leftmost_entry.key,
+        .inserted_leftmost_key = inserted_leftmost_entry.key,
+        .add_cached_leftmost_keys = add_cached_leftmost_keys,
+    };
+}
+
+test "phase1 rbtree cached bench replay keeps leftmost-return semantics explicit" {
+    const replay = runCachedReplay();
+    try std.testing.expectEqual(@as(i32, 5), replay.initial_leftmost_key);
+    try std.testing.expect(replay.erase_non_leftmost_returned_leftmost);
+    try std.testing.expectEqual(@as(i32, 10), replay.promoted_leftmost_key);
+    try std.testing.expectEqual(@as(i32, 10), replay.replacement_leftmost_key);
+    try std.testing.expectEqual(@as(i32, 3), replay.inserted_leftmost_key);
+    try std.testing.expectEqual([3]i32{ 10, 5, 3 }, replay.add_cached_leftmost_keys);
+    try std.testing.expectEqual(@as(u64, 37), replay.checksum);
+}
+
+test "phase1 rbtree cached bench replay matches the bench checksum packet" {
+    var checksum: u64 = 0;
+    var idx: usize = 0;
+    while (idx < iterations_rbtree) : (idx += 1) {
+        checksum +%= runCachedReplay().checksum;
+    }
+    try std.testing.expectEqual(@as(u64, 148_000), checksum);
+}
diff --git a/zigux/tests/phase1_rbtree_cached_bench_replay_build.zig b/zigux/tests/phase1_rbtree_cached_bench_replay_build.zig
new file mode 100644
index 00000000000000..481d71d9d58c10
--- /dev/null
+++ b/zigux/tests/phase1_rbtree_cached_bench_replay_build.zig
@@ -0,0 +1,39 @@
+const std = @import("std");
+
+pub fn build(b: *std.Build) void {
+    const target = b.standardTargetOptions(.{});
+    const optimize = b.standardOptimizeOption(.{});
+
+    const root_module = b.createModule(.{
+        .root_source_file = b.path("phase1_rbtree_cached_bench_replay.zig"),
+        .target = target,
+        .optimize = optimize,
+    });
+    const rbtree_module = b.createModule(.{
+        .root_source_file = b.path("../../tools/lib/rbtree.zig"),
+        .target = target,
+        .optimize = optimize,
+    });
+
+    root_module.addImport("rbtree", rbtree_module);
+
+    const replay_tests = b.addTest(.{
+        .name = "phase1-rbtree-cached-bench-replay",
+        .root_module = root_module,
+    });
+    const run_replay = b.addRunArtifact(replay_tests);
+
+    const replay_step = b.step(
+        "phase1-rbtree-cached-bench-replay",
+        "Run the focused Phase 1 rbtree cached bench replay from zigux/tests",
+    );
+    replay_step.dependOn(&run_replay.step);
+
+    const test_step = b.step(
+        "test",
+        "Run the focused Phase 1 rbtree cached bench replay from zigux/tests",
+    );
+    test_step.dependOn(&run_replay.step);
+
+    b.default_step.dependOn(test_step);
+}