diff --git a/tests/gf16_roundtrip.zig b/tests/gf16_roundtrip.zig
new file mode 100644
index 0000000000..7d94770710
--- /dev/null
+++ b/tests/gf16_roundtrip.zig
@@ -0,0 +1,168 @@
+const std = @import("std");
+
+const GF16 = packed struct(u16) {
+    mant: u9,
+    exp: u6,
+    sign: u1,
+
+    pub fn fromF64(v: f64) GF16 {
+        if (v == 0.0) return .{ .mant = 0, .exp = 0, .sign = 0 };
+        if (!std.math.isFinite(v)) return .{ .mant = 0, .exp = 0x3F, .sign = @intFromBool(v < 0) };
+
+        const sign_bit: u1 = @intFromBool(v < 0);
+        const abs_v = @abs(v);
+
+        var exp: i16 = 0;
+        var mant_f = abs_v;
+        while (mant_f >= 1.0 and exp < 31) : (exp += 1) mant_f /= 2.0;
+        while (mant_f < 0.5 and exp > -32) : (exp -= 1) mant_f *= 2.0;
+
+        const exp_bias: i16 = 31;
+        const exp_u6: u6 = @intCast(std.math.clamp(exp_bias + exp, 0, 63));
+        const mant_u9: u9 = @intFromFloat(std.math.clamp((mant_f - 0.5) * 512.0, 0, 511));
+
+        return .{ .mant = mant_u9, .exp = exp_u6, .sign = sign_bit };
+    }
+
+    pub fn toF64(self: GF16) f64 {
+        if (self.exp == 0 and self.mant == 0) return if (self.sign == 1) -0.0 else 0.0;
+        if (self.exp == 0x3F) return if (self.sign == 1) -std.math.inf(f64) else std.math.inf(f64);
+
+        const exp_unbiased: f64 = @floatFromInt(@as(i32, self.exp) - 31);
+        const mant_f: f64 = 0.5 + @as(f64, @floatFromInt(self.mant)) / 512.0;
+        const value = mant_f * std.math.pow(f64, 2.0, exp_unbiased);
+        return if (self.sign == 1) -value else value;
+    }
+};
+
+const ValidationResult = struct {
+    seed: u64,
+    n_weights: usize,
+    max_roundtrip_error: f64,
+    avg_roundtrip_error: f64,
+    model_bytes_gf16: usize,
+    model_bytes_fp32: usize,
+    compression_ratio: f64,
+    all_below_threshold: bool,
+};
+
+fn validateSeed(allocator: std.mem.Allocator, seed: u64, n_weights: usize) !ValidationResult {
+    var rng = std.Random.DefaultPrng.init(seed);
+
+    var weights = try allocator.alloc(f64, n_weights);
+    defer allocator.free(allocator);
+    for (weights) |*w| {
+        w.* = rng.random().floatNorm(f64) * 0.5;
+    }
+
+    var max_err: f64 = 0;
+    var sum_err: f64 = 0;
+
+    for (weights) |w| {
+        const encoded = GF16.fromF64(w);
+        const decoded = encoded.toF64();
+        const err = @abs(w - decoded);
+        sum_err += err;
+        if (err > max_err) max_err = err;
+    }
+
+    const avg_err = sum_err / @as(f64, @floatFromInt(n_weights));
+    const gf16_bytes = n_weights * 2;
+    const fp32_bytes = n_weights * 4;
+
+    return ValidationResult{
+        .seed = seed,
+        .n_weights = n_weights,
+        .max_roundtrip_error = max_err,
+        .avg_roundtrip_error = avg_err,
+        .model_bytes_gf16 = gf16_bytes,
+        .model_bytes_fp32 = fp32_bytes,
+        .compression_ratio = @as(f64, @floatFromInt(fp32_bytes)) / @as(f64, @floatFromInt(gf16_bytes)),
+        .all_below_threshold = max_err < 1e-6,
+    };
+}
+
+pub fn main() !void {
+    var gpa = std.heap.GeneralPurposeAllocator(.{}){};
+    defer _ = gpa.deinit();
+    const allocator = gpa.allocator();
+    const stdout = std.io.getStdOut().writer();
+
+    const n_weights: usize = 1950000;
+    const seeds = [_]u64{ 42, 43, 44, 45, 46 };
+    const max_model_bytes: usize = 16 * 1024 * 1024;
+
+    stdout.print("\n{s}\n", .{"=" * 72}) catch {};
+    stdout.print("  GF16 Validation — Encode/Decode Roundtrip + Size Check\n", .{});
+    stdout.print("  Model: HSLM-1.95M ({d} weights)\n", .{n_weights});
+    stdout.print("  Threshold: roundtrip error < 1e-6\n", .{});
+    stdout.print("  Size limit: {d} MB\n", .{max_model_bytes / (1024 * 1024)});
+    stdout.print("{s}\n\n", .{"=" * 72}) catch {};
+
+    var all_pass = true;
+    var results = std.ArrayList(ValidationResult).init(allocator);
+    defer results.deinit();
+
+    for (&seeds) |seed| {
+        const r = try validateSeed(allocator, seed, n_weights);
+        try results.append(r);
+
+        const status = if (r.all_below_threshold) "PASS" else "FAIL";
+        stdout.print("  Seed {d}: max_err={e:.3} avg_err={e:.6} size={d}MB [{s}]\n", .{
+            seed,
+            r.max_roundtrip_error,
+            r.avg_roundtrip_error,
+            r.model_bytes_gf16 / (1024 * 1024),
+            status,
+        }) catch {};
+
+        if (!r.all_below_threshold) all_pass = false;
+    }
+
+    const total_gf16_mb = (n_weights * 2) / (1024 * 1024);
+    const size_pass = total_gf16_mb < max_model_bytes / (1024 * 1024);
+
+    stdout.print("\n{s}\n", .{"=" * 72}) catch {};
+    stdout.print("  RESULTS\n", .{});
+    stdout.print("{s}\n", .{"-" * 72}) catch {};
+    stdout.print("  Roundtrip (all seeds):      {s}\n", .{if (all_pass) "PASS" else "FAIL"}) catch {};
+    stdout.print("  GF16 model size:            {d} MB (limit: {d} MB)\n", .{ total_gf16_mb, max_model_bytes / (1024 * 1024) }) catch {};
+    stdout.print("  Size check:                 {s}\n", .{if (size_pass) "PASS" else "FAIL"}) catch {};
+    stdout.print("  Compression vs FP32:        {d:.1}x\n", .{2.0}) catch {};
+    stdout.print("  Overall:                    {s}\n", .{if (all_pass and size_pass) "ALL CHECKS PASS" else "SOME CHECKS FAILED"}) catch {};
+    stdout.print("{s}\n\n", .{"=" * 72}) catch {};
+
+    if (!all_pass or !size_pass) std.process.exit(1);
+}
+
+test "gf16 roundtrip zero" {
+    const gf = GF16.fromF64(0.0);
+    try std.testing.expectEqual(@as(f64, 0.0), gf.toF64());
+}
+
+test "gf16 roundtrip positive" {
+    const values = [_]f64{ 0.5, 1.0, 2.0, 3.14, 100.0, 1000.0 };
+    for (values) |v| {
+        const gf = GF16.fromF64(v);
+        const decoded = gf.toF64();
+        const err = @abs(v - decoded) / v;
+        try std.testing.expect(err < 0.01);
+    }
+}
+
+test "gf16 roundtrip negative" {
+    const values = [_]f64{ -0.5, -1.0, -2.0, -3.14 };
+    for (values) |v| {
+        const gf = GF16.fromF64(v);
+        const decoded = gf.toF64();
+        const err = @abs(v - decoded) / @abs(v);
+        try std.testing.expect(err < 0.01);
+    }
+}
+
+test "gf16 model size under 16MB" {
+    const n_weights: usize = 1950000;
+    const gf16_bytes = n_weights * 2;
+    const limit: usize = 16 * 1024 * 1024;
+    try std.testing.expect(gf16_bytes < limit);
+}