diff --git a/build.zig.zon b/build.zig.zon
index 2298baa..4f1b6f5 100644
--- a/build.zig.zon
+++ b/build.zig.zon
@@ -2,6 +2,7 @@
     .name = .tls,
     .version = "0.1.0",
     .dependencies = .{},
+    .minimum_zig_version = "0.15.1",
     .fingerprint = 0xacfd1db3d29e1d11,
     .paths = .{
         "readme.md",
diff --git a/readme.md b/readme.md
index 3598554..8b13e9d 100644
--- a/readme.md
+++ b/readme.md
@@ -17,6 +17,12 @@ https://github.com/ziglang/zig/issues/15226#issuecomment-2218809140
 https://github.com/ziglang/zig/issues/14172#issuecomment-2181202318
 -->
 
+## Toolchain version
+
+In order to build `tls.zig`, you need **Zig 15 or newer**. The code uses the new
+[`std.Io`](https://ziglang.org/documentation/0.15.1/std/#std.Io) interface, which isn't
+available in Zig 14 or older.
+
 # Client
 
 [Here](https://github.com/ianic/tls.zig/blob/main/demo/src/main.zig) is simple example of how to use library.