diff --git a/build.zig b/build.zig
index 03c9e23..a55bd38 100644
--- a/build.zig
+++ b/build.zig
@@ -20,9 +20,11 @@ pub fn build(b: *std.Build) !void {
 
     const tests = b.addTest(.{
         .name = "objc-test",
-        .root_source_file = b.path("src/main.zig"),
-        .target = target,
-        .optimize = optimize,
+        .root_module = b.createModule(.{
+            .root_source_file = b.path("src/main.zig"),
+            .target = target,
+            .optimize = optimize,
+        }),
     });
     tests.linkSystemLibrary("objc");
     tests.linkFramework("Foundation");
@@ -66,7 +68,7 @@ pub fn addAppleSDK(b: *std.Build, m: *std.Build.Module) !void {
     if (!gop.found_existing) {
         gop.value_ptr.* = std.zig.system.darwin.getSdk(
             b.allocator,
-            m.resolved_target.?.result,
+            &m.resolved_target.?.result,
         );
     }
 
diff --git a/build.zig.zon b/build.zig.zon
index 2720c3a..523777e 100644
--- a/build.zig.zon
+++ b/build.zig.zon
@@ -2,6 +2,7 @@
     .name = .zig_objc,
     .version = "0.0.0",
     .fingerprint = 0x8a91772ba7d2bf22,
+    .minimum_zig_version = "0.15.1",
     .paths = .{
         "src/",
         "build.zig",
diff --git a/flake.lock b/flake.lock
index 6616332..3cb2c5c 100644
--- a/flake.lock
+++ b/flake.lock
@@ -3,11 +3,11 @@
     "flake-compat": {
       "flake": false,
       "locked": {
-        "lastModified": 1668681692,
-        "narHash": "sha256-Ht91NGdewz8IQLtWZ9LCeNXMSXHUss+9COoqu6JLmXU=",
+        "lastModified": 1747046372,
+        "narHash": "sha256-CIVLLkVgvHYbgI2UpXvIIBJ12HWgX+fjA8Xf8PUmqCY=",
         "owner": "edolstra",
         "repo": "flake-compat",
-        "rev": "009399224d5e398d03b22badca40a37ac85412a1",
+        "rev": "9100a0f413b0c601e0533d1d94ffd501ce2e7885",
         "type": "github"
       },
       "original": {
@@ -33,12 +33,15 @@
       }
     },
     "flake-utils": {
+      "inputs": {
+        "systems": "systems"
+      },
       "locked": {
-        "lastModified": 1667395993,
-        "narHash": "sha256-nuEHfE/LcWyuSWnS8t12N1wc105Qtau+/OdUAjtQ0rA=",
+        "lastModified": 1731533236,
+        "narHash": "sha256-l0KFg5HjrsfsO/JpG+r7fRrqm12kzFHyUHqHCVpMMbI=",
         "owner": "numtide",
         "repo": "flake-utils",
-        "rev": "5aed5285a952e0b949eb3ba02c12fa4fcfef535f",
+        "rev": "11707dc2f618dd54ca8739b309ec4fc024de578b",
         "type": "github"
       },
       "original": {
@@ -49,7 +52,7 @@
     },
     "flake-utils_2": {
       "inputs": {
-        "systems": "systems"
+        "systems": "systems_2"
       },
       "locked": {
         "lastModified": 1705309234,
@@ -67,18 +70,15 @@
     },
     "nixpkgs": {
       "locked": {
-        "lastModified": 1717895720,
-        "narHash": "sha256-Dl6JKx1rIDEuv4q9rtlt9QwyerSQbrk1bUtNHzx9bIY=",
-        "owner": "nixos",
-        "repo": "nixpkgs",
-        "rev": "0e0826ec06d2b3db8e28e280d68179f022b1d160",
-        "type": "github"
+        "lastModified": 1755770914,
+        "narHash": "sha256-CPJ7DELncy3IRAs0XDbGuHVyjPjPNDbIu/XASVjIupA=",
+        "rev": "9cb344e96d5b6918e94e1bca2d9f3ea1e9615545",
+        "type": "tarball",
+        "url": "https://releases.nixos.org/nixos/25.05/nixos-25.05.808519.9cb344e96d5b/nixexprs.tar.xz"
       },
       "original": {
-        "owner": "nixos",
-        "ref": "release-24.05",
-        "repo": "nixpkgs",
-        "type": "github"
+        "type": "tarball",
+        "url": "https://channels.nixos.org/nixos-25.05/nixexprs.tar.xz"
       }
     },
     "nixpkgs_2": {
@@ -120,6 +120,21 @@
         "type": "github"
       }
     },
+    "systems_2": {
+      "locked": {
+        "lastModified": 1681028828,
+        "narHash": "sha256-Vy1rq5AaRuLzOxct8nz4T6wlgyUR7zLU309k9mBC768=",
+        "owner": "nix-systems",
+        "repo": "default",
+        "rev": "da67096a3b9bf56a91d16901293e51ba5b49a27e",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nix-systems",
+        "repo": "default",
+        "type": "github"
+      }
+    },
     "zig": {
       "inputs": {
         "flake-compat": "flake-compat_2",
@@ -127,11 +142,11 @@
         "nixpkgs": "nixpkgs_2"
       },
       "locked": {
-        "lastModified": 1741702955,
-        "narHash": "sha256-Emk3VnNByigCBlBN34IH2cMrsUqIhdex02uw9TSDsN0=",
+        "lastModified": 1755909141,
+        "narHash": "sha256-dogbHGpLmwfu0qkM/vntqMYazfi66DXWSQiCR1rxC4M=",
         "owner": "mitchellh",
         "repo": "zig-overlay",
-        "rev": "1152ce759114a94134081270affd970ae1b957ed",
+        "rev": "e010faa4eb9271b81cb6c30c828d972028465deb",
         "type": "github"
       },
       "original": {
diff --git a/flake.nix b/flake.nix
index 6717cab..edde8fd 100644
--- a/flake.nix
+++ b/flake.nix
@@ -2,7 +2,7 @@
   description = "Objective-C runtime bindings for Zig";
 
   inputs = {
-    nixpkgs.url = "github:nixos/nixpkgs/release-24.05";
+    nixpkgs.url = "https://channels.nixos.org/nixos-25.05/nixexprs.tar.xz";
     flake-utils.url = "github:numtide/flake-utils";
     zig.url = "github:mitchellh/zig-overlay";
 
@@ -35,7 +35,7 @@
       in rec {
         devShells.default = pkgs.mkShell {
           nativeBuildInputs = with pkgs; [
-            zigpkgs."0.14.0"
+            zigpkgs."0.15.1"
           ];
         };
 
diff --git a/src/block.zig b/src/block.zig
index 550f8e4..4df6dca 100644
--- a/src/block.zig
+++ b/src/block.zig
@@ -120,7 +120,7 @@ pub fn Block(
             _Block_release(@ptrCast(@alignCast(ctx)));
         }
 
-        fn descCopyHelper(src: *anyopaque, dst: *anyopaque) callconv(.C) void {
+        fn descCopyHelper(src: *anyopaque, dst: *anyopaque) callconv(.c) void {
             const real_src: *Context = @ptrCast(@alignCast(src));
             const real_dst: *Context = @ptrCast(@alignCast(dst));
             inline for (captures_info.fields) |field| {
@@ -134,7 +134,7 @@ pub fn Block(
             }
         }
 
-        fn descDisposeHelper(src: *anyopaque) callconv(.C) void {
+        fn descDisposeHelper(src: *anyopaque) callconv(.c) void {
             const real_src: *Context = @ptrCast(@alignCast(src));
             inline for (captures_info.fields) |field| {
                 if (field.type == objc.c.id) {
@@ -158,7 +158,7 @@ pub fn Block(
 
             return @Type(.{
                 .@"fn" = .{
-                    .calling_convention = .C,
+                    .calling_convention = .c,
                     .is_generic = false,
                     .is_var_args = false,
                     .return_type = Return,
@@ -249,16 +249,16 @@ const BlockFieldFlags = enum(c_int) {
     byref_caller = 128, // BLOCK_BYREF_CALLER
 };
 
-extern "C" fn _Block_copy(src: *const anyopaque) callconv(.c) ?*anyopaque;
-extern "C" fn _Block_release(src: *const anyopaque) callconv(.c) void;
-extern "C" fn _Block_object_assign(dst: *anyopaque, src: *const anyopaque, flag: BlockFieldFlags) void;
-extern "C" fn _Block_object_dispose(src: *const anyopaque, flag: BlockFieldFlags) void;
+extern "c" fn _Block_copy(src: *const anyopaque) callconv(.c) ?*anyopaque;
+extern "c" fn _Block_release(src: *const anyopaque) callconv(.c) void;
+extern "c" fn _Block_object_assign(dst: *anyopaque, src: *const anyopaque, flag: BlockFieldFlags) void;
+extern "c" fn _Block_object_dispose(src: *const anyopaque, flag: BlockFieldFlags) void;
 
 const Descriptor = extern struct {
     reserved: c_ulong = 0,
     size: c_ulong,
-    copy_helper: *const fn (dst: *anyopaque, src: *anyopaque) callconv(.C) void,
-    dispose_helper: *const fn (src: *anyopaque) callconv(.C) void,
+    copy_helper: *const fn (dst: *anyopaque, src: *anyopaque) callconv(.c) void,
+    dispose_helper: *const fn (src: *anyopaque) callconv(.c) void,
     signature: ?[*:0]const u8,
 };
 
@@ -287,7 +287,7 @@ test "Block" {
     };
 
     var block: AddBlock.Context = AddBlock.init(captures, (struct {
-        fn addFn(block: *const AddBlock.Context) callconv(.C) i32 {
+        fn addFn(block: *const AddBlock.Context) callconv(.c) i32 {
             return block.x + block.y;
         }
     }).addFn);
@@ -317,7 +317,7 @@ test "Block copy objc id" {
     var block = TestBlock.init(.{
         .id = obj.value,
     }, (struct {
-        fn addFn(block: *const TestBlock.Context) callconv(.C) i32 {
+        fn addFn(block: *const TestBlock.Context) callconv(.c) i32 {
             _ = block;
             return 0;
         }
diff --git a/src/class.zig b/src/class.zig
index bf92b7c..34feea1 100644
--- a/src/class.zig
+++ b/src/class.zig
@@ -167,7 +167,7 @@ test "allocatecClassPair and replaceMethod" {
     const NSObject = getClass("NSObject").?;
     var my_object = allocateClassPair(NSObject, "my_object").?;
     my_object.replaceMethod("hash", struct {
-        fn inner(target: c.id, sel: c.SEL) callconv(.C) u64 {
+        fn inner(target: c.id, sel: c.SEL) callconv(.c) u64 {
             _ = sel;
             _ = target;
             return 69;
@@ -208,7 +208,7 @@ test "addMethod" {
         const My_Class = allocateClassPair(objc.getClass("NSObject").?, "my_class").?;
         defer registerClassPair(My_Class);
         std.debug.assert(My_Class.addMethod("my_addition", struct {
-            fn imp(target: objc.c.id, sel: objc.c.SEL, a: i32, b: i32) callconv(.C) i32 {
+            fn imp(target: objc.c.id, sel: objc.c.SEL, a: i32, b: i32) callconv(.c) i32 {
                 _ = sel;
                 _ = target;
                 return a + b;
diff --git a/src/encoding.zig b/src/encoding.zig
index 7fdafac..7cfdf1b 100644
--- a/src/encoding.zig
+++ b/src/encoding.zig
@@ -10,9 +10,9 @@ fn comptimeN(comptime T: type) usize {
         const encoding = objc.Encoding.init(T);
 
         // Figure out how much space we need
-        var counting = std.io.countingWriter(std.io.null_writer);
-        try std.fmt.format(counting.writer(), "{}", .{encoding});
-        return counting.bytes_written;
+        var stream: std.io.Writer.Discarding = .init(&.{});
+        stream.writer.print("{f}", .{encoding}) catch unreachable;
+        return stream.count;
     }
 }
 
@@ -23,8 +23,8 @@ pub fn comptimeEncode(comptime T: type) [comptimeN(T):0]u8 {
 
         // Build our final signature
         var buf: [comptimeN(T) + 1]u8 = undefined;
-        var fbs = std.io.fixedBufferStream(buf[0 .. buf.len - 1]);
-        try std.fmt.format(fbs.writer(), "{}", .{encoding});
+        var fbs: std.io.Writer = .fixed(buf[0 .. buf.len - 1]);
+        fbs.print("{f}", .{encoding}) catch unreachable;
         buf[buf.len - 1] = 0;
 
         return buf[0 .. buf.len - 1 :0].*;
@@ -107,9 +107,7 @@ pub const Encoding = union(enum) {
 
     pub fn format(
         comptime self: Encoding,
-        comptime fmt: []const u8,
-        options: std.fmt.FormatOptions,
-        writer: anytype,
+        writer: *std.io.Writer,
     ) !void {
         switch (self) {
             .char => try writer.writeAll("c"),
@@ -133,7 +131,7 @@ pub const Encoding = union(enum) {
             .array => |a| {
                 try writer.print("[{}", .{a.len});
                 const encode_type = init(a.arr_type);
-                try encode_type.format(fmt, options, writer);
+                try encode_type.format(writer);
                 try writer.writeAll("]");
             },
             .structure => |s| {
@@ -152,7 +150,7 @@ pub const Encoding = union(enum) {
                     try writer.writeAll("=");
                     inline for (struct_info.@"struct".fields) |field| {
                         const field_encode = init(field.type);
-                        try field_encode.format(fmt, options, writer);
+                        try field_encode.format(writer);
                     }
                 }
 
@@ -174,7 +172,7 @@ pub const Encoding = union(enum) {
                     try writer.writeAll("=");
                     inline for (union_info.@"union".fields) |field| {
                         const field_encode = init(field.type);
-                        try field_encode.format(fmt, options, writer);
+                        try field_encode.format(writer);
                     }
                 }
 
@@ -209,7 +207,7 @@ pub const Encoding = union(enum) {
                         }
 
                         // call this format function again, this time with the child type encoding
-                        try encoding.format(fmt, options, writer);
+                        try encoding.format(writer);
                     },
                     else => @compileError("Pointer size not supported for encoding"),
                 }
@@ -219,10 +217,10 @@ pub const Encoding = union(enum) {
 
                 // Return type is first in a method encoding
                 const ret_type_enc = init(fn_info.return_type.?);
-                try ret_type_enc.format(fmt, options, writer);
+                try ret_type_enc.format(writer);
                 inline for (fn_info.params) |param| {
                     const param_enc = init(param.type.?);
-                    try param_enc.format(fmt, options, writer);
+                    try param_enc.format(writer);
                 }
             },
             .unknown => {},
@@ -249,7 +247,7 @@ fn indirectionCountAndType(comptime T: type) struct {
 fn encodingMatchesType(comptime T: type, expected_encoding: []const u8) !void {
     var buf: [200]u8 = undefined;
     const enc = Encoding.init(T);
-    const enc_string = try std.fmt.bufPrint(&buf, "{s}", .{enc});
+    const enc_string = try std.fmt.bufPrint(&buf, "{f}", .{enc});
     try testing.expectEqualStrings(expected_encoding, enc_string);
 }
 
@@ -400,7 +398,7 @@ test "**Union to Encoding.union encoding" {
 
 test "Fn to Encoding.function encoding" {
     const test_fn = struct {
-        fn add(_: c.id, _: c.SEL, _: i8) callconv(.C) void {}
+        fn add(_: c.id, _: c.SEL, _: i8) callconv(.c) void {}
     };
 
     try encodingMatchesType(@TypeOf(test_fn.add), "v@:c");
diff --git a/src/msg_send.zig b/src/msg_send.zig
index 39fb5a9..2382e2a 100644
--- a/src/msg_send.zig
+++ b/src/msg_send.zig
@@ -78,7 +78,7 @@ pub fn MsgSend(comptime T: type) type {
         fn msgSendPtr(
             comptime Return: type,
             comptime super: bool,
-        ) *const fn () callconv(.C) void {
+        ) *const fn () callconv(.c) void {
             // See objc/message.h. The high-level is that depending on the
             // target architecture and return type, we must use a different
             // objc_msgSend function.
@@ -204,7 +204,7 @@ fn MsgSendFn(
 
     return @Type(.{
         .@"fn" = .{
-            .calling_convention = .C,
+            .calling_convention = .c,
             .is_generic = false,
             .is_var_args = false,
             .return_type = Return,
@@ -218,8 +218,8 @@ test {
     try testing.expectEqual(fn (
         c.id,
         c.SEL,
-    ) callconv(.C) u64, MsgSendFn(u64, c.id, @TypeOf(.{})));
-    try testing.expectEqual(fn (c.id, c.SEL, u16, u32) callconv(.C) u64, MsgSendFn(u64, c.id, @TypeOf(.{
+    ) callconv(.c) u64, MsgSendFn(u64, c.id, @TypeOf(.{})));
+    try testing.expectEqual(fn (c.id, c.SEL, u16, u32) callconv(.c) u64, MsgSendFn(u64, c.id, @TypeOf(.{
         @as(u16, 0),
         @as(u32, 0),
     })));
@@ -229,7 +229,7 @@ test "subClass" {
     const Subclass = objc.allocateClassPair(objc.getClass("NSObject").?, "subclass").?;
     defer objc.disposeClassPair(Subclass);
     const str = struct {
-        fn inner(target: objc.c.id, sel: objc.c.SEL) callconv(.C) objc.c.id {
+        fn inner(target: objc.c.id, sel: objc.c.SEL) callconv(.c) objc.c.id {
             _ = sel;
             const self = objc.Object.fromId(target);
             self.msgSendSuper(objc.getClass("NSObject").?, void, "init", .{});