diff --git a/Sources/BinaryParseKit/BinaryParseKit.swift b/Sources/BinaryParseKit/BinaryParseKit.swift
index e0b1cec..7c398cd 100644
--- a/Sources/BinaryParseKit/BinaryParseKit.swift
+++ b/Sources/BinaryParseKit/BinaryParseKit.swift
@@ -538,3 +538,23 @@ public macro matchDefault() = #externalMacro(
     module: "BinaryParseKitMacros",
     type: "EmptyPeerMacro",
 )
+
+// MARK: - Bitmask
+
+@attached(peer)
+public macro mask(bitCount: Int) = #externalMacro(
+    module: "BinaryParseKitMacros",
+    type: "EmptyPeerMacro",
+)
+
+@attached(peer)
+public macro mask() = #externalMacro(
+    module: "BinaryParseKitMacros",
+    type: "EmptyPeerMacro",
+)
+
+@attached(extension, conformances: BinaryParseKit.Parsable, names: arbitrary)
+public macro ParseBitmask() = #externalMacro(
+    module: "BinaryParseKitMacros",
+    type: "ConstructParseBitmaskMacro",
+)
diff --git a/Sources/BinaryParseKit/Protocols/BitmaskParsable.swift b/Sources/BinaryParseKit/Protocols/BitmaskParsable.swift
new file mode 100644
index 0000000..e0f7419
--- /dev/null
+++ b/Sources/BinaryParseKit/Protocols/BitmaskParsable.swift
@@ -0,0 +1,10 @@
+//
+//  BitmaskParsable.swift
+//  BinaryParseKit
+//
+//  Created by Larry Zeng on 11/28/25.
+//
+
+public protocol BitmaskParsable: Parsable {
+    static var bitCount: Int { get }
+}
diff --git a/Sources/BinaryParseKitMacros/BinaryParseKitMacro.swift b/Sources/BinaryParseKitMacros/BinaryParseKitMacro.swift
index bf3f18c..c56c7cb 100644
--- a/Sources/BinaryParseKitMacros/BinaryParseKitMacro.swift
+++ b/Sources/BinaryParseKitMacros/BinaryParseKitMacro.swift
@@ -7,5 +7,6 @@ struct BinaryParseKitPlugin: CompilerPlugin {
         EmptyPeerMacro.self,
         ConstructStructParseMacro.self,
         ConstructEnumParseMacro.self,
+        ConstructParseBitmaskMacro.self,
     ]
 }
diff --git a/Sources/BinaryParseKitMacros/Macros/ParseBitmask/ConstructParseBitmaskMacro.swift b/Sources/BinaryParseKitMacros/Macros/ParseBitmask/ConstructParseBitmaskMacro.swift
new file mode 100644
index 0000000..340a4cc
--- /dev/null
+++ b/Sources/BinaryParseKitMacros/Macros/ParseBitmask/ConstructParseBitmaskMacro.swift
@@ -0,0 +1,72 @@
+//
+//  ConstructParseBitmaskMacro.swift
+//  BinaryParseKit
+//
+//  Created by Larry Zeng on 11/28/25.
+//
+import SwiftDiagnostics
+import SwiftSyntax
+import SwiftSyntaxMacros
+
+enum ParseBitmaskMacroError: Error, DiagnosticMessage {
+    case onlyStructsAreSupported
+
+    var message: String {
+        switch self {
+        case .onlyStructsAreSupported: "@ParseBitmask can only be applied to structs."
+        }
+    }
+
+    var diagnosticID: SwiftDiagnostics.MessageID {
+        .init(
+            domain: "observer.universe.BinaryParseKit.ParseBitmaskMacroError",
+            id: "\(self)",
+        )
+    }
+
+    var severity: SwiftDiagnostics.DiagnosticSeverity {
+        switch self {
+        case .onlyStructsAreSupported: .error
+        }
+    }
+}
+
+public struct ConstructParseBitmaskMacro: ExtensionMacro {
+    public static func expansion(
+        of node: SwiftSyntax.AttributeSyntax,
+        attachedTo declaration: some SwiftSyntax.DeclGroupSyntax,
+        providingExtensionsOf type: some SwiftSyntax.TypeSyntaxProtocol,
+        conformingTo _: [SwiftSyntax.TypeSyntax],
+        in context: some SwiftSyntaxMacros.MacroExpansionContext,
+    ) throws -> [SwiftSyntax.ExtensionDeclSyntax] {
+        guard let structDeclaration = declaration.as(StructDeclSyntax.self) else {
+            throw ParseBitmaskMacroError.onlyStructsAreSupported
+        }
+
+        let type = type.trimmed
+
+        let accessorInfo = try extractAccessor(
+            from: node,
+            attachedTo: declaration,
+            in: context,
+        )
+
+        // extract mask macro info
+
+        let bitmaskParsableExtension =
+            try ExtensionDeclSyntax("extension \(type): \(raw: Constants.Protocols.bitmaskParsableProtocol)") {
+                try VariableDeclSyntax("static var bitCount: Int") {
+                    #"fatalError("TODO")"#
+                }
+
+                // FIXME: accessors
+                try InitializerDeclSyntax(
+                    "\(accessorInfo.parsingAccessor) init(parsing span: inout \(raw: Constants.BinaryParsing.parserSpan)) throws(\(raw: Constants.BinaryParsing.thrownParsingError))",
+                ) {
+                    #"fatalError("TODO")"#
+                }
+            }
+
+        return [bitmaskParsableExtension]
+    }
+}
diff --git a/Sources/BinaryParseKitMacros/Macros/Supports/Constants.swift b/Sources/BinaryParseKitMacros/Macros/Supports/Constants.swift
index 834aa79..d080e93 100644
--- a/Sources/BinaryParseKitMacros/Macros/Supports/Constants.swift
+++ b/Sources/BinaryParseKitMacros/Macros/Supports/Constants.swift
@@ -35,6 +35,7 @@ extension Constants {
         static let expressibleByParsingProtocol = PackageMember(name: "ExpressibleByParsing")
         static let matchableProtocol = PackageMember(name: "Matchable")
         static let printableProtocol = PackageMember(name: "Printable")
+        static let bitmaskParsableProtocol = PackageMember(name: "BitmaskParsable")
     }
 }
 
diff --git a/Sources/BinaryParseKitMacros/Macros/Supports/MacroAccessorVisitor.swift b/Sources/BinaryParseKitMacros/Macros/Supports/MacroAccessorVisitor.swift
index 3d8efc3..882aef3 100644
--- a/Sources/BinaryParseKitMacros/Macros/Supports/MacroAccessorVisitor.swift
+++ b/Sources/BinaryParseKitMacros/Macros/Supports/MacroAccessorVisitor.swift
@@ -28,7 +28,7 @@ enum MacroAccessorError: DiagnosticMessage, Error {
 
     var diagnosticID: SwiftDiagnostics.MessageID {
         .init(
-            domain: "BinaryParseKit.MacroACLError",
+            domain: "observer.universe.BinaryParseKit.MacroACLError",
             id: "\(self)",
         )
     }