Initial implementation

Sources/MCP/Base/ID.swift

@@ -0,0 +1,69 @@
+import struct Foundation.UUID
+
+/// A unique identifier for a request.
+public enum ID: Hashable, Sendable {
+    /// A string ID.
+    case string(String)
+
+    /// A number ID.
+    case number(Int)
+
+    /// Generates a random string ID.
+    public static var random: ID {
+        return .string(UUID().uuidString)
+    }
+}
+
+// MARK: - ExpressibleByStringLiteral
+
+extension ID: ExpressibleByStringLiteral {
+    public init(stringLiteral value: String) {
+        self = .string(value)
+    }
+}
+
+// MARK: - ExpressibleByIntegerLiteral
+
+extension ID: ExpressibleByIntegerLiteral {
+    public init(integerLiteral value: Int) {
+        self = .number(value)
+    }
+}
+
+// MARK: - CustomStringConvertible
+
+extension ID: CustomStringConvertible {
+    public var description: String {
+        switch self {
+        case .string(let str): return str
+        case .number(let num): return String(num)
+        }
+    }
+}
+
+// MARK: - Codable
+
+extension ID: Codable {
+    public init(from decoder: Decoder) throws {
+        let container = try decoder.singleValueContainer()
+        if let string = try? container.decode(String.self) {
+            self = .string(string)
+        } else if let number = try? container.decode(Int.self) {
+            self = .number(number)
+        } else if container.decodeNil() {
+            // Handle unspecified/null IDs as empty string
+            self = .string("")
+        } else {
+            throw DecodingError.dataCorruptedError(
+                in: container, debugDescription: "ID must be string or number")
+        }
+    }
+
+    public func encode(to encoder: Encoder) throws {
+        var container = encoder.singleValueContainer()
+        switch self {
+        case .string(let str): try container.encode(str)
+        case .number(let num): try container.encode(num)
+        }
+    }
+}