diff --git a/README.md b/README.md
index 3a75589..aec3ac3 100644
--- a/README.md
+++ b/README.md
@@ -84,10 +84,18 @@ Support levels:
           <summary><code>AttributedString</code></summary>
           <ul>
 <li><code>init()</code></li>
+<li><code>init(_ string: String)</code></li>
+<li><code>init(_ string: String, attributes: AttributeContainer)</code></li>
 <li><code>init(stringLiteral: String)</code></li>
 <li><code>init(markdown: String)</code></li>
 <li><code>init(localized keyAndValue: String.LocalizationValue, table: String? = nil, bundle: Bundle? = nil, locale: Locale? = nil, comment: String? = nil)</code></li>
 <li><code>init(localized key: String, table: String? = nil, bundle: Bundle? = nil, locale: Locale = Locale.current, comment: String? = nil)</code></li>
+<li><code>var characters: String</code></li>
+<li><code>var string: String</code></li>
+<li><code>var runs: Runs</code></li>
+<li><code>func append(_ other: AttributedString)</code></li>
+<li><code>static func +(lhs:rhs:)</code> (Kotlin <code>plus</code> operator)</li>
+<li><code>AttributeContainer</code>, <code>AttributedStringKey</code>, <code>AttributeScopes.FoundationAttributes</code> (link, markdown)</li>
           </ul>
         </details> 
       </td>
diff --git a/Sources/SkipFoundation/AttributeContainer.swift b/Sources/SkipFoundation/AttributeContainer.swift
new file mode 100644
index 0000000..e359f5a
--- /dev/null
+++ b/Sources/SkipFoundation/AttributeContainer.swift
@@ -0,0 +1,86 @@
+// Copyright 2023–2026 Skip
+// SPDX-License-Identifier: MPL-2.0
+#if SKIP
+
+/// A container for attribute keys and values.
+public struct AttributeContainer : Hashable {
+    internal var storage: AttributeStorage
+
+    public init() {
+        storage = AttributeStorage()
+    }
+
+    internal init(_ storage: AttributeStorage) {
+        self.storage = storage
+    }
+
+    public func value(key: String) -> Any? {
+        return storage.value(key: key)
+    }
+
+    public mutating func setValue(_ value: Any?, key: String) {
+        storage.setValue(value, key: key)
+    }
+}
+
+/// Type-erased attribute storage keyed by attribute name.
+struct AttributeStorage : Hashable {
+    private var values: [String: Any] = [:]
+
+    func value(key: String) -> Any? {
+        return values[key]
+    }
+
+    mutating func setValue(_ value: Any?, key: String) {
+        if let value {
+            values[key] = value
+        } else {
+            values.removeValue(forKey: key)
+        }
+    }
+
+    func merging(_ other: AttributeStorage) -> AttributeStorage {
+        var result = self
+        for (key, value) in other.values {
+            result.values[key] = value
+        }
+        return result
+    }
+
+    static func ==(lhs: AttributeStorage, rhs: AttributeStorage) -> Bool {
+        guard lhs.values.count == rhs.values.count else { return false }
+        for (key, lhsValue) in lhs.values {
+            guard let rhsValue = rhs.values[key] else { return false }
+            if !AttributeStorage.valuesEqual(lhsValue, rhsValue) { return false }
+        }
+        return true
+    }
+
+    private static func valuesEqual(_ lhs: Any, _ rhs: Any) -> Bool {
+        if let lhs = lhs as? Bool, let rhs = rhs as? Bool { return lhs == rhs }
+        if let lhs = lhs as? String, let rhs = rhs as? String { return lhs == rhs }
+        if let lhs = lhs as? URL, let rhs = rhs as? URL { return lhs == rhs }
+        return false
+    }
+
+    func hash(into hasher: inout Hasher) {
+        for key in values.keys.sorted() {
+            hasher.combine(key)
+            hasher.combine(values[key])
+        }
+    }
+
+    func attributeNames() -> [String] {
+        return Array(values.keys)
+    }
+
+    func value(forName name: String) -> Any? {
+        return values[name]
+    }
+
+    mutating func setValue(name: String, value: Any) {
+        values[name] = value
+    }
+}
+
+#endif
diff --git a/Sources/SkipFoundation/AttributedString.swift b/Sources/SkipFoundation/AttributedString.swift
index 22afe34..67ee333 100644
--- a/Sources/SkipFoundation/AttributedString.swift
+++ b/Sources/SkipFoundation/AttributedString.swift
@@ -2,49 +2,102 @@
 // SPDX-License-Identifier: MPL-2.0
 #if SKIP
 public struct AttributedString: Hashable {
-    // Allow e.g. SwiftUI to access our state
-    public let string: String
-    public let markdownNode: MarkdownNode?
+    /// An index into an attributed string, measured in UTF-16 code units.
+    public struct Index : Hashable, Comparable {
+        internal let utf16Offset: Int
+
+        internal init(utf16Offset: Int) {
+            self.utf16Offset = utf16Offset
+        }
+
+        public static func < (lhs: Index, rhs: Index) -> Bool {
+            return lhs.utf16Offset < rhs.utf16Offset
+        }
+    }
+
+    /// A run of characters sharing the same attributes.
+    public struct Run : Hashable {
+        public let utf16Range: Range<Int>
+        public let attributes: AttributeContainer
+
+        internal init(utf16Range: Range<Int>, attributes: AttributeContainer) {
+            self.utf16Range = utf16Range
+            self.attributes = attributes
+        }
+    }
+
+    /// The plain text content.
+    public var characters: String
+
+    internal var _runs: [Run]
+
+    /// Plain text for display and compatibility.
+    public var string: String {
+        return characters
+    }
+
+    internal init(characters: String, runs: [Run]) {
+        self.characters = characters
+        self._runs = runs
+    }
 
     public init() {
-        string = ""
-        markdownNode = nil
+        characters = ""
+        _runs = []
     }
 
-    public init(stringLiteral: String) {
-        string = stringLiteral
-        markdownNode = nil
+    public init(stringLiteral value: String) {
+        self.init(value)
     }
 
     public init(markdown: String) throws {
-        string = markdown
-        markdownNode = MarkdownNode.from(string: markdown)
+        if let node = MarkdownNode.from(string: markdown) {
+            let built = Self.from(markdown: node)
+            characters = built.characters
+            _runs = built._runs
+        } else {
+            characters = markdown
+            _runs = Self.singleRun(length: markdown.count, attributes: AttributeContainer())
+        }
     }
 
-    public init(localized keyAndValue: String.LocalizationValue, /* options: AttributedString.FormattingOptions = [], */ table: String? = nil, bundle: Bundle? = nil, locale: Locale? = nil, comment: String? = nil) {
-        let key = keyAndValue.patternFormat // interpolated string: "Hello \(name)" keyed as: "Hello %@"
+    public init(localized keyAndValue: String.LocalizationValue, table: String? = nil, bundle: Bundle? = nil, locale: Locale? = nil, comment: String? = nil) {
+        let key = keyAndValue.patternFormat
         let (_, locfmt, locnode) = (bundle ?? Bundle.main).localizedInfo(forKey: key, value: nil, table: table, locale: locale)
-        // re-interpret the placeholder strings in the resulting localized string with the string interpolation's values
-        self.string = locfmt.format(*keyAndValue.stringInterpolation.values.toTypedArray())
-        self.markdownNode = locnode?.format(keyAndValue.stringInterpolation.values)
+        let characters = locfmt.format(*keyAndValue.stringInterpolation.values.toTypedArray())
+        if let locnode {
+            let built = Self.from(markdown: locnode.format(keyAndValue.stringInterpolation.values), interpolations: keyAndValue.stringInterpolation.values)
+            self.characters = characters
+            _runs = built._runs
+        } else {
+            self.characters = characters
+            _runs = Self.singleRun(length: characters.count, attributes: AttributeContainer())
+        }
     }
 
     public init(localized key: String, table: String? = nil, bundle: Bundle? = nil, locale: Locale? = nil, comment: String? = nil) {
         let (locstring, _, locnode) = (bundle ?? Bundle.main).localizedInfo(forKey: key, value: nil, table: table, locale: locale)
-        self.string = locstring
-        self.markdownNode = locnode
+        if let locnode {
+            let built = Self.from(markdown: locnode)
+            characters = locstring
+            _runs = built._runs
+        } else {
+            characters = locstring
+            _runs = Self.singleRun(length: locstring.count, attributes: AttributeContainer())
+        }
     }
 
     public var description: String {
-        return string
+        return characters
     }
 
-    public static func ==(lhs: AttributedString, rhs: AttributedString) {
-        return lhs.string == rhs.string
+    public static func ==(lhs: AttributedString, rhs: AttributedString) -> Bool {
+        return lhs.characters == rhs.characters && lhs._runs == rhs._runs
     }
 
     public func hash(into hasher: inout Hasher) {
-        hasher.combine(string)
+        hasher.combine(characters)
+        hasher.combine(_runs)
     }
 }
 
diff --git a/Sources/SkipFoundation/AttributedStringKey.swift b/Sources/SkipFoundation/AttributedStringKey.swift
new file mode 100644
index 0000000..a443a2e
--- /dev/null
+++ b/Sources/SkipFoundation/AttributedStringKey.swift
@@ -0,0 +1,15 @@
+// Copyright 2023–2026 Skip
+// SPDX-License-Identifier: MPL-2.0
+#if SKIP
+
+/// A type that defines an attributed string attribute's name and value type.
+public protocol AttributedStringKey {
+    associatedtype Value : Hashable
+    static var name: String { get }
+}
+
+/// A type that collects related attribute keys.
+public protocol AttributeScope {
+}
+
+#endif
diff --git a/Sources/SkipFoundation/AttributedStringMarkdown.swift b/Sources/SkipFoundation/AttributedStringMarkdown.swift
new file mode 100644
index 0000000..323d688
--- /dev/null
+++ b/Sources/SkipFoundation/AttributedStringMarkdown.swift
@@ -0,0 +1,123 @@
+// Copyright 2023–2026 Skip
+// SPDX-License-Identifier: MPL-2.0
+#if SKIP
+
+extension AttributedString {
+    /// Builds an attributed string from a markdown AST node.
+    internal static func from(markdown: MarkdownNode, interpolations: kotlin.collections.List<AnyHashable>? = nil) -> AttributedString {
+        var builder = MarkdownRunBuilder()
+        appendMarkdown(markdown, to: &builder, interpolations: interpolations, isFirstChild: true)
+        return AttributedString(characters: builder.text, runs: builder.runs())
+    }
+
+    private static func appendMarkdown(_ markdown: MarkdownNode, to builder: inout MarkdownRunBuilder, interpolations: kotlin.collections.List<AnyHashable>?, isFirstChild: Bool) {
+        func appendChildren() {
+            markdown.children?.forEachIndexed { appendMarkdown($1, to: &builder, interpolations: interpolations, isFirstChild: $0 == 0) }
+        }
+
+        switch markdown.type {
+        case MarkdownNode.NodeType.bold:
+            builder.pushBold()
+            appendChildren()
+            builder.pop()
+        case MarkdownNode.NodeType.code:
+            builder.pushCode()
+            if let text = markdown.formattedString(interpolations) {
+                builder.append(text)
+            }
+            builder.pop()
+        case MarkdownNode.NodeType.italic:
+            builder.pushItalic()
+            appendChildren()
+            builder.pop()
+        case MarkdownNode.NodeType.link:
+            if let urlString = markdown.formattedString(interpolations), let url = URL(string: urlString) {
+                builder.pushLink(url)
+            }
+            appendChildren()
+            builder.pop()
+        case MarkdownNode.NodeType.paragraph:
+            if !isFirstChild {
+                builder.append("\n\n")
+            }
+            appendChildren()
+        case MarkdownNode.NodeType.root:
+            appendChildren()
+        case MarkdownNode.NodeType.strikethrough:
+            builder.pushStrikethrough()
+            appendChildren()
+            builder.pop()
+        case MarkdownNode.NodeType.text:
+            if let text = markdown.formattedString(interpolations) {
+                builder.append(text)
+            }
+        case MarkdownNode.NodeType.unknown:
+            appendChildren()
+        }
+    }
+}
+
+/// Walks markdown and produces runs with varying attributes per text segment.
+struct MarkdownRunBuilder {
+    private var attributeStack: [AttributeContainer] = [AttributeContainer()]
+    private var segments: [(String, AttributeContainer)] = []
+
+    mutating func pushBold() {
+        push { $0.markdownBold = true }
+    }
+
+    mutating func pushItalic() {
+        push { $0.markdownItalic = true }
+    }
+
+    mutating func pushStrikethrough() {
+        push { $0.markdownStrikethrough = true }
+    }
+
+    mutating func pushCode() {
+        push { $0.markdownCode = true }
+    }
+
+    mutating func pushLink(_ url: URL) {
+        push { $0.link = url }
+    }
+
+    private mutating func push(_ update: (inout AttributeContainer) -> Void) {
+        var container = attributeStack.last ?? AttributeContainer()
+        update(&container)
+        attributeStack.append(container)
+    }
+
+    mutating func pop() {
+        if attributeStack.count > 1 {
+            attributeStack.removeLast()
+        }
+    }
+
+    mutating func append(_ string: String) {
+        guard !string.isEmpty else { return }
+        segments.append((string, attributeStack.last ?? AttributeContainer()))
+    }
+
+    var text: String {
+        return segments.map { $0.0 }.joined()
+    }
+
+    func runs() -> [AttributedString.Run] {
+        var result: [AttributedString.Run] = []
+        var offset = 0
+        for (segment, attributes) in segments {
+            let length = segment.count
+            if length > 0 {
+                result.append(AttributedString.Run(
+                    utf16Range: offset..<(offset + length),
+                    attributes: attributes
+                ))
+                offset += length
+            }
+        }
+        return AttributedString.coalesce(result)
+    }
+}
+
+#endif
diff --git a/Sources/SkipFoundation/AttributedStringMutation.swift b/Sources/SkipFoundation/AttributedStringMutation.swift
new file mode 100644
index 0000000..a6127ea
--- /dev/null
+++ b/Sources/SkipFoundation/AttributedStringMutation.swift
@@ -0,0 +1,70 @@
+// Copyright 2023–2026 Skip
+// SPDX-License-Identifier: MPL-2.0
+#if SKIP
+
+extension AttributedString {
+    public init(_ string: String) {
+        self.init(characters: string, runs: Self.singleRun(length: string.count, attributes: AttributeContainer()))
+    }
+
+    public init(_ string: String, attributes: AttributeContainer) {
+        self.init(characters: string, runs: Self.singleRun(length: string.count, attributes: attributes))
+    }
+
+    public func attributeValue(key: String) -> Any? {
+        guard _runs.count == 1 else {
+            return attributes(in: entireUTF16Range).value(key: key)
+        }
+        return _runs[0].attributes.value(key: key)
+    }
+
+    public mutating func setAttributeValue(_ value: Any?, key: String) {
+        if _runs.isEmpty && !characters.isEmpty {
+            _runs = Self.singleRun(length: characters.count, attributes: AttributeContainer())
+        }
+        _runs = _runs.map { run in
+            var attrs = run.attributes
+            attrs.setValue(value, key: key)
+            return Run(utf16Range: run.utf16Range, attributes: attrs)
+        }
+    }
+
+    // SKIP DECLARE: operator fun plus(other: AttributedString): AttributedString
+    public func plus(other: AttributedString) -> AttributedString {
+        var result = self
+        result.append(other)
+        return result
+    }
+
+    // SKIP DECLARE: operator fun plusAssign(other: AttributedString)
+    public mutating func plusAssign(other: AttributedString) {
+        append(other)
+    }
+
+    public mutating func append(_ other: AttributedString) {
+        let offset = _utf16Length
+        characters += other.characters
+        for run in other._runs {
+            let runUTF16 = run.utf16Range
+            let shifted = Run(
+                utf16Range: (runUTF16.lowerBound + offset)..<(runUTF16.upperBound + offset),
+                attributes: run.attributes
+            )
+            _runs.append(shifted)
+        }
+        _runs = Self.coalesce(_runs)
+    }
+
+    internal func attributes(in utf16Range: Range<Int>) -> AttributeContainer {
+        var merged = AttributeContainer()
+        for run in _runs {
+            let runUTF16 = run.utf16Range
+            if runUTF16.lowerBound < utf16Range.upperBound && runUTF16.upperBound > utf16Range.lowerBound {
+                merged.storage = merged.storage.merging(run.attributes.storage)
+            }
+        }
+        return merged
+    }
+}
+
+#endif
diff --git a/Sources/SkipFoundation/AttributedStringRuns.swift b/Sources/SkipFoundation/AttributedStringRuns.swift
new file mode 100644
index 0000000..ceed5eb
--- /dev/null
+++ b/Sources/SkipFoundation/AttributedStringRuns.swift
@@ -0,0 +1,54 @@
+// Copyright 2023–2026 Skip
+// SPDX-License-Identifier: MPL-2.0
+#if SKIP
+
+extension AttributedString {
+    public var entireUTF16Range: Range<Int> {
+        return 0..<_utf16Length
+    }
+
+    /// Attribute runs in the attributed string.
+    public var runs: [Run] {
+        return _runs
+    }
+
+    public var startIndex: Index {
+        return Index(utf16Offset: 0)
+    }
+
+    public var endIndex: Index {
+        return Index(utf16Offset: _utf16Length)
+    }
+
+    internal var _utf16Length: Int {
+        return characters.count
+    }
+
+    public func substring(in range: Range<Int>) -> String {
+        return String(characters[range.lowerBound..<range.upperBound])
+    }
+
+    internal func index(at utf16Offset: Int) -> Index {
+        return Index(utf16Offset: utf16Offset)
+    }
+
+    internal static func coalesce(_ runs: [Run]) -> [Run] {
+        guard !runs.isEmpty else { return [] }
+        var result: [Run] = []
+        for run in runs {
+            if let last = result.last, last.attributes == run.attributes, last.utf16Range.upperBound == run.utf16Range.lowerBound {
+                result[result.count - 1] = Run(utf16Range: last.utf16Range.lowerBound..<run.utf16Range.upperBound, attributes: last.attributes)
+            } else {
+                result.append(run)
+            }
+        }
+        return result
+    }
+
+    internal static func singleRun(length: Int, attributes: AttributeContainer) -> [Run] {
+        guard length > 0 else { return [] }
+        return [Run(utf16Range: 0..<length, attributes: attributes)]
+    }
+}
+
+#endif
diff --git a/Sources/SkipFoundation/FoundationAttributes.swift b/Sources/SkipFoundation/FoundationAttributes.swift
new file mode 100644
index 0000000..a8544af
--- /dev/null
+++ b/Sources/SkipFoundation/FoundationAttributes.swift
@@ -0,0 +1,74 @@
+// Copyright 2023–2026 Skip
+// SPDX-License-Identifier: MPL-2.0
+#if SKIP
+
+public enum AttributeScopes {
+  public static var foundation: FoundationAttributes.Type { FoundationAttributes.self }
+
+  public struct FoundationAttributes : AttributeScope {
+    public enum LinkAttribute : AttributedStringKey {
+      public typealias Value = URL
+      public static let name = "Link"
+    }
+
+    /// Markdown bold (**text**).
+    public enum MarkdownBoldAttribute : AttributedStringKey {
+      public typealias Value = Bool
+      public static let name = "MarkdownBold"
+    }
+
+    /// Markdown italic (*text*).
+    public enum MarkdownItalicAttribute : AttributedStringKey {
+      public typealias Value = Bool
+      public static let name = "MarkdownItalic"
+    }
+
+    /// Markdown strikethrough (~~text~~).
+    public enum MarkdownStrikethroughAttribute : AttributedStringKey {
+      public typealias Value = Bool
+      public static let name = "MarkdownStrikethrough"
+    }
+
+    /// Markdown inline code (`text`).
+    public enum MarkdownCodeAttribute : AttributedStringKey {
+      public typealias Value = Bool
+      public static let name = "MarkdownCode"
+    }
+  }
+}
+
+extension AttributeContainer {
+  public var link: URL? {
+    get { value(key: AttributeScopes.FoundationAttributes.LinkAttribute.name) as? URL }
+    set { setValue(newValue, key: AttributeScopes.FoundationAttributes.LinkAttribute.name) }
+  }
+
+  public var markdownBold: Bool {
+    get { value(key: AttributeScopes.FoundationAttributes.MarkdownBoldAttribute.name) as? Bool == true }
+    set { setValue(newValue, key: AttributeScopes.FoundationAttributes.MarkdownBoldAttribute.name) }
+  }
+
+  public var markdownItalic: Bool {
+    get { value(key: AttributeScopes.FoundationAttributes.MarkdownItalicAttribute.name) as? Bool == true }
+    set { setValue(newValue, key: AttributeScopes.FoundationAttributes.MarkdownItalicAttribute.name) }
+  }
+
+  public var markdownStrikethrough: Bool {
+    get { value(key: AttributeScopes.FoundationAttributes.MarkdownStrikethroughAttribute.name) as? Bool == true }
+    set { setValue(newValue, key: AttributeScopes.FoundationAttributes.MarkdownStrikethroughAttribute.name) }
+  }
+
+  public var markdownCode: Bool {
+    get { value(key: AttributeScopes.FoundationAttributes.MarkdownCodeAttribute.name) as? Bool == true }
+    set { setValue(newValue, key: AttributeScopes.FoundationAttributes.MarkdownCodeAttribute.name) }
+  }
+}
+
+extension AttributedString {
+  public var link: URL? {
+    get { attributeValue(key: AttributeScopes.FoundationAttributes.LinkAttribute.name) as? URL }
+    mutating set { setAttributeValue(newValue, key: AttributeScopes.FoundationAttributes.LinkAttribute.name) }
+  }
+}
+
+#endif
diff --git a/Tests/SkipFoundationTests/Formats/TestAttributedStringSkip.swift b/Tests/SkipFoundationTests/Formats/TestAttributedStringSkip.swift
new file mode 100644
index 0000000..3ee0745
--- /dev/null
+++ b/Tests/SkipFoundationTests/Formats/TestAttributedStringSkip.swift
@@ -0,0 +1,52 @@
+// Copyright 2023–2026 Skip
+// SPDX-License-Identifier: MPL-2.0
+import Foundation
+import XCTest
+
+#if SKIP
+
+final class TestAttributedStringSkip: XCTestCase {
+    func testEmpty() {
+        let str = AttributedString()
+        XCTAssertEqual(str.characters, "")
+        XCTAssertEqual(str.runs.first, nil)
+    }
+
+    func testInitWithAttributes() {
+        var container = AttributeContainer()
+        container.link = URL(string: "https://example.com")
+        let str = AttributedString("Hello", attributes: container)
+        XCTAssertEqual(str.characters, "Hello")
+        XCTAssertEqual(str.link, URL(string: "https://example.com"))
+    }
+
+    func testAppendAndPlus() {
+        var a = AttributedString("Hello")
+        a.append(AttributedString(" World"))
+        XCTAssertEqual(a.characters, "Hello World")
+        let b = AttributedString("!") + a
+        XCTAssertEqual(b.characters, "!Hello World")
+    }
+
+    func testWholeStringLinkOnMarkdown() throws {
+        var str = try AttributedString(markdown: "**Bold** and plain")
+        str.link = URL(string: "https://example.com")
+        for run in str.runs {
+            XCTAssertEqual(run.attributes.link, URL(string: "https://example.com"))
+        }
+    }
+
+    func testMarkdownInit() throws {
+        let str = try AttributedString(markdown: "**Bold** and *italic*")
+        XCTAssertEqual(str.characters, "Bold and italic")
+        var hasBold = false
+        for run in str.runs {
+            if run.attributes.markdownBold {
+                hasBold = true
+            }
+        }
+        XCTAssertTrue(hasBold)
+    }
+}
+
+#endif