iOS 27 Writing Tools for Developers: Opt-In Mechanics, writingToolsBehavior, and Third-Party AI

At a glance: iOS 27 adds Writing Tools — an AI-powered text editing UI — to every UITextView, NSTextView, and WKWebView by default. Developers control depth via writingToolsBehavior. New in iOS 27: apps with a Siri Extension can power Writing Tools with their own AI, replacing Apple Intelligence for their users. Part of our Builder’s Log.

If your app has a text view, Writing Tools is already in it. That’s the default behavior starting in iOS 27.

Writing Tools is the toolbar that appears when a user selects text — offering Proofread, Rewrite, Summarize, Create Table, and similar operations powered by Apple Intelligence. In iOS 18 and 26, Apple shipped the initial version of this system. In iOS 27, two things changed: the API to control it matured, and third-party AI can now power it.

This guide is for developers who need to understand exactly what Writing Tools does to their text views, how to configure it, and — if you’ve built an AI product — how to hook your AI into the system.

What Writing Tools Does to Your App by Default

When your app runs on iOS 27 with a UITextView, the system automatically:

Adds a Writing Tools button to the text selection menu (the callout that appears on tap-and-hold)
Provides a secondary toolbar strip for quick access to Proofread and Rewrite when text is selected
Shows a Writing Tools panel when the user taps the full Writing Tools button

This happens with zero code changes. Your existing UIKit and SwiftUI text views get Writing Tools for free.

What the user sees depends on whether their device has Apple Intelligence enabled and available. On devices without Apple Intelligence, Writing Tools falls back to a subset of non-AI operations (like basic proofreading using on-device grammar checking). On devices with Apple Intelligence, the full LLM-powered suite is available.

writingToolsBehavior: Controlling What Writing Tools Can Do

UITextView and NSTextView have a writingToolsBehavior property. It’s an enum with three cases:

Value	Behavior
`.default`	Full Writing Tools, including AI-powered rewriting and summarization
`.limited`	Shows Writing Tools UI but restricts to grammar/spell check only — no AI rewriting
`.none`	Removes Writing Tools entirely from this text view

// Exclude Writing Tools from a code editor text view
let codeEditor = UITextView()
codeEditor.writingToolsBehavior = .none

// Allow grammar check but not AI rewriting in a legal document field
let legalField = UITextView()
legalField.writingToolsBehavior = .limited

// Default — full AI Writing Tools (this is the default, no need to set it)
let noteField = UITextView()
noteField.writingToolsBehavior = .default

When to use .none: Code editors, markdown source views, terminal emulators, any view where the content should not be treated as natural language prose. AI rewriting of code or structured data produces wrong results and confuses users.

When to use .limited: Forms where you want the spelling/grammar safety net but don’t want AI to silently restructure user-written content. Legal agreements, medical intake forms, and similar high-stakes text fields fall here.

When to leave it as .default: Notes, messages, email composition, documents — anywhere users write prose and would benefit from AI assistance.

SwiftUI: TextEditor

SwiftUI’s TextEditor surfaces the same control through a modifier:

TextEditor(text: $code)
    .writingToolsBehavior(.none)

TextEditor(text: $documentBody)
    .writingToolsBehavior(.default)

The modifier is available in SwiftUI for iOS 27+, macOS 27+, and visionOS 27+. For earlier OS targets, the modifier silently does nothing — you don’t need to guard it.

WKWebView Support

WKWebView also participates in Writing Tools. Content loaded in a web view gets Writing Tools on editable text fields and contenteditable regions.

Control is through WKWebViewConfiguration:

let config = WKWebViewConfiguration()
config.writingToolsBehavior = .none  // or .limited, .default
let webView = WKWebView(frame: .zero, configuration: config)

Note that this is a configuration-time setting on WKWebView — you cannot change it after the web view is created. Plan your web view initialization accordingly.

For apps that embed rich text editors in a web view (Notion-style editors, draft.js, Prosemirror, etc.), .none is usually the right call: the web-based editor has its own AI integration surface, and Writing Tools layering on top creates a confusing UX with two competing AI edit flows.

What Happens During a Writing Tools Operation

Understanding the event flow helps you handle edge cases.

When a user triggers a Writing Tools operation:

The system reads the text in the range the user selected (or the entire text view if no selection)
Apple Intelligence (or your Siri Extension’s AI — more below) processes it
The result is previewed inline — the user sees the rewritten text in context before accepting
The user confirms or dismisses; on confirm, the text view replaces the content

Your text view delegate receives standard textViewDidChange notifications after confirmed changes. Writing Tools does not bypass your delegate or text storage.

Important edge case: Writing Tools reads the attributed string content of your text view. If you use NSTextStorage with custom attributes for semantic markup (tracked changes, annotation IDs, internal metadata), those attributes may not survive a Writing Tools rewrite if the user accepts. Build your text storage handling to tolerate rewrites that drop custom attributes on rewritten ranges.

Receiving Callbacks During Writing Tools Interactions

If your app needs to know when Writing Tools is active (to pause autosave, disable undo grouping, show a status indicator), use UITextViewDelegate:

extension YourViewController: UITextViewDelegate {
    func textViewWritingToolsWillBegin(_ textView: UITextView) {
        // Pause autosave, disable undo
        autosaveTimer?.invalidate()
        undoManager?.disableUndoRegistration()
    }

    func textViewWritingToolsDidEnd(_ textView: UITextView) {
        // Resume autosave, re-enable undo
        startAutosaveTimer()
        undoManager?.enableUndoRegistration()
    }
}

The analogous NSTextViewDelegate methods exist for macOS.

Third-Party AI Powering Writing Tools: The Siri Extension Path

This is the significant new capability in iOS 27 for AI product builders.

Prior to iOS 27, Writing Tools always used Apple Intelligence — your app had no ability to substitute its own AI model. In iOS 27, if your app implements a Siri Extension with writing capabilities declared, users can choose your AI as the Writing Tools engine.

How the user sees it: In Settings → Apple Intelligence & Siri → Writing Tools, users can select which AI powers their Writing Tools system-wide. If your app’s Siri Extension is installed and declares writing capability, it appears in that list alongside Apple Intelligence. When selected, your AI handles all Writing Tools operations system-wide.

How this works from the extension side:

Your Siri Extension implements WritingToolsIntent from the SiriKit framework (new in iOS 27). The system calls your extension with the selected text, operation type (rewrite, summarize, make friendly, etc.), and context. Your extension returns the modified text.

import SiriKit
import Intents

class WritingToolsIntentHandler: NSObject, WritingToolsIntentHandling {
    func handle(intent: WritingToolsIntent, completion: @escaping (WritingToolsIntentResponse) -> Void) {
        let inputText = intent.selectedText ?? ""
        let operation = intent.operationType  // .rewrite, .summarize, .proofread, .makeList, etc.

        Task {
            let result = try await yourAIEngine.process(
                text: inputText,
                operation: operation,
                context: intent.documentContext
            )
            let response = WritingToolsIntentResponse(code: .success, userActivity: nil)
            response.resultText = result
            completion(response)
        }
    }
}

What operationType values you handle: The system passes the Writing Tools operation the user selected. Your extension should handle at minimum .proofread, .rewrite, and .summarize — these are the three most used operations. If your extension returns an error for an operation type, the system falls back to Apple Intelligence for that operation.

Document context: intent.documentContext is a string with surrounding text beyond the selection — typically a few hundred characters before and after. Use this for coherent rewrites that don’t lose thread-of-thought at selection boundaries.

Declaring Writing Capability in Your Siri Extension

Your Info.plist needs to declare the capability for the system to surface your extension in Settings:

<key>NSExtension</key>
<dict>
    <key>NSExtensionAttributes</key>
    <dict>
        <key>IntentsSupported</key>
        <array>
            <string>WritingToolsIntent</string>
        </array>
        <key>WritingToolsCapabilities</key>
        <array>
            <string>Rewrite</string>
            <string>Proofread</string>
            <string>Summarize</string>
            <string>MakeFriendly</string>
            <string>MakeProfessional</string>
            <string>MakeConcise</string>
        </array>
    </dict>
</dict>

Only declare capabilities you actually implement. The system shows your extension to users in the capabilities you declare.

What Most Apps Actually Need

Most apps do not need a Siri Extension for Writing Tools. The summary:

Notes, documents, messaging apps: Leave writingToolsBehavior as .default. Users get Apple Intelligence Writing Tools for free. No configuration needed.
Code editors, terminal apps, markdown source views: Set writingToolsBehavior = .none. Done.
High-stakes text fields (legal, medical, financial): Consider .limited to get grammar checking without AI rewriting.
Apps with their own AI writing product (Notion AI, Grammarly, etc.): Consider implementing a Siri Extension to surface your AI in the Writing Tools list, giving users the choice.

The only developers who need to read the Siri Extension section are those building an AI writing product that competes with or complements Apple Intelligence.

Writing Tools and Foundation Models: Not the Same

A common confusion: Writing Tools is not the same as the Foundation Models API.

	Foundation Models	Writing Tools
Who calls it	Your app code	The system (on behalf of user)
What it does	General LLM inference	Text editing operations on selected text
Integration point	`LanguageModelSession`	`writingToolsBehavior` + (optionally) Siri Extension
User sees	Your custom UI	System Writing Tools UI

Foundation Models is the direct API — you call the model, you build the UI, you control the experience end-to-end.

Writing Tools is the system-managed experience — Apple controls the UI, the user triggers it, and you either accept the default (Apple Intelligence) or replace the engine (via Siri Extension).

They serve different use cases. Most apps that use Foundation Models directly do not need to touch Writing Tools configuration beyond setting .none on code/structured-data fields.

WWDC 2026 Sessions

“Integrate Writing Tools into your app” — covers writingToolsBehavior, delegate callbacks, and the edge cases
“Extend Writing Tools with your AI” — covers the Siri Extension integration path for third-party AI
“Meet Writing Tools” — user-facing overview, useful for understanding what users experience

The “Extend Writing Tools” session is the one most developers skip and then regret. If you’re building any kind of AI writing product, watch it before deciding not to implement the extension.

iOS 27 Apple Intelligence for Developers: Which Framework Do You Actually Need? — framework decision guide
iOS 27 Siri Extensions API: Builder Guide — full Siri Extensions integration
Apple Foundation Models iOS 27: On-Device LLM API — direct LLM API
App Intents + AssistantSchemas: iOS 27 Builder Guide — Siri action integration

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