Cotypist
A macOS app that suggests inline text completions as you type, in almost any app, using a local model.
The idea is simple: watch what you type, predict the rest, show it as ghost text, accept with Tab. The prediction part is genuinely easy now, a small local model or even a personal n-gram table over your own writing gets you most of the way. The hard part is everything around it: reading the current text field and caret position through the macOS Accessibility API, drawing an overlay that tracks a moving cursor, and not breaking in Electron apps, Chrome, terminals, password fields, or anything that reimplements text editing. You can build something that works in native AppKit fields in a couple of long sessions and feels magic. Getting it to work everywhere, at typing latency, without eating keystrokes or leaking into your bank login, is where the actual product lives.
Build verification: not recorded. How we judge buildability
What you give up
- Coverage: your build will work in native text fields and fail or misbehave in Electron apps, browsers, and terminals
- Latency polish: suggestions that arrive after you already typed the words are worse than nothing
- Safety rails: skipping password fields, secure input mode, and sensitive apps takes deliberate work
- Personalization that actually improves over months of your writing rather than a one-off corpus dump
- Signed, notarized, auto-updating distribution and the permission onboarding that makes it survive OS updates
Why people still pay
Because a text predictor that works in nine apps and glitches in the tenth is a text predictor you turn off. The value here is invisible reliability across an entire OS, which means quietly handling every app that draws its own caret, every accessibility API quirk, and every macOS release that changes the rules. That is maintenance work, not a build, and it is worth a small license fee to hand off.
Your build guide
The stack, security requirements, and agent rules for a focused replacement.
Before you start
- macOS, Xcode and a selected deployment target. Ask for each OS permission only when its feature is enabled; code signing and distribution are separate release tasks.
- Implementation components: Swift/SwiftUI and AppKit for a visible menu-bar state and non-focus-stealing suggestion panel. Accessibility context and prediction request/revision tokens held in memory only; a local n-gram predictor first, with an optional local model after cancellation is reliable. Codable settings contain only app allowlists, pause preferences and shortcut configuration. No persistent text-context database or history.
- Scope boundary: Universal inline completion and language-model quality are not promised.
Use these project rules and optional skill references alongside the prompt. Review each skill before adding it to your agent; the AGENTS.md export includes the same guidance.
Optional external skill: swiftui-expert-skill — Build and review native SwiftUI views, state management, navigation, accessibility and rendering performance. Review its instructions and compatibility before use; it does not grant deployment, data-access or publication permission.
Optional external skill: swift-concurrency — Design Swift async tasks, actors, isolation, cancellation and safe data sharing, including Swift 6 migration. Review its instructions and compatibility before use; it does not grant deployment, data-access or publication permission.
Optional external skill: sharp-edges — Review security-sensitive APIs and configuration for dangerous defaults and easy-to-misuse interfaces. Review its instructions and compatibility before use; it does not grant deployment, data-access or publication permission.
Project rule — data model: allowlisted apps, bounded caret contexts, ephemeral prediction requests and document/caret revision tokens
Project rule — preserve this invariant: Never persist raw surrounding text; secure fields are excluded and late predictions cannot insert into a changed caret or application.
Project rule — acceptance evidence: Switch apps while a prediction is pending and insert nothing; denied Accessibility permission leaves typing untouched and the assistant paused.
Implementation plan
Phase 1
Scope and fixtures. Implement this bounded workflow: Start with a paused menu-bar assistant for one supported text field. Generate local next-word suggestions, show a non-focus-stealing panel and accept only the current suggestion with Tab; Escape or further typing dismisses it. Record prerequisites, select representative user-owned fixtures and document the unsupported features: Universal inline completion and language-model quality are not promised.
Phase 2
Separate settings from ephemeral input. Persist only the app allowlist, shortcut and pause preferences. Keep bounded caret context, suggestions and request/revision tokens in memory with cancellation. There is no context history to migrate, back up or export; secure fields are excluded before prediction.
Phase 3
Complete the first useful path. Implement the workflow's input, review and output interface, with clear controls and explicit empty/error states. Keep caret context, suggestions and request tokens ephemeral. Cancel them on focus/caret/revision change, clear them on pause or app exit, and accept text only if the original target revision still matches.
Phase 4
Permissions and integration failure. Start capture or automation paused. Respect denied permissions, exclude secure fields and sensitive applications, and expose a visible pause/stop control. Request integration credentials and permissions only for the enabled feature; show a disconnected state instead of mock results.
Phase 5
Portable handoff. Export/import only allowlists, shortcut settings and preferences. Never serialize caret text, suggestions, prediction traces or surrounding document contents; restart paused with no restored text context. Include setup, operating limits, fixture walkthrough and shutdown/restart instructions in the README.
Phase 6
Acceptance scenarios. Switch apps while a prediction is pending and insert nothing; denied Accessibility permission leaves typing untouched and the assistant paused. Repeat the workflow after restart and with a denied permission or unavailable dependency; show recoverable failure rather than a success placeholder.
WORKING SLICE Start with a paused menu-bar assistant for one supported text field. Generate local next-word suggestions, show a non-focus-stealing panel and accept only the current suggestion with Tab; Escape or further typing dismisses it. Build this scoped Cotypist-inspired workflow with a documented data model and visible failure states. Architecture - Swift/SwiftUI and AppKit for a visible menu-bar state and non-focus-stealing suggestion panel. - Accessibility context and prediction request/revision tokens held in memory only; a local n-gram predictor first, with an optional local model after cancellation is reliable. - Codable settings contain only app allowlists, pause preferences and shortcut configuration. No persistent text-context database or history. Prerequisites and limits macOS, Xcode and a selected deployment target. Ask for each OS permission only when its feature is enabled; code signing and distribution are separate release tasks. Outside this release: Universal inline completion and language-model quality are not promised. Data model and correctness allowlisted apps, bounded caret contexts, ephemeral prediction requests and document/caret revision tokens Invariant: Never persist raw surrounding text; secure fields are excluded and late predictions cannot insert into a changed caret or application. Keep caret context, suggestions and request tokens ephemeral. Cancel them on focus/caret/revision change, clear them on pause or app exit, and accept text only if the original target revision still matches. Security and privacy Start capture or automation paused. Respect denied permissions, exclude secure fields and sensitive applications, and expose a visible pause/stop control. Recovery and export Export/import only allowlists, shortcut settings and preferences. Never serialize caret text, suggestions, prediction traces or surrounding document contents; restart paused with no restored text context. Implementation order 1. Phase 1 — Scope and fixtures. Implement this bounded workflow: Start with a paused menu-bar assistant for one supported text field. Generate local next-word suggestions, show a non-focus-stealing panel and accept only the current suggestion with Tab; Escape or further typing dismisses it. Record prerequisites, select representative user-owned fixtures and document the unsupported features: Universal inline completion and language-model quality are not promised. 2. Phase 2 — Separate settings from ephemeral input. Persist only the app allowlist, shortcut and pause preferences. Keep bounded caret context, suggestions and request/revision tokens in memory with cancellation. There is no context history to migrate, back up or export; secure fields are excluded before prediction. 3. Phase 3 — Complete the first useful path. Implement the workflow's input, review and output interface, with clear controls and explicit empty/error states. Keep caret context, suggestions and request tokens ephemeral. Cancel them on focus/caret/revision change, clear them on pause or app exit, and accept text only if the original target revision still matches. 4. Phase 4 — Permissions and integration failure. Start capture or automation paused. Respect denied permissions, exclude secure fields and sensitive applications, and expose a visible pause/stop control. Request integration credentials and permissions only for the enabled feature; show a disconnected state instead of mock results. 5. Phase 5 — Portable handoff. Export/import only allowlists, shortcut settings and preferences. Never serialize caret text, suggestions, prediction traces or surrounding document contents; restart paused with no restored text context. Include setup, operating limits, fixture walkthrough and shutdown/restart instructions in the README. 6. Phase 6 — Acceptance scenarios. Switch apps while a prediction is pending and insert nothing; denied Accessibility permission leaves typing untouched and the assistant paused. Repeat the workflow after restart and with a denied permission or unavailable dependency; show recoverable failure rather than a success placeholder. Acceptance Switch apps while a prediction is pending and insert nothing; denied Accessibility permission leaves typing untouched and the assistant paused. Use real source data or clearly labeled fixtures. Explain unsupported input and provider failures; do not fabricate analytics, delivery receipts, accuracy claims or security guarantees. Optional agent guidance Optional external skill: [swiftui-expert-skill](https://github.com/AvdLee/SwiftUI-Agent-Skill/blob/main/skills/swiftui-expert-skill/SKILL.md) — Build and review native SwiftUI views, state management, navigation, accessibility and rendering performance. Review its instructions and compatibility before use; it does not grant deployment, data-access or publication permission. Optional external skill: [swift-concurrency](https://github.com/AvdLee/Swift-Concurrency-Agent-Skill/blob/main/skills/swift-concurrency/SKILL.md) — Design Swift async tasks, actors, isolation, cancellation and safe data sharing, including Swift 6 migration. Review its instructions and compatibility before use; it does not grant deployment, data-access or publication permission. Optional external skill: [sharp-edges](https://github.com/trailofbits/skills/blob/main/plugins/sharp-edges/skills/sharp-edges/SKILL.md) — Review security-sensitive APIs and configuration for dangerous defaults and easy-to-misuse interfaces. Review its instructions and compatibility before use; it does not grant deployment, data-access or publication permission. Project rule — data model: allowlisted apps, bounded caret contexts, ephemeral prediction requests and document/caret revision tokens Project rule — preserve this invariant: Never persist raw surrounding text; secure fields are excluded and late predictions cannot insert into a changed caret or application. Project rule — acceptance evidence: Switch apps while a prediction is pending and insert nothing; denied Accessibility permission leaves typing untouched and the assistant paused.
$ open in your agent (prompt prefilled, you press enter), copy the prompt or copy or download AGENTS.md
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Questions about Cotypist
Can you build your own Cotypist with AI?
Partly. The idea is simple: watch what you type, predict the rest, show it as ghost text, accept with Tab. The prediction part is genuinely easy now, a small local model or even a personal n-gram table over your own writing gets you most of the way. The hard part is everything around it: reading the current text field and caret position through the macOS Accessibility API, drawing an overlay that tracks a moving cursor, and not breaking in Electron apps, Chrome, terminals, password fields, or anything that reimplements text editing. You can build something that works in native AppKit fields in a couple of long sessions and feels magic. Getting it to work everywhere, at typing latency, without eating keystrokes or leaking into your bank login, is where the actual product lives.
What does the Cotypist build prompt cover?
The prompt starts with this scope: Start with a paused menu-bar assistant for one supported text field. Generate local next-word suggestions, show a non-focus-stealing panel and accept only the current suggestion with Tab; Escape or further typing dismisses it. Full-product capabilities excluded from the comparison include: Coverage: your build will work in native text fields and fail or misbehave in Electron apps, browsers, and terminals; Latency polish: suggestions that arrive after you already typed the words are worse than nothing; Safety rails: skipping password fields, secure input mode, and sensitive apps takes deliberate work. Follow the implementation plan and its prerequisites before expanding the build.
How do I use the prompt, AGENTS.md and agent skills?
Start with the Cotypist prerequisites and stack, then copy the prompt into your coding agent. Save the project rules as AGENTS.md in the project root. Linked skills are optional packages or source instructions for specific tasks; review their current contents and install only those matching the chosen stack. A skill does not supply API credentials or verify the finished app.
How long will this Cotypist project take?
The catalogue estimate is multi-day for the limited scope. Setup, integration approvals, debugging, deployment and ongoing maintenance can add time. This is an estimate, not a delivery guarantee.
What would I give up by replacing Cotypist?
Coverage: your build will work in native text fields and fail or misbehave in Electron apps, browsers, and terminals; Latency polish: suggestions that arrive after you already typed the words are worse than nothing; Safety rails: skipping password fields, secure input mode, and sensitive apps takes deliberate work; Personalization that actually improves over months of your writing rather than a one-off corpus dump; Signed, notarized, auto-updating distribution and the permission onboarding that makes it survive OS updates. Because a text predictor that works in nine apps and glitches in the tenth is a text predictor you turn off. The value here is invisible reliability across an entire OS, which means quietly handling every app that draws its own caret, every accessibility API quirk, and every macOS release that changes the rules. That is maintenance work, not a build, and it is worth a small license fee to hand off.
What price is this guide comparing against?
The recorded Plus plan is $6/mo (monthly, billed annually, 1 Mac), checked 2026-08-18. Check the linked pricing source before buying. Building your own also has hosting, API and maintenance costs; the recorded amount is not a guaranteed saving.
What can I use instead of building Cotypist?
No alternative is listed in this entry yet. That is a gap in this catalogue, not proof that no suitable product exists. Compare the paid product and the proposed scope before committing to a build.