OutlookAI — Free Open-Source Outlook AI Add-in

The free open-source alternative to GPT for Outlook, Mailbutler, Lavender, Compose AI, OtterMail, Boomerang Respondable, Mailmaestro, EmailTree, SaneBox AI, and Spike Magic AI — running inside Microsoft Outlook desktop and billed against your own ChatGPT subscription. No per-seat fee, no proxy server, no data sent through a third party.

OutlookAI is a free, open-source Outlook AI add-in that replaces the paid "AI for Outlook" market. Sign in once with your own ChatGPT Plus or Pro account; OutlookAI runs entirely inside your Outlook process — no proxy, no SaaS middleware, no telemetry. You get:

• Chat in the compose pane: WebView2 surface with streaming, multi-round tool dispatch, model + reasoning-effort picker.
• Inbox Copilot taskpane: selection-aware actions, full multi-round chat over the messages you highlighted.
• Inbox Reports taskpane: templated chips that turn your inbox into markdown reports, action items, vendor breakdowns.
• Excel and PDF exports: model-callable export tools plus a per-message Save as PDF button. Files land in Documents\OutlookAI\Reports\ with Open / Show in folder.
• 16 Outlook tools the model can call (12 always on + 4 admin-gated safe writes; no send / delete / move tools by design).
• Voice transcription from the same ChatGPT credential.
• In-app updates and a live model list: admins install new releases and refresh the models ChatGPT offers from Settings, no reinstall needed.
• Open source under MIT, fork-friendly, auditable end-to-end.

WebView2-based chat that lives next to the draft you are writing. Streaming text, multi-round tool dispatch with visible tool cards, cancellation that preserves partial text, and light / dark / high-contrast themes. Per-turn reasoning-effort override. Clear, copy-to-clipboard, and Save-as-PDF buttons on every assistant message.

Open the AI Assistant taskpane on any Outlook explorer window and the same chat surface attaches to your inbox. Highlighting one or more messages feeds them into the context. Quick-action chips for common workflows ("summarize this thread", "draft a reply") and full freeform chat with the entire 16-tool mailbox surface.

A second taskpane focused on generating reports from your mailbox. Six default templated chips (action items, top senders, out-of-office digest, project status, conversations with a specific person, stats by sender). Each chip prompts the model with a structured intent and renders the markdown response inline. Save the report as PDF, or export underlying data as Excel.

Three model-callable export tools:

• outlook_export_excel produces a styled .xlsx with bold/frozen header row, autofilter, and per-column formatting (text / number / currency / date / datetime / boolean) via ClosedXML.
• outlook_export_pdf renders polished markdown through an isolated off-screen WebView2 instance into A4 PDF with header bar and no chat-UI chrome.
• outlook_export_search_results exports the complete list of messages matching a search to Excel, not just the first page of results: it counts the true total, collects up to a server-set ceiling (2,000 rows by default, 10,000 at most), and reports how many it exported out of how many matched.

All three save to ~\Documents\OutlookAI\Reports\ (or %LOCALAPPDATA%\OutlookAI\Reports\ when Documents is redirected to a network share, as on many RDS servers) with auto-generated, timestamped, collision-safe filenames. Tool results surface as inline file cards with Open and Show in folder buttons. Every file action goes through a path-policy gate that rejects any path outside the Reports directory.

Every assistant message gets a small button that exports just that message — the markdown the chat is showing, not the rendered HTML — to PDF. One click, no model round-trip.

Always on (12):

• outlook_get_current_compose_state
• outlook_get_current_selection
• outlook_list_folders
• outlook_search_messages
• outlook_read_message
• outlook_read_messages (bulk)
• outlook_count_messages
• outlook_aggregate_messages (group + top-N)
• outlook_list_recent_threads_with
• outlook_export_excel
• outlook_export_pdf
• outlook_export_search_results (complete list to Excel)

Admin-gated safe writes (4):

• outlook_create_draft
• outlook_mark_as_read
• outlook_flag_message
• outlook_set_category

By design there is no send, delete, move-to-deleted, or permanent-mutation tool. The admin password gates which write tools, if any, the model can call.

The mic button opens a Realtime WebSocket at wss://api.openai.com/v1/realtime?model=gpt-realtime-1.5 using the same OAuth token. Transcription lands back in the prompt textbox. No separate Whisper API key.

The gear icon opens a password-gated Settings dialog. Its choices apply to every user on the machine (saved in C:\ProgramData\OutlookAI\config.xml):

• ChatGPT account: Sign In / Sign Out / Refresh.
• Model picker driven by the model catalog ChatGPT publishes for your account. Update Models fetches the current list and each model's supported reasoning efforts, so new OpenAI models (and new effort levels such as Max) show up without an OutlookAI release. The list is cached in C:\ProgramData\OutlookAI\models.json and shared with every user on the machine; retirement dates from the catalog are shown next to the model, and a retired model automatically falls back to its announced replacement.
• Reasoning effort dropdown, filtered per model. Auto (called None before v2.2.1) sends no effort, so the model's own default applies (medium today). config.xml still stores it as None, which older versions read the same way.
• 4 checkboxes for the safe-write tools (each can be individually enabled).
• Admin password rotation.
• Updates: Check Now / Install Update for new OutlookAI releases (see Updating).

Public list prices as of writing (May 2026), rounded for readability. The paid alternatives charge per user per month. OutlookAI is free and bills inference against your existing ChatGPT subscription.

• Bring your own ChatGPT subscription — no extra monthly fee. OutlookAI uses the OAuth flow Codex CLI uses (client_id app_EMoamEEZ73f0CkXaXp7hrann), so signing in with your ChatGPT account is enough. There is no second OpenAI API key to manage and no vendor middleman charging a per-seat license.
• Runs entirely in your Outlook process. No proxy server sees your mail or your OAuth token. Every paid Outlook AI add-in we know of routes either your email content, your API key, or both through their own servers. OutlookAI talks directly to chatgpt.com/backend-api/codex/responses (text) and wss://api.openai.com/v1/realtime (voice) from inside Outlook.exe.
• Real tools, real reports, real exports. Not just "rewrite this email." The model can search your mailbox, summarize threads, aggregate by sender or day, draft replies, and export the results to Excel or PDF — all from inside the chat window, with explicit tool cards so you can see every read.
• Open source under MIT. Read the code, fork it, audit the security model, contribute. The competitors are black boxes.

Each release has an install bundle, OutlookAI-vX.Y.Z-RDS-Deploy.zip, and its .sha256. Download both, then in an elevated PowerShell in the download folder:

# 1. Check the download (must print True) and extract it
$zip = ".\OutlookAI-vX.Y.Z-RDS-Deploy.zip"
(Get-FileHash $zip -Algorithm SHA256).Hash -eq (Get-Content "$zip.sha256").Trim()
Unblock-File $zip
Expand-Archive $zip -DestinationPath C:\OutlookAI

# 2. Install
Set-ExecutionPolicy -Scope LocalMachine -ExecutionPolicy RemoteSigned
C:\OutlookAI\Install-OutlookAI.ps1 -SourcePath C:\OutlookAI

# 3. Open Outlook → AI Assistant → sign in with your ChatGPT account.

The same bundle covers single workstations, multi-user RDS / Terminal Server and IT-managed images; see Deploy/README.txt, which is also inside the zip. To build the bundle from source instead, see Contributing.

Admins update from inside Outlook: Settings → Updates → Check Now, then Install Update when a newer release is out. It downloads the release, checks its SHA256 and runs the installer with administrator rights. The installer closes Outlook for every user on the machine and leaves it closed, so warn RDS users first; everyone reopens Outlook afterwards. Updates keep the settings.

Outlook.exe
  └── AITaskPane (WinForms tab control)
        ├── Chat                  → ChatController → CodexChatService → OpenAI Codex Responses API
        ├── Inbox Copilot         → InboxCopilotController → CodexChatService → tool catalog
        └── Inbox Reports         → InboxReportsController → CodexChatService → tool catalog

CodexChatService
  └── OutlookToolHost / ToolDispatcher
        └── 16 IOutlookTool implementations
              ├── LiveOutlookSurface (Outlook COM)
              └── Services/Export
                    ├── ExcelWorkbookBuilder       (ClosedXML)
                    ├── PdfRenderer                (off-screen WebView2 + PrintToPdfAsync)
                    └── PrintTemplateRenderer

ExportBridge (WebMessageReceived)
  ├── export_pdf
  ├── open_file              ← path policy
  └── reveal_in_explorer     ← path policy

Key components:

• Services/CodexChatService.cs — Codex Responses request/streaming, multi-round tool dispatch, parallel tool calls, cancellation.
• Services/OutlookToolHost.cs — tool catalog construction and per-tool admin-write gating.
• Services/Tools/LiveOutlookSurface.cs — Outlook COM surface for read/write/search; AdvancedSearch + iterative-folder fallback; budgeted / early-stop scan policy.
• Services/Export/ExcelWorkbookBuilder.cs, Services/Export/ExcelCellCoercer.cs — ClosedXML Excel construction.
• Services/Export/PdfRenderer.cs, Services/Export/PrintTemplateRenderer.cs — off-screen WebView2 PDF rendering with an isolated user-data folder.
• TaskPane/Chat/ExportBridge.cs — WebView2 host-message bridge for export_pdf / open_file / reveal_in_explorer, guarded by IExportPathPolicy.
• WebUI/* — chat.js, markdown.js, styles.css, print-template.html, print-styles.css. Extracted from chat.js so the same markdown renderer drives chat and PDF.
• Services/Models/ — the model catalog behind Update Models: fetch, cache (models.json), per-model reasoning efforts, retirement routing.
• Services/Updates/ — the in-app updater: GitHub Releases lookup, download and SHA256 check, elevated install, update history.
• Config.cs, SettingsForm.cs — the config layers and the Settings dialog; saves are merged into the ProgramData config.xml under a cross-process lock (Services/FileLock.cs).

• OAuth via ChatGPT. Tokens stored at C:\ProgramData\OutlookAI\auth.json (machine, RDS-shared by design).

• Settings (model, effort, write tools and the admin password, in plain text) live in C:\ProgramData\OutlookAI\config.xml, which every signed-in user on the machine can read and change, like auth.json.

• No telemetry. In normal use, outbound traffic is two endpoints from inside Outlook.exe:

  • https://chatgpt.com/backend-api/codex/responses (text inference).
  • wss://api.openai.com/v1/realtime (voice transcription).

  Admin actions in Settings add, only when clicked:

  • Check Now / Install Update: api.github.com and the release download for kirklandsig/OutlookAI.
  • Update Models: https://chatgpt.com/backend-api/codex/models (the model catalog), one rejected /responses call that reports the server's reasoning-effort values, and api.github.com for the latest openai/codex release number. Only the ChatGPT calls carry the sign-in token.

• Path policy on file actions. IExportPathPolicy.RequireInsideReportsDir(...) rejects any open/reveal path that escapes the Reports folder (Documents\OutlookAI\Reports\, or %LOCALAPPDATA%\OutlookAI\Reports\ when Documents is on a network share). Path traversal attempts are logged and never launched.

• No destructive tools. No outlook_send_message, outlook_delete_message, or outlook_move_to_deleted. The admin can additionally disable any safe-write tool from Settings.

• RDS shared-credential. On multi-user servers, auth.json grants Authenticated Users: Modify — explicit accepted risk. Rotation procedure is documented in Deploy/README.txt.

Yes — OutlookAI. It is open-source under MIT, free to use, and bills inference against your existing ChatGPT Plus or Pro subscription instead of charging a separate per-seat fee.

Yes. OutlookAI uses the same OAuth client ID Codex CLI uses, so signing in with your ChatGPT account is enough. There is no second OpenAI API key to manage.

Mailbutler bundles AI drafting with snooze, send-later, tracking, and other email-productivity features that route data through their servers. Lavender focuses on tone scoring and predictive suggestions during typing. Compose AI is a browser autocomplete. OutlookAI is a native VSTO add-in with multi-round chat, taskpane Copilot, taskpane Reports, and Excel/PDF exports — different design point, broader capability surface, $0 vs $5-$89/mo.

Not yet. OutlookAI is a VSTO add-in for Outlook desktop on Windows (Outlook 2016, 2019, 2021, 2024, and Microsoft 365). OWA and Outlook for Mac would require a separate Office.js add-in; that is on the long-term roadmap.

For text, whatever the admin picks in Settings from the models ChatGPT offers your account (the default is the catalog's top model; gpt-6-astra as of September 2026). Voice uses gpt-realtime-1.5. Settings → Update Models refreshes the list.

Two endpoints in normal use, both directly from Outlook.exe, plus the update and model-list checks an admin runs from Settings. No third-party proxy, no telemetry, no analytics. See the Security model section above.

Yes — that is the primary deployment target. See Deploy/README.txt.

In an elevated PowerShell, run the uninstaller from the install bundle (or from Deploy\ in a clone):

C:\OutlookAI\Uninstall-OutlookAI.ps1

It removes the add-in registration, the install folder and the shared ChatGPT sign-in (C:\ProgramData\OutlookAI\auth.json). It keeps the rest of C:\ProgramData\OutlookAI — the Settings in config.xml (including the admin password), models.json and Backups\ — so a reinstall picks them up; delete that folder too to remove everything.

Shipped (details on the Releases page):

• v2 (ChatGPT OAuth): Chat tab in the compose pane, Inbox Copilot and Inbox Reports taskpanes, Excel and PDF export tools, per-message Save as PDF, file cards guarded by the path policy, voice transcription, and the RDS / Terminal Server install path. No API keys to manage.
• v2.1.0: in-app updater (Settings → Updates).
• v2.1.1: exports work when Documents is redirected to a network share; admin Settings are shared through C:\ProgramData\OutlookAI\config.xml.
• v2.1.2 / v2.1.3: outlook_export_search_results for complete-list Excel exports (with a folder column); searches report the true total when they truncate.
• v2.2.0: Update Models — the model list and reasoning efforts come from ChatGPT's catalog, and retired models route to their replacement.
• v2.2.1: updates keep config.xml and per-user settings; the None effort is now called Auto.
• v2.2.2: Settings saves for every user, and the ProgramData file wins over old per-user copies.

Known gaps / explicit follow-ups:

• Multi-sheet Excel workbooks.
• PDF page numbers, footer, table of contents, embedded images.
• Settings UI for picking a custom Reports folder.
• CSV, .docx, and other export formats.
• Automatic model-list refresh (Update Models is a manual button).
• Mac / OWA support (would require an Office.js add-in).

# Clone
git clone https://github.com/kirklandsig/OutlookAI.git
cd OutlookAI

# One-time: a certificate to sign the add-in's manifests. The maintainer's
# isn't in the repo, and installs trust the add-in by location, so any will do.
$cert = New-SelfSignedCertificate -Type CodeSigningCert -Subject "CN=OutlookAI dev" `
  -CertStoreLocation Cert:\CurrentUser\My

# Restore and build (Debug)
& "C:\Program Files\Microsoft Visual Studio\18\Community\MSBuild\Current\Bin\MSBuild.exe" `
  "VSTO2\OutlookAI.sln" /restore /p:RestorePackagesConfig=true /p:Configuration=Debug `
  /p:Platform="Any CPU" /p:ManifestCertificateThumbprint=$($cert.Thumbprint)

# Test (898 tests, all passing)
& "C:\Program Files\Microsoft Visual Studio\18\Community\Common7\IDE\CommonExtensions\Microsoft\TestWindow\vstest.console.exe" `
  "VSTO2\OutlookAI.Tests\bin\Debug\net472\OutlookAI.Tests.dll"

# Build an install bundle (the same zip a release has) into out\
.\Deploy\Make-ReleaseZip.ps1 -Tag v0.0.0-dev -OutDir out -CertThumbprint $cert.Thumbprint

Building needs Visual Studio with the Office/SharePoint development workload. To install your build, run out\staging-v0.0.0-dev\Install-OutlookAI.ps1 -SourcePath out\staging-v0.0.0-dev elevated. In a later session, find the certificate again with Get-ChildItem Cert:\CurrentUser\My -CodeSigningCert.

Branch model:

• master — released code; each release is tagged vX.Y.Z.
• feature/<name> and fix/<name> — work branches, merged with --no-ff. Larger features get a spec under docs/superpowers/specs/ and a plan under docs/superpowers/plans/.

Spec / plan workflow: see docs/superpowers/ for the spec → plan → implementation cycle that drove every phase of OutlookAI development.

• Windows 10 / 11, or Windows Server 2019 / 2022 / 2025.
• Microsoft Outlook 2016 / 2019 / 2021 / 2024 / Microsoft 365 (desktop).
• .NET Framework 4.7.2.
• Visual Studio Tools for Office Runtime.
• Microsoft Edge WebView2 Evergreen Runtime (installed by Install-OutlookAI.ps1 if missing).

MIT — see LICENSE.