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Technology Stack

X21 combines mature Microsoft Office technologies with modern TypeScript tooling to deliver an AI-first Excel workflow. The following table summarises the major components and why they were chosen.

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Runtime & Frameworks

  • Microsoft Office VSTO (.NET Framework 4.8) – Hosts the add-in, exposes COM interop to Excel, and embeds WebView2 for the task pane UI.
  • Deno 1.43+ – Provides a secure TypeScript runtime with built-in tooling, WebSocket support, and native bundling for the orchestration server.
  • React 18 + Vite – Powers the task pane UI with fast refresh and a lightweight bundle suitable for WebView2.
  • shadcn/ui + Tailwind CSS – Supplies accessible UI primitives for lists, dialogs, badges, and tool approval cards.

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AI & Conversation Handling

  • Anthropic Claude Sonnet 4.5 – Primary large language model accessed through the official SDK (@anthropic-ai/sdk). Used for streaming responses, summarisation, and tool planning.
  • Tooling Framework – A hand-rolled tool abstraction where each tool class implements Tool.execute. Tools include read/write operations, formatting, sheet management, formulas, charts, and VBA tasks.
  • Conversation Compacting – When token limits are approached, compacting.ts summarises prior turns using Claude’s messages.create and replaces history with a concise prompt.

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Telemetry & Analytics

  • Langfuse – Captures traces, spans, tool approvals/rejections, scores, and feedback. The Deno service streams metadata and attaches workbook + session IDs.
  • PostHog – Used in both the UI and VSTO layer for behaviour analytics. The VSTO service calls PostHogService.Identify, while the UI reports feature usage via posthog.ts.
  • NLog – Provides structured logging for the .NET side, alongside a custom rotating logger for Deno.

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Packaging & Deployment

  • ClickOnce – Ships the add-in with automatic updates. The Publish.ps1 script orchestrates version bumps, signing, rclone uploads, and optional zip packaging.
  • rclone + Cloudflare R2 – Distributes published artifacts to cloud storage buckets per environment.
  • Supabase – Handles authentication (PKCE), user management, and optional row-level policies.

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Supporting Libraries

  • WebView2 – Embeds the web UI inside Excel and enables message passing through chrome.webview.
  • Oak-style Router – A lightweight custom router built around Deno.serve handles HTTP requests and WebSocket upgrades.
  • Deno Standard Library – Supplies logging, file system utilities, path manipulation, and testing helpers.
These technologies let X21 deliver responsive user experiences, enforce security boundaries, and keep the deployment pipeline manageable for enterprise environments.