YouTube Transcript Monorepo

A lightweight Bun workspace monorepo for downloading YouTube transcripts.

Why this exists

YouTube strictly rate-limits or blocks non-residential IP addresses (like those of Cloudflare Workers, AWS, or other cloud edge hosting). To reliably download transcripts, this project uses a client-side architecture:

1. Userscript: Runs in the browser and injects a "Transcript" button on YouTube videos.
2. Local HTTP Server: A background process listening on localhost:3456. When you click the userscript button, it sends the video ID to this server, which scrapes the transcript using your residential IP address and returns it.

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Package Directory Layout

• packages/scraper: Core library that contacts YouTube's Innertube API, fetches the caption track, and parses it (zero build step in local development).
• packages/server: Background HTTP API server that loads the scraper directly in-memory (no subprocess spawning).
• packages/userscript: Browser userscript built using Vite and vite-plugin-monkey.
• packages/cli: Command-line interfaces, tests, and debugging scripts.

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Getting Started

1. Install Dependencies

Run from the root directory:

bun install

2. Configure Environment Variables

Copy .env.example to .env in the root directory:

cp .env.example .env

(Optionally define a SERVER_API_KEY for authenticating calls between the userscript and the server. If configured, you should also create packages/userscript/.env containing VITE_TRANSCRIPT_API_KEY=your_key)

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Development & Utility Commands

Execute these commands from the root directory:

Running the Server & Userscript

• Start local server (with hot reload):

  bun run dev:server

• Build userscript:

  bun run build:userscript

  (Produces output inside packages/userscript/dist/youtube-copy-transcript.user.js for installation in Tampermonkey/Violentmonkey)

Using CLI & Inspection Tools

• Run CLI to print transcript:

  bun run cli <videoId-or-URL>

• Inspect raw Innertube player response (for debugging):

  bun run inspect <videoId>

Code Quality & Formatting

• Run unit tests:

  bun run test

• Run workspace typechecks:

  bun run typecheck

• Format & Lint code (handled automatically on commit via Lefthook):

  bun run format
  bun run lint

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Data Flow & Workflows

1. Normal Workflow (Browser Userscript + Local Server)

This is the standard workflow when browsing YouTube. The transcript is fetched via the local server using your home IP and copied to your browser's clipboard automatically.

sequenceDiagram
    autonumber
    actor User as Browser User
    participant Script as Userscript (packages/userscript)
    participant Server as Bun Server (packages/server)
    participant Scraper as Scraper Library (packages/scraper)
    participant YouTube as YouTube API

    User->>Script: Click "Transcript" Button
    Script->>Script: Extract video ID from DOM/URL
    Script->>Server: HTTP GET /transcript?videoId=<id> (x-api-key header)
    Server->>Server: Validate x-api-key
    Server->>Scraper: Call fetchTranscript(videoId) in-memory
    Scraper->>YouTube: Fetch watch page HTML (extract Innertube API Key)
    Scraper->>YouTube: POST /youtubei/v1/player (get captionTracks list)
    Scraper->>YouTube: GET caption track URL (fetch raw XML transcript)
    Scraper->>Scraper: Parse XML, convert to Plaintext, format with YAML metadata
    Scraper-->>Server: Return formatted transcript string
    Server-->>Script: HTTP 200 JSON { videoId, transcript }
    Script->>Script: Write transcript text to Clipboard (GM_setClipboard)
    Script->>Script: Persist video ID to storage (GM_setValue)
    Script-->>User: Show "Copied!" button state & green dot indicators

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• Who fetches the transcript? The @youtube-transcript/scraper library (embedded in @youtube-transcript/server) coordinates the requests to YouTube.
• Who copies to the clipboard? The browser-level userscript processes the returned text and writes it to your system clipboard using Tampermonkey's GM_setClipboard.
• Who persists the state? The userscript saves the copied video ID locally via Tampermonkey storage so that the button can reflect a "previously copied" indicator (a green dot) if you revisit the page.

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2. Alternative Workflow (Command-Line Interface / CLI)

This is the developer workflow. You run the scraping pipeline directly in your terminal, and can optionally output transcripts, save debug files, or copy to the clipboard.

sequenceDiagram
    autonumber
    actor Dev as Developer / Terminal
    participant CLI as CLI wrapper (packages/cli)
    participant Scraper as Scraper Library (packages/scraper)
    participant YouTube as YouTube API
    participant System as System Clipboard

    Dev->>CLI: bun run cli <videoId-or-URL> --copy --debug
    CLI->>Scraper: Call fetchTranscript(videoId, config) in-memory
    Scraper->>YouTube: Fetch watch page, caption track list, and XML transcript
    Scraper->>Scraper: Parse and format output
    Scraper-->>CLI: Return transcript and metadata
    CLI->>Dev: Print formatted transcript to stdout
    Note over CLI,System: If --copy / -c flag is passed
    CLI->>System: Copy transcript text using clipboardy library
    Note over Scraper: If --debug flag is passed
    Scraper->>CLI: Write raw logs, HTML, and JSON to ./debug/

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• Who fetches the transcript? The @youtube-transcript/scraper library (imported by @youtube-transcript/cli).
• Who copies to the clipboard? The @youtube-transcript/cli package, utilizing the clipboardy library to interface with your OS clipboard.
• Who writes debug files? The @youtube-transcript/scraper debug handler writes intermediate HTTP responses to a local ./debug directory if the --debug flag is passed.