Fantasy Baseball for Claude & Codex

Manage your Yahoo Fantasy Baseball team from Claude or Codex with built-in skills for daily roster reviews, weekly post-mortems, automated lineup adjustments, and free-agent scouting.

Roster-changing actions are supported through Claude in Chrome or the Codex Browser plugin, while other features use Yahoo Fantasy Sports API.

For deeper player-performance analysis, we use the MLB Stats API, Baseball Savant Statcast, and FanGraphs.

Contents

• Install
  • Claude Desktop
  • Codex desktop app
• Connect your Yahoo team
• Talk to your team
• Good to know
• License

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Install

Pick the app you use — you only need one. Then continue to Connect your Yahoo team.

Claude Desktop

1. Add the extension

1. Download fantasy-baseball-vX.X.X.mcpb from the Releases page.
2. In Claude Desktop, go to Settings → Extensions.
3. Drag the file into the Extensions window and click Install.

Leave the Client ID / Client Secret boxes empty for now — you'll fill them in during Connect your Yahoo team.

2. Add the Fantasy Baseball skill (guided start/sit and roster-review workflow)

1. Download fantasy-baseball-skill-vX.X.X.zip from the Releases page.
2. Go to Settings → Capabilities and turn on code execution.
3. Go to Customize → Skills, click + → Create skill → Upload a skill, and choose the ZIP.

Codex desktop app

Requires Node.js (nodejs.org/download — install the LTS build). The plugin bundles the Yahoo tools and the Fantasy Baseball skill in one step.

1. In the Codex app, go to Settings → Plugins → Add plugin marketplace.

2. Add from a GitHub repo:

   • Source: dingyiyi0226/fantasy-baseball-mcp
   • Git ref: master
   • Sparse paths: (leave blank)

3. Open the Fantasy Baseball marketplace and install the Fantasy Baseball plugin.

4. Sign in to Yahoo in Codex's in-app browser (one time, for lineup and add/drop moves).

   The Yahoo authorization below lets the plugin read your roster. Browser-driven lineup and add/drop moves also need Yahoo Fantasy to be logged in inside Codex's in-app browser. In a Codex chat, ask:

   hljs language-text

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   Open https://baseball.fantasysports.yahoo.com in the in-app browser so I can sign in.
   

   When the in-app browser tab opens, sign in to Yahoo there. After that, adjust-lineup and add-drop-player workflows can use the same in-app browser session. If a workflow opens Yahoo and finds that the tab is not logged in, it will stop and report the login state as an error instead of trying another browser.

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Connect your Yahoo team

You only do this once, and it's the same in both apps: type fantasy start in a chat and follow the prompts.

a) Create your free Yahoo app

Yahoo requires each person to use their own keys. Go to developer.yahoo.com/apps/create and fill in:

• Application Name: anything, e.g. My Fantasy Helper
• Homepage URL: https://localhost:8488
• Redirect URI(s): https://localhost:8488/callback
• OAuth Client Type: Confidential Client
• API Permissions: Fantasy Sports → Read
• Click Create App

Yahoo then gives you a Client ID and a Client Secret.

b) Enter those keys

Paste the Client ID and Client Secret into the chat when asked, then say fantasy start again. (In Claude Desktop you can instead enter them under Settings → Extensions → Fantasy Baseball.)

c) Authorize and finish

Open the authorization link and click Agree. If your browser warns about a self-signed certificate, click Advanced and continue to localhost. Once you see Authorization complete!, say fantasy authorize.

The assistant will find your leagues, set your default team, and finish setup.

Stuck? Type fantasy status to see what's still missing.

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Talk to your team

Browser actions use the signed-in Yahoo session: Codex's in-app browser or Claude's connected Chrome. They stop if Yahoo is not signed in or the requested move is ambiguous. You can always ask for advice first; an add/drop is submitted only after you explicitly approve the exact player or pair.

Good to know

• Your Yahoo app credentials are stored locally in ~/.fantasy-baseball-mcp/config.json.
• For batters, we use season, 14-day, and 30-day splits; xBA, xSLG, xwOBA, and their expected-vs-actual gaps; exit velocity, hard-hit rate, barrel rate, launch angle, sweet-spot rate, and batted-ball distance; WAR, wRC+, wOBA, ISO, BABIP, K%, BB%, SwStr%, GB%, FB%, HR/FB, and LD%.
• For pitchers, we use season, 14-day, and 30-day splits; xBA, xSLG, and xwOBA allowed; exit velocity, hard-hit rate, barrel rate, and launch angle allowed; WAR, ERA, FIP, xFIP, WHIP, K%, BB%, SwStr%, GB%, FB%, HR/FB, BABIP, and LOB%.

License

MIT