GCBC for developers: check a claim against your own files
From your own code over GCBC's local HTTP API, or from Claude Desktop or Cursor over MCP: the file, the lines and the exact quote behind a claim, or a refusal that names its law. Every command below was run on the shipped app by the check that guards this page.
1. Install GCBC
Download GCBC from the link in your email and drag it into Applications. It is signed with a Developer ID and notarized by Apple, so macOS checks it and lets it open.
2. Open GCBC
Open it: the page headed Welcome to GCBC appears in your browser.
3. Paste your key
Paste the key from your email and click Activate; the page says Activated.
4. Answer the setup questions
Answer the eight setup questions and click Save and open the board.
5. Add the folder your code should check against
Click Add documents… and choose the folder. Wait for Added.
6. Print the commands for your copy
GCBC carries its own Node, so nothing needs installing (on an Intel Mac write darwin-x64 in place of darwin-arm64). Open Terminal and run: cd /Applications/GCBC.app/Contents/Resources && ./runtime/darwin-arm64/bin/node gcbc.mjs kit. It prints GCBC developer kit and Three examples, each a line you can paste as it is, with a claim your own documents support.
7. Check a claim over the local HTTP API
While GCBC is open, its local HTTP API answers on GCBC's own 127.0.0.1 address and only this computer can reach it. Run: cd /Applications/GCBC.app/Contents/Resources && GCBC_DATA="$HOME/Library/Application Support/GCBC" ./runtime/darwin-arm64/bin/node developer-kit/verify-http.mjs "What was the pick rate after the rollout?". It prints sources: with your indexed files, then verify: cite with the file and line numbers, and the exact lines quoted from your file. Put your own claim in the last quotes.
8. Add GCBC to Claude Desktop or Cursor
Run: cd /Applications/GCBC.app/Contents/Resources && ./runtime/darwin-arm64/bin/node gcbc.mjs mcp --config. It prints an mcpServers entry named gcbc. Add that entry to claude_desktop_config.json for Claude Desktop, or to .cursor/mcp.json for Cursor, and restart the app. The client then starts GCBC's MCP server itself, over stdio, with three tools: list_sources, get_claims and verify_claim.
9. Check a claim over MCP
To see what the client gets without opening one, run: cd /Applications/GCBC.app/Contents/Resources && GCBC_DATA="$HOME/Library/Application Support/GCBC" ./runtime/darwin-arm64/bin/node developer-kit/verify-mcp.mjs "What was the pick rate after the rollout?". It starts the MCP server exactly as the printed entry tells a client to, lists its tools (verify_claim among them), calls verify_claim and prints verify: cite with the file, the lines and the quote. GCBC does not need to be open for this one.
10. A claim your files do not hold is refused, with its law
Ask about something your files do not cover: cd /Applications/GCBC.app/Contents/Resources && GCBC_DATA="$HOME/Library/Application Support/GCBC" ./runtime/darwin-arm64/bin/node developer-kit/verify-mcp.mjs "What is the parental leave policy in Germany?". It prints REFUSED · Law and the law it follows. Nothing is guessed, so every citation your code does get back can be checked against the file it names.