Guard Core as a ChatGPT plugin

The same tools that power the local MCP server are also served remotely at https://mcp.guard-core.com/mcp and packaged as a ChatGPT plugin, so ChatGPT (and Codex, and any MCP client) can validate Guard configs, search the documentation and run payloads through the real detection engine.

Architecture

ChatGPT (MCP client)
  |
  |  1. POST https://mcp.guard-core.com/mcp          -> 401 + WWW-Authenticate
  |  2. GET  /.well-known/oauth-protected-resource   -> points at the platform API
  |  3. GET  https://api.guard-core.com/.well-known/oauth-authorization-server
  |  4. Browser sign-in + consent (guard-core account, PKCE, RS256)
  |  5. Bearer access token on every MCP request
  v
guard-core-mcp hosted (streamable-http, all nine tools, latest pinned releases)

The hosted server is a pure OAuth 2.1 resource server: it verifies RS256 access tokens against the platform API's JWKS and checks the mcp scope. It keeps no user data; configs and payloads sent to validate_config and check_payload are processed transiently and never persisted or logged.

Environment variables

Variable	Required	Purpose
GUARD_CORE_MCP_PUBLIC_URL	yes	Public origin of the server, advertised as the OAuth resource (https://mcp.guard-core.com)
GUARD_CORE_MCP_OAUTH_ISSUER	yes	Authorization server origin (https://api.guard-core.com)
GUARD_CORE_MCP_OAUTH_JWKS_URL	one of two	Where to fetch signing keys (https://api.guard-core.com/.well-known/jwks.json)
GUARD_CORE_MCP_OAUTH_JWKS	one of two	Inline JWKS JSON, for tests and air-gapped runs; wins over the URL
GUARD_CORE_MCP_OAUTH_AUDIENCE	no	Expected aud claim; defaults to GUARD_CORE_MCP_PUBLIC_URL
GUARD_CORE_MCP_OAUTH_SCOPE	no	Required scope; defaults to mcp
GUARD_CORE_MCP_HTTP_HOST	no	Bind host; defaults to 127.0.0.1
GUARD_CORE_MCP_HTTP_PORT	no	Bind port; defaults to 8020

Running it

uv sync --extra hosted
export GUARD_CORE_MCP_PUBLIC_URL=https://mcp.guard-core.com
export GUARD_CORE_MCP_OAUTH_ISSUER=https://api.guard-core.com
export GUARD_CORE_MCP_OAUTH_JWKS_URL=https://api.guard-core.com/.well-known/jwks.json
make run-http

Or use the container image, which is what the platform deploys:

docker run -p 8020:8020 \
  -e GUARD_CORE_MCP_PUBLIC_URL=https://mcp.guard-core.com \
  -e GUARD_CORE_MCP_OAUTH_ISSUER=https://api.guard-core.com \
  -e GUARD_CORE_MCP_OAUTH_JWKS_URL=https://api.guard-core.com/.well-known/jwks.json \
  ghcr.io/guard-core/guard-core-mcp:v1.4.5

The image installs the hosted extra, so versions reports the exact guard-core, fastapi-guard and guard-agent releases the container was built with. The lockfile pins those at build time; refresh them with uv lock --upgrade when a new engine release ships.

Testing in ChatGPT developer mode

1. Settings -> Security and login -> enable Developer mode.
2. In ChatGPT Plugins, add a connector with the URL https://mcp.guard-core.com/mcp.
3. ChatGPT discovers the OAuth metadata and opens the guard-core sign-in; sign in, approve the consent screen.
4. From the homepage, switch the tab from Chat to Work, type @ and pick Guard Core.
5. Ask: "Would fastapi-guard block POST /login with body user=admin' OR 1=1--?" and confirm check_payload reports a threat.

Local development against a local API

Point every variable at localhost, generate a test signing key on the API side, and pass the public half inline:

GUARD_CORE_MCP_PUBLIC_URL=http://127.0.0.1:8020 \
GUARD_CORE_MCP_OAUTH_ISSUER=http://127.0.0.1:8000 \
GUARD_CORE_MCP_OAUTH_JWKS='{"keys":[ ...public jwk... ]}' \
make run-http

The e2e tests in tests/test_e2e_http.py do exactly this with a throwaway RSA key.

Plugin package

plugin/ holds the directory submission package: plugin.json (identity and the com.openai presentation metadata), mcp.json (the remote server declaration) and skills/guard-core/SKILL.md (tool-selection guidance that ships to the model). The icons under plugin/assets/ are generated placeholders; swap in official brand assets before submitting if you have them.

Submission checklist

• [ ] DNS record for mcp.guard-core.com and the Let's Encrypt certificate
• [ ] Platform API deployed with the OAuth authorization server enabled
• [ ] Signing key generated (scripts/generate_oauth_signing_key.py in guard-core-app) and stored as a secret
• [ ] ChatGPT client registered on the OpenAI platform; its redirect URI and client id added to the API's OAUTH_SERVER_CLIENTS
• [ ] Developer-mode test loop passes end to end
• [ ] Privacy policy at guard-core.com/privacy mentions the plugin connection and transient payload processing
• [ ] Upload plugin/ through the plugin submission portal and pass review