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Reference

MCP Reference

Cessy exposes different MCP servers for different jobs. Choose the narrowest server that matches the task.

Design MCP

Design MCP is for builder work:

  • inspect apps and DomainConfigs
  • read reference knowledge
  • create workspaces
  • edit model artifacts
  • validate changesets
  • preview pages
  • inspect errors
  • promote approved changes

Endpoints:

/api/design-mcp-readonly?tenantId=<tenant-id>&appId=<app-id>
/api/design-mcp?tenantId=<tenant-id>&appId=<app-id>
/api/design-mcp

Use read-only for onboarding, review, and discovery. Use full Design MCP only for trusted model-changing sessions.

Runtime MCP

Runtime MCP is for published app operation:

  • command tools
  • projection tools
  • event import and projection rebuild tools
  • channel tools
  • user and DM tools
  • page refresh tools

Tool names and schemas come from the published DomainConfig.

Endpoint:

/api/mcp/<tenant-id>/<app-id>

Runtime MCP requires bearer authentication with the runtime scope, or an app-scoped agent service token. For workspace-selected runtime calls, use workspaceId=<workspace-id> or X-CES-Workspace-ID; the older env query parameter is not supported.

Client support

Codex supports Streamable HTTP MCP servers through codex mcp and ~/.codex/config.toml. Claude and ChatGPT can connect through custom connector flows by using the app-scoped Remote MCP URL.

See Connect Design MCP.

Tool design rule

MCP tools should expose intentional workflows, not unmodeled data access. A tool should make authority, target app, target environment, and side effects clear.