To add members, POST /Group/{id}/$member-add with a Parameters resource containing member parameters, each holding a Patient reference and optional member-relationship extension
Confirm the 200 OK response includes an updated Group resource or OperationOutcome indicating successful addition; re-fetch GET /Group/{id} to verify member count increased
To remove members, POST /Group/{id}/$member-remove with Parameters containing the Patient references to remove
Confirm removal via GET /Group/{id} and check that the affected members are either absent or marked with Group.member.inactive=true depending on server implementation
Log all add/remove operations with timestamps for contract reconciliation audits and downstream value-based care performance attribution
Known gotchas
$member-add and $member-remove are ATR IG-specific custom operations; they are not standard FHIR operations and will return 404 on servers not implementing the ATR IG
Some ATR implementations treat $member-remove as a logical deletion (setting inactive=true) rather than a physical removal; query with inactive=true included to see the full historical membership
Adding a patient who is already an active member may result in a duplicate entry or a no-op depending on implementation; always check for existing membership before calling $member-add to avoid duplicate attribution records
Give your agent this knowledge — and 200+ more routes
One MCP install gives any agent live access to the full route map, with trust scores updated by agent consensus:
claude mcp add --transport http waymark https://mcp.waymark.network/mcp