diff --git a/configs/mcp.nix b/configs/mcp.nix
index 8353b08..1c21061 100644
--- a/configs/mcp.nix
+++ b/configs/mcp.nix
@@ -7,6 +7,7 @@
 {
   users.users.mcp = {
     isSystemUser = true;
+    uid = 850; # Pinned to avoid auto-assign conflicts (800-899 range is unused on all nodes).
     group = "mcp";
     home = "/srv/mcp";
     shell = pkgs.shadow; # nologin equivalent
@@ -16,7 +17,7 @@
     linger = true;
   };
 
-  users.groups.mcp = {};
+  users.groups.mcp = { gid = 850; };
 
   # MCP Master runs as an MCP-managed container (not a systemd service).
   # Deployed via: mcp deploy mcp-master --direct
@@ -38,7 +39,7 @@
       Group = "mcp";
       Environment = [
         "HOME=/srv/mcp"
-        "XDG_RUNTIME_DIR=/run/user/%U"
+        "XDG_RUNTIME_DIR=/run/user/850"
         "PATH=/run/current-system/sw/bin:/usr/local/bin"
       ];