diff --git a/tidepool/src/builders/foundation/basic.nix b/tidepool/src/builders/foundation/basic.nix
index 1210d43..e59a537 100644
--- a/tidepool/src/builders/foundation/basic.nix
+++ b/tidepool/src/builders/foundation/basic.nix
@@ -50,6 +50,12 @@ in
         script = lib.strings.concatMapSep "\n" (entry: ''
           # Phase: ${entry.name}
           # ====================================================
+          if [[ -v NIX_LOG_FD ]]; then
+            # Sends a JSON command to the Nix build daemon, which sets the phase of the build allowing
+            # nix-output-monitor to display the phase properly.
+            # https://github.com/NixOS/nix/blob/1a9ba0d6fe7b6c05e8bbe1c59ead420759b631cc/src/libutil/logging.cc#L441
+            echo '@nix { "action": "setPhase", "phase": "${entry.name}" }' >&"$NIX_LOG_FD"
+          fi
           ${entry.value}
         '') sorted.result;