diff --git a/modules/homebrew.nix b/modules/homebrew.nix
index 61fb5c8..aadec07 100644
--- a/modules/homebrew.nix
+++ b/modules/homebrew.nix
@@ -476,6 +476,16 @@ let
           linked and keg-only.
         '';
       };
+      postinstall = mkNullOrStrOption {
+        description = ''
+          A shell command to run after the formula is installed or upgraded. The command is passed
+          to the system shell and only executes when the formula actually changed (was freshly
+          installed or upgraded), not on every {command}`brew bundle` run.
+        '';
+      };
+      # `version_file` is intentionally not exposed: it writes the installed version to a file
+      # path relative to the `brew bundle` working directory, which is not meaningful during
+      # nix-darwin system activation.
 
       brewfileLine = mkInternalOption { type = types.nullOr types.str; };
     };
@@ -517,6 +527,13 @@ let
           itself.
         '';
       };
+      postinstall = mkNullOrStrOption {
+        description = ''
+          A shell command to run after the cask is installed or upgraded. The command is passed to
+          the system shell and only executes when the cask was actually installed or upgraded, not
+          on every {command}`brew bundle` run.
+        '';
+      };
 
       brewfileLine = mkInternalOption { type = types.nullOr types.str; };
     };
@@ -681,6 +698,12 @@ in
             link = true;
             conflicts_with = [ "mysql" ];
           }
+
+          # `brew install`, run a post-install command on version changes
+          {
+            name = "postgresql@16";
+            postinstall = "\''${HOMEBREW_PREFIX}/opt/postgresql@16/bin/postgres -D \''${HOMEBREW_PREFIX}/var/postgresql@16";
+          }
         ]
       '';
       description = ''
@@ -712,6 +735,12 @@ in
             name = "opera";
             greedy = true;
           }
+
+          # `brew install --cask`, run a post-install command on install or upgrade
+          {
+            name = "google-cloud-sdk";
+            postinstall = "\''${HOMEBREW_PREFIX}/bin/gcloud components update";
+          }
         ]
       '';
       description = ''
diff --git a/tests/homebrew.nix b/tests/homebrew.nix
index 7815158..beaf5ff 100644
--- a/tests/homebrew.nix
+++ b/tests/homebrew.nix
@@ -50,6 +50,10 @@ in
       link = true;
       conflicts_with = [ "mysql" ];
     }
+    {
+      name = "postgresql@16";
+      postinstall = "\${HOMEBREW_PREFIX}/opt/postgresql@16/bin/postgres -D \${HOMEBREW_PREFIX}/var/postgresql@16";
+    }
   ];
 
   homebrew.casks = [
@@ -62,6 +66,10 @@ in
       name = "opera";
       greedy = true;
     }
+    {
+      name = "google-cloud-sdk";
+      postinstall = "\${HOMEBREW_PREFIX}/bin/gcloud components update";
+    }
   ];
 
   homebrew.masApps = {
@@ -92,11 +100,13 @@ in
     ${mkTest "imagemagick" ''brew "imagemagick"''}
     ${mkTest "denji/nginx/nginx-full" ''brew "denji/nginx/nginx-full", args: ["with-rmtp"], link: :overwrite, restart_service: :always''}
     ${mkTest "mysql@5.6" ''brew "mysql@5.6", conflicts_with: ["mysql"], link: true, restart_service: true''}
+    ${mkTest "postgresql@16" ''brew "postgresql@16", postinstall: "''${HOMEBREW_PREFIX}/opt/postgresql@16/bin/postgres -D ''${HOMEBREW_PREFIX}/var/postgresql@16"''}
 
     echo "checking cask entries in Brewfile" >&2
     ${mkTest "google-chrome" ''cask "google-chrome"''}
     ${mkTest "firefox" ''cask "firefox", args: { appdir: "~/my-apps/Applications" }''}
     ${mkTest "opera" ''cask "opera", greedy: true''}
+    ${mkTest "google-cloud-sdk" ''cask "google-cloud-sdk", postinstall: "''${HOMEBREW_PREFIX}/bin/gcloud components update"''}
 
     echo "checking mas entries in Brewfile" >&2
     ${mkTest "1Password for Safari" ''mas "1Password for Safari", id: 1569813296''}