diff --git a/Library/Homebrew/brew.rb b/Library/Homebrew/brew.rb
index 27b78a0acd..a5fae3c604 100644
--- a/Library/Homebrew/brew.rb
+++ b/Library/Homebrew/brew.rb
@@ -86,44 +86,7 @@ begin
   require "commands"
   require "settings"
 
-  if cmd
-    internal_cmd = Commands.valid_internal_cmd?(cmd)
-    internal_cmd ||= begin
-      internal_dev_cmd = Commands.valid_internal_dev_cmd?(cmd)
-      if internal_dev_cmd && !Homebrew::EnvConfig.developer?
-        if ENV["HOMEBREW_DEV_CMD_RUN"].blank?
-          opoo <<~MESSAGE
-            #{Tty.bold}#{cmd}#{Tty.reset} is a developer command, so
-            Homebrew's developer mode has been automatically turned on.
-            To turn developer mode off, run #{Tty.bold}brew developer off#{Tty.reset}
-
-          MESSAGE
-        end
-
-        Homebrew::Settings.write "devcmdrun", true
-        ENV["HOMEBREW_DEV_CMD_RUN"] = "1"
-      end
-      internal_dev_cmd
-    end
-  end
-
-  developer_mode = if cmd == "developer" && ARGV.include?("on")
-    true
-  elsif cmd == "developer" && ARGV.include?("off")
-    false
-  else
-    Homebrew::EnvConfig.developer? || Homebrew::Settings.read("devcmdrun") == "true"
-  end
-
-  if internal_dev_cmd && Homebrew::EnvConfig.install_from_api?
-    odie "Developer commands cannot be run while HOMEBREW_INSTALL_FROM_API is set!"
-  elsif Homebrew::EnvConfig.install_from_api? && developer_mode
-    opoo <<~MESSAGE
-      Developers should not have HOMEBREW_INSTALL_FROM_API set!
-      Please unset HOMEBREW_INSTALL_FROM_API or turn developer mode off by running:
-        brew developer off
-    MESSAGE
-  end
+  internal_cmd = Commands.valid_internal_cmd?(cmd) || Commands.valid_internal_dev_cmd?(cmd) if cmd
 
   unless internal_cmd
     # Add contributed commands to PATH before checking.
diff --git a/Library/Homebrew/brew.sh b/Library/Homebrew/brew.sh
index 48d080334a..6e2843e309 100644
--- a/Library/Homebrew/brew.sh
+++ b/Library/Homebrew/brew.sh
@@ -718,25 +718,61 @@ then
 fi
 export HOMEBREW_CORE_GIT_REMOTE
 
+# Set HOMEBREW_DEVELOPER_COMMAND if the command being run is a developer command
+if [[ -f "${HOMEBREW_LIBRARY}/Homebrew/dev-cmd/${HOMEBREW_COMMAND}.sh" ]] ||
+   [[ -f "${HOMEBREW_LIBRARY}/Homebrew/dev-cmd/${HOMEBREW_COMMAND}.rb" ]]
+then
+  export HOMEBREW_DEVELOPER_COMMAND="1"
+fi
+
+# Set HOMEBREW_DEVELOPER_MODE if this command will turn (or keep) developer mode on. This is the case if:
+# - The command being run is not `brew developer off`
+# - Any of the following are true
+#   - HOMEBREW_DEVELOPER is set
+#   - HOMEBREW_DEV_CMD_RUN is set
+#   - A developer command is being run
+#   - The command being run is `brew developer on`
+if [[ "${HOMEBREW_COMMAND}" != "developer" || ! $* =~ "off" ]] &&
+   [[ -n "${HOMEBREW_DEVELOPER}" ||
+      -n "${HOMEBREW_DEV_CMD_RUN}" ||
+      -n "${HOMEBREW_DEVELOPER_COMMAND}" ||
+      "${HOMEBREW_COMMAND}" == "developer" && $* =~ "on" ]]
+then
+  export HOMEBREW_DEVELOPER_MODE="1"
+fi
+
+if [[ -n "${HOMEBREW_INSTALL_FROM_API}" && -n "${HOMEBREW_DEVELOPER_COMMAND}" ]]
+then
+  odie "Developer commands cannot be run while HOMEBREW_INSTALL_FROM_API is set!"
+elif [[ -n "${HOMEBREW_INSTALL_FROM_API}" && -n "${HOMEBREW_DEVELOPER_MODE}" ]]
+then
+  message="Developers should not have HOMEBREW_INSTALL_FROM_API set!
+Please unset HOMEBREW_INSTALL_FROM_API or turn developer mode off by running:
+  brew developer off
+"
+  opoo "${message}"
+fi
+
+if [[ -n "${HOMEBREW_DEVELOPER_COMMAND}" && -z "${HOMEBREW_DEVELOPER}" ]]
+then
+  if [[ -z "${HOMEBREW_DEV_CMD_RUN}" ]]
+  then
+    message="$(bold "${HOMEBREW_COMMAND}") is a developer command, so
+Homebrew's developer mode has been automatically turned on.
+To turn developer mode off, run $(bold "brew developer off")
+"
+    opoo "${message}"
+  fi
+
+  git config --file="${HOMEBREW_GIT_CONFIG_FILE}" --replace-all homebrew.devcmdrun true 2>/dev/null
+  export HOMEBREW_DEV_CMD_RUN="1"
+fi
+
 if [[ -f "${HOMEBREW_LIBRARY}/Homebrew/cmd/${HOMEBREW_COMMAND}.sh" ]]
 then
   HOMEBREW_BASH_COMMAND="${HOMEBREW_LIBRARY}/Homebrew/cmd/${HOMEBREW_COMMAND}.sh"
 elif [[ -f "${HOMEBREW_LIBRARY}/Homebrew/dev-cmd/${HOMEBREW_COMMAND}.sh" ]]
 then
-  if [[ -z "${HOMEBREW_DEVELOPER}" ]]
-  then
-    if [[ -z "${HOMEBREW_DEV_CMD_RUN}" ]]
-    then
-      message="$(bold "${HOMEBREW_COMMAND}") is a developer command, so
-Homebrew's developer mode has been automatically turned on.
-To turn developer mode off, run $(bold "brew developer off")
-"
-      opoo "${message}"
-    fi
-
-    git config --file="${HOMEBREW_GIT_CONFIG_FILE}" --replace-all homebrew.devcmdrun true 2>/dev/null
-    export HOMEBREW_DEV_CMD_RUN="1"
-  fi
   HOMEBREW_BASH_COMMAND="${HOMEBREW_LIBRARY}/Homebrew/dev-cmd/${HOMEBREW_COMMAND}.sh"
 fi