diff --git a/README.md b/README.md
index 59f9e17..2a9a187 100644
--- a/README.md
+++ b/README.md
@@ -94,7 +94,8 @@ Python is installed via `actions/setup-python@v7`, unless Homebrew is requested.
 
 #### Whether to Install Python with Homebrew
 
-Install Python via Homebrew instead. This only has effect on MacOS.
+Install Python via Homebrew instead. This only has effect on MacOS. Only `python@<version>` and its own
+dependencies are installed or upgraded; other formulae already present on the runner are left as they are.
 
 ```yaml
   homebrew-python:
diff --git a/action.yml b/action.yml
index 1acb9b9..675d6b9 100644
--- a/action.yml
+++ b/action.yml
@@ -143,15 +143,26 @@ runs:
     - name: Setup Python with Homebrew
       shell: bash
       if: inputs.install-python == 'true' && runner.os == 'macOS' && inputs.homebrew-python == 'true'
+      env:
+        # Only touch Python and its own dependencies, not every formula on the runner image
+        HOMEBREW_NO_AUTO_UPDATE: '1'
+        HOMEBREW_NO_INSTALLED_DEPENDENTS_CHECK: '1'
+        HOMEBREW_NO_INSTALL_CLEANUP: '1'
+        HOMEBREW_NO_ENV_HINTS: '1'
       run: |
         brew update
-        brew upgrade --overwrite
         brew uninstall --force --ignore-dependencies 'python@${{ inputs.python-version }}' || true
+        # Dependencies that `brew install` upgrades implicitly are linked without `--overwrite`,
+        # so upgrade Python's outdated dependencies explicitly first
+        PYTHON_DEPS="$(brew deps 'python@${{ inputs.python-version }}')"
+        OUTDATED_DEPS="$(brew outdated --formula --quiet | grep -Fx -f <(echo "$PYTHON_DEPS") || true)"
+        if [ -n "$OUTDATED_DEPS" ]; then
+          brew upgrade --overwrite $OUTDATED_DEPS
+        fi
         brew install --overwrite 'python@${{ inputs.python-version }}'
         echo "/usr/local/opt/python@${{ inputs.python-version }}/Frameworks/Python.framework/Versions/${{ inputs.python-version }}/bin" >> $GITHUB_PATH
         echo "/usr/local/opt/python@${{ inputs.python-version }}/libexec/bin" >> $GITHUB_PATH
         echo "/usr/local/opt/python@${{ inputs.python-version }}/bin" >> $GITHUB_PATH
-        brew cleanup
 
     - name: Python Pre-VENV Check
       shell: bash