diff --git a/.gitea/workflows/build-flatpak.yml b/.gitea/workflows/build-flatpak.yml
index f581a53c4..6c27d379e 100644
--- a/.gitea/workflows/build-flatpak.yml
+++ b/.gitea/workflows/build-flatpak.yml
@@ -26,9 +26,11 @@ jobs:
       - name: Install build dependencies
         run: |
           apt-get update
+          # cmake + python3: O3DE's get_python.sh (run on the host, not in the
+          # sandbox) uses cmake to fetch its Python runtime.
           DEBIAN_FRONTEND=noninteractive apt-get install -y --no-install-recommends \
-            ca-certificates curl git jq xz-utils zstd binutils \
-            flatpak
+            ca-certificates curl git jq xz-utils zstd binutils tar \
+            flatpak cmake python3
 
       # Done as a plain git clone instead of actions/checkout@v4: the bare
       # ubuntu image has no Node.js, so JavaScript actions fail with exit 127.
diff --git a/README.md b/README.md
index 321aff64f..4aa75d254 100644
--- a/README.md
+++ b/README.md
@@ -87,7 +87,8 @@ the upstream version has no matching `vX.Y.Z` tag yet, so reruns are cheap no-op
 ## Building locally
 
 ```sh
-sudo apt install flatpak      # or your distro's equivalent
+# flatpak + cmake (cmake is used by O3DE's get_python.sh) + curl
+sudo apt install flatpak cmake      # or your distro's equivalent
 ./scripts/build.sh
 ```