diff --git a/rust/src/extensions.rs b/rust/src/extensions.rs
index 349c2f1283..5abccf4708 100644
--- a/rust/src/extensions.rs
+++ b/rust/src/extensions.rs
@@ -8,7 +8,7 @@
 
 use anyhow::{bail, Context, Result};
 use openat_ext::OpenatDirExt;
-use serde_derive::Deserialize;
+use serde_derive::{Deserialize, Serialize};
 use std::collections::HashMap;
 
 use crate::cxxrsutil::*;
@@ -17,17 +17,19 @@ use crate::utils;
 
 const RPMOSTREE_EXTENSIONS_STATE_FILE: &str = ".rpm-ostree-state-chksum";
 
-#[derive(Deserialize, Debug)]
+#[derive(Serialize, Deserialize, Debug)]
 #[serde(rename_all = "kebab-case")]
 pub struct Extensions {
     extensions: HashMap<String, Extension>,
 }
 
-#[derive(Deserialize, Debug)]
+#[derive(Serialize, Deserialize, Debug)]
 #[serde(rename_all = "kebab-case")]
 pub struct Extension {
     packages: Vec<String>,
+    #[serde(skip_serializing_if = "Option::is_none")]
     architectures: Option<Vec<String>>,
+    #[serde(skip_serializing_if = "Option::is_none")]
     match_base_evr: Option<String>,
 }
 
@@ -107,6 +109,15 @@ impl Extensions {
             .write_file_contents(RPMOSTREE_EXTENSIONS_STATE_FILE, 0o644, chksum)
             .with_context(|| format!("updating state file {}", RPMOSTREE_EXTENSIONS_STATE_FILE))?)
     }
+
+    pub(crate) fn serialize_to_dir(&self, output_dir: &str) -> CxxResult<()> {
+        let output_dir = openat::Dir::open(output_dir)?;
+        Ok(output_dir
+            .write_file_with("extensions.json", 0o644, |w| -> Result<_> {
+                Ok(serde_json::to_writer_pretty(w, self)?)
+            })
+            .context("while serializing")?)
+    }
 }
 
 #[cfg(test)]
diff --git a/rust/src/lib.rs b/rust/src/lib.rs
index c90be69169..d01fa89d32 100644
--- a/rust/src/lib.rs
+++ b/rust/src/lib.rs
@@ -196,6 +196,7 @@ mod ffi {
         fn get_packages(&self) -> Vec<String>;
         fn state_checksum_changed(&self, chksum: &str, output_dir: &str) -> Result<bool>;
         fn update_state_checksum(&self, chksum: &str, output_dir: &str) -> Result<()>;
+        fn serialize_to_dir(&self, output_dir: &str) -> Result<()>;
     }
 
     // rpmutils.rs
diff --git a/src/app/rpmostree-compose-builtin-tree.cxx b/src/app/rpmostree-compose-builtin-tree.cxx
index 80eb7efef0..fa07fb8dbf 100644
--- a/src/app/rpmostree-compose-builtin-tree.cxx
+++ b/src/app/rpmostree-compose-builtin-tree.cxx
@@ -1612,6 +1612,7 @@ rpmostree_compose_builtin_extensions (int             argc,
     }
 
   extensions->update_state_checksum (state_checksum, opt_extensions_output_dir);
+  extensions->serialize_to_dir (opt_extensions_output_dir);
   if (!process_touch_if_changed (error))
     return FALSE;
 
diff --git a/tests/compose/test-basic-unified.sh b/tests/compose/test-basic-unified.sh
index 7baf949d6b..0578eb2392 100755
--- a/tests/compose/test-basic-unified.sh
+++ b/tests/compose/test-basic-unified.sh
@@ -95,6 +95,11 @@ extensions:
     packages:
       - dodo
       - solitaire
+  another-arch:
+    packages:
+      - nonexistent
+    architectures:
+      - badarch
 EOF
 
 # we don't actually need root here, but in CI the cache may be in a qcow2 and
@@ -106,6 +111,9 @@ runasroot rpm-ostree compose extensions --repo=${repo} \
 
 ls extensions/{dodo-1.0,dodo-base-1.0,solitaire-1.0}-*.rpm
 test -f extensions-changed
+assert_jq extensions/extensions.json \
+  '.extensions|length == 1' \
+  '.extensions["extinct-birds"]'
 echo "ok extensions"
 
 rm extensions-changed