diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 0d8e665..2ae56dd 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -17,6 +17,15 @@ jobs:
       - name: Setup Bun
         uses: oven-sh/setup-bun@v2
 
+      # The size report shells out to `npm pack --json`, whose output shape
+      # changed in npm 12. Runner images still bundle npm 10, so pin npm 12
+      # to exercise the shape published releases actually run against, and
+      # print the version so the log records which shape was tested.
+      - name: Setup npm 12
+        run: |
+          npm install -g npm@12
+          npm --version
+
       - name: Install dependencies
         run: bun install
 
diff --git a/scripts/size.ts b/scripts/size.ts
index fff2f3a..c30466b 100644
--- a/scripts/size.ts
+++ b/scripts/size.ts
@@ -31,6 +31,13 @@ interface PackSummary {
   unpackedSize: number;
 }
 
+/**
+ * `npm pack --json` returns an array of summaries on npm < 12 and an object
+ * keyed by package name on npm >= 12. Both shapes are accepted so the report
+ * works regardless of the npm bundled with the active Node/CI runner.
+ */
+type PackOutput = PackSummary[] | Record<string, PackSummary>;
+
 interface ConsumerBundlePaths {
   entryPath: string;
   outDir: string;
@@ -83,7 +90,8 @@ const getPackSummary = (): PackSummary => {
     },
     stdio: ["ignore", "pipe", "inherit"],
   });
-  const [summary] = JSON.parse(output) as PackSummary[];
+  const parsed = JSON.parse(output) as PackOutput;
+  const [summary] = Array.isArray(parsed) ? parsed : Object.values(parsed);
 
   if (!summary) {
     throw new Error("npm pack did not return package metadata.");