diff --git a/pr-checks/update-ghes-versions.ts b/pr-checks/update-ghes-versions.ts
index 283601572..72e7fbe2f 100644
--- a/pr-checks/update-ghes-versions.ts
+++ b/pr-checks/update-ghes-versions.ts
@@ -6,9 +6,13 @@
  * an `enterprise-releases` checkout.
  */
 
+import * as fs from "node:fs";
+
 import { type SemVer } from "semver";
 import * as semver from "semver";
 
+import * as json from "../src/json";
+
 import { API_COMPATIBILITY_FILE } from "./config";
 
 /** The first GHES version that included Code Scanning. */
@@ -19,6 +23,36 @@ export enum EnvVar {
   ENTERPRISE_RELEASES_PATH = "ENTERPRISE_RELEASES_PATH",
 }
 
+/** The JSON schema for `API_COMPATIBILITY_FILE`. */
+const apiCompatibilitySchema = {
+  minimumVersion: json.string,
+  maximumVersion: json.string,
+} as const satisfies json.Schema;
+
+/** The type representing the expected contents of `API_COMPATIBILITY_FILE`. */
+type ApiCompatibility = json.FromSchema<typeof apiCompatibilitySchema>;
+
+/** Reads the current contents of the `API_COMPATIBILITY_FILE` file. */
+export function readApiCompatibility(): ApiCompatibility {
+  const apiCompatibilityData: unknown = JSON.parse(
+    fs.readFileSync(API_COMPATIBILITY_FILE, "utf8"),
+  );
+
+  if (!json.isObject(apiCompatibilityData)) {
+    throw new Error(
+      `Expected '${API_COMPATIBILITY_FILE}' to contain an object.`,
+    );
+  }
+  if (!json.validateSchema(apiCompatibilitySchema, apiCompatibilityData)) {
+    throw new Error(
+      `The contents of '${API_COMPATIBILITY_FILE}' do not match the expected JSON schema.`,
+    );
+  }
+
+  return apiCompatibilityData;
+}
+
+
 function main() {
   const enterpriseReleasesPath = process.env[EnvVar.ENTERPRISE_RELEASES_PATH];
   if (!enterpriseReleasesPath) {
@@ -26,6 +60,9 @@ function main() {
       `${EnvVar.ENTERPRISE_RELEASES_PATH} environment variable must be set`,
     );
   }
+
+  // Get the version compatibility data stored in the repo.
+  const apiCompatibilityData = readApiCompatibility();
 }
 
 // Only call `main` if this script was run directly.