diff --git a/pr-checks/bundle-metadata.ts b/pr-checks/bundle-metadata.ts
index 25c282e9a..8a7df630f 100755
--- a/pr-checks/bundle-metadata.ts
+++ b/pr-checks/bundle-metadata.ts
@@ -1,8 +1,43 @@
 #!/usr/bin/env npx tsx
 
 import * as fs from "node:fs/promises";
+import { parseArgs, ParseArgsConfig } from "node:util";
 
-import { BUNDLE_METADATA_FILE } from "./config";
+import * as exec from "@actions/exec";
+
+import {
+  ApiClient,
+  CODEQL_ACTION_REPO,
+  getApiClient,
+  TOKEN_OPTION_CONFIG,
+} from "./api-client";
+import { BASELINE_BUNDLE_METADATA_FILE, BUNDLE_METADATA_FILE } from "./config";
+
+const optionsConfig = {
+  ...TOKEN_OPTION_CONFIG,
+  branch: {
+    type: "string",
+    default: "main",
+  },
+  runner: {
+    type: "string",
+    default: "macos-latest",
+  },
+  "node-version": {
+    type: "string",
+    default: "24",
+  },
+} satisfies ParseArgsConfig["options"];
+
+function parseOptions() {
+  const { values: options } = parseArgs({
+    options: optionsConfig,
+  });
+
+  return options;
+}
+
+type Options = ReturnType<typeof parseOptions>;
 
 interface InputInfo {
   bytesInOutput: number;
@@ -23,7 +58,80 @@ function toMB(bytes: number): string {
   return `${(bytes / (1024 * 1024)).toFixed(2)}MB`;
 }
 
+async function getBaselineFrom(client: ApiClient, options: Options) {
+  const workflowRun = await client.rest.actions.listWorkflowRuns({
+    ...CODEQL_ACTION_REPO,
+    branch: options.branch,
+    workflow_id: "pr-checks.yml",
+    status: "success",
+    per_page: 1,
+    event: "push",
+  });
+
+  if (workflowRun.data.total_count === 0) {
+    throw new Error(
+      `Expected to find a 'pr-checks.yml' run for '${options.branch}', but found none.`,
+    );
+  }
+
+  const expectedArtifactName = `bundle-metadata-${options.runner}-${options["node-version"]}`;
+  const artifacts = await client.rest.actions.listWorkflowRunArtifacts({
+    ...CODEQL_ACTION_REPO,
+    run_id: workflowRun.data.workflow_runs[0].id,
+    name: expectedArtifactName,
+  });
+
+  if (artifacts.data.total_count === 0) {
+    throw new Error(
+      `Expected to find an artifact named '${expectedArtifactName}', but found none.`,
+    );
+  }
+
+  const downloadInfo = await client.rest.actions.downloadArtifact({
+    ...CODEQL_ACTION_REPO,
+    artifact_id: artifacts.data.artifacts[0].id,
+    archive_format: "zip",
+  });
+
+  // This works fine for us with our version of Octokit, so we don't need to
+  // worry about over-complicating this script and handle other possibilities.
+  if (downloadInfo.data instanceof ArrayBuffer) {
+    const archivePath = `${expectedArtifactName}.zip`;
+    await fs.writeFile(archivePath, Buffer.from(downloadInfo.data));
+
+    console.info(`Extracting zip file: ${archivePath}`);
+    await exec.exec("unzip", ["-o", archivePath, "-d", "."]);
+
+    // We no longer need the archive after unzipping it.
+    await fs.rm(archivePath);
+
+    // Check that we have the expected file.
+    try {
+      await fs.stat(BASELINE_BUNDLE_METADATA_FILE);
+    } catch (err) {
+      throw new Error(
+        `Expected '${BASELINE_BUNDLE_METADATA_FILE}' to have been extracted, but it does not exist: ${err}`,
+      );
+    }
+
+    const baselineData = await fs.readFile(BASELINE_BUNDLE_METADATA_FILE);
+    return JSON.parse(String(baselineData)) as Metadata;
+  } else {
+    throw new Error("Expected to receive artifact data, but didn't.");
+  }
+}
+
 async function main() {
+  const options = parseOptions();
+
+  if (options.token === undefined) {
+    throw new Error("Missing --token");
+  }
+
+  // Initialise the API client.
+  const client = getApiClient(options.token);
+  const baselineMetadata = await getBaselineFrom(client, options);
+
   const fileContents = await fs.readFile(BUNDLE_METADATA_FILE);
   const metadata = JSON.parse(String(fileContents)) as Metadata;
 
diff --git a/pr-checks/config.ts b/pr-checks/config.ts
index 253843f22..35acf6ddb 100644
--- a/pr-checks/config.ts
+++ b/pr-checks/config.ts
@@ -11,3 +11,9 @@ export const PR_CHECK_EXCLUDED_FILE = path.join(PR_CHECKS_DIR, "excluded.yml");
 
 /** The path to the esbuild metadata file. */
 export const BUNDLE_METADATA_FILE = path.join(PR_CHECKS_DIR, "..", "meta.json");
+
+/** The path of the baseline esbuild metadata file, once extracted from a workflow artifact. */
+export const BASELINE_BUNDLE_METADATA_FILE = path.join(
+  PR_CHECKS_DIR,
+  "meta.json",
+);