diff --git a/package.json b/package.json
index aee26de..0aaac4d 100644
--- a/package.json
+++ b/package.json
@@ -3,6 +3,7 @@
   "private": true,
   "packageManager": "pnpm@10.33.0",
   "scripts": {
+    "check:agent": "node scripts/check-agent.mjs test",
     "dev": "pnpm --filter playground dev",
     "build": "pnpm -r --filter './packages/*' build",
     "test": "pnpm -r run test",
diff --git a/scripts/check-agent.mjs b/scripts/check-agent.mjs
new file mode 100644
index 0000000..963b695
--- /dev/null
+++ b/scripts/check-agent.mjs
@@ -0,0 +1,58 @@
+#!/usr/bin/env node
+import { spawnSync } from 'node:child_process';
+import { existsSync } from 'node:fs';
+import { dirname, join } from 'node:path';
+import { fileURLToPath } from 'node:url';
+
+const SUBTREE = 'Visimer';
+const ROOT = dirname(dirname(fileURLToPath(import.meta.url)));
+const INSTALLED = 'node_modules/.modules.yaml';
+const INSTALL = `\`pnpm install\` in ${ROOT}`;
+
+function refuse(message, status) {
+  console.error(`check:agent (${SUBTREE}): ${message}`);
+  process.exit(status);
+}
+
+const [script, ...rest] = process.argv.slice(2);
+if (!script || script.startsWith('-')) {
+  refuse(
+    'the manifest must name the script this round runs, as `node scripts/check-agent.mjs <script>`',
+    2
+  );
+}
+let base = null;
+let all = false;
+let because = null;
+for (const arg of rest) {
+  if (arg === '--') continue;
+  if (arg === '--all') all = true;
+  else if (arg.startsWith('--all-because='))
+    because = arg.slice('--all-because='.length).trim() || null;
+  else if (arg.startsWith('--base=')) base = arg.slice('--base='.length).trim() || null;
+  else {
+    refuse(
+      `unrecognized argument '${arg}'. This round accepts --base=<commit>, --all and --all-because=<reason>, or GATE_BASE_SHA, and runs the whole subtree whichever it is given.`,
+      2
+    );
+  }
+}
+const comparison = base ?? ((process.env.GATE_BASE_SHA || '').trim() || null);
+
+if (!existsSync(join(ROOT, INSTALLED))) {
+  refuse(
+    `its dependencies are not installed (${INSTALLED} is missing), so the round cannot run. Install them with ${INSTALL}, then re-run.`,
+    2
+  );
+}
+
+const widened = all
+  ? `; --all${because ? ` (${because})` : ''} changes nothing, since nothing is narrowed`
+  : '';
+console.log(
+  `check:agent (${SUBTREE}): runs \`pnpm run ${script}\` over the whole subtree; the comparison ${comparison ?? '(none given)'} does not narrow it${widened}.`
+);
+const result = spawnSync('pnpm', ['run', script], { cwd: ROOT, stdio: 'inherit' });
+if (result.error) refuse(`pnpm could not be started: ${result.error.message}`, 1);
+if (result.signal) refuse(`\`pnpm run ${script}\` was killed by signal ${result.signal}`, 1);
+process.exit(result.status ?? 1);