diff --git a/stryker.config.mjs b/stryker.config.mjs
index fe14adc..4077997 100644
--- a/stryker.config.mjs
+++ b/stryker.config.mjs
@@ -52,5 +52,6 @@ export default {
   reporters: ['html', 'clear-text', 'progress', 'json'],
   htmlReporter: { fileName: 'reports/mutation/index.html' },
   jsonReporter: { fileName: 'reports/mutation/mutation.json' },
-  thresholds: { break: null, high: 80, low: 60 },
+  // `break` fails the run below this score. Raise it as mutants get killed.
+  thresholds: { break: 60, high: 80, low: 60 },
 };
diff --git a/test/parsePdf.test.ts b/test/parsePdf.test.ts
index 702ff6d..b942e0e 100644
--- a/test/parsePdf.test.ts
+++ b/test/parsePdf.test.ts
@@ -2,6 +2,7 @@ import assert from 'node:assert';
 import { readFile } from 'node:fs/promises';
 import path from 'node:path';
 import { describe, it } from 'node:test';
+import { setTimeout as delay } from 'node:timers/promises';
 
 import { parsePdf } from '#afpp/src/index.js';
 
@@ -126,4 +127,30 @@ describe('parsePdf', () => {
       assert.equal(data.length, 9);
     });
   });
+
+  describe('concurrency limit', () => {
+    it('should run at most `concurrency` pages at the same time', async () => {
+      const input = path.join('test', 'example.pdf');
+      const concurrency = 2;
+      let active = 0;
+      let peak = 0;
+
+      const data = await parsePdf(input, { concurrency }, async (content) => {
+        active++;
+        peak = Math.max(peak, active);
+        // Hold the slot so that pages processed at the same time overlap here.
+        await delay(50);
+        active--;
+        return content;
+      });
+
+      assert.equal(data.length, 9);
+      // Without the limit all nine pages start at once and the peak is 9.
+      assert.equal(
+        peak,
+        concurrency,
+        `expected at most ${concurrency} pages at a time, saw ${peak}`,
+      );
+    });
+  });
 });