diff --git a/apps/worker/src/browser.ts b/apps/worker/src/browser.ts
index 6c4f03044..c10841706 100644
--- a/apps/worker/src/browser.ts
+++ b/apps/worker/src/browser.ts
@@ -3,6 +3,7 @@ import { join } from "node:path";
 import type { BrowserContext, Page } from "playwright";
 import {
   capturePdfDownload,
+  downloadLimitReached,
   MAX_DOWNLOAD_BYTES,
   type PdfDownload,
   readDownloadFailures,
@@ -241,12 +242,14 @@ export async function createBrowserManager(
         void dialog.dismiss();
       });
       page.on("download", (download) => {
+        // The current download is already in instance.pending when this runs,
+        // so the cap admits the 20th PDF and blocks the 21st.
         const pending = downloads(id).then((saved) =>
           capturePdfDownload({
             directory: directory(id),
             tempDirectory,
             download,
-            limitReached: saved.length + instance.pending.size > 20,
+            limitReached: downloadLimitReached(saved.length, instance.pending.size),
           }),
         );
         instance.pending.add(pending);
diff --git a/apps/worker/src/downloads.ts b/apps/worker/src/downloads.ts
index 134838d30..d9f2f0ad3 100644
--- a/apps/worker/src/downloads.ts
+++ b/apps/worker/src/downloads.ts
@@ -60,6 +60,14 @@ export async function readDownloadFailures(directory: string, recoverInterrupted
   return failures.sort((a, b) => b.createdAt.localeCompare(a.createdAt));
 }
 
+/**
+ * Whether a new download exceeds the documented 20-PDF per-session cap.
+ * pendingCount must include the download being decided: the session handler
+ * adds the current transfer to its pending set before this is evaluated.
+ */
+export function downloadLimitReached(savedCount: number, pendingCount: number): boolean {
+  return savedCount + pendingCount > 20;
+}
 export async function capturePdfDownload(options: {
   directory: string;
   tempDirectory: string;
diff --git a/apps/worker/tests/downloads.test.ts b/apps/worker/tests/downloads.test.ts
new file mode 100644
index 000000000..89a6bdd65
--- /dev/null
+++ b/apps/worker/tests/downloads.test.ts
@@ -0,0 +1,81 @@
+import assert from "node:assert/strict";
+import { mkdtemp, readdir, readFile, rm } from "node:fs/promises";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+import { Readable } from "node:stream";
+import test from "node:test";
+import { capturePdfDownload, downloadLimitReached } from "../src/downloads.ts";
+
+test("the session cap admits the twentieth PDF and blocks the twenty-first", () => {
+  // pendingCount includes the download being decided, matching the handler,
+  // which adds the current transfer to its pending set before evaluating.
+  assert.equal(downloadLimitReached(19, 1), false, "the twentieth PDF is allowed");
+  assert.equal(downloadLimitReached(20, 1), true, "the twenty-first PDF is blocked");
+  assert.equal(downloadLimitReached(0, 1), false);
+  assert.equal(downloadLimitReached(20, 0), false, "no pending download decides nothing");
+});
+
+test("concurrent downloads at the boundary keep the cap at 20", () => {
+  // Both handlers add to the pending set before either decision evaluates, so
+  // a racing pair counts each other: at 19 saved both see 21 and are blocked.
+  assert.equal(downloadLimitReached(18, 2), false, "two racing at 18 saved: both admitted");
+  assert.equal(downloadLimitReached(19, 2), true, "two racing at 19 saved: both blocked");
+  assert.equal(downloadLimitReached(18, 3), true, "three racing at 18 saved: all blocked");
+});
+
+test("a reached download limit cancels the PDF and records DOWNLOAD_LIMIT", async (t) => {
+  const directory = await mkdtemp(join(tmpdir(), "openmuse-downloads-"));
+  t.after(() => rm(directory, { recursive: true, force: true }));
+  let cancelled = false;
+  await capturePdfDownload({
+    directory,
+    tempDirectory: join(directory, "tmp"),
+    limitReached: true,
+    download: {
+      suggestedFilename: () => "report.pdf",
+      createReadStream: () => {
+        throw new Error("a limited download must not be read");
+      },
+      cancel: async () => {
+        cancelled = true;
+      },
+      delete: async () => {},
+    },
+  });
+  assert.equal(cancelled, true, "the download is cancelled, not captured");
+  const outcomes = join(directory, "download-outcomes");
+  const files = await readdir(outcomes);
+  assert.equal(files.length, 1);
+  const outcome = JSON.parse(await readFile(join(outcomes, files[0]), "utf8"));
+  assert.equal(outcome.code, "DOWNLOAD_LIMIT");
+  assert.match(outcome.message, /20 PDF download limit/);
+});
+
+test("a download under the limit is captured instead of cancelled", async (t) => {
+  const directory = await mkdtemp(join(tmpdir(), "openmuse-downloads-"));
+  t.after(() => rm(directory, { recursive: true, force: true }));
+  let cancelled = false;
+  await capturePdfDownload({
+    directory,
+    tempDirectory: await mkdtemp(join(tmpdir(), "openmuse-tmp-")),
+    limitReached: false,
+    download: {
+      suggestedFilename: () => "report.pdf",
+      createReadStream: async () => Readable.from(Buffer.from("%PDF-1.4 fake")),
+      cancel: async () => {
+        cancelled = true;
+      },
+      delete: async () => {},
+    },
+  });
+  assert.equal(cancelled, false, "an allowed download must not be cancelled");
+  const saved = await readdir(join(directory, "downloads"));
+  const metadata = JSON.parse(
+    await readFile(
+      join(directory, "downloads", saved.find((file) => file.endsWith(".json")) ?? ""),
+      "utf8",
+    ),
+  );
+  assert.equal(metadata.name, "report.pdf");
+  assert.equal(metadata.mimeType, "application/pdf");
+});