diff --git a/src/lib/server/compute/concurrency-limiter.ts b/src/lib/server/compute/concurrency-limiter.ts
new file mode 100644
index 0000000..08102db
--- /dev/null
+++ b/src/lib/server/compute/concurrency-limiter.ts
@@ -0,0 +1,40 @@
+export class ConcurrencyLimiter {
+  private readonly maxInFlight: number;
+  private inFlight = 0;
+  private readonly queue: Array<() => void> = [];
+
+  constructor(limit: number) {
+    this.maxInFlight = Number.isFinite(limit) && limit >= 1 ? Math.floor(limit) : 1;
+  }
+
+  async run<T>(fn: () => Promise<T>): Promise<T> {
+    await this.acquire();
+    try {
+      return await fn();
+    } finally {
+      this.release();
+    }
+  }
+
+  private acquire(): Promise<void> {
+    if (this.inFlight < this.maxInFlight) {
+      this.inFlight += 1;
+      return Promise.resolve();
+    }
+    return new Promise((resolve) => {
+      this.queue.push(() => {
+        this.inFlight += 1;
+        resolve();
+      });
+    });
+  }
+
+  private release(): void {
+    this.inFlight = Math.max(0, this.inFlight - 1);
+    const next = this.queue.shift();
+    if (next) next();
+  }
+}
+
+export const LOCAL_PDF_LIMITER = new ConcurrencyLimiter(2);
+export const LOCAL_WHISPER_LIMITER = new ConcurrencyLimiter(1);
diff --git a/src/lib/server/compute/local.ts b/src/lib/server/compute/local.ts
index f4e0922..4d3ee0c 100644
--- a/src/lib/server/compute/local.ts
+++ b/src/lib/server/compute/local.ts
@@ -1,4 +1,5 @@
 import type { ComputeBackend, PdfLayoutInput, WhisperAlignInput, WhisperAlignResult } from '@/lib/server/compute/types';
+import { LOCAL_PDF_LIMITER, LOCAL_WHISPER_LIMITER } from '@/lib/server/compute/concurrency-limiter';
 import { getDocumentBlob } from '@/lib/server/documents/blobstore';
 import { getTtsSegmentAudioObject } from '@/lib/server/tts/segments-blobstore';
 import {
@@ -10,42 +11,46 @@ export class LocalComputeBackend implements ComputeBackend {
   readonly mode = 'local' as const;
 
   async alignWords(input: WhisperAlignInput): Promise<WhisperAlignResult> {
-    let audioBuffer = input.audioBuffer ?? null;
-    if (!audioBuffer && input.audioObjectKey) {
-      const bytes = new Uint8Array(await getTtsSegmentAudioObject(input.audioObjectKey));
-      audioBuffer = bytes.buffer.slice(bytes.byteOffset, bytes.byteOffset + bytes.byteLength);
-    }
-    if (!audioBuffer) {
-      throw new Error('Local compute alignment requires audioBuffer or audioObjectKey');
-    }
-    return runWhisperAlignmentFromAudioBuffer({
-      audioBuffer,
-      text: input.text,
-      cacheKey: input.cacheKey,
-      lang: input.lang,
+    return LOCAL_WHISPER_LIMITER.run(async () => {
+      let audioBuffer = input.audioBuffer ?? null;
+      if (!audioBuffer && input.audioObjectKey) {
+        const bytes = new Uint8Array(await getTtsSegmentAudioObject(input.audioObjectKey));
+        audioBuffer = bytes.buffer.slice(bytes.byteOffset, bytes.byteOffset + bytes.byteLength);
+      }
+      if (!audioBuffer) {
+        throw new Error('Local compute alignment requires audioBuffer or audioObjectKey');
+      }
+      return runWhisperAlignmentFromAudioBuffer({
+        audioBuffer,
+        text: input.text,
+        cacheKey: input.cacheKey,
+        lang: input.lang,
+      });
     });
   }
 
   async parsePdfLayout(input: PdfLayoutInput) {
-    let pdfBytes = input.pdfBytes ?? null;
-    if (!pdfBytes && input.documentId && typeof input.namespace !== 'undefined') {
-      const bytes = new Uint8Array(await getDocumentBlob(input.documentId, input.namespace));
-      pdfBytes = bytes.buffer.slice(bytes.byteOffset, bytes.byteOffset + bytes.byteLength);
-    }
-    if (!pdfBytes) {
-      throw new Error('Local compute PDF layout requires pdfBytes or (documentId + namespace)');
-    }
-    return {
-      parsed: (await runPdfLayoutFromPdfBuffer({
-        documentId: input.documentId,
-        pdfBytes,
-        onPageParsed: (page) => input.onProgress?.({
-          totalPages: page.totalPages,
-          pagesParsed: page.pageNumber,
-          currentPage: page.pageNumber,
-          phase: 'infer',
-        }),
-      })).parsed,
-    };
+    return LOCAL_PDF_LIMITER.run(async () => {
+      let pdfBytes = input.pdfBytes ?? null;
+      if (!pdfBytes && input.documentId && typeof input.namespace !== 'undefined') {
+        const bytes = new Uint8Array(await getDocumentBlob(input.documentId, input.namespace));
+        pdfBytes = bytes.buffer.slice(bytes.byteOffset, bytes.byteOffset + bytes.byteLength);
+      }
+      if (!pdfBytes) {
+        throw new Error('Local compute PDF layout requires pdfBytes or (documentId + namespace)');
+      }
+      return {
+        parsed: (await runPdfLayoutFromPdfBuffer({
+          documentId: input.documentId,
+          pdfBytes,
+          onPageParsed: (page) => input.onProgress?.({
+            totalPages: page.totalPages,
+            pagesParsed: page.pageNumber,
+            currentPage: page.pageNumber,
+            phase: 'infer',
+          }),
+        })).parsed,
+      };
+    });
   }
 }
diff --git a/tests/unit/compute-concurrency-limiter.spec.ts b/tests/unit/compute-concurrency-limiter.spec.ts
new file mode 100644
index 0000000..a37349b
--- /dev/null
+++ b/tests/unit/compute-concurrency-limiter.spec.ts
@@ -0,0 +1,65 @@
+import { expect, test } from '@playwright/test';
+import { ConcurrencyLimiter } from '../../src/lib/server/compute/concurrency-limiter';
+
+function sleep(ms: number): Promise<void> {
+  return new Promise((resolve) => setTimeout(resolve, ms));
+}
+
+test.describe('compute-concurrency-limiter', () => {
+  test('caps active jobs at configured limit', async () => {
+    const limiter = new ConcurrencyLimiter(2);
+    let inFlight = 0;
+    let maxInFlightSeen = 0;
+
+    const jobs = Array.from({ length: 6 }, (_, i) => limiter.run(async () => {
+      inFlight += 1;
+      maxInFlightSeen = Math.max(maxInFlightSeen, inFlight);
+      await sleep(20 + (i % 2) * 5);
+      inFlight -= 1;
+      return i;
+    }));
+
+    const result = await Promise.all(jobs);
+    expect(result).toEqual([0, 1, 2, 3, 4, 5]);
+    expect(maxInFlightSeen).toBe(2);
+  });
+
+  test('queues in FIFO order when saturated', async () => {
+    const limiter = new ConcurrencyLimiter(1);
+    const starts: number[] = [];
+
+    const one = limiter.run(async () => {
+      starts.push(1);
+      await sleep(25);
+    });
+    const two = limiter.run(async () => {
+      starts.push(2);
+      await sleep(5);
+    });
+    const three = limiter.run(async () => {
+      starts.push(3);
+      await sleep(1);
+    });
+
+    await Promise.all([one, two, three]);
+    expect(starts).toEqual([1, 2, 3]);
+  });
+
+  test('releases slot after failure', async () => {
+    const limiter = new ConcurrencyLimiter(1);
+    let startedSecond = false;
+
+    const first = limiter.run(async () => {
+      await sleep(10);
+      throw new Error('boom');
+    });
+    const second = limiter.run(async () => {
+      startedSecond = true;
+      return 'ok';
+    });
+
+    await expect(first).rejects.toThrow('boom');
+    await expect(second).resolves.toBe('ok');
+    expect(startedSecond).toBeTruthy();
+  });
+});