diff --git a/backend/src/lib/error-mapping.test.ts b/backend/src/lib/error-mapping.test.ts
new file mode 100644
index 0000000..e7ed3fb
--- /dev/null
+++ b/backend/src/lib/error-mapping.test.ts
@@ -0,0 +1,71 @@
+import { describe, expect, it } from 'vitest';
+import { BadRequestError, ConflictError, ForbiddenError, NotFoundError, UnauthorizedError } from '@bytelyst/errors';
+import { createServiceApp } from '@bytelyst/fastify-core';
+import { mapBlobFailure, mapExtractionFailure, mapLlmFailure } from './error-mapping.js';
+
+async function buildErrorApp() {
+  const app = await createServiceApp({
+    name: 'error-mapping-test',
+    version: '0.1.0',
+    logger: false,
+  });
+
+  app.get('/validation', async () => {
+    throw new BadRequestError('Invalid request', { code: 'VALIDATION_ERROR', field: 'title' });
+  });
+  app.get('/auth', async () => {
+    throw new UnauthorizedError('Unauthorized', { code: 'AUTH_REQUIRED' });
+  });
+  app.get('/forbidden', async () => {
+    throw new ForbiddenError('Forbidden', { code: 'FORBIDDEN' });
+  });
+  app.get('/not-found', async () => {
+    throw new NotFoundError('Missing resource', { code: 'NOT_FOUND' });
+  });
+  app.get('/conflict', async () => {
+    throw new ConflictError('Duplicate idempotency key', { code: 'CONFLICT' });
+  });
+  app.get('/extraction-failure', async () => {
+    throw mapExtractionFailure(new Error('HTTP 503 Service Unavailable'));
+  });
+  app.get('/llm-timeout', async () => {
+    throw mapLlmFailure(new Error('LLM request timed out'));
+  });
+  app.get('/blob-failure', async () => {
+    throw mapBlobFailure(new Error('SAS token request failed'));
+  });
+
+  await app.ready();
+  return app;
+}
+
+describe('structured error mapping', () => {
+  it.each([
+    ['/validation', 400, 'Invalid request', 'VALIDATION_ERROR'],
+    ['/auth', 401, 'Unauthorized', 'AUTH_REQUIRED'],
+    ['/forbidden', 403, 'Forbidden', 'FORBIDDEN'],
+    ['/not-found', 404, 'Missing resource', 'NOT_FOUND'],
+    ['/conflict', 409, 'Duplicate idempotency key', 'CONFLICT'],
+    ['/extraction-failure', 502, 'Extraction service failed', 'EXTRACTION_SERVICE_FAILURE'],
+    ['/llm-timeout', 504, 'LLM request timed out', 'LLM_TIMEOUT'],
+    ['/blob-failure', 502, 'Blob service failed', 'BLOB_SERVICE_FAILURE'],
+  ])('maps %s to a stable error response', async (url, statusCode, message, code) => {
+    const app = await buildErrorApp();
+    try {
+      const res = await app.inject({
+        method: 'GET',
+        url,
+        headers: { 'x-request-id': 'req-errors-1' },
+      });
+
+      expect(res.statusCode).toBe(statusCode);
+      expect(res.json()).toMatchObject({
+        error: message,
+        requestId: 'req-errors-1',
+        details: { code },
+      });
+    } finally {
+      await app.close();
+    }
+  });
+});
diff --git a/backend/src/lib/error-mapping.ts b/backend/src/lib/error-mapping.ts
new file mode 100644
index 0000000..8a2a71e
--- /dev/null
+++ b/backend/src/lib/error-mapping.ts
@@ -0,0 +1,54 @@
+import { ServiceError } from '@bytelyst/errors';
+
+type DependencyKind = 'extraction-service' | 'llm' | 'blob';
+
+function causeMessage(error: unknown): string {
+  return error instanceof Error ? error.message : String(error);
+}
+
+function dependencyError(
+  dependency: DependencyKind,
+  statusCode: number,
+  message: string,
+  code: string,
+  error: unknown,
+): ServiceError {
+  return new ServiceError(statusCode, message, {
+    code,
+    dependency,
+    cause: causeMessage(error),
+  });
+}
+
+export function mapExtractionFailure(error: unknown): ServiceError {
+  return dependencyError(
+    'extraction-service',
+    502,
+    'Extraction service failed',
+    'EXTRACTION_SERVICE_FAILURE',
+    error,
+  );
+}
+
+export function mapLlmFailure(error: unknown): ServiceError {
+  const message = causeMessage(error).toLowerCase();
+  const timedOut = message.includes('timed out') || message.includes('timeout');
+
+  return dependencyError(
+    'llm',
+    timedOut ? 504 : 502,
+    timedOut ? 'LLM request timed out' : 'LLM request failed',
+    timedOut ? 'LLM_TIMEOUT' : 'LLM_FAILURE',
+    error,
+  );
+}
+
+export function mapBlobFailure(error: unknown): ServiceError {
+  return dependencyError(
+    'blob',
+    502,
+    'Blob service failed',
+    'BLOB_SERVICE_FAILURE',
+    error,
+  );
+}
diff --git a/backend/src/lib/extraction-client.test.ts b/backend/src/lib/extraction-client.test.ts
index 58e3780..ad45486 100644
--- a/backend/src/lib/extraction-client.test.ts
+++ b/backend/src/lib/extraction-client.test.ts
@@ -19,4 +19,20 @@ describe('extraction client request propagation', () => {
       'x-request-id': 'req-propagated',
     });
   });
+
+  it('maps extraction-service failures to a structured dependency error', async () => {
+    vi.stubGlobal(
+      'fetch',
+      vi.fn(async () => new Response(JSON.stringify({ error: 'down' }), { status: 503, statusText: 'Service Unavailable' })),
+    );
+
+    await expect(extractFromText('hello', 'summarization')).rejects.toMatchObject({
+      statusCode: 502,
+      message: 'Extraction service failed',
+      details: {
+        code: 'EXTRACTION_SERVICE_FAILURE',
+        dependency: 'extraction-service',
+      },
+    });
+  });
 });
diff --git a/backend/src/lib/extraction-client.ts b/backend/src/lib/extraction-client.ts
index f755525..c942703 100644
--- a/backend/src/lib/extraction-client.ts
+++ b/backend/src/lib/extraction-client.ts
@@ -1,4 +1,6 @@
+import { ServiceError } from '@bytelyst/errors';
 import { config } from './config.js';
+import { mapExtractionFailure } from './error-mapping.js';
 import { requestIdHeaders } from './request-context.js';
 
 export interface ExtractionResult {
@@ -14,15 +16,22 @@ export async function extractFromText(
 ): Promise<ExtractionResult> {
   const url = `${config.EXTRACTION_SERVICE_URL}/api/extract`;
 
-  const res = await fetch(url, {
-    method: 'POST',
-    headers: { 'Content-Type': 'application/json', ...requestIdHeaders(options.requestId) },
-    body: JSON.stringify({ text, task: taskType }),
-  });
+  try {
+    const res = await fetch(url, {
+      method: 'POST',
+      headers: { 'Content-Type': 'application/json', ...requestIdHeaders(options.requestId) },
+      body: JSON.stringify({ text, task: taskType }),
+    });
 
-  if (!res.ok) {
-    throw new Error(`Extraction service error: ${res.status} ${res.statusText}`);
+    if (!res.ok) {
+      throw mapExtractionFailure(new Error(`HTTP ${res.status} ${res.statusText}`));
+    }
+
+    return res.json() as Promise<ExtractionResult>;
+  } catch (error) {
+    if (error instanceof ServiceError) {
+      throw error;
+    }
+    throw mapExtractionFailure(error);
   }
-
-  return res.json() as Promise<ExtractionResult>;
 }
diff --git a/backend/src/modules/note-prompts/runner.test.ts b/backend/src/modules/note-prompts/runner.test.ts
index 93c6fc5..7496395 100644
--- a/backend/src/modules/note-prompts/runner.test.ts
+++ b/backend/src/modules/note-prompts/runner.test.ts
@@ -182,6 +182,20 @@ describe('executePrompt', () => {
     expect(mockChatCompletion).toHaveBeenCalledTimes(1);
   });
 
+  it('maps LLM timeouts to structured dependency errors', async () => {
+    mockChatCompletion.mockRejectedValue(new Error('LLM request timed out'));
+
+    await expect(executePrompt(makeTemplate(), makeInput(), 'body')).rejects.toMatchObject({
+      statusCode: 504,
+      message: 'LLM request timed out',
+      details: {
+        code: 'LLM_TIMEOUT',
+        dependency: 'llm',
+      },
+    });
+    expect(mockChatCompletion).toHaveBeenCalledTimes(1);
+  });
+
   it('uses custom model from template if provided', async () => {
     const tpl = makeTemplate({ model: 'gpt-4-turbo' });
     await executePrompt(tpl, makeInput(), 'body');
diff --git a/backend/src/modules/note-prompts/runner.ts b/backend/src/modules/note-prompts/runner.ts
index 8c92387..de2cd24 100644
--- a/backend/src/modules/note-prompts/runner.ts
+++ b/backend/src/modules/note-prompts/runner.ts
@@ -4,6 +4,7 @@
 
 import { llm } from '../../lib/llm.js';
 import { config } from '../../lib/config.js';
+import { mapLlmFailure } from '../../lib/error-mapping.js';
 import { trackEvent } from '../../lib/telemetry.js';
 import { buildNoteLettWakeUp } from '../palace/wakeup.js';
 import { PRODUCT_ID } from '../../lib/product-config.js';
@@ -141,5 +142,9 @@ export async function executePrompt(
     }
   }
 
-  throw lastError instanceof Error ? lastError : new Error('LLM call failed after retries');
+  if (lastError instanceof Error && lastError.message.toLowerCase().includes('timed out')) {
+    throw mapLlmFailure(lastError);
+  }
+
+  throw lastError instanceof Error ? lastError : mapLlmFailure(lastError);
 }