diff --git a/packages/backend/tests/integration/api/api-security.test.ts b/packages/backend/tests/integration/api/api-security.test.ts
index 3dac342..4f87d10 100644
--- a/packages/backend/tests/integration/api/api-security.test.ts
+++ b/packages/backend/tests/integration/api/api-security.test.ts
@@ -106,6 +106,82 @@ describe('API Security', () => {
       const body = JSON.parse(response.body);
       expect(body).toHaveProperty('bodyErrors');
     });
+
+    it('should sanitize XSS attempts in project name', async () => {
+      const response = await app.inject({
+        method: 'POST',
+        url: '/api/v1/scans',
+        payload: {
+          url: `${baseUrl}/test-page`,
+          name: '<script>alert("XSS")</script>',
+          sync: false,
+        },
+      });
+
+      // Should either reject (400) or sanitize the input
+      expect([200, 202, 400]).toContain(response.statusCode);
+
+      // If accepted, verify the script tag is not stored verbatim
+      if (response.statusCode === 202 || response.statusCode === 200) {
+        const body = JSON.parse(response.body);
+        // Name should not contain script tags
+        expect(JSON.stringify(body)).not.toContain('<script>');
+      }
+    });
+
+    it('should sanitize XSS attempts in project description', async () => {
+      const response = await app.inject({
+        method: 'POST',
+        url: '/api/v1/scans',
+        payload: {
+          url: `${baseUrl}/test-page`,
+          description: '<img src=x onerror="alert(1)">',
+          sync: false,
+        },
+      });
+
+      // Should either reject (400) or sanitize the input
+      expect([200, 202, 400]).toContain(response.statusCode);
+
+      // If accepted, verify the malicious markup is not stored
+      if (response.statusCode === 202 || response.statusCode === 200) {
+        const body = JSON.parse(response.body);
+        expect(JSON.stringify(body)).not.toContain('onerror=');
+      }
+    });
+  });
+
+  describe('Command Injection Prevention', () => {
+    it('should prevent command injection via URL parameter', async () => {
+      const response = await app.inject({
+        method: 'POST',
+        url: '/api/v1/scans',
+        payload: {
+          url: 'http://example.com; ls -la',
+          sync: false,
+        },
+      });
+
+      // Should reject malicious URL
+      expect(response.statusCode).toBe(400);
+      const body = JSON.parse(response.body);
+      expect(body).toHaveProperty('bodyErrors');
+    });
+
+    it('should prevent command injection via pipe character', async () => {
+      const response = await app.inject({
+        method: 'POST',
+        url: '/api/v1/scans',
+        payload: {
+          url: 'http://example.com | cat /etc/passwd',
+          sync: false,
+        },
+      });
+
+      expect(response.statusCode).toBe(400);
+      const body = JSON.parse(response.body);
+      expect(body).toHaveProperty('bodyErrors');
+    });
   });
 
   describe('Malformed JSON Handling', () => {
diff --git a/packages/backend/tests/integration/api/error-handling.test.ts b/packages/backend/tests/integration/api/error-handling.test.ts
new file mode 100644
index 0000000..14b2c00
--- /dev/null
+++ b/packages/backend/tests/integration/api/error-handling.test.ts
@@ -0,0 +1,119 @@
+/**
+ * API Error Handling tests
+ * Tests for error responses, payload limits, and concurrent operations
+ */
+
+import { mkdir, rm } from 'node:fs/promises';
+import { join } from 'node:path';
+import type { FastifyInstance } from 'fastify';
+import * as tmp from 'tmp';
+import { afterAll, beforeAll, describe, expect, it } from 'vitest';
+import { createApp } from '../../../src/api/app.js';
+import {
+  closeDatabase,
+  createDatabase,
+  type DatabaseInstance,
+} from '../../../src/storage/database.js';
+import { createDrizzleDb } from '../../../src/storage/drizzle/db.js';
+import { HTML_FIXTURES } from '../../fixtures/html/index.js';
+import { MockServer } from '../../helpers/mock-server.js';
+
+describe('API Error Handling', () => {
+  let app: FastifyInstance;
+  let db: DatabaseInstance;
+  let mockServer: MockServer;
+  let baseUrl: string;
+  let testDir: string;
+
+  beforeAll(async () => {
+    testDir = tmp.dirSync({ unsafeCleanup: true, prefix: 'diff-voyager-error-test-' }).name;
+
+    const dbPath = join(testDir, 'test.db');
+    const artifactsDir = join(testDir, 'artifacts');
+    await mkdir(artifactsDir, { recursive: true });
+
+    db = createDatabase({ dbPath, baseDir: testDir, artifactsDir });
+    const drizzleDb = createDrizzleDb(db);
+
+    app = await createApp({ db, drizzleDb, artifactsDir, disableLogging: true });
+
+    mockServer = new MockServer({
+      routes: [{ path: '/test-page', body: HTML_FIXTURES.baseline.simple }],
+    });
+    baseUrl = await mockServer.start();
+  });
+
+  afterAll(async () => {
+    await mockServer.stop();
+    closeDatabase(db);
+    await rm(testDir, { recursive: true, force: true });
+  });
+
+  describe('Payload Size Limits', () => {
+    it('should handle large but valid payloads', async () => {
+      // Create a large but reasonable payload (100KB description)
+      const largeDescription = 'x'.repeat(100000);
+      const response = await app.inject({
+        method: 'POST',
+        url: '/api/v1/scans',
+        payload: {
+          url: `${baseUrl}/test-page`,
+          name: 'Test Project',
+          description: largeDescription,
+          sync: false,
+        },
+      });
+
+      // Should accept large but valid payloads (or reject gracefully)
+      expect([200, 202, 400, 413]).toContain(response.statusCode);
+    });
+
+    it('should reject extremely large payloads', async () => {
+      // Create an extremely large payload (10MB)
+      const hugePayload = { url: `${baseUrl}/test-page`, data: 'x'.repeat(10000000) };
+      const response = await app.inject({
+        method: 'POST',
+        url: '/api/v1/scans',
+        payload: hugePayload,
+      });
+
+      // Should reject with 413 (Payload Too Large) or 400 (Bad Request)
+      expect([400, 413]).toContain(response.statusCode);
+    });
+  });
+
+  describe('Concurrent Operations', () => {
+    it('should handle concurrent scan creations', async () => {
+      // Two simultaneous scan creations
+      const [response1, response2] = await Promise.all([
+        app.inject({
+          method: 'POST',
+          url: '/api/v1/scans',
+          payload: {
+            url: `${baseUrl}/test-page`,
+            name: 'Project A',
+            sync: false,
+          },
+        }),
+        app.inject({
+          method: 'POST',
+          url: '/api/v1/scans',
+          payload: {
+            url: `${baseUrl}/test-page`,
+            name: 'Project B',
+            sync: false,
+          },
+        }),
+      ]);
+
+      // Both should succeed (different projects)
+      expect([200, 202]).toContain(response1.statusCode);
+      expect([200, 202]).toContain(response2.statusCode);
+
+      // Should create different projects
+      const body1 = JSON.parse(response1.body);
+      const body2 = JSON.parse(response2.body);
+      expect(body1.projectId).not.toBe(body2.projectId);
+    });
+  });
+});
diff --git a/packages/backend/tests/integration/security/path-traversal-advanced.test.ts b/packages/backend/tests/integration/security/path-traversal-advanced.test.ts
index fba8cf6..cda6b3c 100644
--- a/packages/backend/tests/integration/security/path-traversal-advanced.test.ts
+++ b/packages/backend/tests/integration/security/path-traversal-advanced.test.ts
@@ -505,4 +505,24 @@ describe('Advanced Path Traversal Security', () => {
       expect(invalidResponses.length).toBe(5);
     });
   });
+
+  describe('Additional security edge cases', () => {
+    it('should reject pageId with control characters', async () => {
+      const response = await app.inject({
+        method: 'GET',
+        url: '/api/v1/artifacts/page-\x00-123/screenshot',
+      });
+      expect(response.statusCode).toBe(400);
+    });
+
+    it('should reject oversized path segments', async () => {
+      const longPath = 'a'.repeat(5000);
+      const response = await app.inject({
+        method: 'GET',
+        url: `/api/v1/artifacts/${longPath}/screenshot`,
+      });
+      // Oversized paths can be rejected as either 400 (bad request) or 404 (not found)
+      expect([400, 404]).toContain(response.statusCode);
+    });
+  });
 });