diff --git a/services/platform-service/src/modules/diagnostics/diagnostics.test.ts b/services/platform-service/src/modules/diagnostics/diagnostics.test.ts
new file mode 100644
index 00000000..cb1d82ea
--- /dev/null
+++ b/services/platform-service/src/modules/diagnostics/diagnostics.test.ts
@@ -0,0 +1,351 @@
+/**
+ * Diagnostics module tests — Phase 1: Server Foundation
+ *
+ * @module diagnostics
+ */
+
+import { describe, it, expect, beforeEach } from 'vitest';
+import { randomUUID } from 'node:crypto';
+import * as repo from './repository.js';
+import {
+  CreateDebugSessionSchema,
+  UpdateDebugSessionSchema,
+  IngestTracesSchema,
+  IngestLogsSchema,
+  type DebugSessionDoc,
+  type DebugTraceDoc,
+  type DebugLogEntryDoc,
+  type CreateDebugSessionInput,
+  type UpdateDebugSessionInput,
+  type IngestTracesInput,
+  type IngestLogsInput,
+} from './types.js';
+
+// Test helpers
+function generateId(prefix: string): string {
+  return `${prefix}_${randomUUID().replace(/-/g, '')}`;
+}
+
+function createTestSession(productId: string, overrides?: Partial<DebugSessionDoc>): DebugSessionDoc {
+  const now = new Date().toISOString();
+  return {
+    id: generateId('ds'),
+    productId,
+    targetUserId: `user_${randomUUID()}`,
+    status: 'pending',
+    collectionLevel: 'debug',
+    captureLogs: true,
+    captureNetwork: true,
+    captureScreenshots: false,
+    screenshotOnError: true,
+    maxDurationMinutes: 60,
+    createdAt: now,
+    updatedAt: now,
+    expiresAt: new Date(Date.now() + 60 * 60000).toISOString(),
+    logCount: 0,
+    traceCount: 0,
+    screenshotCount: 0,
+    createdBy: 'test_admin',
+    ...overrides,
+  };
+}
+
+// ─── Session CRUD Tests ────────────────────────────────────────────────────
+
+describe('Session CRUD', () => {
+  const productId = 'test_product';
+
+  it('should create a session', async () => {
+    const session = createTestSession(productId);
+    const created = await repo.createSession(session);
+    expect(created.id).toBe(session.id);
+    expect(created.status).toBe('pending');
+  });
+
+  it('should get a session by id', async () => {
+    const session = createTestSession(productId);
+    await repo.createSession(session);
+
+    const found = await repo.getSession(session.id);
+    expect(found).not.toBeNull();
+    expect(found?.id).toBe(session.id);
+  });
+
+  it('should return null for non-existent session', async () => {
+    const found = await repo.getSession('non_existent_id');
+    expect(found).toBeNull();
+  });
+
+  it('should update a session', async () => {
+    const session = createTestSession(productId);
+    await repo.createSession(session);
+
+    const updated = await repo.updateSession(session.id, { status: 'active' });
+    expect(updated).not.toBeNull();
+    expect(updated?.status).toBe('active');
+    expect(updated?.updatedAt).not.toBe(session.updatedAt);
+  });
+
+  it('should return null when updating non-existent session', async () => {
+    const updated = await repo.updateSession('non_existent', { status: 'active' });
+    expect(updated).toBeNull();
+  });
+
+  it('should soft delete (cancel) a session', async () => {
+    const session = createTestSession(productId);
+    await repo.createSession(session);
+
+    const result = await repo.deleteSession(session.id);
+    expect(result).toBe(true);
+
+    const found = await repo.getSession(session.id);
+    expect(found?.status).toBe('cancelled');
+  });
+
+  it('should list sessions with filters', async () => {
+    const session1 = createTestSession(productId, { status: 'active' });
+    const session2 = createTestSession(productId, { status: 'pending' });
+    await repo.createSession(session1);
+    await repo.createSession(session2);
+
+    const { sessions } = await repo.listSessions({
+      productId,
+      status: 'active',
+      limit: 10,
+      offset: 0,
+    });
+
+    expect(sessions.length).toBeGreaterThanOrEqual(1);
+    expect(sessions.some((s) => s.id === session1.id)).toBe(true);
+  });
+});
+
+// ─── Trace Ingest Tests ────────────────────────────────────────────────────
+
+describe('Trace Ingest', () => {
+  const productId = 'test_product';
+  const sessionId = generateId('ds');
+
+  it('should ingest traces', async () => {
+    const traces: DebugTraceDoc[] = [
+      {
+        id: generateId('tr'),
+        pk: `${productId}:${sessionId}`,
+        sessionId,
+        productId,
+        traceId: randomUUID(),
+        spanId: randomUUID(),
+        name: 'test_operation',
+        startTime: new Date().toISOString(),
+        durationMs: 100,
+        attributes: {},
+        status: 'ok',
+      },
+    ];
+
+    await repo.ingestTraces(productId, sessionId, traces);
+
+    const { traces: found } = await repo.getTraces(productId, sessionId, { limit: 10 });
+    expect(found.length).toBeGreaterThanOrEqual(1);
+  });
+
+  it('should query traces for a session', async () => {
+    const traceId = randomUUID();
+    const spanId = randomUUID();
+
+    const traces: DebugTraceDoc[] = [
+      {
+        id: generateId('tr'),
+        pk: `${productId}:${sessionId}`,
+        sessionId,
+        productId,
+        traceId,
+        spanId,
+        name: 'test_query',
+        startTime: new Date().toISOString(),
+        durationMs: 50,
+        attributes: { key: 'value' },
+        status: 'ok',
+      },
+    ];
+
+    await repo.ingestTraces(productId, sessionId, traces);
+
+    const { traces: found } = await repo.getTraces(productId, sessionId, { limit: 10 });
+    expect(found.some((t) => t.name === 'test_query')).toBe(true);
+  });
+});
+
+// ─── Log Ingest Tests ────────────────────────────────────────────────────
+
+describe('Log Ingest', () => {
+  const productId = 'test_product';
+  const sessionId = generateId('ds');
+
+  it('should ingest logs', async () => {
+    const logs: DebugLogEntryDoc[] = [
+      {
+        id: generateId('log'),
+        pk: `${productId}:${sessionId}`,
+        sessionId,
+        productId,
+        level: 'info',
+        message: 'Test log message',
+        timestamp: new Date().toISOString(),
+        module: 'TestModule',
+        context: {},
+      },
+    ];
+
+    await repo.ingestLogs(productId, sessionId, logs);
+
+    const { logs: found } = await repo.getLogs(productId, sessionId, { limit: 10 });
+    expect(found.length).toBeGreaterThanOrEqual(1);
+  });
+
+  it('should filter logs by level', async () => {
+    const logs: DebugLogEntryDoc[] = [
+      {
+        id: generateId('log'),
+        pk: `${productId}:${sessionId}`,
+        sessionId,
+        productId,
+        level: 'error',
+        message: 'Error message',
+        timestamp: new Date().toISOString(),
+        module: 'TestModule',
+        context: {},
+      },
+      {
+        id: generateId('log'),
+        pk: `${productId}:${sessionId}`,
+        sessionId,
+        productId,
+        level: 'info',
+        message: 'Info message',
+        timestamp: new Date().toISOString(),
+        module: 'TestModule',
+        context: {},
+      },
+    ];
+
+    await repo.ingestLogs(productId, sessionId, logs);
+
+    const { logs: found } = await repo.getLogs(productId, sessionId, {
+      level: 'error',
+      limit: 10,
+    });
+
+    expect(found.every((l) => l.level === 'error')).toBe(true);
+  });
+
+  it('should search logs by message content', async () => {
+    const logs: DebugLogEntryDoc[] = [
+      {
+        id: generateId('log'),
+        pk: `${productId}:${sessionId}`,
+        sessionId,
+        productId,
+        level: 'info',
+        message: 'Searchable unique message content',
+        timestamp: new Date().toISOString(),
+        module: 'TestModule',
+        context: {},
+      },
+    ];
+
+    await repo.ingestLogs(productId, sessionId, logs);
+
+    const { logs: found } = await repo.getLogs(productId, sessionId, {
+      search: 'unique message',
+      limit: 10,
+    });
+
+    expect(found.some((l) => l.message.includes('unique message'))).toBe(true);
+  });
+});
+
+// ─── Schema Validation Tests ───────────────────────────────────────────────
+
+describe('Schema Validation', () => {
+  it('should validate CreateDebugSessionSchema', () => {
+    const input: CreateDebugSessionInput = {
+      productId: 'test_product',
+      targetUserId: 'user_123',
+      collectionLevel: 'debug',
+      captureLogs: true,
+      captureNetwork: true,
+      captureScreenshots: false,
+      screenshotOnError: true,
+      maxDurationMinutes: 60,
+    };
+
+    const result = CreateDebugSessionSchema.parse(input);
+    expect(result.productId).toBe('test_product');
+    expect(result.maxDurationMinutes).toBe(60);
+  });
+
+  it('should validate UpdateDebugSessionSchema', () => {
+    const input: UpdateDebugSessionInput = {
+      status: 'active',
+      collectionLevel: 'trace',
+    };
+
+    const result = UpdateDebugSessionSchema.parse(input);
+    expect(result.status).toBe('active');
+  });
+
+  it('should validate IngestTracesSchema', () => {
+    const input: IngestTracesInput = {
+      sessionId: 'ds_123',
+      traces: [
+        {
+          traceId: randomUUID(),
+          spanId: randomUUID(),
+          name: 'test',
+          startTime: new Date().toISOString(),
+          status: 'ok',
+          attributes: {},
+        },
+      ],
+    };
+
+    const result = IngestTracesSchema.parse(input);
+    expect(result.traces).toHaveLength(1);
+  });
+
+  it('should validate IngestLogsSchema', () => {
+    const input: IngestLogsInput = {
+      sessionId: 'ds_123',
+      logs: [
+        {
+          level: 'info',
+          message: 'Test message',
+          timestamp: new Date().toISOString(),
+          module: 'Test',
+          context: {},
+        },
+      ],
+    };
+
+    const result = IngestLogsSchema.parse(input);
+    expect(result.logs).toHaveLength(1);
+  });
+
+  it('should reject invalid log level', () => {
+    expect(() =>
+      IngestLogsSchema.parse({
+        sessionId: 'ds_123',
+        logs: [
+          {
+            level: 'invalid_level',
+            message: 'Test',
+            timestamp: new Date().toISOString(),
+            module: 'Test',
+            context: {},
+          },
+        ],
+      })
+    ).toThrow();
+  });
+});