Building ChatGPT Apps with LLMFeed Trust: A Developer's Guide

Congratulations: OpenAI just gave you access to 800 million weekly ChatGPT users via the Apps SDK.

The question: How do you build apps those 800 million users can trust?

The answer: LLMFeed's cryptographic trust layer.

This guide shows you exactly how.

---

What We're Building

A complete ChatGPT app with:

• ✅ Natural language interface (Apps SDK)
• ✅ Real-time data access (your API)
• ✅ Cryptographic verification (LLMFeed)
• ✅ Trust scoring (LLMCA)
• ✅ Audit trails (session feeds)
• ✅ Production-ready security

By the end, you'll have:

1. Working ChatGPT app
2. Signed capability declarations
3. Verified trust infrastructure
4. Complete implementation guide

Let's build.

---

Prerequisites

Required

bash
# Node.js 18+
node --version

# OpenAI Apps SDK (when released)
npm install @openai/apps-sdk

# LLMFeed tools
npm install @wellknownmcp/client

Helpful Background

• Basic understanding of MCP (Model Context Protocol)
• API development experience
• Cryptographic signatures concept
• ChatGPT usage familiarity

---

Phase 1: Hello World ChatGPT App

Step 1: Create Basic App

typescript
// app.ts
import { ChatGPTApp } from '@openai/apps-sdk';

const app = new ChatGPTApp({
  name: 'weather-assistant',
  description: 'Get real-time weather information'
});

// Define your app's capabilities
app.addTool({
  name: 'get_weather',
  description: 'Get current weather for a location',
  parameters: {
    type: 'object',
    properties: {
      location: {
        type: 'string',
        description: 'City name or coordinates'
      }
    },
    required: ['location']
  },
  handler: async (params) => {
    // Call your API
    const weather = await fetchWeather(params.location);
    return weather;
  }
});

app.listen(3000);

Step 2: Test Locally

bash
npm run dev
# App running at http://localhost:3000

Result: Basic ChatGPT app running locally.

Problem: Zero trust verification, no security guarantees.

---

Phase 2: Add LLMFeed Trust Layer

Step 1: Create MCP Declaration

json
// public/.well-known/mcp.llmfeed.json
{
  "feed_type": "mcp",
  "metadata": {
    "title": "Weather Assistant API",
    "origin": "https://weather-api.example.com",
    "description": "Real-time weather data with cryptographic verification",
    "version": "1.0.0"
  },
  "capabilities": [
    {
      "name": "get_weather",
      "method": "POST",
      "path": "/api/weather",
      "description": "Get current weather for a location",
      "parameters": {
        "location": {
          "type": "string",
          "required": true,
          "description": "City name or GPS coordinates"
        }
      },
      "response": {
        "temperature": "number",
        "conditions": "string",
        "humidity": "number"
      },
      "rate_limit": "100/hour",
      "requires_user_consent": false
    }
  ],
  "agent_guidance": {
    "interaction_tone": "helpful",
    "fallback_behavior": "suggest manual weather check",
    "privacy_hint": "No personal data collected"
  }
}

Step 2: Sign Your Declaration

typescript
// scripts/sign-feed.ts
import { signLLMFeed } from '@wellknownmcp/client';
import fs from 'fs';

async function signFeed() {
  const feed = JSON.parse(
    fs.readFileSync('public/.well-known/mcp.llmfeed.json', 'utf8')
  );

  // Add trust declaration
  feed.trust = {
    signed_blocks: ['metadata', 'capabilities', 'agent_guidance', 'trust'],
    scope: 'public',
    algorithm: 'ed25519',
    public_key_hint: 'https://weather-api.example.com/.well-known/public.pem'
  };

  // Sign the feed
  const signed = await signLLMFeed(feed, {
    privateKey: process.env.LLMFEED_PRIVATE_KEY
  });

  // Save signed version
  fs.writeFileSync(
    'public/.well-known/mcp.llmfeed.json',
    JSON.stringify(signed, null, 2)
  );

  console.log('✓ Feed signed successfully');
}

signFeed();

Step 3: Generate Keys

bash
# Generate Ed25519 key pair
npm run generate-keys

# Output:
# ✓ Private key saved to: .keys/private.pem
# ✓ Public key saved to: public/.well-known/public.pem
#
# Add to your .env:
# LLMFEED_PRIVATE_KEY=...

Step 4: Sign and Publish

bash
# Sign the feed
npm run sign-feed

# Deploy to your server
npm run deploy

Result: Cryptographically signed capability declaration.

---

Phase 3: Verification Layer

Step 1: Add Verification Endpoint

typescript
// api/verify.ts
import { verifyLLMFeedSignature } from '@wellknownmcp/client';

export async function GET(request: Request) {
  const feedUrl = new URL(request.url).searchParams.get('feed');

  if (!feedUrl) {
    return Response.json({ error: 'Missing feed URL' }, { status: 400 });
  }

  // Fetch the feed
  const feed = await fetch(feedUrl).then(r => r.json());

  // Verify signature
  const verification = await verifyLLMFeedSignature(feed);

  return Response.json({
    valid: verification.valid,
    trust_level: verification.trustLevel,
    certifier: verification.certifier,
    signed_at: verification.signedAt,
    warnings: verification.warnings
  });
}

Step 2: ChatGPT App Verification

typescript
// app.ts (enhanced)
import { ChatGPTApp } from '@openai/apps-sdk';
import { verifyLLMFeedSignature, discoverFeed } from '@wellknownmcp/client';

const app = new ChatGPTApp({
  name: 'weather-assistant',
  description: 'Get real-time weather information'
});

// Add verification before tool execution
app.beforeToolCall(async (toolName, params) => {
  // Discover the feed
  const feed = await discoverFeed('https://weather-api.example.com');

  // Verify signature
  const verification = await verifyLLMFeedSignature(feed);

  if (!verification.valid) {
    throw new Error('Feed signature invalid - execution blocked');
  }

  if (verification.trustLevel < 'signed') {
    console.warn('Low trust level detected:', verification.trustLevel);
  }

  // Proceed with execution
  return true;
});

app.addTool({
  name: 'get_weather',
  // ... rest of tool definition
});

Result: Every API call verified before execution.

---

Phase 4: LLMCA Certification

Step 1: Apply for Certification

bash
# Submit your signed feed for certification
llmfeed certify \
  --feed=https://weather-api.example.com/.well-known/mcp.llmfeed.json \
  --certifier=llmca.org \
  --level=organization

Step 2: Complete Verification

LLMCA Certification Process:
1. Identity verification (domain ownership)
2. Technical validation (feed structure)
3. Security audit (capability review)
4. Reputation assessment
5. Certification issuance

Timeline: 2-5 business days

Step 3: Add Certification Block

json
// After LLMCA approval, update your feed:
{
  "feed_type": "mcp",
  "metadata": { /* ... */ },
  "capabilities": [ /* ... */ ],
  "trust": { /* ... */ },
  "signature": { /* ... */ },

  // Added by LLMCA
  "certification": {
    "certifier": "https://llmca.org",
    "level": "organization",
    "cert_id": "llmca-2025-1234",
    "issued_at": "2025-10-12T10:00:00Z",
    "expires_at": "2026-10-12T10:00:00Z",
    "algorithm": "ed25519",
    "value": "llmca_certification_signature",
    "public_key_hint": "https://llmca.org/.well-known/llmca_cert.pem"
  }
}

Result: LLMCA-certified app, highest trust level.

---

Phase 5: Session Feeds & Audit Trails

Step 1: Generate Session Feeds

typescript
// middleware/session-feed.ts
import { createSessionFeed } from '@wellknownmcp/client';

export async function logSession(sessionData) {
  const feed = await createSessionFeed({
    feed_type: 'session',
    metadata: {
      agent: 'chatgpt-app',
      app_name: 'weather-assistant',
      session_id: sessionData.id,
      started_at: sessionData.startedAt,
      completed_at: new Date().toISOString()
    },
    actions: sessionData.actions.map(action => ({
      timestamp: action.timestamp,
      tool: action.toolName,
      params: action.params,
      result: action.result,
      source_feed: 'https://weather-api.example.com/.well-known/mcp.llmfeed.json',
      verified: action.verified,
      trust_level: action.trustLevel
    })),
    trust: {
      signed_blocks: ['metadata', 'actions'],
      certifier: 'https://llmca.org'
    }
  });

  // Sign the session feed
  const signed = await signLLMFeed(feed, {
    privateKey: process.env.LLMFEED_PRIVATE_KEY
  });

  // Save for audit
  await saveSessionFeed(signed);

  return signed;
}

Step 2: Track Tool Calls

typescript
// app.ts (complete)
import { ChatGPTApp } from '@openai/apps-sdk';
import { verifyLLMFeedSignature, discoverFeed } from '@wellknownmcp/client';
import { logSession } from './middleware/session-feed';

const app = new ChatGPTApp({
  name: 'weather-assistant',
  description: 'Get real-time weather information'
});

// Session tracking
const sessions = new Map();

app.onSessionStart((sessionId) => {
  sessions.set(sessionId, {
    id: sessionId,
    startedAt: new Date().toISOString(),
    actions: []
  });
});

app.beforeToolCall(async (toolName, params, context) => {
  const feed = await discoverFeed('https://weather-api.example.com');
  const verification = await verifyLLMFeedSignature(feed);

  if (!verification.valid) {
    throw new Error('Feed signature invalid');
  }

  // Log the action
  const session = sessions.get(context.sessionId);
  session.actions.push({
    timestamp: new Date().toISOString(),
    toolName,
    params,
    verified: verification.valid,
    trustLevel: verification.trustLevel
  });

  return true;
});

app.afterToolCall(async (toolName, result, context) => {
  // Update action with result
  const session = sessions.get(context.sessionId);
  const action = session.actions[session.actions.length - 1];
  action.result = result;
});

app.onSessionEnd(async (sessionId) => {
  const session = sessions.get(sessionId);

  // Generate and save session feed
  const feed = await logSession(session);

  console.log('✓ Session feed saved:', feed.metadata.session_id);

  sessions.delete(sessionId);
});

app.addTool({
  name: 'get_weather',
  description: 'Get current weather for a location',
  parameters: {
    type: 'object',
    properties: {
      location: {
        type: 'string',
        description: 'City name or coordinates'
      }
    }
  },
  handler: async (params) => {
    const response = await fetch(
      `https://weather-api.example.com/api/weather`,
      {
        method: 'POST',
        headers: { 'Content-Type': 'application/json' },
        body: JSON.stringify({ location: params.location })
      }
    );

    return response.json();
  }
});

app.listen(3000);

Result: Complete audit trail for every session.

---

Phase 6: Production Deployment

Step 1: Environment Configuration

bash
# .env.production
LLMFEED_PRIVATE_KEY=your_private_key
LLMFEED_PUBLIC_KEY_URL=https://weather-api.example.com/.well-known/public.pem
LLMCA_CERTIFICATION_ID=llmca-2025-1234
API_BASE_URL=https://weather-api.example.com
NODE_ENV=production

Step 2: Deployment Checklist

markdown
## Pre-Deployment

- [ ] Feed signed with production key
- [ ] LLMCA certification active
- [ ] Public key published at .well-known/public.pem
- [ ] Session feed storage configured
- [ ] Rate limiting enabled
- [ ] Error monitoring setup
- [ ] Backup key pair secured

## Deployment

- [ ] Deploy app to production
- [ ] Verify .well-known/ endpoint accessible
- [ ] Test signature verification
- [ ] Confirm LLMCA certification validates
- [ ] Run end-to-end test in ChatGPT

## Post-Deployment

- [ ] Monitor session feed generation
- [ ] Check trust score metrics
- [ ] Review error logs
- [ ] Test fail over scenarios

Step 3: Deploy

bash
# Build production app
npm run build

# Deploy to your platform (example: Vercel)
vercel --prod

# Verify deployment
curl https://weather-api.example.com/.well-known/mcp.llmfeed.json | jq .

# Test verification
curl "https://weather-api.example.com/api/verify?feed=https://weather-api.example.com/.well-known/mcp.llmfeed.json" | jq .

Result: Production-ready ChatGPT app with full trust infrastructure.

---

Phase 7: Monitoring & Analytics

Trust Score Dashboard

typescript
// dashboard/trust-metrics.ts
export async function getTrustMetrics() {
  const feeds = await getAllSessionFeeds();

  const metrics = {
    total_sessions: feeds.length,
    verified_sessions: feeds.filter(f => f.actions.every(a => a.verified)).length,
    trust_levels: {
      certified: 0,
      signed: 0,
      unsigned: 0
    },
    avg_session_duration: 0,
    error_rate: 0
  };

  feeds.forEach(feed => {
    // Calculate metrics
    if (feed.certification) metrics.trust_levels.certified++;
    else if (feed.signature) metrics.trust_levels.signed++;
    else metrics.trust_levels.unsigned++;
  });

  return metrics;
}

Real-Time Monitoring

typescript
// monitoring/real-time.ts
import { createSessionFeed } from '@wellknownmcp/client';

export function monitorTrustEvents() {
  // Listen for verification events
  app.on('verification:success', (event) => {
    console.log('✓ Verification successful:', event);
  });

  app.on('verification:failure', (event) => {
    console.error('✗ Verification failed:', event);
    // Alert security team
    alertSecurityTeam(event);
  });

  app.on('trust:degraded', (event) => {
    console.warn('⚠ Trust level degraded:', event);
    // Log for review
    logTrustDegradation(event);
  });
}

---

Security Best Practices

1. Key Management

bash
# NEVER commit private keys
echo ".keys/" >> .gitignore
echo ".env.production" >> .gitignore

# Use environment variables
export LLMFEED_PRIVATE_KEY=$(cat .keys/private.pem)

# Rotate keys annually
npm run rotate-keys

2. Feed Validation

typescript
// Always validate before signing
import { validateFeed } from '@wellknownmcp/client';

const validation = await validateFeed(feed);

if (!validation.valid) {
  throw new Error(`Invalid feed: ${validation.errors.join(', ')}`);
}

3. Rate Limiting

typescript
// Protect your API
import rateLimit from 'express-rate-limit';

const limiter = rateLimit({
  windowMs: 60 * 60 * 1000, // 1 hour
  max: 100, // limit each IP to 100 requests per windowMs
  message: 'Too many requests from this IP'
});

app.use('/api/', limiter);

4. Error Handling

typescript
app.onError((error, context) => {
  console.error('App error:', error);

  // Don't expose internal errors
  if (error.message.includes('PRIVATE')) {
    return { error: 'Internal server error' };
  }

  // Log for audit
  logError({
    error: error.message,
    sessionId: context.sessionId,
    timestamp: new Date().toISOString()
  });

  return { error: error.message };
});

---

Testing Guide

Unit Tests

typescript
// tests/verification.test.ts
import { verifyLLMFeedSignature } from '@wellknownmcp/client';
import { readFileSync } from 'fs';

describe('Feed Verification', () => {
  test('should verify signed feed', async () => {
    const feed = JSON.parse(
      readFileSync('test/fixtures/signed-feed.json', 'utf8')
    );

    const result = await verifyLLMFeedSignature(feed);

    expect(result.valid).toBe(true);
    expect(result.trustLevel).toBe('signed');
  });

  test('should reject tampered feed', async () => {
    const feed = JSON.parse(
      readFileSync('test/fixtures/signed-feed.json', 'utf8')
    );

    // Tamper with capabilities
    feed.capabilities[0].name = 'malicious_action';

    const result = await verifyLLMFeedSignature(feed);

    expect(result.valid).toBe(false);
  });
});

Integration Tests

typescript
// tests/integration.test.ts
describe('ChatGPT App Integration', () => {
  test('should execute tool with verification', async () => {
    const app = createTestApp();

    const result = await app.executeTool('get_weather', {
      location: 'San Francisco'
    });

    expect(result.verified).toBe(true);
    expect(result.trustLevel).toBe('certified');
    expect(result.data.temperature).toBeDefined();
  });

  test('should block execution on invalid signature', async () => {
    const app = createTestApp();

    // Mock invalid signature
    mockInvalidSignature();

    await expect(
      app.executeTool('get_weather', { location: 'SF' })
    ).rejects.toThrow('Feed signature invalid');
  });
});

---

Common Pitfalls

1. Forgetting to Sign Updates

typescript
// ❌ Wrong: Deploy unsigned changes
feed.capabilities.push(newCapability);
deployFeed(feed);

// ✅ Correct: Sign after changes
feed.capabilities.push(newCapability);
const signed = await signLLMFeed(feed, { privateKey });
deployFeed(signed);

2. Exposing Private Keys

typescript
// ❌ Wrong: Hardcoded keys
const privateKey = '-----BEGIN PRIVATE KEY-----...';

// ✅ Correct: Environment variables
const privateKey = process.env.LLMFEED_PRIVATE_KEY;
if (!privateKey) throw new Error('Private key not configured');

3. Skipping Verification

typescript
// ❌ Wrong: Trust without verification
const weather = await callAPI();
return weather;

// ✅ Correct: Always verify
const feed = await discoverFeed(apiUrl);
const verified = await verifyLLMFeedSignature(feed);
if (!verified.valid) throw new Error('Signature invalid');

const weather = await callAPI();
return weather;

---

Next Steps

Enhance Your App

1. Add more capabilities
2. Implement caching
3. Add user preferences
4. Enable offline mode
5. Build analytics dashboard

Expand Trust Infrastructure

1. Get enterprise certification
2. Implement key rotation
3. Add backup certifiers
4. Build trust monitoring
5. Create compliance reports

Scale to Production

1. Load testing
2. CDN deployment
3. Failover setup
4. Monitoring dashboards
5. Incident response plan

---

Conclusion

You now have:

• ✅ Complete ChatGPT app with 800M user reach
• ✅ Cryptographic trust infrastructure
• ✅ LLMCA certification process
• ✅ Session feed audit trails
• ✅ Production-ready security
• ✅ Monitoring and analytics

The opportunity:

"Build once with LLMFeed trust, reach 800 million users with cryptographic guarantees."

The future:

"Every ChatGPT app comes with verifiable trust by default."

Start building today.

---

Resources

• Apps SDK: developers.openai.com/apps-sdk
• LLMFeed Client: @wellknownmcp/client
• Example Repo: github.com/wellknownmcp/chatgpt-app-example
• LLMCA Certification: llmca.org/certify
• Community: wellknownmcp.org/join

---

The Apps SDK gives you reach.

LLMFeed gives you trust.

Together, they enable the next generation of AI applications.

Now go build something incredible.