Custom GPT vs Assistants API: Which Should You Build?
Short answer: Custom GPT if you want a no‑code experience inside ChatGPT. Assistants API if you need to embed AI inside your own app, control UX, or run server‑side workflows.
Also see: Add Knowledge • GPT Actions • Pillar guide
Side‑by‑side comparison
| Aspect | Custom GPT | Assistants API |
|---|---|---|
| Setup | No‑code in ChatGPT (editor) | Code + SDKs in your app |
| Distribution | Inside ChatGPT (users log in) | Your product UI (web/mobile/backend) |
| Knowledge | Upload files per GPT | Use your own storage/indexes |
| Integrations | Actions via OpenAPI | Any API/service you code |
| Auth & Users | Handled by ChatGPT | Your auth, tokens, roles |
| Brand/UX control | Medium | Full control |
| Billing | Via ChatGPT plans/usage | Your OpenAI API billing |
| Rate limits | Platform‑managed | You manage |
| Best for | Creators, support, internal tooling | SaaS features, programmatic workflows |
Choose Custom GPT if…
- You want to publish a helper inside ChatGPT fast.
- Your “integration” is simple (fetch/read data, no heavy auth flows).
- You prefer maintaining content (Instructions/Knowledge) over code.
- You don’t need fine‑grained user analytics in your own stack.
Choose Assistants API if…
- You need to embed AI into your app or backend.
- You require custom memory, databases, or vector search.
- You need SSO/roles, webhooks, queues, or background jobs.
- You want full control of telemetry, costs, and UX.
Decision tree
- Do users live in ChatGPT? → Yes: Custom GPT. No: Assistants API.
- Do you need multi‑tenant auth/analytics/billing? → Assistants API.
- Is your integration simple read‑only? → Custom GPT + Actions.
- Do you need programmatic control or batch jobs? → Assistants API.
Example stacks
Custom GPT stack
- Instructions + Knowledge files
- Optional: Actions (OpenAPI → your API)
- Optional: Public listing + simple analytics via UTM links
Assistants API stack
- Backend (Node/Python) + OpenAI SDK
- Vector DB (e.g., pgvector, Pinecone) for retrieval
- Auth (JWT/SSO), billing, queue/workers, observability
Migration path (start simple, scale later)
- Prototype the experience as a Custom GPT (ship in a day).
- Prove demand → collect FAQs, friction points, and KPIs.
- Rebuild the core flow with Assistants API in your app.
- Keep the Custom GPT as a lead‑gen/tutor that links to your app.
FAQ
Can I call my Custom GPT via API?
No — use the Assistants API if you need programmatic control.
Can both use external APIs?
Yes. Custom GPTs use Actions; Assistants API uses your code directly.
Which is cheaper?
It depends on usage and plan. Custom GPTs bill via ChatGPT; Assistants API bills your API calls. Start with the simplest option.
SEO note
If you’re building a public tool/site, Assistants API lets you own the domain/SEO surface.