Understand the stack.
Build better agents.
Explore how the Agents API, models, tools, and infrastructure connect. From your first session to production.
Explore the agent ecosystem
Agents API
OpenAI runs the Codex harness: the model-and-tool loop, durable sessions, context compaction, and recovery.
- UPSTREAM
- Application requests and configuration
- DOWNSTREAM
- Models, tools, optional environments
A clear path through the ecosystem
Start with the big picture, choose your runtime, or go straight to code.
Get the big picture
Agents, sessions, tools, and environments. Understand what each piece does.
Explore the fundamentalsChoose your runtime
Agents API, Agents SDK, or Responses? Compare who runs what.
Compare the optionsMake your first call
Install the SDK, create a session, and follow the agent’s progress.
Open the quickstartWhat happens after you send a task?
Walk through the request, tool call, and result one event at a time.
Submit a task
Your server creates a session and supplies instructions, input, and permitted tools.
Application → Agents APIGo deeper into the stack
Common questions, clear answers.
Is the Agents API the same as the Agents SDK?+
No. The Agents API runs a managed Codex harness at OpenAI. The SDK runs its agent loop in your application runtime. They have different resource and deployment models.
Do I need a sandbox for every agent?+
No. Use environment.type: none for tasks that can use remote MCP and application functions without a filesystem or command execution.
Can I use my own infrastructure?+
Yes. A self-hosted environment connects your compute to the managed harness through an executor. You own that environment’s provisioning and lifecycle.
Is this an official OpenAI website?+
No. Agents API Encyclopedia is an independent educational resource. Each topic links to the relevant official documentation.