Side by side
Agent Runtime & Agent Harness
The harness runs the agent loop around a model; the runtime executes that work durably in production, keeping sessions alive across failures, restarts, and long-running tasks.
Neither term has a canonical definition; check each author’s scope before comparing products.
OpenAI’s Agents API ships both layers in one product, which is why the terms are frequently conflated.
Credal’s analysis is vendor content; its layer split matches independent documentation but serves its product narrative.
Swap columns ⇄| Dimension | Agent Runtime | Agent Harness |
|---|---|---|
| Purpose | Executes agent work reliably: sessions, checkpointing, recovery, and isolation for long-running tasks. | Runs the agent loop: assembles context, routes tool requests, and coordinates subagents. [LangChain][Credal] |
| Role in the system | Infrastructure layer, typically owned by a platform team; hosted options are provider-managed. | Application layer, typically owned by the team building the agent. [Credal][OpenAI] |
| Inputs and outputs | Receives dispatched actions; runs them in isolated environments and returns results, telemetry, and audit logs. | Takes model responses and tool results; dispatches prompts and tool calls. [Credal] |
| Persistence | Persists state across restarts, deploys, and failures so interrupted runs can resume. | Manages conversation context inside a session, including compaction and summarization. [OpenAI][Credal] |
| Example | The same API pairs that harness with managed sandboxes and durable sessions that run for hours or days. | OpenAI hosts and maintains the Codex harness: compaction, tool search, and subagent coordination. [OpenAI] |
| Boundary caveats | Scopes differ by author: LangChain treats LangGraph as a runtime, while Credal separates the layers and attributes subagent spawning to the harness. | Vendors bundle both layers: OpenAI describes its bundle as harness and infrastructure and never uses the word runtime. [OpenAI][LangChain][Credal] |
Evidence for this distinction
- Agent Harness vs Agent Runtime: What’s the difference? ↗ (opens in new tab)Credal · 2026-04-21
Relevant section: What is an agent runtime?; How harness and runtime fit together - Runtimes, frameworks, and harnesses ↗ (opens in new tab)LangChain · Publication date unknown
Relevant section: Agent runtimes (like LangGraph); Agent harnesses (like the Deep Agents SDK) - Introducing the Agents API ↗ (opens in new tab)OpenAI · 2026-09-10
Relevant section: Build with an evolving Codex harness; Keep agents working across long sessions
Last editorial review: 2026-09-18.