---
title: "Agent runtime architecture"
summary: "How OpenClaw structures the built-in agent runtime: code layout, boundaries, resource manifests, and runtime selection."
---

OpenClaw owns the built-in agent runtime. Runtime code lives under `src/agents/`, model/provider transport lives under `src/llm/`, and plugin-facing contracts are exposed through `openclaw/plugin-sdk/*` barrels.

## Runtime Layout

| Path                                | Owns                                                                                                                                                                                                                      |
| ----------------------------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| `src/agents/embedded-agent-runner/` | Built-in attempt loop (`run.ts`, `run/`), model selection and provider normalization (`model*.ts`), per-provider request params (`extra-params.*`), compaction, transcript and session wiring.                            |
| `src/agents/sessions/`              | Session persistence (`session-manager.ts`), resource discovery (`package-manager.ts`, `resource-loader.ts`), in-session `extensions` loading, prompt templates, skills, themes, and TUI-backed tool renderers (`tools/`). |
| `packages/agent-core/`              | Reusable agent core (`@openclaw/agent-core`): agent loop, harness types, messages, compaction helpers, prompt templates, skills, and session storage contracts.                                                           |
| `src/agents/runtime/`               | OpenClaw facade that wires `@openclaw/agent-core` to the plugin SDK LLM runtime and re-exports it plus local proxy utilities.                                                                                             |
| `src/agents/agent-tools*.ts`        | OpenClaw-owned tool definitions, parameter schemas, tool policy, before/after tool-call adapters, and host/sandbox edit tools.                                                                                            |
| `src/agents/agent-hooks/`           | Built-in runtime hooks: compaction safeguard, compaction instructions, context pruning.                                                                                                                                   |
| `src/agents/harness/`               | Harness registry, selection policy, and lifecycle for the built-in and plugin-registered harnesses.                                                                                                                       |
| `src/llm/`                          | Model/provider registry, transport helpers, and provider-specific stream implementations (`src/llm/providers/`).                                                                                                          |

## Boundaries

Core calls the built-in runtime through OpenClaw modules and SDK barrels; no external agent framework packages remain. Plugins use documented `openclaw/plugin-sdk/*` entrypoints and do not import `src/**` internals.

`@earendil-works/pi-tui` remains a third-party dependency: a terminal component toolkit used by the local TUI and session tool renderers. Internalizing it would be a separate vendoring effort.

## Manifests

Resource packages declare OpenClaw resources in `package.json` metadata. Entries are file paths or globs relative to the package root:

```json
{
  "openclaw": {
    "extensions": ["extensions/index.ts"],
    "skills": ["skills/*.md"],
    "prompts": ["prompts/*.md"],
    "themes": ["themes/*.json"]
  }
}
```

Resource types not listed in a manifest fall back to discovery of conventional `extensions/`, `skills/`, `prompts/`, and `themes/` directories.

## Runtime Selection

- The built-in runtime id is `openclaw`. The legacy alias `pi` normalizes to `openclaw`; `codex-app-server` normalizes to `codex`.
- Plugin harnesses register additional runtime ids (for example `codex`).
- Runtime policy is model/provider-scoped `agentRuntime.id` config (model entry wins over provider entry). Unset or `default` resolves to `auto`.
- `auto` selects a registered plugin harness that supports the effective provider route, otherwise the built-in OpenClaw runtime. A provider or model prefix alone never selects a harness.
- OpenAI may select `codex` implicitly only for an exact official HTTPS Platform Responses or ChatGPT Responses route with no authored request override. Completions adapters, custom endpoints, and routes with authored request behavior stay on `openclaw`; plaintext official HTTP endpoints are rejected. See [OpenAI implicit agent runtime](/providers/openai#implicit-agent-runtime).

## Model Runtime Generations

Gateway startup and config, plugin, or auth publication build one prepared model runtime generation per configured agent. Each generation owns the discovered auth template, model registry, and projected model catalog as one atomic snapshot. Agent runs fork mutable auth and registry stores from that snapshot; browse, status, cron, doctor, TUI, PDF, and image paths read the published catalog instead of repeating filesystem discovery.

Standalone embedded runtimes publish the same snapshot shape at their activation boundary. A failed or stale generation is never served alongside a newer partial generation; the lifecycle owner must publish a complete replacement first.

## Related

- [OpenClaw agent runtime workflow](/openclaw-agent-runtime)
- [Agent runtimes](/concepts/agent-runtimes)