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Agent Harness Plugins

An agent harness is the low level executor for one prepared OpenClaw agent turn. It is not a model provider, not a channel, and not a tool registry. Use this surface only for bundled or trusted native plugins. The contract is still experimental because the parameter types intentionally mirror the current embedded runner.

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When to use a harness

Register an agent harness when a model family has its own native session runtime and the normal OpenClaw provider transport is the wrong abstraction. Examples:
  • a native coding-agent server that owns threads and compaction
  • a local CLI or daemon that must stream native plan/reasoning/tool events
  • a model runtime that needs its own resume id in addition to the OpenClaw session transcript
Do not register a harness just to add a new LLM API. For normal HTTP or WebSocket model APIs, build a provider plugin.

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What core still owns

Before a harness is selected, OpenClaw has already resolved:
  • provider and model
  • runtime auth state
  • thinking level and context budget
  • the OpenClaw transcript/session file
  • workspace, sandbox, and tool policy
  • channel reply callbacks and streaming callbacks
  • model fallback and live model switching policy
That split is intentional. A harness runs a prepared attempt; it does not pick providers, replace channel delivery, or silently switch models.

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Register a harness

Import: openclaw/plugin-sdk/agent-harness 
import type { AgentHarness } from "openclaw/plugin-sdk/agent-harness";
import { definePluginEntry } from "openclaw/plugin-sdk/plugin-entry";

const myHarness: AgentHarness = {
  id: "my-harness",
  label: "My native agent harness",

  supports(ctx) {
    return ctx.provider === "my-provider"
      ? { supported: true, priority: 100 }
      : { supported: false };
  },

  async runAttempt(params) {
    // Start or resume your native thread.
    // Use params.prompt, params.tools, params.images, params.onPartialReply,
    // params.onAgentEvent, and the other prepared attempt fields.
    return await runMyNativeTurn(params);
  },
};

export default definePluginEntry({
  id: "my-native-agent",
  name: "My Native Agent",
  description: "Runs selected models through a native agent daemon.",
  register(api) {
    api.registerAgentHarness(myHarness);
  },
});

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Selection policy

OpenClaw chooses a harness after provider/model resolution:
  1. OPENCLAW_AGENT_RUNTIME=<id> forces a registered harness with that id.
  2. OPENCLAW_AGENT_RUNTIME=pi forces the built-in PI harness.
  3. OPENCLAW_AGENT_RUNTIME=auto asks registered harnesses if they support the resolved provider/model.
  4. If no registered harness matches, OpenClaw uses PI unless PI fallback is disabled.
Forced plugin harness failures surface as run failures. In auto mode, OpenClaw may fall back to PI when the selected plugin harness fails before a turn has produced side effects. Set OPENCLAW_AGENT_HARNESS_FALLBACK=none or embeddedHarness.fallback: "none" to make that fallback a hard failure instead. The bundled Codex plugin registers codex as its harness id. For compatibility, codex-app-server and app-server also resolve to that same harness when you set OPENCLAW_AGENT_RUNTIME manually.

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Provider plus harness pairing

Most harnesses should also register a provider. The provider makes model refs, auth status, model metadata, and /model selection visible to the rest of OpenClaw. The harness then claims that provider in supports(...). The bundled Codex plugin follows this pattern:
  • provider id: codex
  • user model refs: codex/gpt-5.4, codex/gpt-5.2, or another model returned by the Codex app server
  • harness id: codex
  • auth: synthetic provider availability, because the Codex harness owns the native Codex login/session
  • app-server request: OpenClaw sends the bare model id to Codex and lets the harness talk to the native app-server protocol
The Codex plugin is additive. Plain openai/gpt-* refs remain OpenAI provider refs and continue to use the normal OpenClaw provider path. Select codex/gpt-* when you want Codex-managed auth, Codex model discovery, native threads, and Codex app-server execution. /model can switch among the Codex models returned by the Codex app server without requiring OpenAI provider credentials. For operator setup, model prefix examples, and Codex-only configs, see Codex Harness. OpenClaw requires Codex app-server 0.118.0 or newer. The Codex plugin checks the app-server initialize handshake and blocks older or unversioned servers so OpenClaw only runs against the protocol surface it has been tested with.

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Disable PI fallback

By default, OpenClaw runs embedded agents with agents.defaults.embeddedHarness set to { runtime: "auto", fallback: "pi" }. In auto mode, registered plugin harnesses can claim a provider/model pair. If none match, or if an auto-selected plugin harness fails before producing output, OpenClaw falls back to PI. Set fallback: "none" when you need to prove that a plugin harness is the only runtime being exercised. This disables automatic PI fallback; it does not block an explicit runtime: "pi" or OPENCLAW_AGENT_RUNTIME=pi. For Codex-only embedded runs: 
{
  "agents": {
    "defaults": {
      "model": "codex/gpt-5.4",
      "embeddedHarness": {
        "runtime": "codex",
        "fallback": "none"
      }
    }
  }
}
If you want any registered plugin harness to claim matching models but never want OpenClaw to silently fall back to PI, keep runtime: "auto" and disable the fallback: 
{
  "agents": {
    "defaults": {
      "embeddedHarness": {
        "runtime": "auto",
        "fallback": "none"
      }
    }
  }
}
Per-agent overrides use the same shape: 
{
  "agents": {
    "defaults": {
      "embeddedHarness": {
        "runtime": "auto",
        "fallback": "pi"
      }
    },
    "list": [
      {
        "id": "codex-only",
        "model": "codex/gpt-5.4",
        "embeddedHarness": {
          "runtime": "codex",
          "fallback": "none"
        }
      }
    ]
  }
}
OPENCLAW_AGENT_RUNTIME still overrides the configured runtime. Use OPENCLAW_AGENT_HARNESS_FALLBACK=none to disable PI fallback from the environment. 
OPENCLAW_AGENT_RUNTIME=codex \
OPENCLAW_AGENT_HARNESS_FALLBACK=none \
openclaw gateway run
With fallback disabled, a session fails early when the requested harness is not registered, does not support the resolved provider/model, or fails before producing turn side effects. That is intentional for Codex-only deployments and for live tests that must prove the Codex app-server path is actually in use. This setting only controls the embedded agent harness. It does not disable image, video, music, TTS, PDF, or other provider-specific model routing.

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Native sessions and transcript mirror

A harness may keep a native session id, thread id, or daemon-side resume token. Keep that binding explicitly associated with the OpenClaw session, and keep mirroring user-visible assistant/tool output into the OpenClaw transcript. The OpenClaw transcript remains the compatibility layer for:
  • channel-visible session history
  • transcript search and indexing
  • switching back to the built-in PI harness on a later turn
  • generic /new, /reset, and session deletion behavior
If your harness stores a sidecar binding, implement reset(...) so OpenClaw can clear it when the owning OpenClaw session is reset.

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Tool and media results

Core constructs the OpenClaw tool list and passes it into the prepared attempt. When a harness executes a dynamic tool call, return the tool result back through the harness result shape instead of sending channel media yourself. This keeps text, image, video, music, TTS, approval, and messaging-tool outputs on the same delivery path as PI-backed runs.

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Current limitations

  • The public import path is generic, but some attempt/result type aliases still carry Pi names for compatibility.
  • Third-party harness installation is experimental. Prefer provider plugins until you need a native session runtime.
  • Harness switching is supported across turns. Do not switch harnesses in the middle of a turn after native tools, approvals, assistant text, or message sends have started.