---
summary: "Experimental SDK surface for plugins that replace the low level embedded agent executor"
title: "Agent harness plugins"
sidebarTitle: "Agent Harness"
read_when:
  - You are changing the embedded agent runtime or harness registry
  - You are registering an agent harness from a bundled or trusted plugin
  - You need to understand how the Codex plugin relates to model providers
---

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. For
the user-facing mental model, see [Agent runtimes](/concepts/agent-runtimes).

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.

## 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:

- 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](/plugins/sdk-provider-plugins).

## What core still owns

Before a harness is selected, OpenClaw has already resolved:

- provider and model
- runtime auth state, unless the harness declares that it owns auth bootstrap
- 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

A harness runs a prepared attempt; it does not pick providers, replace channel
delivery, or silently switch models.

### Harness-owned auth bootstrap

By default, core resolves provider credentials before calling a harness. A
trusted harness that can authenticate through its own native runtime may set
`authBootstrap: "harness"` on its static `AgentHarness` registration. Core then
skips its generic provider credential bootstrap and missing-credential failure
for every attempt claimed by that harness.

Core still forwards a compatible, explicitly selected or ordered OpenClaw auth
profile and its scoped store when one exists. The harness must resolve that
profile or its native credentials before issuing model requests, keep secrets
scoped to the attempt, and surface actionable authentication failures. Do not
set this capability on a harness that only sometimes owns authentication.

The prepared attempt also includes `params.runtimePlan`, an OpenClaw-owned
policy bundle for runtime decisions that must stay shared across OpenClaw and
native harnesses:

- `runtimePlan.tools.normalize(...)` and `runtimePlan.tools.logDiagnostics(...)`
  for provider-aware tool schema policy
- `runtimePlan.transcript.resolvePolicy(...)` for transcript sanitization and
  tool-call repair policy
- `runtimePlan.delivery.isSilentPayload(...)` for shared `NO_REPLY` and media
  delivery suppression
- `runtimePlan.outcome.classifyRunResult(...)` for model fallback
  classification
- `runtimePlan.observability` for resolved provider/model/harness metadata

Harnesses may use the plan for decisions that need to match OpenClaw behavior,
but treat it as host-owned attempt state: do not mutate it or use it to switch
providers/models inside a turn.

## Register a harness

**Import:** `openclaw/plugin-sdk/agent-harness`

```typescript
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);
  },
});
```

`authBootstrap` is intentionally absent from this generic example. Add
`authBootstrap: "harness"` only when the harness meets the contract above.

## Selection policy

OpenClaw chooses a harness after provider/model resolution:

1. Model-scoped runtime policy wins.
2. Provider-scoped runtime policy comes next.
3. `auto` asks registered harnesses if they support the resolved
   provider/model.
4. If no registered harness matches, OpenClaw uses its embedded runtime.

Plugin harness failures surface as run failures. In `auto` mode, embedded
fallback only applies when no registered plugin harness supports the resolved
provider/model. Once a plugin harness has claimed a run, OpenClaw does not
replay that same turn through another runtime, because that can change
auth/runtime semantics or duplicate side effects.

Whole-session and whole-agent runtime pins are ignored by selection. That
includes stale session `agentHarnessId` values, `agents.defaults.agentRuntime`,
`agents.list[].agentRuntime`, and `OPENCLAW_AGENT_RUNTIME`. `/status` shows the
effective runtime selected from the provider/model route.

If the selected harness is surprising, enable `agents/harness` debug logging
and inspect the gateway's structured `agent harness selected` record: it
includes the selected harness id, selection reason, runtime/fallback policy,
and, in `auto` mode, each plugin candidate's support result.

The bundled Codex plugin registers `codex` as its harness id. Core treats that
as an ordinary plugin harness id; Codex-specific aliases belong in the plugin
or operator config, not in the shared runtime selector.

## 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:

- preferred user model refs: `openai/gpt-5.5`
- compatibility refs: legacy `codex/gpt-*` refs remain accepted, but new
  configs should not use them as normal provider/model refs
- 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-*` agent refs on the official
OpenAI API endpoint (`api.openai.com`) select the Codex harness by default;
custom OpenAI-compatible base URLs keep their configured provider behavior
instead. Older `codex/gpt-*` refs still select the Codex provider and harness
for compatibility.

For operator setup, model prefix examples, and Codex-only configs, see
[Codex Harness](/plugins/codex-harness).

The Codex plugin enforces the minimum app-server version documented in
[Codex Harness](/plugins/codex-harness). It checks the initialize handshake and
blocks older or unversioned servers, so OpenClaw only runs against the protocol
surface it has tested.

### Tool-result middleware

Bundled plugins and explicitly enabled installed plugins with matching
manifest contracts can attach runtime-neutral tool-result middleware through
`api.registerAgentToolResultMiddleware(...)` when their manifest declares the
targeted runtime ids in `contracts.agentToolResultMiddleware`. This trusted
seam is for async tool-result transforms that must run before OpenClaw or
Codex feeds tool output back into the model.

Legacy bundled plugins can still use
`api.registerCodexAppServerExtensionFactory(...)` for Codex app-server-only
middleware, but new result transforms should use the runtime-neutral API. The
embedded-runner-only `api.registerEmbeddedExtensionFactory(...)` hook has been
removed; embedded tool-result transforms must use runtime-neutral middleware.

### Terminal outcome classification

Native harnesses that own their own protocol projection can use
`classifyAgentHarnessTerminalOutcome(...)` from
`openclaw/plugin-sdk/agent-harness-runtime` when a completed turn produced no
visible assistant text. The helper returns `empty`, `reasoning-only`, or
`planning-only` so OpenClaw's fallback policy can decide whether to retry on a
different model. `planning-only` requires the harness's explicit `planText`
field; OpenClaw does not infer it from assistant prose. The helper
intentionally leaves prompt errors, in-flight turns, and intentional silent
replies such as `NO_REPLY` unclassified.

### Agent-end side effects

Native harnesses must call `runAgentEndSideEffects(...)` from
`openclaw/plugin-sdk/agent-harness-runtime` after they finalize an attempt. It
dispatches the portable `agent_end` hook and OpenClaw's research capture
without delaying interactive replies. Use `awaitAgentEndSideEffects(...)` for
local, non-interactive runs where the attempt must not resolve until those
side effects finish. Both helpers accept the same `{ event, ctx }` payload as
`runAgentHarnessAgentEndHook(...)`; their failures do not alter the completed
attempt result.

### User input and tool surfaces

Native harnesses that expose a runtime-level user-input request should use the
user-input helpers from `openclaw/plugin-sdk/agent-harness-runtime` to format
the prompt, deliver it through OpenClaw's blocking reply path, and normalize
choice/free-form answers back into the runtime's native response shape. The
helper keeps channel/TUI presentation consistent while each harness keeps its
own protocol parsing and pending-request lifecycle.

Native harnesses that need PI-like compact tool routing should use
`createAgentHarnessToolSurfaceRuntime(...)` from
`openclaw/plugin-sdk/agent-harness-tool-runtime`. It owns
tool-search/code-mode control selection, local-model lean defaults,
runtime-compatible schema filtering, hidden catalog execution, directory
hydration, and catalog cleanup. Harnesses still own their SDK-specific tool
conversion and native execution callback.

### Native Codex harness mode

The bundled `codex` harness is the native Codex mode for embedded OpenClaw
agent turns. Enable the bundled `codex` plugin first, and include `codex` in
`plugins.allow` if your config uses a restrictive allowlist. Native app-server
configs should use `openai/gpt-*`; OpenAI agent turns select the Codex harness
by default. Legacy Codex model refs routes should be repaired with
`openclaw doctor --fix`, and legacy `codex/*` model refs remain compatibility
aliases for the native harness.

When this mode runs, Codex owns the native thread id, resume behavior,
compaction, and app-server execution. OpenClaw still owns the chat channel,
visible transcript mirror, tool policy, approvals, media delivery, and session
selection. Use provider/model `agentRuntime.id: "codex"` when you need to
prove that only the Codex app-server path can claim the run. Explicit plugin
runtimes fail closed; Codex app-server selection failures and runtime failures
are not retried through another runtime.

## Runtime strictness

By default, OpenClaw uses `auto` provider/model runtime policy: registered
plugin harnesses can claim a provider/model pair, and the embedded runtime
handles the turn when none match. OpenAI agent refs on the official OpenAI
provider default to Codex. Use an explicit provider/model plugin runtime such
as `agentRuntime.id: "codex"` when missing harness selection should fail
instead of routing through the embedded runtime. Selected plugin harness
failures always fail hard. This does not block an explicit provider/model
`agentRuntime.id: "openclaw"`.

For Codex-only embedded runs:

```json
{
  "models": {
    "providers": {
      "openai": {
        "agentRuntime": {
          "id": "codex"
        }
      }
    }
  },
  "agents": {
    "defaults": {
      "model": "openai/gpt-5.5"
    }
  }
}
```

If you want a CLI backend for one canonical model, put the runtime on that
model entry:

```json
{
  "agents": {
    "defaults": {
      "model": "anthropic/claude-opus-4-8",
      "models": {
        "anthropic/claude-opus-4-8": {
          "agentRuntime": {
            "id": "claude-cli"
          }
        }
      }
    }
  }
}
```

Per-agent overrides use the same model-scoped shape:

```json
{
  "agents": {
    "list": [
      {
        "id": "codex-only",
        "model": "openai/gpt-5.5",
        "models": {
          "openai/gpt-5.5": {
            "agentRuntime": { "id": "codex" }
          }
        }
      }
    ]
  }
}
```

Legacy whole-agent runtime examples like this are ignored:

```json
{
  "agents": {
    "defaults": {
      "agentRuntime": {
        "id": "codex"
      }
    }
  }
}
```

With an explicit plugin runtime, 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.

## 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 OpenClaw 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.

## 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 OpenClaw-backed runs.

## Current limitations

- The public import path is generic, but some attempt/result type aliases
  still carry legacy 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.

## Related

- [SDK Overview](/plugins/sdk-overview)
- [Runtime Helpers](/plugins/sdk-runtime)
- [Provider Plugins](/plugins/sdk-provider-plugins)
- [Codex Harness](/plugins/codex-harness)
- [Model Providers](/concepts/model-providers)