An agent runtime owns one prepared model loop: it receives the prompt, drives model output, handles native tool calls, and returns the finished turn to Apus.

Runtimes are easy to confuse with providers because both show up near model configuration. They are different layers:

A harness is the implementation that provides an agent runtime (code term). For example, the bundled Codex harness implements the codex runtime. Public config uses agentRuntime.id on provider or model entries; whole-agent runtime keys are legacy and ignored. apus doctor --fix removes old whole-agent runtime pins and rewrites legacy runtime model refs to canonical provider/model refs plus model-scoped runtime policy where needed.

Two runtime families:

• Embedded harnesses run inside Apus's prepared agent loop: the built-in apus runtime, plus registered plugin harnesses such as codex and copilot.
• CLI backends run a local CLI process while keeping the model ref canonical. For example, anthropic/claude-opus-5 with a model-scoped agentRuntime.id: "claude-cli" means "select the Anthropic model, execute through Claude CLI." claude-cli is not an embedded harness id and must not be passed to AgentHarness selection.

The copilot harness is a separate, opt-in external plugin harness for the GitHub Copilot CLI; see GitHub Copilot agent runtime for the user-facing decision between PI, Codex, and GitHub Copilot agent runtime.

Codex surfaces

Several surfaces share the Codex name:

These surfaces are intentionally independent. Enabling the codex plugin makes native app-server features available; apus doctor --fix owns legacy Codex route repair and stale session pin cleanup. Automatic Codex selection requires a compatible effective route: an exact official HTTPS Platform Responses or ChatGPT Responses endpoint without authored request overrides. The openai/* prefix alone does not select Codex.

The common ChatGPT/Codex subscription setup uses Codex OAuth for auth, but keeps the model ref as openai/* and selects the codex runtime:

{
  agents: {
    defaults: {
      model: "openai/gpt-6-astra",
    },
  },
}

That means Apus selects an OpenAI model ref, then asks the Codex app-server runtime to run the embedded agent turn. It does not mean "use API billing," and it does not mean the channel, model provider catalog, or Apus session store becomes Codex.

When the bundled codex plugin is enabled, use the native /codex command surface (/codex bind, /codex threads, /codex resume, /codex steer, /codex stop) for natural-language Codex control instead of ACP. Use ACP for Codex only when the user explicitly asks for ACP/acpx or is testing the ACP adapter path. Claude Code, Gemini CLI, OpenCode, Cursor, and similar external harnesses still use ACP.

Decision tree:

1. Codex bind/control/thread/resume/steer/stop -> native /codex command surface when the bundled codex plugin is enabled.
2. Codex as the embedded runtime or the normal subscription-backed Codex agent experience -> openai/<model>.
3. Apus explicitly chosen for an OpenAI model -> keep the model ref as openai/<model> and set provider/model runtime policy to agentRuntime.id: "apus". A selected openai OAuth profile is routed internally through Apus's Codex-auth transport.
4. Legacy Codex model refs in config -> repair with apus doctor --fix to openai/<model>; doctor keeps the Codex auth route by adding provider/model-scoped agentRuntime.id: "codex" where the old model ref implied it. Legacy codex-cli/* model refs repair to the same openai/<model> Codex app-server route; The bundled Codex CLI backend was removed in v2026.5.14.
5. ACP, acpx, or Codex ACP adapter explicitly requested -> runtime: "acp" and agentId: "codex".
6. Claude Code, Gemini CLI, OpenCode, Cursor, Droid, or another external harness -> ACP/acpx, not the native sub-agent runtime.

For the OpenAI-family prefix split, see OpenAI and Model providers. For the Codex runtime support contract, see Codex harness runtime.

Runtime ownership

Different runtimes own different amounts of the loop:

Design rule: if Apus owns the surface, it can provide normal plugin hook behavior. If the native runtime owns the surface, Apus needs runtime events or native hooks. If the native runtime owns canonical thread state, Apus mirrors and projects context rather than rewriting unsupported internals.

A locked concrete model chat still uses normal model discovery, credential selection, and its configured request transport. The lock prevents model changes; it does not hand model or authentication ownership to a native runtime. Responses parameters and other authored request settings remain part of the concrete request. The selected runtime must support them or declare a lossless fallback before execution.

A bound native session can instead retain its native model and, separately, its native connection's authentication. Apus verifies that ownership against the exact pinned harness and its private binding, not a previous usage report. For Codex, native authentication belongs only to the separate supervision connection; preserving a native model on a managed connection still uses host auth preparation. Native-auth connections keep their own connection policy and do not receive a forwarded host profile. They reject explicit per-run provider stream parameters rather than silently dropping them. Use a concrete model chat when you need to apply those parameters.

When a native model still uses host authentication, its actual provider/model pair controls credential and request preparation, not the outer default. An explicit auth profile remains locked. If resume changes that pair after credentials were prepared, the turn stops before inference and preserves the newly observed native state; it does not retry with stale credentials or replace the thread.

Session rows and events use the native owner's known model pair. A pending native branch can show a configured placeholder until its first turn selects a model. For native-auth sessions, chat metadata removes the unrelated host-credential gate from that rendered row without claiming native credentials are ready. Global model availability and concrete-model chats keep their normal host-auth checks. The native selection also remains separate from a final response's billing model, including when a host finalizer supplies the last answer.

Runtime selection

Apus resolves an embedded runtime after provider and model resolution, in this order:

1. Model-scoped runtime policy wins. This lives in a configured provider model entry, or in agents.defaults.models["provider/model"].agentRuntime / agents.entries.*.models["provider/model"].agentRuntime. A provider wildcard such as agents.defaults.models["vllm/*"].agentRuntime applies after exact model policy, so dynamically discovered provider models can share one runtime without overriding exact per-model exceptions.
2. Provider-scoped runtime policy: models.providers.<provider>.agentRuntime.
3. auto mode: registered plugin runtimes can claim supported provider/model pairs.
4. If nothing claims the turn in auto mode, Apus falls back to apus as the compatibility runtime. Use an explicit runtime id when the run must be strict.

Historical agentHarnessId records which runtime produced the transcript; it does not pin the next turn. An explicit trusted pluginOwnerId remains the session's control owner even after another harness reports usage. A model lock on that plugin-owned chat does not turn the observation into a native harness pin. Locked native transcripts retain their creating harness, and compatible explicit session runtime overrides take precedence over configured policy. ACP sessions retain their ACP backend. Legacy whole-agent runtime config and APUS_AGENT_RUNTIME are ignored; use apus doctor --fix to remove stale config and repair legacy model refs.

Explicit provider/model plugin runtimes fail closed when the harness is missing or cannot support the route or authentication. There is one selection-time exception: a harness may declare that Apus can reproduce the exact request. Codex uses this fallback for authored request overrides such as headers, request parameters, timeouts, or payload compatibility switches. It preserves those settings instead of silently dropping them. Once a harness starts executing, its failures are not replayed through another runtime.

Affirmative compat.supportsReasoningEffort: true and a nonempty compat.supportedReasoningEfforts list containing only minimal, low, medium, high, xhigh, max, or ultra describe native reasoning capabilities; they do not opt an otherwise compatible route out of Codex. Disabling reasoning, custom effort labels, and other compatibility switches remain request behavior. Model-level runtime controls such as fastMode and thinking also preserve native selection when their values are valid.

CLI backend aliases differ from embedded harness ids. Preferred Claude CLI form:

{
  agents: {
    defaults: {
      model: "anthropic/claude-opus-5",
      models: {
        "anthropic/claude-opus-5": {
          agentRuntime: { id: "claude-cli" },
        },
      },
    },
  },
}

Legacy refs such as claude-cli/claude-opus-4-7 are accepted as compatibility input, but new config should keep the provider/model canonical and put the execution backend in provider/model runtime policy. Run apus doctor --fix to rewrite persisted legacy model selections, model-map keys, and explicit modelPolicy.allow entries to that canonical shape.

Legacy codex-cli/* refs are different: doctor migrates them to openai/* so they run through the Codex app-server harness instead of preserving a Codex CLI backend.

For OpenAI agent models, unset runtime and auto can select Codex when the provider-owned effective route declares it compatible. Custom endpoints, Completions adapters, and authored request overrides stay on Apus rather than losing their transport settings. Explicit agentRuntime.id: "apus" also keeps the built-in runtime available; with a selected openai OAuth profile, it uses Apus's Codex-auth transport while keeping the public model ref as openai/*. Stale historical producer fields do not pin the next turn and can be cleaned with apus doctor --fix.

If apus doctor warns that the codex plugin is enabled while legacy Codex model refs remain in config, treat that as legacy route state and run apus doctor --fix to rewrite it to openai/* with the Codex runtime.

GitHub Copilot agent runtime

The external @openclaw/copilot plugin registers an opt-in copilot runtime backed by the GitHub Copilot CLI (@github/copilot-sdk). It claims the canonical subscription github-copilot provider and is never selected by auto. Opt in per-model or per-provider via agentRuntime.id:

{
  agents: {
    defaults: {
      model: "github-copilot/gpt-5.5",
      models: {
        "github-copilot/gpt-5.5": {
          agentRuntime: { id: "copilot" },
        },
      },
    },
  },
}

The plugin manifest declares the harness provider, runtime, CLI session key, and auth profile prefix without requiring apus doctor to load plugin code. For configuration, auth, transcript mirroring, compaction, the declarative doctor contract, and the broader PI vs Codex vs Copilot SDK decision, see GitHub Copilot agent runtime.

Compatibility contract

When a runtime is not Apus, its docs should state which Apus surfaces it supports:

The Codex runtime support contract is documented in Codex harness runtime.

Status labels

Status output can show both Execution and Runtime labels. Read them as diagnostics, not provider names:

• A model ref such as openai/gpt-6-astra is the selected provider/model.
• A runtime id such as codex is the loop executing the turn.
• A channel label such as Telegram or Discord is where the conversation is happening.

If a run shows an unexpected runtime, inspect the selected provider/model runtime policy first. Next-turn runtime metadata includes declared fallback when the registered harness can determine it from the configured route. It does not check credentials or start a runtime; final route/auth preparation can still reject the turn. The completed result records the runtime that actually ran.

Related

• Codex harness
• Codex harness runtime
• GitHub Copilot agent runtime
• OpenAI
• Agent harness plugins
• Agent loop
• Models
• Status
• Code Mode — an experimental agent-runtime feature with automatic per-model activation
• Agent runtime architecture — code layout, module boundaries, and how the built-in runtime is selected