CLI Backends

CLI backends (fallback runtime)

Velaclaw can run local AI CLIs as a text-only fallback when API providers are down, rate-limited, or temporarily misbehaving. This is intentionally conservative:

• Velaclaw tools are not injected directly, but backends with bundleMcp: true can receive gateway tools via a loopback MCP bridge.
• JSONL streaming for CLIs that support it.
• Sessions are supported (so follow-up turns stay coherent).
• Images can be passed through if the CLI accepts image paths.

This is designed as a safety net rather than a primary path. Use it when you want “always works” text responses without relying on external APIs.

If you want a full harness runtime with ACP session controls, background tasks, thread/conversation binding, and persistent external coding sessions, use ACP Agents instead. CLI backends are not ACP.

Beginner-friendly quick start

You can use Codex CLI without any config (the bundled OpenAI plugin registers a default backend):

velaclaw agent --message "hi" --model codex-cli/gpt-5.4

If your gateway runs under launchd/systemd and PATH is minimal, add just the command path:

{
  agents: {
    defaults: {
      cliBackends: {
        "codex-cli": {
          command: "/opt/homebrew/bin/codex",
        },
      },
    },
  },
}

That’s it. No keys, no extra auth config needed beyond the CLI itself.

If you use a bundled CLI backend as the primary message provider on a gateway host, Velaclaw now auto-loads the owning bundled plugin when your config explicitly references that backend in a model ref or under agents.defaults.cliBackends.

Using it as a fallback

Add a CLI backend to your fallback list so it only runs when primary models fail:

{
  agents: {
    defaults: {
      model: {
        primary: "anthropic/claude-opus-4-6",
        fallbacks: ["codex-cli/gpt-5.4"],
      },
      models: {
        "anthropic/claude-opus-4-6": { alias: "Opus" },
        "codex-cli/gpt-5.4": {},
      },
    },
  },
}

Notes:

• If you use agents.defaults.models (allowlist), you must include your CLI backend models there too.
• If the primary provider fails (auth, rate limits, timeouts), Velaclaw will try the CLI backend next.

Configuration overview

All CLI backends live under:

agents.defaults.cliBackends

Each entry is keyed by a provider id (e.g. codex-cli, my-cli). The provider id becomes the left side of your model ref:

<provider>/<model>

Example configuration

{
  agents: {
    defaults: {
      cliBackends: {
        "codex-cli": {
          command: "/opt/homebrew/bin/codex",
        },
        "my-cli": {
          command: "my-cli",
          args: ["--json"],
          output: "json",
          input: "arg",
          modelArg: "--model",
          modelAliases: {
            "claude-opus-4-6": "opus",
            "claude-sonnet-4-6": "sonnet",
          },
          sessionArg: "--session",
          sessionMode: "existing",
          sessionIdFields: ["session_id", "conversation_id"],
          systemPromptArg: "--system",
          // Codex-style CLIs can point at a prompt file instead:
          // systemPromptFileConfigArg: "-c",
          // systemPromptFileConfigKey: "model_instructions_file",
          systemPromptWhen: "first",
          imageArg: "--image",
          imageMode: "repeat",
          serialize: true,
        },
      },
    },
  },
}

How it works

1. Selects a backend based on the provider prefix (codex-cli/...).
2. Builds a system prompt using the same Velaclaw prompt + workspace context.
3. Executes the CLI with a session id (if supported) so history stays consistent.
4. Parses output (JSON or plain text) and returns the final text.
5. Persists session ids per backend, so follow-ups reuse the same CLI session.

The bundled Anthropic claude-cli backend is supported again. Anthropic staff told us Velaclaw-style Claude CLI usage is allowed again, so Velaclaw treats claude -p usage as sanctioned for this integration unless Anthropic publishes a new policy.

The bundled OpenAI codex-cli backend passes Velaclaw’s system prompt through Codex’s model_instructions_file config override (-c model_instructions_file="..."). Codex does not expose a Claude-style --append-system-prompt flag, so Velaclaw writes the assembled prompt to a temporary file for each fresh Codex CLI session.

The bundled Anthropic claude-cli backend receives the Velaclaw skills snapshot two ways: the compact Velaclaw skills catalog in the appended system prompt, and a temporary Claude Code plugin passed with --plugin-dir. The plugin contains only the eligible skills for that agent/session, so Claude Code’s native skill resolver sees the same filtered set that Velaclaw would otherwise advertise in the prompt. Skill env/API key overrides are still applied by Velaclaw to the child process environment for the run.

Sessions

• If the CLI supports sessions, set sessionArg (e.g. --session-id) or sessionArgs (placeholder {sessionId}) when the ID needs to be inserted into multiple flags.
• If the CLI uses a resume subcommand with different flags, set resumeArgs (replaces args when resuming) and optionally resumeOutput (for non-JSON resumes).
• sessionMode:
  • always: always send a session id (new UUID if none stored).
  • existing: only send a session id if one was stored before.
  • none: never send a session id.

Serialization notes:

• serialize: true keeps same-lane runs ordered.
• Most CLIs serialize on one provider lane.
• Velaclaw drops stored CLI session reuse when the backend auth state changes, including relogin, token rotation, or a changed auth profile credential.

Images (pass-through)

If your CLI accepts image paths, set imageArg:

imageArg: "--image",
imageMode: "repeat"

Velaclaw will write base64 images to temp files. If imageArg is set, those paths are passed as CLI args. If imageArg is missing, Velaclaw appends the file paths to the prompt (path injection), which is enough for CLIs that auto- load local files from plain paths.

Inputs / outputs

• output: "json" (default) tries to parse JSON and extract text + session id.
• For Gemini CLI JSON output, Velaclaw reads reply text from response and usage from stats when usage is missing or empty.
• output: "jsonl" parses JSONL streams (for example Codex CLI --json) and extracts the final agent message plus session identifiers when present.
• output: "text" treats stdout as the final response.

Input modes:

• input: "arg" (default) passes the prompt as the last CLI arg.
• input: "stdin" sends the prompt via stdin.
• If the prompt is very long and maxPromptArgChars is set, stdin is used.

Defaults (plugin-owned)

The bundled OpenAI plugin also registers a default for codex-cli:

• command: "codex"
• args: ["exec","--json","--color","never","--sandbox","workspace-write","--skip-git-repo-check"]
• resumeArgs: ["exec","resume","{sessionId}","--color","never","--sandbox","workspace-write","--skip-git-repo-check"]
• output: "jsonl"
• resumeOutput: "text"
• modelArg: "--model"
• imageArg: "--image"
• sessionMode: "existing"

The bundled Google plugin also registers a default for google-gemini-cli:

• command: "gemini"
• args: ["--output-format", "json", "--prompt", "{prompt}"]
• resumeArgs: ["--resume", "{sessionId}", "--output-format", "json", "--prompt", "{prompt}"]
• imageArg: "@"
• imagePathScope: "workspace"
• modelArg: "--model"
• sessionMode: "existing"
• sessionIdFields: ["session_id", "sessionId"]

Prerequisite: the local Gemini CLI must be installed and available as gemini on PATH (brew install gemini-cli or npm install -g @google/gemini-cli).

Gemini CLI JSON notes:

• Reply text is read from the JSON response field.
• Usage falls back to stats when usage is absent or empty.
• stats.cached is normalized into Velaclaw cacheRead.
• If stats.input is missing, Velaclaw derives input tokens from stats.input_tokens - stats.cached.

Override only if needed (common: absolute command path).

Plugin-owned defaults

CLI backend defaults are now part of the plugin surface:

• Plugins register them with api.registerCliBackend(...).
• The backend id becomes the provider prefix in model refs.
• User config in agents.defaults.cliBackends.<id> still overrides the plugin default.
• Backend-specific config cleanup stays plugin-owned through the optional normalizeConfig hook.

Plugins that need tiny prompt/message compatibility shims can declare bidirectional text transforms without replacing a provider or CLI backend:

api.registerTextTransforms({
  input: [
    { from: /red basket/g, to: "blue basket" },
    { from: /paper ticket/g, to: "digital ticket" },
    { from: /left shelf/g, to: "right shelf" },
  ],
  output: [
    { from: /blue basket/g, to: "red basket" },
    { from: /digital ticket/g, to: "paper ticket" },
    { from: /right shelf/g, to: "left shelf" },
  ],
});

input rewrites the system prompt and user prompt passed to the CLI. output rewrites streamed assistant deltas and parsed final text before Velaclaw handles its own control markers and channel delivery.

For CLIs that emit Claude Code stream-json compatible JSONL, set jsonlDialect: "claude-stream-json" on that backend’s config.

Bundle MCP overlays

CLI backends do not receive Velaclaw tool calls directly, but a backend can opt into a generated MCP config overlay with bundleMcp: true.

Current bundled behavior:

• claude-cli: generated strict MCP config file
• codex-cli: inline config overrides for mcp_servers
• google-gemini-cli: generated Gemini system settings file

When bundle MCP is enabled, Velaclaw:

• spawns a loopback HTTP MCP server that exposes gateway tools to the CLI process
• authenticates the bridge with a per-session token (VELACLAW_MCP_TOKEN)
• scopes tool access to the current session, account, and channel context
• loads enabled bundle-MCP servers for the current workspace
• merges them with any existing backend MCP config/settings shape
• rewrites the launch config using the backend-owned integration mode from the owning extension

If no MCP servers are enabled, Velaclaw still injects a strict config when a backend opts into bundle MCP so background runs stay isolated.

Limitations

• No direct Velaclaw tool calls. Velaclaw does not inject tool calls into the CLI backend protocol. Backends only see gateway tools when they opt into bundleMcp: true.
• Streaming is backend-specific. Some backends stream JSONL; others buffer until exit.
• Structured outputs depend on the CLI’s JSON format.
• Codex CLI sessions resume via text output (no JSONL), which is less structured than the initial --json run. Velaclaw sessions still work normally.

Troubleshooting

• CLI not found: set command to a full path.
• Wrong model name: use modelAliases to map provider/model → CLI model.
• No session continuity: ensure sessionArg is set and sessionMode is not none (Codex CLI currently cannot resume with JSON output).
• Images ignored: set imageArg (and verify CLI supports file paths).