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Agent Canvas can drive your conversations with the built-in OpenHands agent or with an external ACP agent — Claude Code, Codex, or Gemini CLI. This guide explains what ACP agents are, how to onboard one, and how to switch agents or models later.

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What is an ACP agent?

The Agent Client Protocol (ACP) is a standard for talking to coding agents over JSON-RPC on stdio. Instead of Agent Canvas calling an LLM directly, the Agent Server spawns the agent’s own CLI as a subprocess and relays each turn to it. The external agent manages its own LLM, tools, and execution; Agent Canvas sends messages and renders what comes back. The Agent Server owns the subprocess and the credentials; Agent Canvas only records which agent to run and surfaces a form for the secrets it needs. The agent choice is stored per backend, so switching backends can switch agents.

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Supported providers

ProviderDefault command
Claude Codenpx -y @agentclientprotocol/claude-agent-acp
Codexnpx -y @zed-industries/codex-acp
Gemini CLInpx -y @google/gemini-cli --acp
The provider list is sourced from the OpenHands SDK registry (openhands.sdk.settings.acp_providers, mirrored into @openhands/typescript-client) and enriched with Canvas UI metadata. Adding or changing a provider happens upstream in the SDK.

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Authentication

ACP agents authenticate two ways: a subscription login, or an API key — and the onboarding fields are optional. If you’re already signed in to the provider’s CLI on the machine the agent runs on, it reuses that login automatically, so locally you often don’t need a key at all. The login takes priority over an API key: while you’re signed in, a key set in the environment isn’t used — so the onboarding key fields do nothing and can be left blank.
A “subscription login” is the credential the provider’s own CLI stores when you sign in once — a file in your home directory, or, for Claude Code on macOS, the system Keychain. When the Agent Server runs on that same machine (a local or self-hosted backend), the provider CLI finds that login automatically — no API key required. On a clean cloud sandbox there’s no stored login, so an API key is needed instead.
ProviderSubscription login (auto-detected)API key
Claude CodeA Claude Code login (Pro/Max), from Claude Code’s own credential store: the macOS Keychain, or ~/.claude/.credentials.json on LinuxANTHROPIC_API_KEY
CodexA ChatGPT login (codex login) cached at ~/.codex/auth.jsonOPENAI_API_KEY
Gemini CLIYour Google login (gemini/gemini --acp) cached at ~/.gemini/oauth_creds.jsonGEMINI_API_KEY
All three collect an optional API key (plus base URL) in onboarding. As noted above, a subscription / OAuth login takes priority over an API key — when the provider’s CLI is signed in, a key set in the environment is not used:
  • Codex — codex login status keeps reporting the ChatGPT login even with OPENAI_API_KEY set.
  • Gemini CLI — uses the OAuth auth type chosen at gemini login; GEMINI_API_KEY is only consulted if you switch the auth type. The free Google login is the common no-key path locally — sign in once and it just works.
  • Claude Code — with both present, claude auth status reports it is authenticated via the subscription (claude.ai), not the key. The login is auto-detected from the macOS Keychain (or ~/.claude/.credentials.json on Linux); CLAUDE_CONFIG_DIR is not required for it — it only relocates Claude Code’s config directory (settings/history, not the token) and signals the SDK to strip a conflicting ANTHROPIC_API_KEY / ANTHROPIC_BASE_URL.
The one exception is the base URL (*_BASE_URL): a custom value points the CLI at a different endpoint (a proxy or gateway) and does take effect even under a login — for Gemini it rides the ACP gateway param. It’s an advanced override, not needed for normal use.

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Onboarding an ACP agent

First-time users get a four-step onboarding modal. To onboard an ACP agent:
  1. Choose agent — pick Claude Code, Codex, or Gemini CLI instead of OpenHands. The choice is saved immediately to your backend’s settings.
  2. Check backend — confirms Agent Canvas can reach the Agent Server.
  3. Set up credentials — enter the provider’s API key (and, optionally, a custom base URL for a proxy or gateway). All three providers collect these here, and every field is optional.
  4. Say hello — creates your first conversation and closes the modal.
Every credential field is optional and the step is skippable. Leave a field blank to reuse a key already set on the backend, or to authenticate the agent through a subscription / OAuth login instead.

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How credentials reach the agent

Each credential you enter is saved as a global secret whose name is exactly the environment variable the Agent Server exports into the ACP subprocess (e.g. ANTHROPIC_API_KEY). Saving in onboarding is identical to adding the secret under Settings → Secrets, where you can edit or remove it anytime. Keeping the secret name equal to the env var is what makes a saved key actually reach the provider CLI.

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Switching agent or model later

Open Settings → Agent at any time:
  • Agent — switch between OpenHands and ACP.
  • Preset — pick a built-in provider (Claude Code, Codex, Gemini CLI) or Custom to point at any other ACP server.
  • Command — the command line used to spawn the subprocess. Selecting a preset fills this in; editing it to match another preset re-detects that provider. API keys are not entered here — they live in the Secrets panel.
  • Model — choose a suggested model for the provider or enter a custom model override. Built-in providers save a concrete model rather than leaving it blank.
Saving writes an agent_settings_diff (agent_kind, acp_server, acp_command, acp_model) to PATCH /api/settings. A running conversation keeps the agent it started with; the new choice applies to conversations you start afterward.

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Custom ACP servers

Any stdio ACP server works: choose Custom in Settings → Agent and enter its launch command. Custom servers have no curated model list, so enter the model ID the server expects (if any) as a custom model. Pass credentials by adding the env vars the server reads as global secrets under Settings → Secrets.