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BERD TASK · 0003 · SET UP BERD

Choose a harness for a session

Every chat in Berd runs on a harness — the agent runtime that actually reads files, runs commands and calls the model. You pick it when the session starts, and three Berd features quietly depend on which one you picked.

Steps at a glance

  1. Know that the harness is chosen per session
  2. Default to Goose unless you need a harness's own tooling
  3. Know where Berd stops and the harness begins
  4. Install a missing harness from onboarding or Settings

1. Know that the harness is chosen per session

Berd is the app around the agent, not the agent. The thing doing the work in a chat is one of seven runtimes at v0.6.4: Goose (bundled with Berd, Block's own), Claude Code, Codex, Copilot, Amp, Cursor Agent and Pi. Internally these are goose, claude-acp, codex-acp, copilot-acp, amp-acp, cursor-agent and pi-acp; you will see the friendly names.

The choice is made per session, not per install. Two chats side by side can run on two different harnesses. The composer toolbar carries a provider choice next to the agent and model choices, and that is the place to look when starting a new chat. Exactly how the harness is presented in the composer today, and whether a running session can be moved from one harness to another in place, is not verified from source. Assume a change of harness means a new chat until someone shows you otherwise.

[SCREENSHOT: T03-01 — harness choice in a new chat]

2. Default to Goose unless you need a harness's own tooling

Goose is the default for a reason beyond being Block's. Three things in Berd only work on it:

  • Voice Conversation. The composer says so in as many words: Voice Conversation requires Goose. See Hold a voice conversation.
  • Steering a queued message into the running turn. The settings label reads Steer (Goose harness only), and even on Goose it needs a steering-capable backend — the error text talks about restarting with "the steering backend branch", which suggests not every Goose build has it. See Run a working session.
  • Auto-compaction of older context, likewise labelled Goose harness only.

Goose is also the harness that reads the hints and memory Berdy saves for you. Choose another harness when you want its tooling — your existing Claude Code or Codex setup, a subscription you already pay for, or a model only that harness exposes (GPT-6 Astra is selectable when starting a Codex session, since v0.6.4). Amp is the odd one out: it has no model list in Berd, and you set its model with the Amp CLI.

[SCREENSHOT: T03-02 — the voice-requires-Goose pill]

3. Know where Berd stops and the harness begins

Berd's own help draws a line it will not cross: a question about the app around the harness — where a setting lives, how a session started, how to export a chat — is Berd's to answer. A question about what a specific harness did inside the session, its output or its errors, is not. It will tell you to take that to the harness.

Adopt the same line yourself. When a Claude Code session behaves oddly, the fix is in Claude Code's own documentation and configuration, and reporting it to Berd will get it bounced. When the composer, the right rail or the session list behaves oddly, that is Berd. Knowing which side of the line you are on saves an afternoon.

4. Install a missing harness from onboarding or Settings

Harnesses that are not present can be installed from Berd: at onboarding (Pi got one-click install there in v0.6.4) or from the harnesses part of the providers section in Settings, which offers Install, Sign in and Fix per harness, and Troubleshoot in chat when setup snags. Installation runs in numbered steps and verifies at the end. If the verification says the CLI is not on your PATH, restart your terminal and check again.

A harness that needs a sign-in (Claude Code, Codex, Copilot, Amp, Cursor Agent) completes that sign-in in its own flow; Pi installs without one. Once installed, the harness appears as a choice for new sessions.

Last checked: Berd v0.6.4, 2026-09-11 — read from source and release notes, not yet tested in the app.

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Reference

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Berd v0.6.4 · 2026-09-11 · source-read
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Guide built with Berd, Claude and Codex · verified against Berd v0.6.4, 2026-09-11 · read from source, not yet tested in the app · Buzz Guide · About · not affiliated with Block