Chat Backends¶
MCP Chat runs supported CLI tools through a local Python relay. Install and sign in to a CLI outside Unity first; Chat reuses that CLI's cached authentication. There is no API-key field in Unity.
Supported Backends¶
| Backend | CLI | Process model | Resume argument | Chat-start MCP configuration |
|---|---|---|---|---|
| Claude | claude | One process; turns are written to stdin | --resume | Temporary Claude MCP JSON |
| Codex | codex | New codex exec process for each turn | exec resume | Inline Codex configuration |
| Kimi | kimi | New process for each turn | Not defined | Temporary Kimi MCP JSON |
| Antigravity | agy | New process for each turn | Not defined | Temporary Antigravity settings |
| OpenCode | opencode | New opencode run process for each turn | -s | Temporary config through OPENCODE_CONFIG |
The model selector and launch options come from MCP > Settings > Chat Settings. Model availability and authentication are owned by the selected CLI, so verify custom model IDs in that CLI before using them in Chat.
Process Lifecycle¶
Claude¶
The relay starts Claude with stream-json input and output. The process remains available between turns, and Chat writes each new turn to stdin. A returned session ID can be passed back with --resume.
Codex, Kimi, Antigravity, and OpenCode¶
These CLIs receive the prompt as a command argument instead of reading turns from stdin. The relay therefore defers startup until a prompt is available, starts one process for that turn, and closes stdin.
Switching the backend or model stops the current backend. Stopping a turn also kills its process; Chat creates a fresh backend for the next send.
Ask, Agent, and Approvals¶
Ask is the default Chat mode. When a backend emits a permission request, Chat shows Allow, Deny, Session, and Always choices. Agent automatically approves permission requests emitted by the backend.
Mode behavior remains backend-specific. Some CLIs use their own non-interactive or permission-bypass flags and do not emit the same prompts as Claude. Ask and Agent are not a replacement for server-side tool visibility or code-execution security; see Settings.
Sessions¶
The backend definitions accept resume IDs for Claude, Codex, and OpenCode. The Chat session picker scans local stores for Claude, Codex, Kimi, and Antigravity; it does not scan OpenCode's SQLite store.
These are separate capabilities: a picker entry means Chat can find an ID, while a resume argument means the backend can consume one. A resume ID selected for a per-turn backend is not currently applied to the next turn. Reliable in-Chat resume is therefore limited to Claude.
When a backend returns a session ID, To CLI copies that backend's resume command. See Using MCP Chat for the user workflow.
Authentication and Configuration¶
- Sign in with the backend CLI outside Unity.
- Ensure the CLI executable is available on the login-shell
PATH, then restart Unity after installing it. - Claude exposes a cached-auth status check through
claude auth status. - Chat writes temporary or inline MCP configuration at backend start; this does not configure the same CLI as a standalone external MCP client.
- For external-client setup, follow the matching client guide.
Antigravity¶
The Antigravity backend requires the agy executable and its CLI authentication/configuration to be available before Chat starts. Verify that agy --help runs in a login shell, restart Unity, then select Antigravity in MCP Chat. The relay writes temporary MCP settings for each turn.
For common recovery steps, see Using MCP Chat. If a custom model is rejected, test its ID with the backend CLI before saving it in Chat Settings.