Add opencode as a connectable MCP agent and a chat harness

[askmaddyy]

Adds opencode to the two places Diffusion Studio connects to an agent.

MCP settings -> Agents

A new opencode row writes the app's diffusion entry into ~/.config/opencode/opencode.json, under opencode's own mcp root:

{ "mcp": { "diffusion": { "type": "remote", "url": "http://127.0.0.1:3274/mcp" } } }

This needs a new ConfigFormat ("mcp"); the JSON merge / remove / read paths are shared with the existing JSON agents, so only the root key and the entry shape differ. enabled is omitted because opencode defaults it to true.

Chat agent picker

OpenCodeHarness spawns opencode serve on a loopback port it picks, creates (or resumes) a session, streams the turn from the server's SSE /event feed, and maps text, reasoning, tool calls, and permission.asked / question.asked onto the existing Item and QuestionBox types. Interrupt, close and resume follow the codex adapter.

One child server per live session, so its event stream carries only that project's work. The MCP entry and the chat instructions are injected as OPENCODE_CONFIG_CONTENT and a per-prompt system, so nothing is written to the user's own config.

Picker

• Models are grouped by model.group into a submenu per provider, labelled with the provider's display name.
• probe reads GET /provider from a short-lived opencode serve (~400ms), so each model carries its real display name (Claude Sonnet 4.6, not claude-sonnet-4-6) and its provider's name as the group (OpenCode Zen). Only connected providers are listed.
• The submenu caps at max-h-96, so a provider's seventy-odd models scroll instead of filling the screen.
• The harness icon is the official opencode mark.

HarnessInfo.models gained an optional group; models without one list flat, so Claude Code and Codex are unchanged.

Testing

• npm run check - agent-chat, desktop, web all pass
• npm test - agent-chat 42 pass; desktop 58 pass (the 3 projects.watch.test.ts failures are a pre-existing flake, reproduced on a clean checkout)
• npm run lint - clean
• End to end: the generated opencode.json is read by opencode itself (opencode mcp list reports diffusion connected), and a live turn against opencode 1.18.31 streams item.started -> item.delta -> item.completed and resolves completed, with the diffusion MCP injected inline

Pre-existing and unrelated: packages/encoder (and its dependents) fail typecheck on showSaveFilePicker in packages/assets/src/browser.ts.

[vercel]

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