Status: verified against the official docs (2026-09-16) — opencode.ai /docs, /docs/acp, /docs/config, /docs/providers. The rig E2E section at the end records what the live install confirmed. OpenCode is SST's open-source terminal coding agent (GitHub sst/opencode, npm opencode-ai, binary opencode). This note is the ground truth the W12 implementation is built on — same role as the T1 dsh note for deepseek.

• npm package opencode-ai (NOT opencode); npm i -g opencode-ai ships a platform binary via postinstall (needs network on the machine).
• Other channels exist (official install script, brew, direct download) — we use npm only for install/upgrade/pin, same as codex. No native self-update exception in our job table (opencode upgrade exists upstream but the npm channel covers all three actions uniformly).
• Version pinning = npm install -g opencode-ai@<exact>; default channel tag latest.
• opencode --version prints a semver — feeds the W1 runtime probe (first line, ≤64 chars, 5s timeout — our standard shape).

OpenCode speaks the Agent Client Protocol natively: the command is opencode acp (official docs /docs/acp; also listed in the ACP registry used by Zed). No third-party wrapper (contrast: claude-code rides @agentclientprotocol/claude-agent-acp, codex rides zed-industries/codex-acp).

• Dialect: standard ACP (Zed-ecosystem shaped) — the daemon's existing mapAcpUpdate standard arm applies. Rig-verified end to end (§5).
• session/list-shaped native session surfaces are NOT exposed over ACP here → the chat rail stays supported:false (honest default; the sessions handler's generic fall-through already answers that).

• Global config: ~/.config/opencode/opencode.json (JSON; JSONC with comments accepted by opencode itself). Project config opencode.json walks up to the git root; we only touch the global file. Env overrides exist (OPENCODE_CONFIG, OPENCODE_CONFIG_DIR) — we do not set them; the machine user's default layout wins.

• Config files MERGE (remote → global → env → project → managed); our writes are therefore additive blocks inside the global file.

• Custom provider block (provider.<id>):

  {
    "$schema": "https://opencode.ai/config.json",
    "provider": {
      "<id>": {
        "npm": "@ai-sdk/openai-compatible",
        "name": "Display Name",
        "options": { "baseURL": "https://api.example.com/v1", "apiKey": "{env:VAR}" },
        "models": {
          "<model-id>": { "name": "Model Display Name", "limit": { "context": 200000 } }
        }
      }
    }
  }

  • npm — the AI SDK package driving the wire: @ai-sdk/openai-compatible for /v1/chat/completions, @ai-sdk/openai for /v1/responses, @ai-sdk/anthropic for the anthropic wire. This is a FINER axis than our W3 api enum → our PROVIDER_API_KIND maps onto it exactly (openai-chat → compatible, openai-responses → openai, anthropic → anthropic).
  • options.baseURL — custom endpoint; omitted → the built-in provider default (models.dev).
  • options.apiKey — variable substitution: {env:VAR} (resolved from opencode's process env — misses the user's own TUI runs) or {file:~/path} (resolved by every opencode invocation — TUI, ACP, headless). We use {file:} + a 0600 key file so the pushed key works for ALL launch modes, not just our daemon's spawns.
  • models is an object map (not an array); unknown models default to {} entries. limit.context optional (unknown → opencode treats as unlimited; we do not invent numbers).

• Default model: top-level model = "provider/model-id" (also small_model for cheap tasks — we leave that alone).

• /connect credentials live in ~/.local/share/opencode/auth.json — never read or written by us (it is the user's own auth store).

• MCP: top-level mcp block in opencode.json — local stdio entries (command/args/env) and remote (url) servers. This is the C3 scanner's MCP source.

Global (OPENCODE_CONFIG_DIR default) ~/.config/opencode/:

• command/*.md — slash commands (frontmatter description).
• agent/*.md — custom agents (maps to our sub_agent kind).
• Skills: opencode added Agent-Skills-style support (skill/<name>/SKILL.md in the config dir); the scanner reads it if present and skips cleanly on versions without it (dir absent).

See docs/dev/test-rig.md for rig mechanics. What the live E2E confirmed — including two corrections to the docs-derived assumptions:

• npm i -g opencode-ai lands opencode on PATH; opencode --version prints a clean semver line (probe shape OK); realpath classifies npm.
• opencode acp initializes over stdio with the standard dialect; a chat turn streams through our unmodified mapAcpUpdate (portal + headless).
• baseURL must end with /v1 (rig-found, the session's one real bug). @ai-sdk/anthropic / @ai-sdk/openai-compatible expect the base to end with /v1 (their defaults do) and append only the method path. Ark's /api/coding — which claude-code takes VERBATIM, it appends /v1/… itself — therefore posted to /api/coding/messages and the gateway auth-checks BEFORE routing, so the failure reads "Unauthorized", not 404. The writer now normalizes (opencodeSdkBaseURL). Verified by curling both paths with the same key: right path 200, /v1-less 401 with the EXACT error opencode surfaced.
• The key file must be the RAW secret, no trailing newline. opencode reads the {file:…} target verbatim; a rides the key and the gateway rejects it (46-byte key + = 47 bytes = 401).
• options.apiKey IS wired for custom providers (provider.ts:1774 — config apiKey with provider.key fallback); auth.json is NOT needed for our flow. (~/.local/share/opencode/auth.json remains the user's own /connect store; {type:"api", key} shape, 0600 — read from source packages/opencode/src/auth/index.ts, which also documents the OPENCODE_AUTH_CONTENT env override.)
• The provider block + {file:} key reference drive live turns on the pushed route (headless opencode run + a portal ACP turn both answered), and the viewer shows opencode.json masked where secrets appear with the .key file wholesale-redacted.

• Profile install adapter (deploy axis) — opencode is runtime-managed + chatable; local-write profile installs would be a separate wave (it has no hooks surface to install into anyway — plugins are TS code).
• Native sessions rail — no session/list surface over ACP here; supported:false.
• ~/.local/share/opencode/auth.json — the user's own credential store; our key file is separate by design.
• JSONC tolerance — if the user's global opencode.json contains comments and fails JSON.parse, the apply-config job FAILS with an explicit error instead of guessing; hand-managed files are documented as out of bounds for the writer.