ADR 0116: Add OpenCode Go as a Fixed Provider Preset

• Status: Accepted
• Date: 2026-08-24
• Deciders: PI-Desktop core
• Updates ADR 0012 and ADR 0020

Context

The OpenCode Go service exposes an OpenAI-compatible Chat Completions API, but its endpoint is service-owned and should not be edited like a generic gateway. Users should be able to configure it without copying an endpoint from separate documentation, while the existing provider studio must continue to support arbitrary OpenAI-compatible services.

Decision

Add opencode_go as a persisted API-style value and expose it in the custom provider dialog. Selecting it applies these immutable connection defaults:

name:    OpenCode Go
baseUrl: https://opencode.ai/zen/go/v1

The renderer keeps the name and endpoint visible but read-only, accepts the API key as the only editable connection field, and continues to use the normal discovered-model selector. Save-time normalization also enforces the fixed values so stale or manually constructed form state cannot override them.

The runtime maps opencode_go to pi-ai's OpenAI Chat Completions adapter unless a model-level catalog entry pins another wire API (see #105). Model discovery calls /models with a Bearer key. Secrets remain owned by the existing Rust host secret store; no OpenCode-specific secret or database table is introduced.

OpenCode Go requires a stable x-opencode-session header on LLM requests so the gateway can pin a conversation to one backend. pi-ai does not emit that header. Agent-runtime injects it (plus x-opencode-client: pi-desktop and a PI-Desktop User-Agent) on session, subagent, prompt-enhancement, and plugin one-shot streams, using the durable conversation id. Detection matches apiStyle: opencode_go, vendor/provider ids opencode / opencode-go, or an opencode.ai base URL so a UUID provider row and a custom Completions row pointed at Go both work. This stays in the agent layer, matching the official Pi coding-agent attribution helper.

Consequences

• OpenCode Go is recognizable in the provider row and durable configuration.
• The service cannot be accidentally pointed at a different endpoint while the preset is selected.
• Generic OpenAI-compatible configuration remains available as a separate API style for user-controlled gateways.
• The preset does not create a second wire adapter or constrain the service's model list to a hardcoded catalog.
• OpenCode Go chat, subagent, and one-shot requests carry a stable session routing header without waiting on a pi-ai change.