openchamber/packages/web/server/lib/small-model/DOCUMENTATION.md

Small Model

Server-side direct LLM calls that reuse the user's existing OpenCode provider logins (~/.local/share/opencode/auth.json). OpenCode uses a "small model" internally (titles, summaries) but does not expose it through the SDK or plugins — this module replicates that mechanism as an OpenChamber runtime API.

Security boundary

Credentials never leave the server process. The client sends only a prompt; auth resolution, OAuth refresh, and provider dispatch all happen server-side. Routes live under /api/* and are gated by the ui-auth middleware like every other runtime API.

Files

• index.js — orchestration: generateSmallModelText() / describeSmallModel().
• resolve.js — model selection, mirroring OpenCode's getSmallModel chain: 0. OpenChamber's own settings override (Settings → Sessions → Small Model): when smallModelUseDefault is false, smallModelOverride (provider/model) outranks everything below. Sanitized in settings-helpers.js (server), persistence.ts (client), and bridge-settings-runtime.ts (VS Code).
  1. small_model from the merged OpenCode config layers (provider/model).
  2. Family-priority scan (gemini-flash → gpt-nano → claude-haiku) within the session's provider first (preferredProviderID, like OpenCode resolves within the current provider), then over the other providers with a usable auth entry, newest release_date first.
  3. GitHub Copilot hidden utility models (gpt-*-nano/mini) — these never appear in the catalog, so they participate as the gpt-nano family entry and as a final utility fallback.
  4. Last resort: the session's own model (preferredModelID) when no small model resolves anywhere — costlier, but always valid.
• Input clamp: the prompt is truncated to the resolved model's catalog limit.context (minus an output reserve, ~4 chars/token estimate; conservative default when the model is not in the catalog). Truncation is reported as inputTruncated: true in the response.
• call.js — wire formats and per-provider auth, replicating OpenCode's plugin auth loaders:
  • GitHub Copilot: OpenAI-compatible /chat/completions on https://api.githubcopilot.com (or copilot-api.<enterprise>) with the stored device-OAuth token as the bearer — no token exchange, no expiry.
  • OpenAI OAuth (ChatGPT plan): streaming Responses API on https://chatgpt.com/backend-api/codex/responses with ChatGPT-Account-Id; expired tokens are refreshed against auth.openai.com (single-flight) and written back to auth.json.
  • Anthropic (type: api): /v1/messages with x-api-key.
  • Google (type: api): generateContent with x-goog-api-key; Gemini 3 uses thinkingLevel while older Flash models use thinkingBudget: 0.
  • Everything else: OpenAI-compatible /chat/completions against the provider's base URL, resolved from (1) provider.<id>.options.baseURL in the OpenCode config, (2) the hardcoded https://api.openai.com/v1 endpoint, or (3) the provider's api field from the models.dev catalog.
  • [small-model:diagnostic] logs record provider/model, input character counts, output budget, thinking toggle, HTTP/finish status, and content/reasoning lengths without logging prompts, response text, or credentials. Goal audit parsing similarly emits [session-goal:diagnostic] structural verdict metadata.
• catalog.js — models.dev catalog via the shared in-process cache (../opencode/models-metadata.js, also serving /api/openchamber/models-metadata).
• routes.js — GET /api/small-model (resolution preview) and POST /api/small-model/generate ({ prompt, system?, maxOutputTokens?, model?, directory? } → { text, providerID, modelID, source }).

Registration

Mounted lazily from feature-routes-runtime.js (same pattern as quota): the module is imported on first request, not at server startup.

Known limitations

• OpenCode's free models (opencode/big-pickle, *-free) work without a token only through OpenCode's own server — direct calls are rejected, and piggybacking on their subsidized infra is out of bounds by design. Every resolution step therefore requires a usable auth entry for the provider: a session on an unauthenticated opencode provider falls through to the global scan (or a clean 404 on a vanilla setup with no logins).

• Anthropic OAuth (Claude Pro/Max) entries are not supported — OpenCode itself keeps those outside auth.json in this generation; only type: api keys work for Anthropic.

• Amazon Bedrock, GitLab, Azure and other credential-chain providers are out of scope; they need more than a key/token (regions, resource names).

• Responses from the codex backend are collected from the SSE stream; the endpoint itself is non-streaming by design (small utility calls).