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ADR-0002 — Provider-agnostic model layer ​

Status: superseded by ADR-0018 · Date: 2026-09-14

Context ​

A hard requirement is that any model can be connected later. The obvious path is calling a vendor SDK directly, which is the shortest route to a working call and the fastest route to lock-in — vendor SDKs differ in message shape, tool schema, streaming, and error types, so swapping one later touches every call site.

Decision ​

Use the Vercel AI SDK's createProviderRegistry over the Anthropic, OpenAI, and Google provider packages. The active model is a single environment variable in provider:model form. No module outside src/agent/registry.ts imports a provider package (Constitution C2).

Above that sits an AgentRunner port, so the call site depends on our interface rather than on the SDK.

Consequences ​

  • Switching provider is an env edit and a restart; the diff is zero lines.
  • Three provider packages are installed but only the active one's key is needed.
  • The registry is also the natural seam for middleware — retries, fallback, telemetry — without touching callers.
  • Accepts a dependency on the AI SDK's abstraction. Judged worthwhile: it is the most used TypeScript AI library, and the AgentRunner port means replacing even the SDK itself is contained to one module.