# ADR 0017: AgentSession Is Provider-Bound

## Status

Accepted.

## Context

ADR-0002 says a long-lived `AgentSession` can be reused by many runs, but never
addressed the provider/model dimension. The agent layer is provider-agnostic
(ADR-0001..0003 never required Claude specifically; the `@Claude` trigger name
is a product brand, not a provider commitment). In practice the Hub routes
model calls through an OpenAI-compatible client (e.g. OpenRouter), so a run may
target different models — and the motivation for multi-model is **role-based
routing**: drafting, reviewing, and triage suit different models.

That raises the question ADR-0002 left open: when a teacher switches model
mid-project, does the `AgentSession` carry live context across the switch, or
is it bound to a single provider/model?

## Decision

An `AgentSession` is **provider/model-bound**. A model or provider switch
starts a new `AgentSession`. The `AgentSession` does not carry live context
across a switch.

Cross-session continuity is not the session's job — it is carried by ADR-0003's
project memory and anchors, which the Hub reads on demand when seeding a new
run. This keeps B consistent with ADR-0003 ("the Hub avoids becoming a full
chat history store"): session continuity and history continuity are the same
mechanism, both via memory/anchors, never via a live cross-provider session.

Per-run model selection (the run carries its model) is the switching point.

## Consequences

- Switching model mid-project = new session; the new run seeds from project
  memory/anchors (ADR-0003), not the prior session's live context.
- The agent layer is provider-agnostic: model calls go through an
  OpenAI-compatible client, and the tool-calling loop is delegated to the
  Vercel AI SDK (`generateText` + `tool`). The provider seam is the SDK's
  `LanguageModel` interface, not a hand-rolled loop; swapping provider is
  swapping the `createOpenAICompatible` factory, not rewriting the loop. No
  hard dependency on a single-vendor agent SDK.
- Role-based model routing (different run kinds default to different models) is
  a product/admin configuration concern, not a spec invariant.
- "AgentSession reused by many runs" (ADR-0002) holds *within* one
  provider/model; it does not span a switch.
- The `@Claude` trigger name remains the product mention brand regardless of the
  backing model.