# RFC: Two LLM adapters as a design-verification twin

Status: implemented

## Problem

`dsh-llm` owns a provider-neutral streaming vocabulary — the `StreamChunk` protocol (`block-start`, `text-delta`, `reasoning-delta`, `tool-call-delta`, `block-end`, `usage`, `finish`) and the content-block types ([the content-block vocabulary](2026-06-11-content-block-vocabulary.md)). A vocabulary defined against a single adapter risks baking that adapter's quirks into the "neutral" contract: anything the one implementation happens to do becomes the de-facto spec, and the abstraction is unverified until a second provider arrives — by which point the leak is expensive to fix.

## Decision

Ship **two** adapters against the one contract from the start, deliberately built on different internals:

- `dsh-llm-deepseek` — hand-rolled `fetch` + SSE parsing against the DeepSeek API.
- `dsh-llm-pi-ai` — the same endpoint through the `@earendil-works/pi-ai` library (its own event vocabulary).

The rule they enforce: **anything the StreamChunk vocabulary cannot express for BOTH implementations is a core-vocabulary bug**, caught immediately rather than at the next provider. The pair pinned down conventions now documented on `StreamChunk` in `dsh-llm/src/types.ts`: usage emitted before finish, nothing after finish, tool-call `arguments` as raw JSON strings end-to-end, and the two sanctioned error paths (throw from `stream()` *or* end with `finish {kind:'error'|'aborted'}`) that a consumer must handle on both sides — a divergence the library-backed adapter surfaced that a single hand-rolled adapter would have hidden.

## Alternatives considered

- **A single adapter** — less code and half the e2e cost, but leaves the "provider-neutral" claim unverified; the vocabulary would encode DeepSeek-via-fetch assumptions silently.
- **A mock second adapter** — cheaper but doesn't exercise a real provider's wire quirks, so it proves little. The twin is real-on-real.

## Consequences

Double the adapter maintenance and double the key-gated e2e surface (both adapters cover V4 Flash and Pro across representative thinking/effort modes). Bought: a continuously-verified neutrality guarantee for the most leak-prone abstraction in the codebase, and a worked second example for adapter authors. The two share the core Config shape (`apiKey`/`baseURL`/`models`) so a deployment swaps mostly one line, but the reasoning knob differs — `dsh-llm-deepseek` takes `thinking`/`reasoningEffort`, `dsh-llm-pi-ai` takes a single `reasoning` level — so a swap translates that field. If the maintenance cost ever outweighs the verification value (e.g. once conformance tests from [architectural conformance](../../proposed/process/2026-06-11-architectural-conformance.md) cover the contract mechanically), retiring the twin to a single adapter + the conformance kit would be a new RFC superseding this one.
