deepseek-harness/docs/rfc/implemented/architecture/2026-06-13-twin-llm-adapters.md

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). 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

The twin doubles adapter and key-gated e2e maintenance—both cover V4 Flash and Pro across representative reasoning modes—in exchange for continuous seam-neutrality validation and a second implementation example. Both use apiKey, baseURL, and models; the hand-rolled adapter exposes thinking/reasoningEffort, while pi-ai exposes one reasoning level. A future conformance suite could justify retiring one adapter through a superseding RFC.