Bug: vLLM self-hosted deployments stream thinking as delta.reasoning — adapter drops it (fix available)

[DiGuStudent]

Summary

Self-hosted DeepSeek deployments on vLLM stream thinking-mode CoT as delta.reasoning instead of the official delta.reasoning_content. dsh-llm-deepseek reads only the official field, so against such gateways every reasoning delta is silently dropped.

Observed consequences (measured against a vLLM 0.25.2 deployment serving deepseek-v4-flash)

1. Thinking is invisible — no reasoning block reaches the session log or the UI.
2. TTFT is inflated by the entire reasoning phase — "first token" lands on the first content delta. One turn in a real session recorded a 199.9 s "TTFT"; a direct wire measurement of the same endpoint shows the first SSE chunk (a reasoning delta) arriving at 0.51 s.
3. Displayed tok/s is inflated — outputTokens comes from completion_tokens (which includes reasoning tokens), while the decode window only spans visible text.

Wire capture

data: {"choices":[{"delta":{"reasoning":"We"}, ...}]}        <- dropped today
data: {"choices":[{"delta":{"reasoning":" need answer"}, ...}]}

vs. the official API, which sends delta.reasoning_content.

Root cause

packages/llm/llm-deepseek/src/translate.ts: const reasoning = delta?.reasoning_content — the alias is never consulted.

Fix available

Two-line fallback plus the WireDelta field declaration:

const reasoning = delta?.reasoning_content ?? delta?.reasoning

Branch with unit tests (alias accepted, official field preferred when both arrive), README note, and an Agent Note per repo policy: https://github.com/DiGuStudent/deepseek-harness/tree/fix/vllm-reasoning-field

Verified end-to-end against the vLLM endpoint after the patch: reasoning blocks are recorded, TTFT reflects the real first token, and the decode window spans the full generation.

I know external PRs are not accepted at the moment — filing this as a bug report per CONTRIBUTING.md; happy to open a PR with this branch whenever that changes, or adjust the approach if you'd rather handle the alias elsewhere.