# US-020 — Manual route selection + hop-penalty benchmarking

## Context

The tracker auto-selects inference routes based on synthetic benchmark scores. To measure
the real cost of adding hops (latency per node boundary), we need:

1. A way to pin a request to a specific route so we control the variable.
2. A benchmark endpoint that runs the same prompt through 1-node, 2-node, and 3-node
   routes and records per-hop latency.

Results are stored to disk. Routing algorithm is **not** changed in this story — this is
data collection only. The data will inform a future routing optimisation story.

## Design decisions (grilled 2026-07-01)

| Decision | Choice |
|---|---|
| Route spec | Optional `route` field in JSON request body (list of node IDs) |
| Trigger | Explicit only — `POST /v1/benchmark/hop-penalty` endpoint |
| Auth | Header-presence stub (`Authorization` must be non-empty); real auth in future story |
| Routing integration | Store data only; routing algorithm unchanged |
| Persistence | Append to `benchmark_results.json` in tracker working dir; in-memory queryable |

## Acceptance criteria

- `POST /v1/chat/completions` accepts optional `"route": ["<node_id>", ...]` in the
  request body. If present, the tracker uses those nodes in order instead of auto-selecting.
  If absent, existing routing is unchanged (no breaking change for unaware clients).
- Missing or invalid node IDs in `route` return HTTP 400 with a descriptive error.
- `POST /v1/benchmark/hop-penalty` is auth-gated: requests without a non-empty
  `Authorization` header return HTTP 401. Body: `{"model": "...", "prompt": "...",
  "max_new_tokens": 64}`.
- Benchmark fans out to up to three routes: 1-node (single node covering all layers),
  2-node (two consecutive shard nodes), 3-node (three nodes) — using whatever is
  currently registered. Routes with insufficient coverage are skipped, not errored.
- Response includes per-route breakdown: `total_ms`, `per_hop_ms: [...]`,
  `tokens_generated`, `route: [node_id, ...]`.
- Results are appended to `<tracker_working_dir>/benchmark_results.json` (created if
  absent) as a JSON array. Each entry includes timestamp, model, prompt hash, and the
  per-route breakdown.
- `GET /v1/benchmark/results` returns the stored results array. Also auth-gated.
- Clients that never send `route` or call `/v1/benchmark/*` are completely unaffected.
- Integration test: send the same prompt via a pinned 1-node route and a pinned 2-node
  route; assert 2-node result has 2 entries in `per_hop_ms`; assert both records appear
  in `benchmark_results.json`.
- `python -m pytest` passes from repo root.
- Commit only this story's changes.