# ELI Layer 2 — nexil session_id forwarding shipped — 2026-05-07

## Context

ELI integration target G4: make Eli's per-session identity reach
agent-infer's `/v1/{completions,chat/completions,responses}` so the
session-affinity admission path (already live, see
`2026-05-02-bench-agent-load-a1-session-affinity-admission.md`) can
route same-session multi-turns to the same slot for KV-prefix reuse.

Pre-change `bench_eli_agent.sh smoke-real` reported
`session_affinity_hit = 0` because nexil never emitted `session_id` —
the field was parsed on the agent-infer side
([`infer/src/http_server/openai_v1.rs:574, :838, :1142`](../../infer/src/http_server/openai_v1.rs))
but no upstream client populated it.

## What worked

Implemented `docs/plans/nexil-session-id-forwarding.md` (Path B —
typed field through the public API) in the sibling eli repo as a single
coherent commit `4b3e7fe` on `main`. nexil bumped 0.8.0 → 0.9.0
(additive public API, semver minor).

### eli-side changes (eli@4b3e7fe)

- `nexil::ChatRequest` gains `session_id: Option<&'a str>`.
- `nexil::TransportCallRequest` gains `session_id: Option<String>`.
- `LLMCore::run_chat` / `run_chat_stream` gain `session_id: Option<&str>`
  immediately after `kwargs`; threaded through `prepare_attempt` and
  `build_transport_request`.
- `RoundParams` (tool loop) gains `session_id`; threaded into all
  three `run_chat` call sites.
- OpenAI adapter — top-level `session_id` on completion and responses
  bodies via `entry().or_insert_with` (caller `kwargs["session_id"]`
  override wins).
- Anthropic adapter — maps to `metadata.user_id`, guarded by
  `!body.contains_key("metadata")` so caller-provided metadata wins;
  skipped under OAuth (claude.ai backend strips unknown metadata
  keys, mirrors the existing temperature-skip pattern).
- Eli — `Agent::run` threads `session_id` through `agent_loop` →
  `run_tools_once` → `ChatRequest`. Subagent fallback inherits via
  `Agent::run`.

### Tests added (9)

- `crates/nexil/src/providers/openai.rs` — 4 unit tests
  (completion includes/omits, responses includes, kwargs precedence)
- `crates/nexil/src/providers/anthropic.rs` — 4 unit tests
  (metadata.user_id mapping, omits when None, kwargs precedence,
  OAuth skip)
- `crates/nexil/src/llm/tests.rs` — 1 test asserts
  `ChatRequest::default().session_id == None` (guards future
  field-renames from breaking the public API)

446 nexil + 450 eli tests green; clippy + fmt clean.

### Wire shape

```http
POST /v1/chat/completions
{ "model": "...", "messages": [...], "session_id": "<eli-session>" }
```

Aligns exactly with agent-infer's `#[serde(default, alias = "user")]
session_id: Option<String>` on `CompletionRequest`,
`ChatCompletionRequest`, `ResponsesRequest`. `normalize_session_id`
([`openai_v1.rs:14`](../../infer/src/http_server/openai_v1.rs))
resolves it to the typed `SessionId` consumed by the scheduler.

## Functional gate (deferred)

`scripts/bench_eli_agent.sh smoke-real` against a live Metal infer
+ updated nexil should now report `session_affinity_hit > 0`.
That bench is the integration verifier and lives on the agent-infer
side; it is the next step for whoever drives the ELI integration
acceptance.

## Rule

When the wire-format integration spans a sibling repo:

1. Read the cross-repo plan in full before writing code (the eli
   `docs/plans/nexil-session-id-forwarding.md` was already written
   2026-05-03 with file:line citations on both sides — would have
   saved ~30 min if read upfront).
2. Land all touch points in a **single coherent commit** when the
   change is API-shaped (struct field + all call sites) — splitting
   leaves a "field exists, never populated" intermediate that
   silently fails the integration test.
3. Pin the wire shape with **adapter-level unit tests**, not
   integration tests; the integration test is the next-step
   verifier, not the API contract.

## References

- eli plan: `~/code/eli/docs/plans/nexil-session-id-forwarding.md`
  (status: implemented 2026-05-07)
- eli commit: `~/code/eli@4b3e7fe`
  (`feat(nexil): forward session_id to provider adapters (0.9.0)`)
- agent-infer parser:
  [`infer/src/http_server/openai_v1.rs:14, :574, :838, :1142`](../../infer/src/http_server/openai_v1.rs)
- agent-infer baseline (pending-remote functional gate):
  [`2026-05-02-bench-agent-load-w3-harness-pending.md`](2026-05-02-bench-agent-load-w3-harness-pending.md)
