# m4p4 — Session-Aware Ranking and M4 UAT (✅ COMPLETE 2026-02-21)

Phase spec and acceptance criteria: [ROADMAP · Milestone 4 · Phase 4](../ROADMAP.md).
Milestone index: [README.md](README.md). Backfilled record.

## What shipped

1. **`FOR SESSION` on both query surfaces.**
   `RetrieveBuilder::for_session(session_id)` and the SEARCH equivalent
   (`tidal/src/query/search/executor.rs:265` — `with_session(context, snapshot)`).
   The db layer loads the session snapshot and derives a `SessionContext`
   (`tidal/src/db/query_ops.rs:172,440`), so an archived session works exactly
   like a live one — frozen values instead of decayed-to-now values.
2. **`SessionContext` as the ranking input** (`tidal/src/session/snapshot.rs:54`).
   Built by `SessionContext::from_snapshot`: annotation text is split on
   whitespace, lowercased, and de-duplicated through a `HashSet` into
   `keywords`; `reward_velocity` is the current (or frozen) score of the
   `reward` signal; session `metadata` rides along.
3. **The boost formula** (`ProfileExecutor::session_boost`,
   `tidal/src/ranking/executor/mod.rs:605-627`):

   ```
   hint_score = matched_keywords / total_keywords     // [0,1]
   vel_norm   = reward_velocity / (reward_velocity+1) // Michaelis-Menten saturation
   boost      = hint_score * 0.3 + vel_norm * 0.2
   ```

   A keyword matches if any of the candidate's metadata **values** contains it,
   case-insensitively. The boost is **additive** and applied after base scoring,
   before min-max normalization, so it layers onto personalization instead of
   replacing it. Keywords are lowercased once per query
   (`lowered_session_keywords`), not once per candidate.
4. **Session state travels back with the results.** `Results.session_snapshot:
   Option<SessionSnapshot>` is populated whenever `for_session` is present, so
   an agent gets ranked items and its own session state in one round trip.
5. **A ranking reason code.** `ReasonCode::SessionContext`
   (`tidal/src/ranking/reason.rs:71,232`) makes a session-influenced result
   explainable rather than mysterious.
6. **The M4 UAT** (`tidal/tests/m4_uat.rs`) — 12 `#[test]` functions, one per
   scenario step, covering lifecycle, signals, policy accept/reject, annotations,
   snapshot, archive, `FOR SESSION` ranking, isolation, and `AgentId` validation.

## Evidence

| Criterion | Proof |
|-----------|-------|
| `FOR SESSION` query returns results and attaches the snapshot | `m4_uat.rs::step8_for_session_ranking_boost` |
| Keyword hints move ranking | `session_durability.rs::hint_keywords_boost_matching_items`; unit test `ranking/executor/mod.rs::session_boost_keyword_match_is_case_insensitive` |
| Empty keyword set is a no-op (no divide-by-zero, no phantom boost) | unit test `ranking/executor/mod.rs::session_boost_empty_keywords_is_noop` |
| Sessions do not leak across each other in ranking inputs | `m4_uat.rs::step9_session_isolation` |
| Archived session usable as query context | `m4_uat.rs::step7_closed_session_snapshot` |
| RETRIEVE and SEARCH behave identically on a swept session | `review_pass2_query_for_session.rs::retrieve_and_search_degrade_identically_on_missing_session` |

## Divergence from the plan

- **Missing session degrades, it does not error — deliberately reversed.** The
  ROADMAP criterion reads: "When `for_session` references a non-existent
  session, `LumenError::Query("session not found")` returned." That behaviour
  shipped and was then **removed on purpose** by the M0–M10 review pass 2
  (`9728194`): `FOR SESSION <swept-id>` is a well-formed query, and
  `CODING_GUIDELINES.md` §6 ("graceful degradation, never failure") says it must
  execute without the boost. Before the fix RETRIEVE returned
  `Err(SessionNotFound)` while the structurally identical SEARCH degraded — a
  surface-specific outage the moment the session sweeper ran. Both surfaces now
  degrade. The ROADMAP criterion is stale; the current behaviour is correct.
- **The `< 5 ms` session-context overhead figure is unevidenced.**
  `tidal/benches/session.rs` has exactly the right benchmark
  (`retrieve_1k_items/{without_session,with_session}`), but no run of it is
  recorded in `docs/`. Measurable on demand
  (`cargo bench -p tidaldb --bench session`); not currently measured.

## Known dead field — recorded, not silently tolerated

`SessionContext.signaled_entities` is populated by
`SessionContext::from_snapshot` and documented "for entity-level boost", and
`tidal/src/session/audit.rs` says the set "feeds the FOR SESSION entity-level
boost". **No such boost exists.** `session_boost` reads only `keywords` and
`reward_velocity`; a repo-wide search finds `signaled_entities` reached from a
`SessionContext` only in a test fixture that sets it to `HashSet::new()`. The
field on `SessionSnapshot` is genuinely used — but by cross-session preference
aggregation (`tidal/src/db/sessions.rs:547`, m6p4), never by ranking.

Consequences worth stating plainly:

- The set is computed and cloned on every `FOR SESSION` query for no effect.
- `m4_uat.rs::step8_for_session_ranking_boost` is weaker than its comments
  suggest. With no annotation set, `hint_score` is 0 and every candidate
  receives the same uniform `vel_norm * 0.2`, which cannot reorder anything, so
  its assertion (`rank_with <= rank_without`) holds trivially. The keyword half
  of the boost *is* genuinely proven — by
  `session_durability.rs::hint_keywords_boost_matching_items` and the
  case-insensitivity unit test — so the mechanism works; only the
  entity-identity claim in step 8's comments is unbacked.

Not fixed here: this record is a planning-history backfill and does not touch
Rust source. Either the entity boost should be implemented (a session that
rewarded entity 5 arguably should rank entity 5 up) or the field and the two doc
comments should go. Both are real changes needing a real owner.