# m6p1 — Cohort Engine + Cohort-Scoped Trending (✅ COMPLETE 2026-02-23)

Phase spec and acceptance criteria: [ROADMAP · Milestone 6 · Phase 1](../ROADMAP.md).
Milestone index: [README.md](README.md). Spec: [docs/specs/05-cohorts.md](../../specs/05-cohorts.md).
Backfilled record.

## What shipped

1. **Cohort definitions as data** (`tidal/src/cohort/types.rs`). `CohortDef {
   name, predicate }` with a five-arm `Predicate` enum — `Eq { field, value }`,
   `Any { field, values }`, `Range { field, lo, hi }` (inclusive, `f64`-parsed),
   `And(Vec<Self>)`, `Or(Vec<Self>)`. Both types are `Serialize`/`Deserialize`,
   which is what makes cohort state checkpointable rather than rebuild-only.
2. **`db.define_cohort(CohortDef)`** (`tidal/src/db/cohorts.rs:19`) registers a
   named cohort at runtime, not only at schema build time.
3. **Membership resolution at write time** (`tidal/src/cohort/resolver.rs`).
   User metadata is evaluated against every registered predicate; the result is
   cached and invalidated when the user's metadata is rewritten, so a signal
   write pays a lookup rather than a predicate sweep.
4. **Per-cohort signal aggregation** (`tidal/src/cohort/ledger.rs` —
   `CohortSignalLedger`). Same decay / windowed-count / velocity semantics as the
   global ledger, keyed by cohort as well as entity and signal type, updated on
   the same write.
5. **Query surface.** `RetrieveBuilder::cohort(name)` scopes signal reads to the
   cohort ledger; `cohort_predicate(Predicate)` answers ad-hoc cohorts by
   resolving matching users at query time (slower, no pre-materialized state).
   A `cohort_trending` built-in profile ships in `tidal/src/ranking/builtins.rs`.
6. **Durability** (`tidal/src/cohort/checkpoint.rs`). Cohort ledger state is
   checkpointed alongside the global ledger and restored on open.

## Evidence

- `tidal/tests/m6_cohort.rs` — 12 `#[test]` functions, including the divergence
  case the ROADMAP calls for: cohort-scoped trending must differ from global
  trending when cohort velocity diverges from global velocity.
- `tidal/tests/m6_crash_surfaces.rs` and `tidal/tests/m7_crash_m6.rs` — M7
  crash-fences the cohort state this phase introduced.
- `tidal/src/testing/crash_injector.rs` references `CohortSignalLedger`, i.e.
  the cohort tier is a first-class target of fault injection, not an untested
  side structure.

## Note on the "< 50 ms at 500 items / 20 users" criterion

Marked `[x]` in the ROADMAP with no recorded run. The functional criteria are
proven by `m6_cohort.rs`; the latency figure has no in-repo measurement. Unlike
M11/M12, this milestone published no `docs/profiling/` record for its own gates.