# Milestone 4 · Agent Memory (✅ COMPLETE 2026-02-21)

Milestone spec, thesis, UAT scenario, and per-phase acceptance criteria:
[ROADMAP · Milestone 4](../ROADMAP.md). Status row: `m4: Agent Session Layer`.

> **These are backfilled records, not the original plans.** M4 shipped in the
> squashed commit `39ada28` (2026-02-21) before this project kept per-milestone
> planning directories, so the four phase docs here were written after the fact
> (2026-08-30) from the ROADMAP acceptance criteria, the shipped source tree, and
> the M4 test suite. Every claim below names the artifact that proves it. Where
> the shipped surface diverges from the ROADMAP text, the phase doc says so
> rather than restating the plan.

## What the milestone proves

An agent — not a human clicking a UI — can own a scoped memory lane inside the
same embedded process: open a session bound to a `(user, agent, policy)` triple,
write short-lived signals that the schema-declared policy accepts or rejects,
read a live snapshot of that session's decayed state, blend the session into a
`RETRIEVE` ranking, then close it and read the frozen archive. No Redis, no
feature store, no policy middleware.

## Phases

| Phase | Name | Record |
|-------|------|--------|
| m4p1 | Session Schema and Lifecycle | [phase-1.md](phase-1.md) |
| m4p2 | Session Signal Engine | [phase-2.md](phase-2.md) |
| m4p3 | Policy Enforcement and Audit | [phase-3.md](phase-3.md) |
| m4p4 | Session-Aware Ranking and M4 UAT | [phase-4.md](phase-4.md) |

The four phases were strictly sequential (each needed the previous phase's write
path), and all four landed in one commit; the phase split below is therefore a
split of the *delivered surface*, traced from the ROADMAP's phase definitions to
the modules and tests that implement each one.

## Shipped surface (verified against the working tree)

| Concern | Artifact |
|---------|----------|
| Session types | `tidal/src/session/types.rs` — `SessionId`, `AgentId`, `SessionHandle` |
| Live session state | `tidal/src/session/state.rs` — per-session accumulators |
| Session signal decay | `tidal/src/session/signal_state.rs` |
| Snapshot / archive read model | `tidal/src/session/snapshot.rs` |
| Policy evaluation | `tidal/src/session/policy.rs` |
| Audit log | `tidal/src/session/audit.rs` |
| Archive serialization | `tidal/src/session/serde/mod.rs`, `serde/start_record.rs` |
| Policy declaration in schema | `tidal/src/schema/validation/policies.rs` — `AgentPolicy` |
| `db.*` session API | `tidal/src/db/sessions.rs` |
| WAL durability | `WalCommand::SessionStart` / `SessionSignal` / `SessionClose` in `tidal/src/wal/writer.rs` |

## Evidence

| Claim | Proof |
|-------|-------|
| Full agent workflow works end to end | `tidal/tests/m4_uat.rs` — 12 `#[test]` steps, "M4 User Acceptance Test: Agent Session Layer". Added by `39ada28`, the M4 commit. |
| Sessions survive a crash | `tidal/tests/session_durability.rs` — 10 tests incl. `active_session_state_restored_after_crash`, `metadata_survives_crash`, `wal_replay_restores_signal_counts_exactly`. Added one commit later by `192c473` (M5), so read it as M4-surface hardening rather than M4 itself. |
| Session semantics still hold after later reworks | `tidal/tests/review_pass2_zone_a_sessions.rs`, `tidal/tests/review_pass2_query_for_session.rs` — added by `9728194` (M0–M10 review pass 2, 2026-06-09) |
| Sessions survive M7 crash hardening | `tidal/tests/m7_crash_m6.rs`, `tidal/tests/m7_crash_invariant.rs` (session-touching cases) |
| Recorded at close | ROADMAP status row: 607 lib + 12 m4_uat + prior UATs passing |

All 49 M4 acceptance criteria in [ROADMAP · Milestone 4](../ROADMAP.md) are
marked `[x]`.

## Naming drift worth knowing

The ROADMAP's M4 text names errors `LumenError::PolicyViolation` /
`LumenError::SessionExpired`. The shipped enum is `TidalError` — the `Lumen`
prefix is a pre-rename artifact left in the planning prose. Real variants:
`TidalError::PolicyViolation` and `TidalError::SessionExpired`
(`tidal/src/schema/error.rs:123,130`), plus the session-internal
`PolicyViolation { kind: PolicyViolationKind }` re-exported from
`tidal/src/lib.rs`.

## What M4 deliberately did not do

The ROADMAP's "Deferred to Later Milestones" list for M4 held nine items. Their
real fates:

- Cross-session aggregation → shipped in m6p4 (see [milestone-6/phase-4.md](../milestone-6/phase-4.md)).
- Session signal influence on the global preference vector → shipped in m6p4.
- Semantic hint matching → M4 shipped keyword matching; embeddings arrived with M5.
- RLHF training-data export, per-agent QPS rate limiting, session TTL sweeper → shipped in M7 (m7p2/m7p4).
- User revocation of agent-contributed signals, multi-agent sessions → shipped in M9/M10; the read-path and profile-override fields on `AgentPolicy` (`allowed_read_signals`, `denied_read_signals`, `allowed_user_attributes`, `denied_user_attributes`, `allowed_profile_overrides`) were added by commit `d8e4083` for that work, not by M4.
- Session forking and merging → still not built. No commit, no test, no issue; it remains an open idea, not a shipped feature.