Status: implemented (proposed and accepted 2026-06-19)
## Context
The [session-persistence seam](2026-06-14-session-persistence.md) split a session's out-of-log metadata into two types owned by `dsh-session`: an immutable `SessionHeader` (`version`, `id`, `createdAt`, `cwd?`, `parentSession?`) written once at creation, and a mutable `SessionSummary` (`updatedAt`, `title?`, `firstPrompt?`) "updateable without touching the append-only log". Their union was `SessionMeta = SessionHeader & SessionSummary`, and the abstract `SessionPersistence` service carried a seventh method — `update(id, summary)` — for rewriting the summary. Each backend implemented the mutable store its own way: JSONL wrote a separate atomic `.summary.json`**sidecar** beside the log (temp-write + rename, best-effort), SQLite kept `updated_at`/`title`/`first_prompt`**columns** bumped inside the append transaction.
The summary was designed for a future session picker (recency ordering via `updatedAt`, a `title`/`firstPrompt` preview). That picker was never built. An audit of the whole repo found the entire `SessionSummary` surface is **dead state**:
-`SessionPersistence.update()` has **zero production callers** (every `.update(` hit is `createHash().update()` or a test).
-`firstPrompt` is **never read** anywhere in production.
-`title`*is* read in the ACP bridge — but from a tool-call **presenter** (`present.title`), never from stored session metadata.
-`updatedAt` has **no consumer**: the only production caller of `list()` reads `meta.cwd` (a `SessionHeader` field) to validate a workspace on `session/load`; resume reads `createdAt`/`cwd`/`parentSession` — all header fields.
- Decisively: the live `Session.header` was already typed `SessionHeader`, not `SessionMeta` — the summary never existed on the live session object; it lived only in the persistence layer, written and read by nothing but its own contract test.
## Decision
Delete the mutable session summary entirely. `SessionSummary` and the `SessionMeta` name are removed; the metadata a backend stores and returns is just `SessionHeader`. `SessionPersistence.update()` is removed from the abstract service and every backend. JSONL loses the whole sidecar machinery (`writeSidecar`/`readSidecar`/`touchSummary`/`removeSidecars`/`sidecarPath` and the load/list overlays); SQLite drops the `updated_at`/`title`/`first_prompt` columns and the per-append `updated_at` bump, and its `SCHEMA_VERSION` goes `1 → 2`.
Anything the summary was meant to provide is **derivable from the append-only log** when a consumer actually needs it (`firstPrompt` = first `user/message`; recency = the last event's `time` or the file mtime) or already lives in the immutable header (`createdAt`, `cwd`). The one thing *not* derivable — a user-*edited* title — had no implementation and is pure YAGNI; it can return as its own log event or header field if a real feature ever needs it.
This is recorded as a decision because it is **durable** (it narrows a public service contract and an on-disk format across two backends), **contested** (the summary was a deliberate forward-looking design, not an accident), and **surprising** (a future reader finding `SessionHeader` where the original RFC describes `SessionMeta` would otherwise ask why the summary vanished). It also unblocks the [shared persistence write coordinator](2026-06-18-shared-persistence-write-coordinator.md): with no mutable summary, the coordinator's hook interface needs no `updateSummary` hook and the JSONL-sidecar-vs-SQLite-column durability divergence disappears, so the two backends' write paths converge.
This is unreleased software (see [root AGENTS.md](../../../AGENTS.md) § "Pre-release stance: foundation over blast radius"), so there are no on-disk databases or logs to preserve. SQLite does not migrate a v1 database: the `openDatabase` guard now rejects any non-current on-disk `user_version` (`onDisk !== 0 && onDisk !== SCHEMA_VERSION`) — older *or* newer — so a stale v1 DB is cleanly rejected rather than half-read against the new column set. A fresh database stamps the current version; that is the only path that needs to work.
## What we gave up
A future session picker now has to derive its preview/ordering from the log (or reintroduce a typed field) rather than reading a ready-made summary row. That is the correct cost: a cache for a feature that does not exist is dead weight that every backend pays to maintain and every contract test pays to assert. The principle — **a passing test pins current behavior, not necessarily correct behavior; behavior can be an artifact of a past compromise** — is now recorded as a standalone convention in [root AGENTS.md](../../../AGENTS.md), with this change as its worked example.
