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2026-06-19-drop-mutable-session-summary.md

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Agent Note: Drop the mutable session summary

Status: implemented

English | 中文

Problem

The session-persistence seam 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 API 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.
  • Session titles come from durable session/title events, while tool-card titles come from tool presenters; neither reads mutable session metadata.
  • Persistence-list consumers use immutable header identity, creation, lineage, and cwd fields. Recency and previews derive from the log rather than an updatedAt summary.
  • 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.

The removal narrows a public service contract and an on-disk format across two backends; the summary was a deliberate forward-looking design, not an accident; and SessionHeader now stands where the original Agent Note described SessionMeta, which is why the summary vanished. It also unblocks the shared persistence write coordinator: 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.

No migration

This is unreleased software (see root 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.

Consequences

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, with this change as its worked example.