The JSONL durable session-persistence backend — a concrete SessionPersistence (the dsh-session-persistence seam). One append-only .jsonl event log per session.

On-disk layout

<root>/
  cwd-<sha256(cwd)[:12]>/        # per-project bucket (or _no-cwd/ when no cwd)
    <encoded-id>.jsonl           # header line + one SessionEvent per line (verbatim)

The first .jsonl line is the immutable SessionHeader tagged { type: 'session', version, id, cwd?, createdAt, parentSession?, seedLength? }; every subsequent line is one SessionEvent JSON, verbatim including assistant/chunk so seq stays contiguous (events[i].seq === i).
Session ids are unvalidated branded strings, so they are percent-encoded to a single safe path segment before use (no traversal, no collision).

Config

Key	Type	Notes
`root`	`string` (required)	Root directory for all session files. No default — a `process.cwd()` default would scatter files as the process's cwd changes (bash calls, subprocesses).

Durability and crash semantics

Lazy materialization. create(meta) writes nothing; on the first append, the backend writes and fsyncs a temporary file, publishes it without overwrite via a hard link, then fsyncs the directory. A created-but-never-appended session leaves nothing on disk and is absent from list.
Append-only. Committed events (at or below a flushed turn/end) are never rewritten. Subsequent appends are line appends at EOF + fsync.
Crash recovery — preserve valid tail work. load keeps the contiguous valid prefix of an interrupted final turn. It truncates from the first unparsable or sequence-gapped uncommitted record, then appends the synthetic tool, step, and turn closers required by the shared persistence contract; the same defect at or before the last committed turn/end rejects.
Contiguous-seq. append rejects a batch whose first seq does not continue the stored log, and rejects non-JSON-serializable event.data naming the offending event type.

Write path

The plugin buffers frozen session events and drains them on flush or disposal. A per-session cursor prevents resumed sessions from re-appending stored events, and live sessions are seeded when the plugin loads. Operations for one session are serialized; disposal waits for initialization and the final drain so no write lands after teardown.

Model Experience

Resumed conversation history

What the model sees: JSONL storage contributes no live prompt or schema. Loading restores stored surface history and preserves prior request headers for reconstruction; the new loop composes its current envelope. Each unanswered call in an interrupted tail is balanced with the exact error text Tool call interrupted by a crash; no result was recorded. Raw assistant/chunk records do not duplicate messages.

Token effect: Zero live-request tokens. A resumed agent pays for retained history and its current envelope, plus the quoted repair result for each interrupted call.

Known Limitations and Deferred Work

Only the current SESSION_FORMAT_VERSION (v0) loads — the on-disk format is pre-release/unstable: a breaking format change is absorbed at v0 and non-current logs are rejected; there is no migration.
Nothing deletes session files — logs accumulate under root until removed externally (the seam has no deletion surface).
Single-process assumption — per-session serialization and the write cursor live in this process; two processes appending to the same root are not coordinated.
Initial materialization requires hard-link support — first append uses link() so same-id races fail instead of overwriting a committed log; a filesystem that cannot create hard links cannot host this backend.