# @deepseek-ai/dsh-session-persistence-jsonl

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). |

`locate(meta)` returns `{ kind: 'jsonl', path }` using the resolved absolute root and the same cwd-bucket/id encoding as materialization. It performs no filesystem I/O: the target can be returned before the file exists, and an existing file contains only the last flushed prefix.

## Durability and crash semantics

- **Lazy materialization.** `create(meta)` writes nothing; on the first `append`, the backend writes and `fsync`s a temporary file, publishes it without overwrite via a hard link, then `fsync`s the directory when the host supports it. 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](../../../docs/rfc/implemented/architecture/2026-06-14-session-persistence.md); 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.

#### KV Cache effect

JSONL storage does not mutate live request prefixes. A resumed loop can reuse provider cache only when its reconstructed history, current envelope, and model route match; crash-repair results append.

## 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.
- **Windows cannot `fsync` directory handles through Node** — the backend tolerates only Windows `EPERM` from directory `fsync`; file-content `fsync` remains mandatory, but a crash can lose a newly published directory entry on a host without an equivalent directory-sync primitive.
