deepseek-harness/packages/session-persistence/session-persistence-jsonl/README.md

@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).

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 — close, don't truncate. A crash can leave a log whose final turn never closed (real events after the last turn/end). load PRESERVES those events (a turn can be huge — they are real work) and closes the orphaned turn by durably appending synthetic boundary events: an error tool/result for every tool-call the crash left unanswered (the loop logs the assistant message before running the tools, so a mid-tool crash leaves dangling calls — and deriveMessages() would replay an assistant tool-call with no result, which providers reject), then a step/end if a step was open, then turn/end {kind:'interrupted'}, returning a balanced log. Only a never-fully-written torn tail fragment (a final line with no newline / unparseable) is ftruncated away before the closers are written. See session persistence.
• Contiguous-seq. load rejects a mid-log parse error or seq gap (unloadable); 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 generalizes the example session-jsonl.ts: it subscribes to session/created (capture the header; persist a fork's seed once), session/event (copy each already-frozen event into the persistence-owned write-behind buffer), and session/flush/dispose (drain that buffer through append). A per-session write cursor means a resumed session never re-appends already-stored events. Existing live sessions are seeded on plugin apply (HMR does not replay session/created). All backend operations for one session are serialized, and disposal awaits quiescence (every init + final drain) before returning, 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.