# Agent Note: Web session-log export as a host-streamed ZIP download

Status: implemented

English | [中文](2026-08-10-web-session-log-export.zh.md)

## Problem

The Trajectory view had no way to hand a debugging artifact to a human: the raw session log lived on disk and in the host, the client history face served folded projections (not raw entries), and a session with subagents spans many independent session logs. A bug report needs the complete raw log of the whole tree, in a shape that survives being emailed around.

## Decision

- **The export is a host-only download, not an RPC**: `GET /api/session.export?sessionId=…&includeDescendants=true` streams one ZIP attachment. Every file is a session's **stored artifact text verbatim**: `readRaw` on the persistence service reads the backend's own durable bytes (the JSONL backend decodes its physical zstd frames, or returns plaintext) — never a reconstruction from parsed events, so packed-chunk rows, key order, and line breaks survive byte-for-byte — under its original base name (`session.jsonl` at the root, `subagents/<id>/session.jsonl` for descendants). Compression runs on the host with fflate's streaming `Zip`/`ZipDeflate` API at validated `sessionExportCompressionLevel` 0–9 (default 6), letting deployments trade CPU and latency against archive size; each entry is deflated in bounded chunks as it is produced, so the response is chunked as it is generated and the host never holds the whole archive in one buffer (at most one descendant's artifact text beyond the preloaded root). At the 64 KiB response byte high-water mark, production waits for consumer pull to restore capacity; fflate's synchronous callback can add at most one bounded input push beyond that queue bound. No manifest is written — every file is byte-identical to the durable artifact and self-describing through its own header line.
- **Error vocabulary is HTTP-native**: missing services → 500, a backend without per-session raw artifacts → 501, missing root session → 404 (all decided before any byte streams), and a descendant without a stored artifact → the stream errors (fail-loud, never silent under-export). Request abort remains cancellation instead of being rewritten as 500; request and response-consumer cancellation converge on the producer signal, which reaches lineage, persistence, and attachment reads and terminates the active compressor. The carrier (`toFetchHandler`) already applies the `/api` trust fence; the GET branch sits beside the existing SSE GET routes, and `ApiProxy.downloads.sessionLog` (host-only, no wire envelope, absent from `IApiClient`) implements it.
- **The UI just downloads**: browser consumers may issue a bodyless `HEAD` preflight for preparation errors, then hand the GET endpoint to the browser's native download manager, so JavaScript never buffers the ZIP. The `session.log` RPC that an earlier iteration shipped was removed — the download endpoint is its only consumer, and the repo rule is no public interface without a current owner. The client bundle carries no archive implementation.
- The current Header and `/export` consumers are defined by the [Web export command and dialog decision](2026-08-11-web-export-command-and-dialog.md).

## Alternatives considered

- **`session.log` data RPC + client-side zip** — shipped first, rejected with the user: the browser pulls the full raw JSON (≈10× the final zip size) and compresses on the main thread; for the 23 MB sessions in real use the host-side stream is strictly better. The RPC was deleted with the migration rather than left as a dead public surface.
- **Single JSONL with envelope lines for multiple sessions** — rejected with the user: mixing sessions in one JSONL loses clean per-file boundaries; a ZIP keeps one canonical file per session.
- **jszip** — heavier (~100 kB) and its dependency graph pulls readable-stream browser mappings; fflate is purpose-built and small.
- **Vendoring fflate's browser entry** — the repo vendoring procedure targets cordis-scale pinned sources; a resolveId alias keeps the maintained dependency without shipping a copy (and host-side fflate needs no alias at all).

## Consequences

- Export fidelity: immediately before reading each live root or descendant, the exporter crosses the authoritative `SessionStore.flush` durability barrier; every exported file is byte-identical to that resulting durable artifact. A live session may append again after its read, so the archive is a per-session read-boundary snapshot rather than one atomic tree snapshot. The archive name is `dsh-session-<sanitized-id>.zip` and archive paths sanitize ids before they can shape entries.
- `supportsRawArtifacts` explicitly separates backend capability from session absence: unsupported backends such as SQLite report `false` and the concrete `readRaw` default rejects, while the JSONL override reports `true`, owns physical decoding, and reserves `undefined` for an absent artifact. `ApiProxy.downloads.sessionLog` adds one host-only member to the contract plus a host-side query schema and a GET branch in the fetch handler — no RPC map row, envelope schema, or client `IApiClient` surface.
- Fixture mode (no host) answers 404 for the export, which the browser reports as a failed download; the navigation-panes golden snapshot includes the 导出 button.
- Deferred: transcript.md and a report/feedback bundle remain future work; the byte-faithful, manifest-free shape keeps the v2 bundle extension cheap.
