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+# Workspace Selective Download — Design
+
+Date: 2026-08-09
+Status: Approved (approach A)
+
+## Goal
+
+Let users select individual files and folders in the workspace Tree view and
+download the selection as a single zip archive. Today only "download
+everything" (`/api/workspace/download`) and single-file download
+(`/api/workspace/file?download=1`) exist.
+
+## UX
+
+- **Tree view only.** Each row gets a tri-state checkbox in front of the icon
+  (checked / unchecked / indeterminate). The "By type" view is unchanged.
+- Checking a folder selects its entire subtree; unchecking a descendant of a
+  checked folder makes the folder partial (indeterminate) without needing to
+  load the subtree.
+- Toolbar keeps the existing **All** download link and gains a
+  **Selected (N)** button, disabled when N = 0. N is the number of
+  effectively-selected top-level entries (collapsed `checked` set size).
+- Clicking **Selected (N)** triggers a native browser download via an
+  `<a href>` built from the selection (no fetch + blob, so large archives
+  stream to disk).
+- Selection is cleared when the conversation (`threadId`) changes. It survives
+  panel refreshes; stale paths that no longer exist are pruned lazily (the
+  server returns a friendly error if nothing ends up in the archive).
+
+## Client state model — `src/lib/workspaceSelection.ts` (new, pure)
+
+Two sets represent the full tri-state selection without requiring unloaded
+subtrees to be materialized:
+
+- `checked: Set<string>` — explicitly checked paths, **collapsed**: checking a
+  folder stores only the folder path and drops any descendants already in the
+  set. Invariant: no entry is a descendant of another entry.
+- `excluded: Set<string>` — paths explicitly unchecked underneath a collapsed
+  checked ancestor.
+
+Derived state:
+
+- `isChecked(path)` — some `checked` entry equals or is an ancestor of `path`,
+  and no `excluded` entry equals or is an ancestor of `path`.
+- `isIndeterminate(dir)` — dir is not fully checked, but has at least one
+  `checked` or `excluded` descendant.
+
+Mutations (all pure, return new state):
+
+- `check(path)` — if covered by a collapsed checked ancestor: remove `path`
+  (and its `excluded` descendants) from `excluded`. Otherwise add `path` to
+  `checked`, dropping existing `checked`/`excluded` descendants of `path`.
+- `uncheck(path)` — if `path` itself is in `checked`: remove it (and any
+  `excluded` descendants). If covered by a collapsed checked ancestor: add
+  `path` to `excluded` (dropping `checked`/`excluded` descendants of `path`,
+  which are moot). Otherwise no-op.
+- `toParams(state)` — `{ include: string[], exclude: string[] }` for the API.
+
+The module has no React or fetch dependencies and is exhaustively unit-tested.
+
+## Component changes — `WorkspacePanel.tsx`
+
+- Hold `selection: { checked: Set<string>; excluded: Set<string> }` in state;
+  reset on `threadId` change (alongside the existing resets).
+- Tree rows render a checkbox whose state comes from
+  `isChecked`/`isIndeterminate`; `onChange` applies `check`/`uncheck`.
+- Toolbar: new **Selected (N)** anchor button next to **All**:
+  `href = /api/workspace/download?threadId=…&include=…&include=…&exclude=…`
+  (repeated params, `URLSearchParams.append`).
+- Guard: if the generated URL would exceed ~7000 characters, show a hint to
+  select fewer, higher-level entries instead of issuing a request the server
+  would reject. (Realistic selections stay far below this; the cap is a
+  safety valve, not a feature.)
+
+## API — extend `GET /api/workspace/download`
+
+New optional query params (repeated): `include=<relPath>` and
+`exclude=<relPath>`.
+
+- **No `include` params** → existing behavior (zip the whole workspace,
+  filename `workspace.zip`). Fully backwards compatible.
+- **With `include` params:**
+  - Every `include`/`exclude` path is validated with `safeResolve`
+    (containment, no hidden/internal entries). Any failure → 400.
+  - Each `exclude` must be a strict descendant of at least one `include`;
+    otherwise 400 (`exclude` outside the selection is a client bug).
+  - Duplicate and ancestor-shadowed `include` entries are deduplicated
+    server-side (collapse to the topmost path).
+  - Zip command: same `spawn("zip", …)` as today, cwd = workspace dir, but the
+    `.` argument is replaced by the deduplicated include list. Excludes are
+    appended as extra `-x` patterns **after** glob-escaping `*`, `?`, `[`,
+    `]`, `\` in the exact relative path, followed by the existing
+    `zipExcludeArgs()` (dotfiles / ignored dirs / ignored suffixes still
+    apply inside selected folders).
+  - Filename: single include → `<basename>.zip`; multiple →
+    `workspace-selection.zip`.
+  - Everything selected-but-excluded → `zip` exit code 12 → friendly error
+    ("The selection is empty."), same as the existing empty-workspace path.
+- Temp file lifecycle, `request.signal` abort handling, streaming response,
+  and `close`-based cleanup are unchanged.
+
+### Why server-side zip (rejected alternatives)
+
+- *New dedicated endpoint*: would duplicate the temp-file/abort/cleanup/error
+  logic; a query-param extension keeps one code path.
+- *Client-side JSZip*: buffers the archive in browser memory and fetches every
+  file individually — unworkable for multi-GB research artifacts.
+
+## Error handling
+
+| Case | Behavior |
+|---|---|
+| Missing/invalid path (traversal, hidden entry) | 400 JSON error |
+| `exclude` not under any `include` | 400 JSON error |
+| Empty result after excludes | 400 JSON error ("The selection is empty.") |
+| Client disconnects mid-archive | child `zip` killed via `SIGKILL`, temp file removed (existing) |
+| `zip` binary missing | friendly error (existing) |
+
+## Testing
+
+- `src/lib/workspaceSelection.test.ts` (new):
+  - check/uncheck of files, folders, and nested combinations
+  - collapse invariant (checking a folder drops checked descendants)
+  - `isChecked` / `isIndeterminate` derivations, including unloaded subtrees
+  - `toParams` output for mixed include/exclude selections
+- `src/app/api/workspace/download/route.test.ts` (new):
+  - single-folder include produces a zip with exactly that folder's
+    (non-ignored) contents
+  - include + exclude removes the excluded file from the archive
+  - multi-include produces entries under each top-level path
+  - `../` traversal in include/exclude → 400
+  - exclude outside all includes → 400
+  - filename with glob metacharacters (`[`, `*`) in exclude handled literally
+  - no include params → whole-workspace zip (regression guard)
+  - all-excluded selection → 400 "selection is empty"
+  - Tests inspect the produced archive by spawning `unzip -l` (or `zipinfo`)
+    on the temp file, consistent with how the repo already shells out to `zip`.
+
+## Out of scope
+
+- Multi-select in the "By type" view
+- Per-row inline folder download buttons
+- Resumable/partial downloads, download progress UI