The workspace-directory picking seam for the web-GUI host: an abstract DirectoryPicker service (ctx.directoryPicker) whose single contract method capability() returns a discriminated capability describing how an operator selects a directory. Backends differ in interaction shape, not just mechanism, so the seam models the shapes explicitly instead of one method set: { kind: 'native', pick(signal) } opens one native OS chooser on the host display (-native); { kind: 'browse', list(path?), createDirectory(path, name) } serves listing/creation primitives an in-app browser drives, which works for remote clients no OS chooser can reach (-browse). Consumers switch on capability().kind; the union derives from the merge-extensible DirectoryPickerCapabilities map (a new backend declaration-merges its shape there), and the documented default for an unknown kind is to hide the picking affordance rather than fail. The capability object must be stable for the service lifetime. The client side mirrors the seam without a wire advertisement: each backend package is dual-face, its browser half registering the matching picking interaction into ui-workspace's directory-flow slots — so one composition row swaps both the host capability and the client flow together. A composition that should not pin an interaction mounts the -auto chooser instead, which resolves the host's situation once at boot and mounts the matching backend row itself.

Browse primitives fail with the typed DirectoryPickerError (directory-unreadable / directory-exists / directory-create-failed, each carrying the subject path), which the consuming gateway maps 1:1 onto wire error codes. DirectoryEntry rows carry a host-owned hidden flag (POSIX dot convention) so display policy stays client-side; DirectoryListing.crumbs is the ancestor chain from the filesystem root, every crumb a jump target. Design rationale, the ctx.fs separation, and the policy decisions live in the directory-picker capability seam Agent Note.

Model Experience

None, as the seam serves the GUI host's directory selection; nothing here reaches a model request.

KV Cache effect

None; this package neither assembles nor sends a provider request.

Known Limitations and Deferred Work

No multi-root vocabulary — the browse contract exposes one ancestry chain per listing; per-deployment root scoping (and Windows drive-root enumeration above a drive) waits for a consumer that needs it, per the seam Agent Note.