deepseek-harness/.agents/notes/implemented/architecture/2026-07-28-directory-picker-capability-seam.md

Agent Note: A capability-discriminated directory-picker seam for the web-GUI host

Status: implemented

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Problem

The web GUI's "Open local folder" flow was hardwired to one interaction: host.pickDirectory invoked a native OS chooser compiled into dsh-host-apiproxy (private module, test-only injection seam). That shape cannot serve remote deployments — no OS dialog reaches a browser on another machine — and the planned in-app directory browser (Figma Harness 802-56979) needs listing/creation primitives, which are a different interaction contract, not a different implementation of the same one. Swapping interactions required editing gateway source, against the repo's everything-is-a-plugin stance.

Decision

A three-package capability seam in packages/host/ — directory-picker (interface), directory-picker-native, directory-picker-browse (backends) — with one contract method: capability() returns a discriminated union, { kind: 'native', pick(signal) } or { kind: 'browse', list(path?), createDirectory(path, name) }. The gateway (dsh-host-apiproxy) injects directoryPicker, advertises the kind through host.describe.directoryPicker, serves the matching RPCs, and answers directory-picker-unavailable for the other kind; the client branches on the advertised kind and hides the affordance for unknown kinds (merge-extensible default). Composition (cordis.yml) is the swap point; the union is discriminated because the backends differ in interaction shape — flattening them into one method set would force every backend to fake the other's shape.

Placement and policy rulings folded into this decision:

• Not the ctx.fs seam. packages/fs/ is the model/session-facing storage stack (policy events, sandbox-swappable backends). Riding it would couple GUI browsing to the model's confinement backend — swapping fs-sandbox for the model must never change GUI behavior — and OS facts (home anchoring, hidden conventions) are not storage primitives. The picker seam stays presentation-free and model-free; packages/host/ is its consumer-domain home.
• Dependency survey (hand-roll vs adopt). Node's stdlib is the maintained cross-platform OS layer (readdir(withFileTypes), homedir, path semantics); surveyed alternatives fail the dependency bar — file-manager packages (node-file-manager, files-and-folders, Syncfusion's provider) are whole HTTP apps (fit), drive-letter helpers (drivelist native addon, windows-drive-letters ~7y stale) fail health/proportionality. The browse backend is a thin adapter over stdlib.
• Hidden entries: return-and-flag. The host stamps hidden (POSIX dot convention) and returns everything; the client filters. Display policy stays client-side, and the planned show-hidden toggle becomes a client-only change. Windows' FILE_ATTRIBUTE_HIDDEN is not exposed by dirents — documented limitation until a native probe pays for itself.
• Symlinks: follow for enterability. stat probes symlinks (broken/cyclic → skipped); crumbs keep the logical path the operator navigated, and workspace.create already canonicalizes via realpath at adoption.
• Whole-filesystem scope, no roots config. workspace.create accepts arbitrary paths and the API serves bash-driving methods, so a browse root would be UX scoping, not a boundary; configurability without a consumer fails the evidence bar. Deferred until a deployment needs it.
• The native backend stays. Plugin-form was the point: multiple providers can serve the seam (an Electron shell would provide the native interaction through its own dialog API). Kind naming: dialog was the first pick and was dropped — the browse interaction also presents a dialog (the in-app modal), so the word failed to discriminate; native names where the chooser runs.

Alternatives considered

• Extend ctx.fs with browse methods. Rejected: authority-domain coupling above; also a listing-for-display contract (hidden flags, crumbs, home anchor) does not belong on a storage seam.
• One uniform seam method set (pick(): path). Rejected: an in-app browser cannot be served behind a single host-side call — the browsing loop lives in the client and needs primitives on the wire; the native chooser cannot implement primitives. The interaction difference is irreducible, hence the discriminant.
• Direct stdlib calls inside apiproxy (no seam). Rejected: keeps the gateway the only swap point (source edits), loses fixture/test backends, and contradicts the plugin doctrine that motivated the work.
• Adopting a file-manager/drive-enumeration dependency. Rejected per the survey above; recorded here as the dependency policy requires.

Consequences

• cordis.yml chooses the interaction; apps/cli currently mounts -native (unchanged behavior). The GUI already gates its picking affordance on describe.directoryPicker (non-native kinds hide it); the in-app browser PR flips the default to -browse and adds the browse UI.
• The wire gains host.listDirectory/host.createDirectory, four error codes, and the describe.directoryPicker field; the connection fixture serves a deterministic browse tree for keyless assembled tests.
• A future interaction (or an Electron provider of the native interaction) is one backend package plus a client branch — no gateway surgery.
• ApiProxyDefaults.pickDirectory (test-only injection) is gone; tests provide a stub ctx.directoryPicker like any other service.