Summary

Addresses #6204 (Hands settings bug). The issue is a screen-recording with no text, so I traced the Hands settings surfaces for the strongest concrete, code-grounded bug.

Root cause

The Hand settings editor builds its PUT payload only from draft — the per-session diff containing just the keys whose input fired onChange (HandsPage.tsx). update_hand_settings forwards that partial map straight to HandRegistry::update_config, which does a wholesale replace (entry.config = config, registry.rs:1355, pinned by the update_config_replaces_and_refreshes unit test). So for a Hand with more than one setting, saving one field overwrote the instance config with just that key and reverted every other previously-saved setting to its default. Invisible for single-setting Hands; data-loss for multi-setting Hands.

Fix

• crates/librefang-api/src/routes/skills/hands.rs — update_hand_settings now reads the active instance's current config and merges the incoming keys over it, making the endpoint a true partial update. The registry's update_config replace contract is left unchanged (it has one other caller and a unit test pinning it).
• crates/librefang-api/dashboard/src/pages/HandsPage.tsx — the editor seeds its payload from the saved current_values before overlaying the draft, as defense in depth.

Confidence / scope note

Medium confidence this is the bug the (text-less) video depicts — it's the strongest concrete settings-save defect in this area. If the video instead shows the agent-config tab (system prompt / tools), that path persists and refetches correctly; the only residual there is the state-resync useEffect potentially wiping an unsaved draft on a window-focus refetch — flagging for whoever has seen the video.

No backend integration test was added: the hands test harness installs no fixture Hand, so the settings happy-path is unreachable without new fixture infrastructure (out of scope for this fix). Recommend a follow-up that adds a fixture Hand with ≥2 settings and asserts PUT{a}→PUT{b}→GET keeps both.

Verification

⚠️ Local verification was not possible (host disk full, OrbStack down → no cargo; disk too tight to risk pnpm). Both changes are static-reviewed: backend — HandInstance.config and the incoming body are both HashMap<String, Value>, so read→extend→update_config(merged) type-checks; frontend — current_values?: Record<string, unknown> spreads cleanly into the payload. CI is the verifier — cargo check --workspace --lib, cargo test -p librefang-api, cargo test -p librefang-hands (replace contract unchanged), and pnpm typecheck.