What

A new private package @assistant-ui/harness: a minimal client for deep-agent backends (todos, files, subagents, PTC, interrupts, queued sends) speaking the assistant-transport wire. Implements task uh-2 from the harness design doc.

Architecture

Built on tap, following the harness-sdk pattern: all logic lives in a single tap resource, and every public surface is a thin binding over it.

• HarnessResource: the core. A plain HarnessCore class owns the command queue, single-flight run loop, optimistic sends, and cooperative cancel; the resource hook wires it to React state and derives views.
• useHarness(options): useResource(HarnessResource(options)), SSR-safe.
• new Harness(options): declarative alternative via createTapRoot, with getState/subscribe/dispose and flushTapSync-wrapped methods.
• HarnessManager: keyed multi-harness lookup with stable identity per id.

State and commands

• Canonical server-owned HarnessState: id-keyed message tree, queue, files, todos, status, interrupt, error, title, typed extras. Snapshots replace wholesale.
• One nesting primitive: a message's parentId may be a tool call id, rooting a nested transcript. Task subagents and programmatic tool-call sub-calls both stream through this path; the client derives a recursive subagents record. No separate PTC concept.
• Command vocabulary: send-message (with queue/steer behavior), add-tool-result, resume, cancel, cancel-queued, send-now, plus a HarnessCustomCommands module-augmentation slot.
• Echo-based optimistic reconciliation: client-generated send ids make optimistic-to-committed an identity merge; optimistic entries drop when their id appears in messages or queue, or when the carrying batch settles.
• Run loop: single-flight with same-tick batching and follow-up coalescing; stop() closes the stream locally and delivers {type:"cancel"} on the immediate follow-up request.

Transport

HarnessTransport is two methods (run, optional resume) yielding state snapshots. HttpHarnessTransport implements the assistant-transport wire (POST {commands, state, threadId}, data-stream or assistant-transport protocol) via assistant-stream's decoder and accumulator.

Deliberately out of scope

• The assistant-ui ExternalThread bridge component (separate task per the design doc)
• WebSocket transport and auto-reconnect
• Python backend changes

Cuts for minimalism

Dropped from the original sketch: regenerate/edit (no command for it yet), threads/switchTo views (the recursive subagents record covers nesting), approve() sugar (resume covers it), separate enqueue/steer commands (send-message.behavior carries it), a status-probe endpoint (resume() covers snapshot-on-connect), attachment/image parts, initialExtras, a React subpath export, and TExtras generics on options/transport (generics only where state is read).

Testing

• 9 vitest tests against a mock transport: optimistic flow and echo merge, mid-run queue reconciliation, same-tick batching, error surfacing and clearing, stop/cancel, interrupt/resume, resume-on-mount, recursive subagent derivation, manager identity/dispose.
• pnpm turbo build --filter=@assistant-ui/harness, pnpm --filter @assistant-ui/harness test, pnpm lint, and pnpm check:resource-memo all pass.

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