feat(app): the harness switcher in the chat box — composer control riding the solver-mode trio, entitlement-gated, harness-aware degradation

[aarontrowbridge]

Important

Problem — Switching harnesses today means the command palette (amicode.selectHarness) and a settings read. The registry, the ADR-0011 Harness Contract, and the telaio descriptor all exist; the affordance is buried where nobody touches it mid-flow. The composer is where the work happens — the harness choice belongs there.
Approach — Ride the solver-mode trio, the proven in-product switcher pattern: a harness control in the composer (a fourth select-slot alongside agent/model/variant) → its selection POSTs to a new engine route /amicode/harness → the extension half does the registry resolution + server restart, with the same switching → ready file-handshake shape solver-mode uses and a banner narrating the switch. The picker's honest disclosure carries over verbatim: switching harnesses switches session history — sessions are harness-local by design.
Approaches considered — Status-popover placement only (rejected as the primary: the ask is composer-level; the popover can link to it later); per-session harness identity (rejected for this slice: no engine multiplexing machinery exists — named follow-up); silent restart (rejected: solver-mode's narrated banner is the honest pattern).
Scope — in: the composer harness control (honest availability — unresolvable harnesses render disabled with the reason), the engine route + extension handshake + restart, the harness.telaio entitlement read-side gate enforced at the UI, harness-aware degradation of composer controls under telaio (contract v1 accepts no agent/model/variant → those slots dim with a why). out: per-session harness identity, cross-harness session continuity, upstream_mode changes, new harness descriptors.
Assumptions — The harness.ts registry and amicode.selectHarness palette command stay the source of truth; the composer control is a new front on the same state, not a competing one.

Acceptance Criteria

• The composer shows a harness select; choosing a different harness restarts the server on it, with the switching → ready narration (the solver-mode pattern end to end)
• An unresolvable or unentitled harness renders disabled with its reason — never silently absent, never selectable-then-failing
• Under telaio, agent/model/variant slots dim with an honest "not accepted by this harness" affordance; under opencode everything behaves as today
• The palette command and the composer control are one source of truth (same registry, same setting) — switching from either lands the same state, and a test pins it
• The disclosure copy ("switching harnesses switches session history") shows at the point of switching

Testing Decisions

Extend the existing extension harness tests (registry resolution + the picker) and the app's composer-component tests (the select-slot pattern has coverage to mirror); the engine route gets route-level tests in the solver-mode route's file; the handshake state machine reuses the solver-mode tests' shape.

Key Decisions

• Process-level switching stays — no schema change, no per-session multiplexing in this slice; the follow-up (per-session harness) is named, not smuggled in.
• One registry, two fronts: palette and composer read/write the same setting through the same resolution path; divergence is a red.
• The restart is narrated, not hidden: banner while switching, disclosure before committing.
• The entitlement gate lives at the UI read (disabled-with-reason), not as a hidden failure at spawn time.

Constraints & Invariants

• harness.telaio entitlement enforcement must not regress the palette command's behavior for entitled users.
• The opencode default path must be byte-for-byte behaviorally unchanged when the control is untouched.
• No new session-identity fields; sessions remain harness-local.

Prior Art

• The solver-mode trio (status-popover control → /amicode/solver-mode engine route → extension handshake + restart + banner) — the pattern to mirror
• The harness registry module and the amicode.selectHarness palette command (the seams this feature fronts)
• ADR-0011 — the Harness Contract and the descriptor registry (availability, entitlement, resolve semantics)
• The composer's existing select slots (agent/model/variant) — the UI pattern and test coverage to follow
• The solver-mode engine route — the route/handshake shape to clone

Source

Aaron's ask, 2026-09-24 ("i really want a harness switcher in the chat box"), in the re-land campaign (sessions/session-20260924-research-engine-reland.md).