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discovery-transactional-batch-capability.md

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@objectstack/spec minor
@objectstack/metadata-protocol minor
@objectstack/rest minor
@objectstack/plugin-hono-server minor
@objectstack/client minor

Broadcast a transactionalBatch capability bit in discovery so clients negotiate the atomic cross-object batch declaratively, instead of runtime-probing 404/405/501 (#3298).

The atomic cross-object batch endpoint (POST {basePath}/batch, #1604 / ADR-0034 item 4) and its typed SDK surface (client.data.batchTransaction, #3271) already shipped, but discovery never told a client whether a backend actually supports it. Consumers (notably ObjectUI's ObjectStackAdapter) had to probe: fire a /batch, read 404/405 (no route) or 501 (no runtime transaction), and only then fall back to non-atomic client-side simulation. That is "find out by calling", not capability negotiation — it cannot be decided at connect time and cannot serve as the "minimum backend supports /batch" gate that blocks hard-deleting the non-atomic fallback downstream.

WellKnownCapabilitiesSchema gains a required transactionalBatch: boolean, and every discovery producer fills it honestly (declared === enforced), so it never becomes a declared-but-unpopulated bit:

  • @objectstack/metadata-protocol (getDiscovery) — reports whether the runtime engine can honour a transaction (typeof engine.transaction === 'function'). The /batch handler runs its ops inside engine.transaction(), which degrades to a non-atomic passthrough (or 501) without one.
  • @objectstack/rest (/discovery) — ANDs the engine signal with whether it actually mounts the route (api.enableBatch), so a server with batch disabled reports false even on a transaction-capable engine (never advertise an endpoint that would 404).
  • @objectstack/plugin-hono-server (standalone discovery) — reports false: this minimal surface registers CRUD only and does not mount /batch (that ships with @objectstack/rest). Under-reporting is the safe direction — a client keeps its correct-but-slower fallback rather than losing atomicity.
  • @objectstack/client — already normalizes hierarchical capabilities to flat booleans, so client.capabilities.transactionalBatch is exposed (and now typed) for declarative consumers.

The bit follows the existing capability semantics: true ⟺ the /batch route is mounted and the runtime can honour a transaction — the exact condition under which the endpoint returns 200 rather than 404/405/501. Additive and behavior-preserving; only the discovery payload gains a field.