MCP _meta channels

Normative conventions for ToolResult._meta on gbrain's MCP surfaces (WP2 amendment 9 / decision D3). _meta is the structured, out-of-band channel for tool-call responses; the response BODY contract never changes shape for it.

Rules

1. One producer per top-level key. A producer owns exactly one namespaced key and never writes another producer's key. The dispatch layer (src/mcp/dispatch.ts) merges per top-level key — never wholesale _meta assignment.
2. Additive-forever within a key. Fields inside a key may be added, never renamed or removed — the RESPONSE_SCHEMAS discipline applied to _meta. Consumers must tolerate unknown fields.
3. Producer isolation. Every producer attaches inside its own try/catch. A failing producer degrades to its key being absent; it never drops another producer's key and never errors the tool call.
4. Merge precedence. Handler-emitted keys (via OperationContext.emitResponseMeta) attach first; transport hooks (metaHook) attach after and may add keys but shadow nothing that matters — key ownership (rule 1) makes ordering a non-event.
5. Model visibility caveat. Mainstream harnesses do NOT feed _meta to the model. Anything the model must SEE rides a content block (see the D8 second text block on empty retrievals); _meta serves structured programmatic consumers (thin clients, harness plumbing, tests).

Registered keys

Key	Producer	Contents
brain_hot_memory	serve-http metaHook (getBrainHotMemoryMeta)	Hot-memory facts relevant to the call (v0.31 eD3)
retrieval	search/query op handlers	returned_count, retrieved_count, vector_enabled, expansion_applied, cache, token_budget, degraded[] (closed stage vocabulary, D6), hint (non-contractual prose, E1)
warnings	dispatch strict-params warn mode (WP3)	[{code: 'unknown_param', param, suggestion?}]

Inbound _meta (e.g. _meta.session_id inside tool ARGUMENTS, CX2-11) is a separate, client-to-server plane. The eval-report _meta.metric_glossary lives in JSON BODIES of eval commands — a third, unrelated plane. Ambient recall (#4028) rides content/hooks, not _meta.

Adding a key: register it in the table above, one producer, additive-forever.