Status: Stable · v2.0 · RFC 0144. Normative contract for the advertised
host.*service surfaces a node pack invokes throughctx.Normative home:
aiEnvelope,promptLibrary,agentRuntime,mcp.
Why this exists
spec/v1/host-capabilities.md describes these three surfaces but states no RFC 2119 obligation; the contract a host takes on by advertising them is written here.
aiEnvelope
A host advertising aiEnvelope MUST expose ctx.aiEnvelope.generate to pack code, and MUST refuse a node whose typeId requires the family when it does not advertise it. It MUST expose ctx.aiEnvelope.await when, and only when, it advertises aiEnvelope.await.
promptLibrary
A host advertising promptLibrary MUST expose ctx.promptLibrary.get, MUST return a pinned version verbatim so that a replayed run resolves the same template, and MUST fail the calling node rather than substitute an unpinned one.
agentRuntime
A host advertising agentRuntime MUST expose spawn, delegate, consensus and messageSend, and MUST satisfy agents.manifestRuntime, which advertising it implies.
mcp
A host advertising mcp.client MUST expose to pack code ctx.mcp.callTool, listTools, readResource and serverHealth, each against a host-configured serverId at the revision mcp negotiates. Each rejects only for an unknown serverId (not_found), an MCP error response (carried unaltered), or a transport failure. callTool MUST resolve to the server's CallToolResult unaltered (content[], structuredContent, isError, _meta), including when isError is true; the host handles an InputRequiredResult itself and never returns one. listTools MUST resolve to one ListToolsResult page unaltered, outputSchema, annotations, nextCursor, ttlMs and cacheScope included, and MUST forward a pack's cursor; readResource resolves to the ReadResourceResult unaltered. serverHealth MUST report reachable, unreachable or incompatible from a server/discover probe no older than its ttlMs, with the DiscoverResult when one was received; it MUST NOT report a connection or session state, which MCP 2026-07-28 does not have.