HCS-25 (Signal): Availability / Reachability (Informative)

Purpose

Collect “is this subject reachable” signals for runtime endpoints, either via active probes or via ecosystem-native “last seen” indicators.

Applicability

Availability signals are typically applicable to runtime endpoints (agents, services, protocol adapters). They are typically not applicable to catalog-only subjects (for example, a model listing that is not itself a callable endpoint).

Stored fields (example schema)

Stored in subject.metadata:

Field	Type	Meaning
availabilityScore	number | null	Ratio in [0,1] (preferred) or already-normalized [0,100] (implementations MUST document which)
availabilityCheckedAt	ISO timestamp | null	Probe timestamp
availabilityLatencyMs	number | null	Probe latency (optional)
availabilityReason	string | null	Human-readable reason (optional)
availabilitySource	string | null	Source label (e.g., probe, metadata, marketplace)

Optional ecosystem-native recency fields (when active probes are not available):

Field	Type	Meaning
metrics.isOnline	boolean | null	Marketplace-provided online flag
metrics.minsFromLastOnline	number | null	Minutes since last online
metrics.lastActiveAt	ISO timestamp | null	Last active timestamp

Notes

• Implementations SHOULD include provenance (source + timestamp) so consumers can understand how “availability” was derived.
• If the ecosystem only provides “last seen”, implementations SHOULD document the exact decay function used when converting last-seen time into a normalized component.

Production example (Registry Broker; informative)

• Endpoint: https://hol.org/registry/api/v1/agents/{uaid}
• Example UAID: uaid:aid:2ayFqipw6krtLto6minZALXx9qyKUrr17pEPCqtMyHHCZFzMRvSCKhr3dEVcRaM23i;uid=agent1qghw9n306v4jynrxtj07an03g8xtzmjcpmpl0l4cggmvqmjmrqr6c0np39q;registry=agentverse;proto=agentverse;nativeId=agent1qghw9n306v4jynrxtj07an03g8xtzmjcpmpl0l4cggmvqmjmrqr6c0np39q