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HCS-21: Package Declaration Registry

The HCS-21 module in the Standards SDK lets you publish package declarations (npm, PyPI, OCI, Composer, CLI plugins, etc.) straight to Hedera Consensus Service. Every declaration pairs a registry namespace with the package’s HCS-2 topic ID (t_id) plus a short human summary (n, d, a, tags). Optional HCS-1 metadata pointers let downstream systems (Registry Broker, HPM, compliance scanners) hydrate richer manifests without trusting centralized APIs.

What’s Included​

  • HCS21Client (Node.js) — creates registry topics, inscribes metadata, publishes declarations, and streams them back using mirror nodes.
  • HCS21BrowserClient (WalletConnect) — enables dApps to publish declarations directly from a user’s wallet without sharing private keys.
  • Transaction helpers — buildHcs21CreateRegistryTx and buildHcs21MessageTx let you embed HCS-21 flows into custom transaction managers.
  • Typed schemas — PackageDeclaration, PackageMetadataRecord, and validation helpers keep payloads inside the 1 KB limit and aligned with HCS-21.

Key Capabilities​

  • Multi-registry namespace support (npm, pypi, oci, composer, cargo, etc.).
  • Strict metadata pointers (hcs://1/<topic>/<sequence>) with optional t_id for HPM-style version feeds.
  • Mirror-node streaming helpers that automatically filter hcs-21 payloads.
  • Works with submit keys, HIP-991 fee gates, or public topics.

Where to Next​

  • Server SDK — instantiate HCS21Client, inscribe metadata, and publish declarations from Node.js services.
  • Browser SDK — wire HCS-21 into wallet-connected front-ends.
  • Transaction Helpers — build custom workflows using low-level builders.
  • API Reference — quick lookup for available types and classes.