v0.3.0 — A2A AgentCard + DNS TXT, four-language parity

[0.3.0] - 2026-05-14

Added

Four-language parity (Rust, JavaScript, Python, Go) for the A2A AgentCard
extension surface and DNS TXT cross-verification. Cards signed in any of the
four SDKs verify cleanly in the other three; signature canonicalisation is
byte-identical across implementations.

A2A AgentCard extension types, signed builder, and verifier

• A2aAgentCard + supporting types — minimal A2A AgentCard subset
  (A2aAgentCard, A2aAgentCapabilities, A2aAgentSkill) plus the
  AgentPin-specific AgentpinExtension payload (agentpin_endpoint,
  public_key_jwk, signature). Inline definition rather than depending on
  the upstream a2a-types crate while the A2A spec is still draft.
• A2aAgentCardBuilder (Rust) / buildAndSignAgentCard (JS) /
  build_and_sign_agent_card (Python) / a2a.BuildAndSignAgentCard (Go) —
  turns an AgentDeclaration into a signed A2aAgentCard. Maps capabilities
  to skills via capability_to_skill; propagates allowed_domains from the
  source constraints into A2aAgentCapabilities.allowed_domains. Detached
  ECDSA P-256 signature covers the canonical bytes of the AgentCard with the
  extension cleared.
• verify_agentpin_extension(card) — verifies the AgentPin extension
  signature against the JWK embedded in the extension. Sorted-key canonical
  JSON shared across all four SDKs; mirrors the canonicalisation pattern
  used by SchemaPin.
• AllowedDomains typed wrapper — extracted from
  Constraints::allowed_domains and exposed for cross-protocol use
  (SchemaPin v1.4's A2aVerificationContext scopes tool verification to the
  intersection of caller and provider domains). Empty list = no restriction
  (all domains trusted) by convention; intersect(unrestricted, X) = X.
• a2a_endpoint field on DiscoveryDocument — optional URL of the
  entity's A2A AgentCard endpoint, enabling cross-protocol discovery.

Discovery resolvers for A2A AgentCards

• LocalAgentCardStore (all SDKs) — in-memory store of pre-registered
  AgentCards keyed by their AgentPin discovery domain. Verifies the
  AgentPin extension signature at registration time and pre-derives a
  DiscoveryDocument so the rest of the AgentPin verification stack runs
  unchanged. Supports Symbiont's push-based external-agent registration
  flow where the coordinator receives AgentCard JSON inline rather than
  fetching it from a .well-known endpoint.
• A2aAgentCardResolver (all SDKs; gated on fetch in Rust) — fetches
  https://{domain}/.well-known/agent-card.json, verifies the AgentPin
  extension, cross-checks that the embedded agentpin_endpoint host
  matches the fetched domain, and derives a DiscoveryDocument. Exposes
  the original A2A representation alongside the derived doc for callers
  that want both.

DNS TXT cross-verification at _agentpin.{domain}

• dns module (all SDKs) — parse_txt_record, verify_dns_match,
  txt_record_name. Wire format mirrors SchemaPin's _schemapin.{domain}
  record with the version tag changed:
  "v=agentpin1; kid=...; fp=sha256:<hex>". Whitespace-tolerant parser,
  case-insensitive on fp, ignores unknown fields for forward
  compatibility.
• fetch_dns_txt(domain) — Rust: async lookup behind the new dns
  Cargo feature (hickory-resolver, tokio, async-trait). JavaScript:
  uses Node's built-in dns/promises. Python: uses the optional
  dnspython package. Go: dns.LookupTxt(ctx, resolver, domain) over the
  standard-library net.Resolver.
• Multi-key match semantics — AgentPin discovery docs may carry several
  keys for rotation; a published TXT record need only match one of them.
  When the TXT carries an explicit kid, the matching key MUST also carry
  the same kid.
• Fail-closed on mismatch — a publisher who intentionally publishes a
  TXT record has signaled DNS is part of their trust chain. Divergence
  between DNS and .well-known indicates compromise of one channel and is
  treated as a hard failure.

Go SDK (fourth-language port)

• Initial go/ SDK — wire-compatible with Rust, JavaScript, and Python.
  Mirrors the package layout of the SchemaPin Go SDK. Module path:
  github.com/ThirdKeyAi/agentpin/go.
• Packages: crypto, jwk, jwt, types, discovery, credential,
  verification, revocation, pinning, delegation, mutual, nonce,
  bundle, resolver, plus the new a2a and dns packages added in this
  release.
• CLI: cmd/agentpin with keygen, issue, verify, bundle
  subcommands matching the Rust binary.
• ES256-only enforcement implemented inline using crypto/ecdsa. The
  JWT verifier rejects none, HS256, RS256, ES384, and any other
  algorithm before any signature work. No third-party JWT dependency.
• CI: .github/workflows/go.yml runs go test, go vet, and
  gofmt -l on every PR touching go/**. The version-consistency check
  in .github/workflows/release.yml validates the Go SDK's declared
  version against the Rust/JavaScript/Python versions.

Changed

• Cross-SDK version coordination — Rust, JavaScript, Python, and Go SDKs
  all release as 0.3.0 together. The earlier 0.3.0-alpha.1 Rust
  preview is superseded by this entry.
• Rust MSRV bumped from 1.70 to 1.86. Downstream ecosystem crates
  (getrandom, clap_builder, the icu_* family) have moved to edition
  2024 and/or to declared rust-version 1.86, making the previously-
  declared 1.70 floor unbuildable from scratch in practice. The CI
  matrix's MSRV row now tests against 1.86.

Notes

• This release is the unblock for Symbiont v1.8.0 Phase 3 (AgentPin-
  verified AgentCards, A2A auth middleware) and SchemaPin v1.4.0's
  A2aVerificationContext (which consumes AllowedDomains for tool-
  verification scoping). Both downstream releases were waiting on this
  surface.
• DNS TXT defends against HTTPS-origin compromise (compromised hosting
  account, expired domain not removed from CDN, ACME ownership-validation
  bypass) and TLS cert mis-issuance — the DNS credential chain (registrar,
  DNS provider, optionally DNSSEC) is independent of the HTTPS hosting
  chain. Spec § 4.8.3 reserved this slot in v0.1; this release ships the
  implementation across all SDKs.
• All additions are purely additive — v0.2.0 callers are unaffected.
  Discovery documents without a2a_endpoint, AgentCards without an
  agentpin extension, and absent _agentpin TXT records all behave
  exactly as before.