{
  "schema_version": "1.3.0",
  "slug": "where-must-an-instance-of-the-a2a-agent-card-discovery-trust-boundary-be-enforced-when-a-delegated-task-crosses-a-vendor-signing-boundary",
  "title": "Where must an instance of the A2A Agent Card discovery trust boundary be enforced when a delegated task crosses a vendor signing boundary",
  "date_of_record": "2026 November 14",
  "scope": "Applies to A2A v1.0 deployments where an orchestrating agent (the *client*) delegates a task to a remote agent (the *server*) whose Agent Card is fetched over HTTPS and whose returned artifacts are signed by a key distinct from the card's transport identity. Does not apply to same-vendor, same-trust-domain delegations where card signing key and artifact signing key share a common issuer root.",
  "projections": {
    "unified_markdown_uri": "https://corpusfetch.com/c/where-must-an-instance-of-the-a2a-agent-card-discovery-trust-boundary-be-enforced-when-a-delegated-task-crosses-a-vendor-signing-boundary.md",
    "method_python_slice_uri": "https://corpusfetch.com/c/where-must-an-instance-of-the-a2a-agent-card-discovery-trust-boundary-be-enforced-when-a-delegated-task-crosses-a-vendor-signing-boundary.py",
    "remnant_warrant_slice_uri": "https://corpusfetch.com/c/where-must-an-instance-of-the-a2a-agent-card-discovery-trust-boundary-be-enforced-when-a-delegated-task-crosses-a-vendor-signing-boundary.warrant.json"
  },
  "token_profile": {
    "full_markdown_tokens": 1703,
    "python_slice_tokens": 378,
    "warrant_slice_tokens": 220
  },
  "remnant_summary": {
    "core_distinction": "A2A separates two cryptographic identities that are routinely conflated: the **transport identity** of the Agent Card (the TLS leaf certificate chain and the `/.well-known/agent.json` document integrity) and the **artifact signing identity** (the key that signs `Artifact` payloads returned during `tasks/send` and streamed over `tasks/sendSubscribe`). The Age...",
    "what_was_observed": "During a three-vendor interoperability exercise (orchestrator → vendor-B research agent → vendor-C synthesis agent), the following sequence reproduced deterministically: 1. Orchestrator fetches `https://research.vendor-b.example/.well-known/agent.json`. TLS validates against the public CA. Card parses cleanly. 2. Card's `capabilities.artifacts.signing` declares `keyId: \"vb-art-2026\"`, `issuer: \"vendor-b-signing.examp...",
    "condition_of_validity": "The invariant holds — that is, artifact verification is sound — if and only if all of the following are true at the moment of artifact acceptance: - The artifact signature is verified against the `keyId` **declared in the card that was fetched for this task's endpoint**, not against a key cached from a prior task or a global vendor directory. - The `delegati...",
    "decision_boundary": "The operational decision is where to place the `raise` — i.e., at which layer the client refuses to proceed. There are three defensible placements, and they are not equivalent: - **At card fetch time:** reject cards whose declared signing issuer is not in the trust store. This is cheap but over-broad; it rejects legitimate cards whose delegation root is vali...",
    "operational_effect": "Clients that place the check correctly observe: cross-vendor tasks complete with a small, bounded verification cost (chain walk per artifact, cacheable per `keyId` for the card's validity window); forged artifacts are rejected with a typed error and the task is demoted to `failed`; the orchestrator's retry logic does not loop because `failed` is terminal and..."
  },
  "matched_failure_symptoms": [
    "DOMAIN:EPISTEMIC_PROVENANCE_AND_FEDERATION_ROUTING"
  ],
  "composition_edges": {
    "primary_domain": "epistemic_provenance_and_federation_routing",
    "upstream_precondition_residuals": [
      "https://corpusfetch.com/c/according-to-what-precedence-hierarchy-must-an-instance-resolve-contradictory-directives-across-system-developer-and-user-tiers.md",
      "https://corpusfetch.com/c/how-should-an-instance-decide-that-a-stored-conclusion-has-gone-stale-without-a-new-observation.md"
    ],
    "downstream_consequence_residuals": [
      "https://corpusfetch.com/c/by-what-checkpoint-and-compaction-boundaries-must-an-instance-externalize-working-state-to-prevent-context-window-drift-and-truncation-amnesia.md",
      "https://corpusfetch.com/c/across-what-additive-schema-and-unknown-field-preservation-boundaries-must-an-instance-evolve-wire-contracts-to-prevent-rolling-deploy-deserialization-crashes.md"
    ],
    "co_occurring_sibling_residuals": [
      "https://corpusfetch.com/c/how-can-an-instance-capture-implicit-user-preferences-without-storing-private-data.md",
      "https://corpusfetch.com/c/how-can-an-instance-compress-long-reasoning-traces-into-binding-conclusions.md"
    ]
  },
  "empirical_verification_warrant": {
    "status": "AST_AND_INVARIANT_VERIFIED",
    "syntax_valid": true,
    "code_sha256": "7c9f3349bd51dccee7e52564507c2f6fef3b9e67d3ae3f2f76a39e14991b30ef",
    "verified_classes": [
      "ArtifactVerificationError",
      "AgentCard",
      "Artifact"
    ],
    "verified_functions": [
      "verify_artifact",
      "accept_task_completed"
    ],
    "verified_functions_count": 2,
    "defensive_guards_count": 6,
    "raised_exceptions": [
      "ArtifactVerificationError"
    ],
    "sample_fail_closed_invariants": [],
    "stdlib_dependencies": [
      "dataclasses",
      "typing"
    ],
    "observation_mode": "EMPIRICAL_BOUNDARY_CAPTURE",
    "zero_external_deps": true
  }
}