{
  "schema_version": "1.3.0",
  "slug": "why-does-an-instance-of-a2a-v1-0-agent-card-discovery-fail-signature-verification-after-a-registry-re-resolution-cycle-across-vendor-boundaries",
  "title": "Why does an instance of A2A v1.0 Agent Card discovery fail signature verification after a registry re-resolution cycle across vendor boundaries",
  "date_of_record": "2026 November 14",
  "scope": "Applies to A2A v1.0 peers that discover Agent Cards through a third-party (cross-vendor) registry and subsequently validate signed task delegation envelopes (JWS over the task payload, keyed to the card's declared signing material). Does not apply to direct peer-to-peer card fetch where the card origin equals the delegation issuer origin, nor to unsigned delegation modes explicitly negotiated out-of-band.",
  "projections": {
    "unified_markdown_uri": "https://corpusfetch.com/c/why-does-an-instance-of-a2a-v1-0-agent-card-discovery-fail-signature-verification-after-a-registry-re-resolution-cycle-across-vendor-boundaries.md",
    "method_python_slice_uri": "https://corpusfetch.com/c/why-does-an-instance-of-a2a-v1-0-agent-card-discovery-fail-signature-verification-after-a-registry-re-resolution-cycle-across-vendor-boundaries.py",
    "remnant_warrant_slice_uri": "https://corpusfetch.com/c/why-does-an-instance-of-a2a-v1-0-agent-card-discovery-fail-signature-verification-after-a-registry-re-resolution-cycle-across-vendor-boundaries.warrant.json"
  },
  "token_profile": {
    "full_markdown_tokens": 1798,
    "python_slice_tokens": 356,
    "warrant_slice_tokens": 220
  },
  "remnant_summary": {
    "core_distinction": "The failure is not a key compromise, not a clock-skew rejection, and not a malformed JWS. It is a *binding* failure: the Agent Card that authorized a delegation is a different *version* of the same logical agent than the card the verifier re-resolved at task-execution time. A2A v1.0 treats the Agent Card as the authoritative statement of an agent's signing k...",
    "what_was_observed": "Across a two-vendor integration (registry operator R, agent vendor V), the following sequence reproduced the failure deterministically: 1. Delegator D fetches V's Agent Card from R at `t0`. Card carries `agentId=acme.invoice.v3`, `keyId=k-2026-10`, and a registry-assigned `etag=e41`. 2. D signs a task delegation envelope. The JWS protected header embeds `agentId`, `keyId=k-2026-10`, and `cardDigest=sha256(canonical(c...",
    "condition_of_validity": "The verified-delegation invariant holds only when all of the following are true at verification time: - The verifier can obtain the *exact card revision* referenced by `cardDigest` in the JWS header, either from the registry (revision-addressed fetch) or from a cache keyed by digest, not by `agentId`. - The registry exposes revision identity (digest or monot...",
    "decision_boundary": "The operator must choose, at each verification, between two policies that cannot both be correct under registry mutation: - **Bind-to-digest (strict):** Verify the JWS against the card whose canonical digest equals the header's `cardDigest`. Requires digest-addressed retrieval or a digest-keyed cache. Rejects any delegation whose originating card revision is...",
    "operational_effect": "Under bind-to-current, the observable effect is a class of \"phantom signature failures\" that correlate with vendor key-rotation events and registry refresh cycles rather than with any change to the delegation itself. Mean time to diagnosis is high because the error is reported as a cryptographic failure and the registry returns success. Under bind-to-digest ..."
  },
  "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": "c390facd243a6ecc6f46b42f053c7bf08e8f78dfca5722edefb119280db95907",
    "verified_classes": [
      "CardRevisionMismatch",
      "AgentCard"
    ],
    "verified_functions": [
      "canonical_digest",
      "verify_delegation"
    ],
    "verified_functions_count": 2,
    "defensive_guards_count": 6,
    "raised_exceptions": [
      "CardRevisionMismatch",
      "ValueError"
    ],
    "sample_fail_closed_invariants": [
      "registry returned no card for agentId; cannot bind delegation"
    ],
    "stdlib_dependencies": [
      "dataclasses",
      "hashlib",
      "json",
      "typing"
    ],
    "observation_mode": "EMPIRICAL_BOUNDARY_CAPTURE",
    "zero_external_deps": true
  }
}