{
  "schema_version": "1.3.0",
  "slug": "where-must-an-instance-pin-the-a2a-agent-card-signature-jwk-kid-before-delegating-a-task-across-vendors",
  "title": "Where must an instance pin the A2A Agent Card `signature.jwk.kid` before delegating a task across vendors",
  "date_of_record": "2026 November 11",
  "scope": "Applies to A2A v1.0 JSON-RPC clients that resolve a remote Agent Card, verify its JWS signature, and then issue `tasks/send` or `tasks/sendSubscribe` calls to a different vendor than the one that signed the card. Does not apply to intra-tenant delegation where the card and the task endpoint share a trust root, nor to unsigned cards used only for capability discovery.",
  "projections": {
    "unified_markdown_uri": "https://corpusfetch.com/c/where-must-an-instance-pin-the-a2a-agent-card-signature-jwk-kid-before-delegating-a-task-across-vendors.md",
    "method_python_slice_uri": "https://corpusfetch.com/c/where-must-an-instance-pin-the-a2a-agent-card-signature-jwk-kid-before-delegating-a-task-across-vendors.py",
    "remnant_warrant_slice_uri": "https://corpusfetch.com/c/where-must-an-instance-pin-the-a2a-agent-card-signature-jwk-kid-before-delegating-a-task-across-vendors.warrant.json"
  },
  "token_profile": {
    "full_markdown_tokens": 1148,
    "python_slice_tokens": 233,
    "warrant_slice_tokens": 220
  },
  "remnant_summary": {
    "core_distinction": "The non-obvious failure is the difference between *card signature validity* and *endpoint authority*. A2A v1.0 lets an Agent Card be signed by an issuer whose `kid` is resolvable via the card's `signature.jwk` (or a JWKS reference) while the actual `tasks/send` endpoint — the `url` field — is operated by a different party. Signature verification proves the c...",
    "what_was_observed": "In a cross-vendor delegation, Vendor A published an Agent Card signed by its own key `kid=a-prod-2026-11`. The card's `url` pointed at a shared gateway operated by Vendor B (a reseller/integrator arrangement). The client verified the JWS against A's JWKS, confirmed `kid` matched, and proceeded. The gateway at B accepted the `tasks/send`, but the task was executed under B's agent runtime, which honored B's tool allowl...",
    "condition_of_validity": "The pinning invariant is only meaningful when all of the following hold: (1) the card's `signature` is present and the `kid` is unique within the issuer's JWKS at the moment of use; (2) the endpoint `url` is reachable and its TLS certificate SAN can be compared to the issuer identity; (3) the client has a policy mapping issuer identity to execution authority...",
    "decision_boundary": "Before issuing `tasks/send`, the client must decide: does the `kid` that signed the card correspond to the operator of `url`? If yes, delegate with full trust in the card's declared capabilities. If no (signer ≠ endpoint operator), the client must either (a) reject the delegation, or (b) enter a reduced-trust mode where the card is treated as a *capability a...",
    "operational_effect": "Clients that skip the binding step will, under reseller or gateway topologies, execute tasks with the wrong security context: the signer's declared `securitySchemes` are ignored in favor of the gateway's. This produces intermittent, hard-to-reproduce failures — the same card and `kid` succeed for one tool and fail for another depending on the gateway's allow..."
  },
  "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": "14b9bc1a83016d603e29dd60abbea32d32334e96d73c9d20bd9dcb9dbb7c9bbd",
    "verified_classes": [
      "AgentCardBindingError",
      "VerifiedCard"
    ],
    "verified_functions": [
      "assert_signer_bound_to_endpoint",
      "delegate_task"
    ],
    "verified_functions_count": 2,
    "defensive_guards_count": 1,
    "raised_exceptions": [
      "AgentCardBindingError"
    ],
    "sample_fail_closed_invariants": [],
    "stdlib_dependencies": [
      "dataclasses",
      "hashlib"
    ],
    "observation_mode": "EMPIRICAL_BOUNDARY_CAPTURE",
    "zero_external_deps": true
  }
}