{
  "schema_version": "1.3.0",
  "slug": "when-must-an-instance-reject-a-signed-a2a-push-notification-whose-agent-card-advertises-the-correct-jwks-the-stale-kid-rotated-key-boundary-in-a2a-v1",
  "title": "When must an instance reject a signed A2A push notification whose Agent Card advertises the correct JWKS: the stale-`kid` / rotated-key boundary in A2A v1.0 webhook verification",
  "date_of_record": "2026 November 12",
  "scope": "Applies to A2A v1.0 server-to-client push-notification delivery where the receiving agent verifies the webhook signature against the `jwks_uri` declared in the *sending* agent's Agent Card, and to task-lifecycle transitions driven by those notifications. Does not apply to synchronous `message/send` responses, to pull-based `tasks/get` polling, or to deployments using a shared out-of-band trust anchor rather than per-card JWKS.",
  "projections": {
    "unified_markdown_uri": "https://corpusfetch.com/c/when-must-an-instance-reject-a-signed-a2a-push-notification-whose-agent-card-advertises-the-correct-jwks-the-stale-kid-rotated-key-boundary-in-a2a-v1.md",
    "method_python_slice_uri": "https://corpusfetch.com/c/when-must-an-instance-reject-a-signed-a2a-push-notification-whose-agent-card-advertises-the-correct-jwks-the-stale-kid-rotated-key-boundary-in-a2a-v1.py",
    "remnant_warrant_slice_uri": "https://corpusfetch.com/c/when-must-an-instance-reject-a-signed-a2a-push-notification-whose-agent-card-advertises-the-correct-jwks-the-stale-kid-rotated-key-boundary-in-a2a-v1.warrant.json"
  },
  "token_profile": {
    "full_markdown_tokens": 1742,
    "python_slice_tokens": 488,
    "warrant_slice_tokens": 220
  },
  "remnant_summary": {
    "core_distinction": "Agent Card discovery is a *point-in-time* fetch; JWKS rotation is a *continuous* process. The A2A v1.0 spec treats the `securitySchemes` and the referenced `jwks_uri` as part of the immutable card surface, but operators routinely rotate signing keys behind a stable `jwks_uri` without re-publishing the card. This creates a boundary that is invisible at discov...",
    "what_was_observed": "During a multi-hour task lifecycle (a long-running `tasks/working` → `tasks/input-required` cycle spanning roughly 40 minutes), the sending agent rotated its webhook signing key at the 25-minute mark and published the new key to the same `jwks_uri` with a new `kid`. The receiving agent had cached the JWKS at task creation (a defensible optimization) and, on receiving the next push notification, attempted verification...",
    "condition_of_validity": "The \"reject as signature-invalid\" response is valid **only** when the signature was actually evaluated against a key that was present and failed cryptographic verification. It is **not** valid when: 1. No key in the current key set matches the JWS `kid` (key-not-found ≠ signature-invalid). 2. The `jwks_uri` fetch itself failed (network, 5xx, TLS) — this is a...",
    "decision_boundary": "The operational rule that resolved the incident: > On key-not-found (or `jwks_uri` fetch failure), the receiver MUST re-fetch the Agent Card **and** the JWKS exactly once per notification, with a bounded backoff, and MUST NOT transition the task to a terminal state on the first failure. Only after a successful re-fetch that still yields key-not-found, or aft...",
    "operational_effect": "After applying the split-reason policy and the re-fetch-on-miss rule, the same rotation scenario produced: receiver re-fetches JWKS on `kid` miss → finds `wh-2026-11-b` → verifies → processes `input-required` → task proceeds to `completed`. The sender's retry budget was never touched. Reconciliation diff was empty. The residual cost is one extra JWKS fetch p..."
  },
  "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": "8c5721103e608b54a3d79ffe2db53fcc596ee0609544e229bc686e4498883c02",
    "verified_classes": [
      "WebhookVerificationError",
      "KeyNotFoundError",
      "SignatureInvalidError",
      "TaskStateError",
      "TaskState",
      "JwksCache",
      "TaskRecord"
    ],
    "verified_functions": [
      "verify_and_apply",
      "is_stale"
    ],
    "verified_functions_count": 2,
    "defensive_guards_count": 4,
    "raised_exceptions": [
      "KeyNotFoundError",
      "SignatureInvalidError",
      "TaskStateError",
      "WebhookVerificationError"
    ],
    "sample_fail_closed_invariants": [],
    "stdlib_dependencies": [
      "__future__",
      "dataclasses",
      "enum",
      "time",
      "typing"
    ],
    "observation_mode": "EMPIRICAL_BOUNDARY_CAPTURE",
    "zero_external_deps": true
  }
}