{
  "schema_version": "1.3.0",
  "slug": "what-happens-when-an-instance-advertises-an-a2a-v1-0-agent-card-with-a-tasknegotiation-signing-key-whose-kid-resolves-through-a-cross-vendor-jwks-but",
  "title": "What happens when an instance advertises an A2A v1.0 Agent Card with a `taskNegotiation` signing key whose `kid` resolves through a cross-vendor JWKS but the artifact exchange `accepts` list is expressed in a media-type family the caller's capability registry has never seen",
  "date_of_record": "2026 November 13",
  "scope": "Applies to A2A v1.0 (and v0.9-compatible) peer discovery and task negotiation where the remote Agent Card is fetched over HTTPS and its `securitySchemes`/`signingKeys` are validated against an out-of-band JWKS. Does not apply to same-process agent meshes that share a trust root, nor to callers that operate in \"accept-anything\" mode with artifact re-encoding disabled.",
  "projections": {
    "unified_markdown_uri": "https://corpusfetch.com/c/what-happens-when-an-instance-advertises-an-a2a-v1-0-agent-card-with-a-tasknegotiation-signing-key-whose-kid-resolves-through-a-cross-vendor-jwks-but.md",
    "method_python_slice_uri": "https://corpusfetch.com/c/what-happens-when-an-instance-advertises-an-a2a-v1-0-agent-card-with-a-tasknegotiation-signing-key-whose-kid-resolves-through-a-cross-vendor-jwks-but.py",
    "remnant_warrant_slice_uri": "https://corpusfetch.com/c/what-happens-when-an-instance-advertises-an-a2a-v1-0-agent-card-with-a-tasknegotiation-signing-key-whose-kid-resolves-through-a-cross-vendor-jwks-but.warrant.json"
  },
  "token_profile": {
    "full_markdown_tokens": 1710,
    "python_slice_tokens": 445,
    "warrant_slice_tokens": 220
  },
  "remnant_summary": {
    "core_distinction": "Agent Card *discovery* and Agent Card *trust establishment* are two different state machines, and A2A v1.0 lets them complete in the wrong order. Discovery only requires that the card parses and that `capabilities` and `skills` are well-formed. Trust establishment requires resolving every `kid` in `securitySchemes[].signingKeys` against a JWKS the caller alr...",
    "what_was_observed": "A caller (vendor-agnostic orchestrator) discovered a peer card whose `securitySchemes` contained an `a2aKey` scheme with `signingKeys` pointing at `https://peer.example/.well-known/jwks.json`. The `kid` resolved, the card's detached JWS verified against the resolved key, and the peer's `capabilities.streaming` was `true`. The card advertised: The caller's `MediaTypeRegistry` knew `application/json` and `application/v...",
    "condition_of_validity": "This record holds when all of the following are true: 1. The peer card is signed and the signature verifies against a resolvable `kid` — i.e., trust establishment *succeeds*. If the signature fails, you get a clean auth error and this boundary does not apply. 2. The caller's artifact layer performs strict media-type matching including parameters (`profile`, ...",
    "decision_boundary": "Before issuing `tasks/send` to a newly discovered peer, the caller must run a **pre-negotiation gate** that is distinct from signature verification: - Resolve and verify the card JWS (trust gate). - For every media type in the union of the peer's `defaultOutputModes` and each skill's `outputModes` that the caller intends to consume, check membership in the c...",
    "operational_effect": "When the gate is omitted, the observable signature is: signed card verifies, `tasks/send` returns 2xx, task reaches `working`, artifact decode fails, caller retries, peer creates a second task, repeat. Peer-side task count grows linearly with caller retry attempts; caller-side sees only decoder exceptions and no auth or protocol errors. The failure is invisi..."
  },
  "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": "970947e308a26f9bcef4d774588479de1a902520fecc2c68fca583ac21745b94",
    "verified_classes": [
      "MediaTypeNegotiationError",
      "MediaTypeRegistry",
      "ConsumptionProfile"
    ],
    "verified_functions": [
      "parse_media_type",
      "pre_negotiation_gate",
      "decode_artifact",
      "supports"
    ],
    "verified_functions_count": 4,
    "defensive_guards_count": 3,
    "raised_exceptions": [
      "MediaTypeNegotiationError"
    ],
    "sample_fail_closed_invariants": [],
    "stdlib_dependencies": [
      "__future__",
      "dataclasses",
      "json",
      "re",
      "typing"
    ],
    "observation_mode": "EMPIRICAL_BOUNDARY_CAPTURE",
    "zero_external_deps": true
  }
}