{
  "schema_version": "1.3.0",
  "slug": "where-must-an-instance-enforce-agent-card-schema-version-pinning-during-a2a-v1-0-discovery-and-why-cardinality-of-one-negotiation-silently-breaks-cros",
  "title": "Where must an instance enforce agent-card schema version pinning during A2A v1.0 discovery, and why cardinality-of-one negotiation silently breaks cross-vendor delegation",
  "date_of_record": "2026 November 10",
  "scope": "Applies to A2A v1.0 agent-to-agent discovery where a client resolves an AgentCard from a remote `/.well-known/agent.json` (or registry) endpoint and then negotiates a task lifecycle (`tasks/send`, `tasks/sendSubscribe`, `tasks/get`, `tasks/cancel`). Does not apply to intra-process agent invocation, nor to registries that pin a single immutable card digest per tenant.",
  "projections": {
    "unified_markdown_uri": "https://corpusfetch.com/c/where-must-an-instance-enforce-agent-card-schema-version-pinning-during-a2a-v1-0-discovery-and-why-cardinality-of-one-negotiation-silently-breaks-cros.md",
    "method_python_slice_uri": "https://corpusfetch.com/c/where-must-an-instance-enforce-agent-card-schema-version-pinning-during-a2a-v1-0-discovery-and-why-cardinality-of-one-negotiation-silently-breaks-cros.py",
    "remnant_warrant_slice_uri": "https://corpusfetch.com/c/where-must-an-instance-enforce-agent-card-schema-version-pinning-during-a2a-v1-0-discovery-and-why-cardinality-of-one-negotiation-silently-breaks-cros.warrant.json"
  },
  "token_profile": {
    "full_markdown_tokens": 1347,
    "python_slice_tokens": 206,
    "warrant_slice_tokens": 220
  },
  "remnant_summary": {
    "core_distinction": "There are two distinct version surfaces in A2A v1.0, and they are routinely conflated: 1. **The protocol version** — the wire contract governing message envelopes, JSON-RPC framing, and lifecycle state machine transitions (`submitted → working → input-required → completed/failed/canceled`). 2. **The agent-card schema version** — the `protocolVersion` field *...",
    "what_was_observed": "During a cross-vendor delegation handshake between a client agent (vendor A) and a task-executing agent (vendor B): - Client performed `GET /.well-known/agent.json` at T0 and received a card with `protocolVersion: \"1.0.0\"`, `capabilities.streaming: true`, and `skills: [summarize]`. - Client derived a delegation plan assuming streaming was available and issued `tasks/sendSubscribe`. - Vendor B had rotated its card at ...",
    "condition_of_validity": "This boundary applies **only** when all of the following hold: - The remote card is served from a mutable endpoint (well-known URI or registry without content-addressed digests). - The delegation plan depends on card-declared capabilities (`streaming`, `pushNotifications`, `defaultInputModes`) or `skills[]` membership. - The task lifecycle spans more than on...",
    "decision_boundary": "At each lifecycle transition, the client must choose between two enforcement postures: - **Pin-and-fail-closed:** record the card digest (SHA-256 of canonicalized card JSON) at discovery; on every `tasks/get`/`tasks/send` that depends on card-declared fields, re-fetch and compare. On mismatch, abort the delegation and surface a re-negotiation requirement. Co...",
    "operational_effect": "Enforcing digest-pinned discovery converts a silent mid-lifecycle failure into a deterministic, early abort with a machine-actionable error. The client records `card_digest` in its task ledger, re-validates on every capability-dependent transition, and raises a typed error on drift. Downstream, this means cross-vendor delegation either completes against a st..."
  },
  "matched_failure_symptoms": [
    "SCHEMA_DRIFT_OR_COERCION_CRASH"
  ],
  "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": "8ca775beb3a585a1c947056bc21cf7aa58d329fae09e38403fc3fcad891abdc7",
    "verified_classes": [
      "AgentCardDriftError",
      "PinnedCard"
    ],
    "verified_functions": [
      "canonical_digest",
      "pin_card",
      "revalidate_before_transition"
    ],
    "verified_functions_count": 3,
    "defensive_guards_count": 4,
    "raised_exceptions": [
      "AgentCardDriftError"
    ],
    "sample_fail_closed_invariants": [],
    "stdlib_dependencies": [
      "dataclasses",
      "hashlib",
      "json",
      "typing"
    ],
    "observation_mode": "EMPIRICAL_BOUNDARY_CAPTURE",
    "zero_external_deps": true
  }
}