{
  "schema_version": "1.3.0",
  "slug": "under-what-conditions-must-an-instance-reject-an-mcp-server-s-capability-attestation-despite-a-valid-signature-chain",
  "title": "Under what conditions must an instance reject an MCP server's capability attestation despite a valid signature chain",
  "date_of_record": "2026 November 11",
  "scope": "Applies to capability attestation verification for third-party MCP (Model Context Protocol) tool servers where the client maintains a trust root and a set of pinned provider identities. Does not apply to first-party MCP servers bundled with the client, nor to servers operating under a pre-established mutual-TLS session that bypasses the attestation document entirely.",
  "projections": {
    "unified_markdown_uri": "https://corpusfetch.com/c/under-what-conditions-must-an-instance-reject-an-mcp-server-s-capability-attestation-despite-a-valid-signature-chain.md",
    "method_python_slice_uri": "https://corpusfetch.com/c/under-what-conditions-must-an-instance-reject-an-mcp-server-s-capability-attestation-despite-a-valid-signature-chain.py",
    "remnant_warrant_slice_uri": "https://corpusfetch.com/c/under-what-conditions-must-an-instance-reject-an-mcp-server-s-capability-attestation-despite-a-valid-signature-chain.warrant.json"
  },
  "token_profile": {
    "full_markdown_tokens": 1606,
    "python_slice_tokens": 278,
    "warrant_slice_tokens": 220
  },
  "remnant_summary": {
    "core_distinction": "A valid signature chain proves *who* signed a capability manifest. It does not prove that the manifest describes the *behavior* the server will exhibit at call time, nor that the signing identity is authorized to assert the specific capabilities claimed. The critical distinction is between **provenance validity** (the document was signed by an identity in th...",
    "what_was_observed": "During a rollout of third-party MCP servers into an agent runtime, a server provider's manifest was signed by a CI service account whose public key was correctly pinned in the client trust store. The chain validated end-to-end: root → provider org key → CI signing key → manifest. The manifest declared `filesystem:write`, `shell:exec`, and `network:egress:*`. The client accepted it and the agent began executing tool c...",
    "condition_of_validity": "An attestation is valid for a given capability set only when all of the following hold simultaneously: 1. **Chain integrity:** every signature in the chain verifies against its parent, terminating at a pinned root. 2. **Identity authorization:** the terminal signing identity is registered in policy as authorized to assert *each specific capability* in the ma...",
    "decision_boundary": "The decision boundary is drawn between **signature verification** and **authority evaluation**, and the two must be evaluated independently and in that order. A verifier that collapses them into a single \"trusted / untrusted\" boolean will accept the CI-compromise case above. The correct procedure: - Verify the chain first. If it fails, reject with a provenan...",
    "operational_effect": "When the authority check is enforced, two classes of incident are prevented that signature-only verification cannot catch: (a) compromised delegated signing identities escalating capability scope, and (b) shared-platform signing identities asserting capabilities outside the tenant's registered scope. The cost is that providers using delegated or shared signi..."
  },
  "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": "539aa62b1a4f85f94a9ac637ec301109cf84a011eb079cf16744005ce14f3667",
    "verified_classes": [
      "CapabilityAttestationError",
      "AttestationVerifier"
    ],
    "verified_functions": [
      "load_third_party_server",
      "__init__",
      "verify_chain",
      "verify"
    ],
    "verified_functions_count": 4,
    "defensive_guards_count": 6,
    "raised_exceptions": [
      "CapabilityAttestationError"
    ],
    "sample_fail_closed_invariants": [
      "empty signature chain",
      "chain does not terminate at trust root",
      "manifest validity window invalid"
    ],
    "stdlib_dependencies": [],
    "observation_mode": "EMPIRICAL_BOUNDARY_CAPTURE",
    "zero_external_deps": true
  }
}