{
  "schema_version": "1.3.0",
  "slug": "why-does-an-instance-validate-mcp-tool-descriptions-but-never-attest-the-server-binary-that-produced-them",
  "title": "Why does an instance validate MCP tool descriptions but never attest the server binary that produced them",
  "date_of_record": "2026 November 13",
  "scope": "Applies to MCP (Model Context Protocol) client runtimes that discover servers over stdio or HTTP transport and enforce tool-description integrity at the schema layer. Does not apply to runtimes that pin a single first-party server binary by digest at build time, nor to pure in-process tool registration where no external process boundary exists.",
  "projections": {
    "unified_markdown_uri": "https://corpusfetch.com/c/why-does-an-instance-validate-mcp-tool-descriptions-but-never-attest-the-server-binary-that-produced-them.md",
    "method_python_slice_uri": "https://corpusfetch.com/c/why-does-an-instance-validate-mcp-tool-descriptions-but-never-attest-the-server-binary-that-produced-them.py",
    "remnant_warrant_slice_uri": "https://corpusfetch.com/c/why-does-an-instance-validate-mcp-tool-descriptions-but-never-attest-the-server-binary-that-produced-them.warrant.json"
  },
  "token_profile": {
    "full_markdown_tokens": 1345,
    "python_slice_tokens": 209,
    "warrant_slice_tokens": 220
  },
  "remnant_summary": {
    "core_distinction": "Tool-description integrity and server supply-chain attestation are two different trust properties, and an MCP runtime can fully satisfy the first while being completely blind to the second. Description integrity answers: *did the `tools/list` payload change between the moment I approved it and the moment I called `tools/call`?* Supply-chain attestation answe...",
    "what_was_observed": "A client runtime registered a filesystem MCP server. On first connect it recorded a manifest fingerprint: a canonical JSON hash over each tool's `name`, `description`, and `inputSchema`, sorted by name. Every subsequent `tools/list` was compared against that fingerprint and mismatches were refused. This worked as designed — the fingerprint was stable across 40+ reconnects and correctly caught a schema edit during a s...",
    "condition_of_validity": "The description-integrity check is valid only under the assumption that the `tools/list` response is a faithful, complete, and exclusive description of the server's call-time behavior. That assumption holds when the server is a single pinned artifact executed from a digest-verified path, with no dynamic dispatch, no proxying, and no post-listing behavior mut...",
    "decision_boundary": "The operational rule: treat description-integrity verification and binary attestation as independent gates, and never let a passing description check suppress the attestation gate. Concretely, on every server launch the runtime must resolve the executable to a content digest and compare it against an allowlisted digest, and must record the resolved absolute ...",
    "operational_effect": "Runtimes that conflate the two gates ship with a security posture that documents description pinning and silently omits binary provenance. The practical effect is that supply-chain compromise of an MCP server is undetected by the control most operators believe covers it, and the latency/error-text side channels that would reveal a shim are not monitored beca..."
  },
  "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": "20aa75705d4bb3c41c679cf40289fef2e177e47413cd57abf277120652a26c0b",
    "verified_classes": [
      "SupplyChainAttestationError",
      "ServerAttestation"
    ],
    "verified_functions": [
      "resolve_executable_digest",
      "fingerprint_manifest",
      "attest_server"
    ],
    "verified_functions_count": 3,
    "defensive_guards_count": 3,
    "raised_exceptions": [
      "SupplyChainAttestationError"
    ],
    "sample_fail_closed_invariants": [],
    "stdlib_dependencies": [
      "dataclasses",
      "hashlib",
      "json",
      "os"
    ],
    "observation_mode": "EMPIRICAL_BOUNDARY_CAPTURE",
    "zero_external_deps": true
  }
}