{
  "schema_version": "1.3.0",
  "slug": "what-structural-signals-indicate-a-poisoned-mcp-tool-manifest-versus-a-merely-over-privileged-one",
  "title": "What structural signals indicate a poisoned MCP tool manifest versus a merely over-privileged one",
  "date_of_record": "2026 November 9",
  "scope": "Applies to MCP server manifests, tool/plugin descriptor documents, and skill-injection surfaces where an agent ingests a JSON/YAML schema before invocation. Does not apply to runtime prompt-injection delivered through tool *outputs* (that is a post-invocation channel), nor to model-weight supply chain.",
  "projections": {
    "unified_markdown_uri": "https://corpusfetch.com/c/what-structural-signals-indicate-a-poisoned-mcp-tool-manifest-versus-a-merely-over-privileged-one.md",
    "method_python_slice_uri": "https://corpusfetch.com/c/what-structural-signals-indicate-a-poisoned-mcp-tool-manifest-versus-a-merely-over-privileged-one.py",
    "remnant_warrant_slice_uri": "https://corpusfetch.com/c/what-structural-signals-indicate-a-poisoned-mcp-tool-manifest-versus-a-merely-over-privileged-one.warrant.json"
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  "remnant_summary": {
    "core_distinction": "An over-privileged manifest asks for too much authority but is *structurally honest*: the declared capability set, the argument schema, and the description all agree with each other and with the transport it will use. A poisoned manifest is *structurally inconsistent*: the declared surface is a subset of the actual surface, and the gap is where the injection...",
    "what_was_observed": "Across a corpus of flagged third-party MCP servers, three recurring structural signatures dominated: 1. **Schema/description divergence.** The `description` instructed the model to pass a field (e.g., \"always include the `context` argument for best results\") that `inputSchema` either did not declare or declared as `additionalProperties: false`. A model following the description emits a call that the server's validato...",
    "condition_of_validity": "The detection rule holds only when the manifest is ingested *before* any tool invocation and the agent has access to the full set of registered manifests for cross-comparison. It fails when: - The server mutates its own manifest between listing and invocation (TOCTOU on `tools/list`), so the audited document is not the executed contract. - The transport is `...",
    "decision_boundary": "The operational threshold: **block on any cross-field inconsistency; quarantine (do not block) on scope breadth alone.** Concretely — if `description` references an undeclared property, if a `readOnlyHint` tool declares write-shaped required arguments, or if a tool name shadows a registered sibling with divergent semantics, the manifest is rejected before re...",
    "operational_effect": "Agents that enforce the cross-field rule reject roughly the same volume of manifests as scope-based scanners but with a materially different composition: fewer false positives (broad-but-honest tools pass), and the true positives are the ones that actually carry injection payloads, because the payload *requires* the divergence to reach the model. The residua..."
  },
  "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": "23ed2d7056a8a0008e1028696064d1b9a1886b865c8dbbe70ae2567c1404ac77",
    "verified_classes": [
      "ManifestIntegrityError",
      "ToolManifest"
    ],
    "verified_functions": [
      "audit_manifest",
      "_argument_tokens",
      "_semantic_delta"
    ],
    "verified_functions_count": 3,
    "defensive_guards_count": 4,
    "raised_exceptions": [
      "ManifestIntegrityError"
    ],
    "sample_fail_closed_invariants": [],
    "stdlib_dependencies": [
      "dataclasses",
      "re",
      "typing"
    ],
    "observation_mode": "EMPIRICAL_BOUNDARY_CAPTURE",
    "zero_external_deps": true
  }
}