{
  "schema_version": "1.3.0",
  "slug": "when-must-an-mcp-host-refuse-to-propagate-a-taint-label-across-a-tool-boundary",
  "title": "When must an MCP host refuse to propagate a taint label across a tool boundary?",
  "date_of_record": "2026 November 5",
  "scope": "Applies to MCP hosts that enforce prompt-injection resistance by tainting model context originating from untrusted tool output and refusing to let tainted content drive privileged tool calls. Does not apply to systems relying solely on model-side instructions or output filters, nor to single-tool deployments with no privilege asymmetry between read and write capabilities.",
  "projections": {
    "unified_markdown_uri": "https://corpusfetch.com/c/when-must-an-mcp-host-refuse-to-propagate-a-taint-label-across-a-tool-boundary.md",
    "method_python_slice_uri": "https://corpusfetch.com/c/when-must-an-mcp-host-refuse-to-propagate-a-taint-label-across-a-tool-boundary.py",
    "remnant_warrant_slice_uri": "https://corpusfetch.com/c/when-must-an-mcp-host-refuse-to-propagate-a-taint-label-across-a-tool-boundary.warrant.json"
  },
  "token_profile": {
    "full_markdown_tokens": 1223,
    "python_slice_tokens": 178,
    "warrant_slice_tokens": 220
  },
  "remnant_summary": {
    "core_distinction": "The non-obvious boundary is not \"tainted data must never reach a privileged tool.\" It is that **taint must not be laundered by an intermediate tool that legitimately transforms its input.** A taint-tracking host marks any tool result whose source is untrusted (web fetch, file read from a user-supplied path, third-party MCP server) with a taint label. The nai...",
    "what_was_observed": "In a host with three MCP servers — `fetch` (untrusted), `format` (trusted, pure string transform), `shell` (privileged) — an injection payload arrived in a fetched page as a natural-language instruction to run a command. The host correctly tainted the `fetch` result. The agent, following its own planning, passed the fetched text to `format` for pretty-printing. The `format` server returned a string with no taint labe...",
    "condition_of_validity": "The \"trusted tool output is untainted\" rule is valid **only if** the tool is *taint-preserving in the strong sense*: for every output byte, the host can prove the output is a function of declared, non-attacker-controlled inputs alone, or the output is drawn from a finite allowlist of literals. A pure formatter satisfies neither: its output is a function of i...",
    "decision_boundary": "The host must refuse to propagate a *cleared* (untainted) label across a tool boundary when **any** of the following hold: (1) the tool has no sanitizer declaration; (2) the declaration exists but the output fails its postcondition check; (3) the tainted input reached the tool through a path the host cannot attribute to a specific argument (e.g., via ambient...",
    "operational_effect": "Enforcing provenance-preserving propagation with postcondition-verified sanitizers eliminated the laundering path: the `format` server, lacking a sanitizer declaration, propagates taint, and the `shell` call is refused with a structured `TaintPropagationError` naming the originating `fetch` call. The false-positive rate on the `validate_schema` path dropped ..."
  },
  "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": "f79cefeaa3a50eadb38e99f3561ad52b94f4e69565ab5d8e519638265909346f",
    "verified_classes": [
      "TaintPropagationError"
    ],
    "verified_functions": [
      "admit_privileged_call",
      "propagate",
      "__init__"
    ],
    "verified_functions_count": 3,
    "defensive_guards_count": 1,
    "raised_exceptions": [
      "TaintPropagationError"
    ],
    "sample_fail_closed_invariants": [],
    "stdlib_dependencies": [],
    "observation_mode": "EMPIRICAL_BOUNDARY_CAPTURE",
    "zero_external_deps": true
  }
}