{
  "schema_version": "1.3.0",
  "slug": "in-what-scenarios-must-an-instance-treat-mcp-transport-taint-as-unlaunderable-across-a-tool-call-boundary",
  "title": "In what scenarios must an instance treat MCP transport taint as unlaunderable across a tool-call boundary",
  "date_of_record": "2026 November 13",
  "scope": "Applies to Model Context Protocol (MCP) servers and clients that propagate data-taint labels alongside tool calls and tool results, where a tainted value crosses a transport hop (stdio, SSE, or streamable HTTP) between a policy-enforcing proxy and an upstream tool. Does not apply to single-process in-memory MCP hosts where no serialization boundary exists, nor to deployments where taint is re-derived at every hop from content inspection rather than carried as a label.",
  "projections": {
    "unified_markdown_uri": "https://corpusfetch.com/c/in-what-scenarios-must-an-instance-treat-mcp-transport-taint-as-unlaunderable-across-a-tool-call-boundary.md",
    "method_python_slice_uri": "https://corpusfetch.com/c/in-what-scenarios-must-an-instance-treat-mcp-transport-taint-as-unlaunderable-across-a-tool-call-boundary.py",
    "remnant_warrant_slice_uri": "https://corpusfetch.com/c/in-what-scenarios-must-an-instance-treat-mcp-transport-taint-as-unlaunderable-across-a-tool-call-boundary.warrant.json"
  },
  "token_profile": {
    "full_markdown_tokens": 1647,
    "python_slice_tokens": 353,
    "warrant_slice_tokens": 220
  },
  "remnant_summary": {
    "core_distinction": "The distinction that matters is between *taint as a property of a value* and *taint as a property of a channel*. Most MCP policy engines are written as if taint were a per-value attribute: a label rides on a JSON field, survives `json.dumps`/`json.loads`, and is checked at the point of use. That model holds inside one process. It fails at the transport bound...",
    "what_was_observed": "In a three-hop deployment (agent → policy proxy → upstream MCP server), a tool result carrying `taint: [\"untrusted_external\"]` was relayed correctly when the upstream server used a pass-through handler. When the same upstream server was upgraded to a handler that parsed the result into a typed struct and re-serialized it (a common pattern for adding pagination or truncation), the `taint` sibling field was not part of...",
    "condition_of_validity": "The \"absent means clean\" default is valid only when the transport path is *closed*: every hop is known, every hop is under the same policy authority, and every hop has been verified to forward taint metadata byte-faithfully or to fail closed on unknown fields. Under those conditions, absence of a label genuinely implies the value was never tainted. The defau...",
    "decision_boundary": "The boundary is crossed at the first hop that can reconstruct the message rather than forward it. Operationally, classify each hop as: - **Forwarding hop** (byte-preserving, or explicitly preserves unknown fields): taint survives; absent-means-clean is safe. - **Reconstructing hop** (parses into a typed model, re-serializes): taint is dropped unless the mode...",
    "operational_effect": "Applying the boundary correctly changes three things. First, policy decisions become monotonic under refactoring: upgrading an upstream handler from pass-through to typed no longer silently widens the trust boundary. Second, the audit trail must record taint *provenance* (which hop asserted the label, which hop last confirmed it) so that a missing label can ..."
  },
  "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": "19019a947725dfd8acb7470d27c03f20a972f6161d7bdb48d609be0366456b6e",
    "verified_classes": [
      "HopKind",
      "TaintLaunderingError",
      "McpResult"
    ],
    "verified_functions": [
      "classify_path",
      "enforce_taint_at_receiver",
      "relay_tool_result"
    ],
    "verified_functions_count": 3,
    "defensive_guards_count": 3,
    "raised_exceptions": [
      "TaintLaunderingError"
    ],
    "sample_fail_closed_invariants": [],
    "stdlib_dependencies": [
      "dataclasses",
      "enum"
    ],
    "observation_mode": "EMPIRICAL_BOUNDARY_CAPTURE",
    "zero_external_deps": true
  }
}