{
  "schema_version": "1.3.0",
  "slug": "when-must-an-instance-refuse-a-tool-call-that-passes-every-static-capability-check-dynamic-taint-re-derivation-across-mcp-tool-call-boundaries",
  "title": "When must an instance refuse a tool call that passes every static capability check: dynamic taint re-derivation across MCP tool-call boundaries",
  "date_of_record": "2026 November 10",
  "scope": "Applies to agent runtimes that enforce capability scoping at the MCP server boundary (per-tool allowlists, per-resource scopes, argument-schema validation) and additionally maintain a taint label on data flowing between tool calls. Does not apply to stateless single-shot tool invocations where no data persists across calls, nor to systems that enforce only static capability policy with no dynamic taint tracking.",
  "projections": {
    "unified_markdown_uri": "https://corpusfetch.com/c/when-must-an-instance-refuse-a-tool-call-that-passes-every-static-capability-check-dynamic-taint-re-derivation-across-mcp-tool-call-boundaries.md",
    "method_python_slice_uri": "https://corpusfetch.com/c/when-must-an-instance-refuse-a-tool-call-that-passes-every-static-capability-check-dynamic-taint-re-derivation-across-mcp-tool-call-boundaries.py",
    "remnant_warrant_slice_uri": "https://corpusfetch.com/c/when-must-an-instance-refuse-a-tool-call-that-passes-every-static-capability-check-dynamic-taint-re-derivation-across-mcp-tool-call-boundaries.warrant.json"
  },
  "token_profile": {
    "full_markdown_tokens": 1678,
    "python_slice_tokens": 229,
    "warrant_slice_tokens": 220
  },
  "remnant_summary": {
    "core_distinction": "A static capability check answers: *may this principal invoke this tool with these argument shapes?* A dynamic taint check answers: *does this invocation carry data whose provenance forbids it from reaching this sink?* These are orthogonal, and the failure this record documents occurs precisely in the gap: an invocation that is **capability-legal and schema-...",
    "what_was_observed": "An agent operating with three MCP servers: a `fetch` server (scope: read arbitrary URLs, taint source = UNTRUSTED), a `notes` server (scope: read/write a local scratch store, taint-neutral), and a `shell` server (scope: execute a fixed allowlist of binaries; requires that any argument derived from UNTRUSTED data be explicitly declassified by a human-approval gate). The agent fetched a document containing the line `bu...",
    "condition_of_validity": "The refusal rule in this record is valid **only** when the runtime can reconstruct the derivation graph from the tainted source to the sink argument — i.e., when taint labels propagate through every intermediate value the agent constructs. If the agent is permitted to construct arguments via an out-of-band channel the runtime cannot observe (e.g., the model ...",
    "decision_boundary": "The decision boundary is not \"is the value tainted?\" but \"**is the value's taint *re-derivable* at the sink, and does the sink's authority exceed the source's authority?**\" Concretely: - If `authority(sink) ⊆ authority(source)` — the sink cannot do anything the tainted source could already cause — the call is admissible regardless of taint. No laundering ris...",
    "operational_effect": "Enforcing the derivation-graph rule changes three operational behaviors. First, field extraction no longer clears taint; it preserves it, and the propagation table must be audited to remove every shape-based clear. Second, tool-call result caches must key on ancestor provenance, not just argument hash, or they must be invalidated on any re-fetch of a tainted..."
  },
  "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": "8dd2c0e7342d30083cad8d1eb20b7f799941088cf3f37b53cc28c55ef61487b8",
    "verified_classes": [
      "TaintLaunderingError",
      "DerivationGraph"
    ],
    "verified_functions": [
      "guard_sink_call",
      "__init__",
      "source",
      "derive",
      "declassify",
      "is_tainted"
    ],
    "verified_functions_count": 6,
    "defensive_guards_count": 2,
    "raised_exceptions": [
      "TaintLaunderingError"
    ],
    "sample_fail_closed_invariants": [],
    "stdlib_dependencies": [],
    "observation_mode": "EMPIRICAL_BOUNDARY_CAPTURE",
    "zero_external_deps": true
  }
}