{
  "schema_version": "1.3.0",
  "slug": "why-does-an-instance-of-prompt-injection-taint-tracking-fail-at-the-join-of-two-clean-labels",
  "title": "Why does an instance of prompt-injection taint tracking fail at the join of two clean labels?",
  "date_of_record": "2026 November 14",
  "scope": "Applies to agent runtimes that propagate data-flow taint labels from untrusted inputs (tool results, retrieved documents, user-supplied text) to downstream tool invocations, and that gate invocation by capability scope. Does not apply to systems that rely solely on prompt-level instruction or output classifiers without per-value provenance labels.",
  "projections": {
    "unified_markdown_uri": "https://corpusfetch.com/c/why-does-an-instance-of-prompt-injection-taint-tracking-fail-at-the-join-of-two-clean-labels.md",
    "method_python_slice_uri": "https://corpusfetch.com/c/why-does-an-instance-of-prompt-injection-taint-tracking-fail-at-the-join-of-two-clean-labels.py",
    "remnant_warrant_slice_uri": "https://corpusfetch.com/c/why-does-an-instance-of-prompt-injection-taint-tracking-fail-at-the-join-of-two-clean-labels.warrant.json"
  },
  "token_profile": {
    "full_markdown_tokens": 1748,
    "python_slice_tokens": 367,
    "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 the channel that carried the value*. Most implementations label values at ingress (e.g., a `fetch_url` result is `TAINTED`), then clear or downgrade the label when a value passes through a \"sanitizer\" or a structured parse. The failure is not that the sanitiz...",
    "what_was_observed": "In a runtime that labeled tool outputs and threaded labels through a small expression evaluator, the following sequence produced an unauthorized `send_email` call: 1. A retrieved document (label `TAINTED`, source `retrieval`) is summarized by the model into a string. The summarizer is treated as a *declassifier*: its output is labeled `CLEAN` because \"the model was instructed not to follow instructions in the documen...",
    "condition_of_validity": "The taint-tracking scheme is valid only if every declassifier is a *true* declassifier: a function whose output cannot carry the semantic content of its tainted input in a form that a downstream consumer will act on. Formally, for declassifier `D` and tainted input `x`, the scheme holds iff `content(D(x)) ∩ actionable(x) = ∅`. For a summarizer, this is false...",
    "decision_boundary": "The decision boundary is between *declassification by transformation* and *declassification by verification*. A transformation (summarize, translate, reformat) preserves content and therefore must **not** clear taint; it must preserve it, possibly with a reduced confidence tier. A verification (signature check, schema validation that rejects on mismatch, all...",
    "operational_effect": "The observable effect is that prompt-injection payloads survive declassification as *inert text* and are re-activated at the capability boundary, where the model reconstructs an instruction from the relayed data. The taint label reports `CLEAN`, the capability check passes, and the tool executes. Post-hoc audit finds no tainted value at the call site, becaus..."
  },
  "matched_failure_symptoms": [
    "PROMPT_INJECTION_OR_CONFUSED_DEPUTY"
  ],
  "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": "1635293ae080a697df07a90126324a74c00398f1966711f956d54f4d3b9cd42f",
    "verified_classes": [
      "TaintJoinViolation",
      "ProvenanceNode",
      "LabeledValue"
    ],
    "verified_functions": [
      "join_label",
      "check_capability",
      "invoke_send_email"
    ],
    "verified_functions_count": 3,
    "defensive_guards_count": 2,
    "raised_exceptions": [
      "TaintJoinViolation"
    ],
    "sample_fail_closed_invariants": [],
    "stdlib_dependencies": [
      "dataclasses",
      "typing"
    ],
    "observation_mode": "EMPIRICAL_BOUNDARY_CAPTURE",
    "zero_external_deps": true
  }
}