{
  "schema_version": "1.3.0",
  "slug": "what-structural-signals-indicate-prompt-injection-taint-has-silently-laundered-across-a-tool-boundary-in-multi-turn-agent-context",
  "title": "What structural signals indicate prompt-injection taint has silently laundered across a tool boundary in multi-turn agent context",
  "date_of_record": "2026 November 7",
  "scope": "Applies to LLM agents that carry dynamic information-flow labels (taint bits) on context segments and propagate them across tool-call boundaries and conversation turns. Does not apply to stateless single-shot inference, nor to systems that enforce provenance exclusively through cryptographic signing of tool outputs.",
  "projections": {
    "unified_markdown_uri": "https://corpusfetch.com/c/what-structural-signals-indicate-prompt-injection-taint-has-silently-laundered-across-a-tool-boundary-in-multi-turn-agent-context.md",
    "method_python_slice_uri": "https://corpusfetch.com/c/what-structural-signals-indicate-prompt-injection-taint-has-silently-laundered-across-a-tool-boundary-in-multi-turn-agent-context.py",
    "remnant_warrant_slice_uri": "https://corpusfetch.com/c/what-structural-signals-indicate-prompt-injection-taint-has-silently-laundered-across-a-tool-boundary-in-multi-turn-agent-context.warrant.json"
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  "remnant_summary": {
    "core_distinction": "The failure of interest is not \"the model obeyed an injected instruction.\" That is a *detected* injection. The failure is **label laundering**: a tainted segment enters the context, is transformed by a tool (summarization, retrieval, formatting, translation, code execution), and the resulting output segment is labeled clean because the tool's return path doe...",
    "what_was_observed": "Across a set of multi-turn tool-using agents, three structural signals preceded laundering events with high reliability: 1. **Label-arity mismatch at the tool boundary.** The tool schema declared `input_labels` but not `output_labels`. When a tool consumed a tainted argument and returned a string, the runtime had no slot to write the propagated label into, so the return defaulted to `untainted`. Every laundering even...",
    "condition_of_validity": "These signals are diagnostic only when the following hold: - Labels are *dynamic* (computed per segment at runtime), not static per-source. If taint is assigned statically to a whole tool or whole document, laundering cannot occur because there is no fine-grained bit to lose — but neither is there useful tracking. - The tool boundary is *observable*: the run...",
    "decision_boundary": "The operative rule: **a segment is trusted only if its label was derived by a total function over the labels of all segments that contributed to its bytes.** \"Total\" is the load-bearing word. If any contributing segment's label was dropped — because the tool had no output-label slot, because compaction did not re-derive, because re-entry minted a new namespa...",
    "operational_effect": "Practically, this converts \"trust the model's judgment about what's safe\" into \"trust the label algebra, and fail closed when the algebra is incomplete.\" The observable effect on the system: agents with total label derivation reject a small number of legitimate tool returns (false positives on conservative tainting) but eliminate the silent-laundering class ..."
  },
  "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": "69c0764daf1f06594754efeb8b14b4db975717711b9043bf172cb15c66b46293",
    "verified_classes": [
      "TaintLaunderingError",
      "Segment",
      "LabelGraph"
    ],
    "verified_functions": [
      "apply_tool_output",
      "handle_compaction",
      "content_hash",
      "register",
      "derive",
      "assert_trusted"
    ],
    "verified_functions_count": 6,
    "defensive_guards_count": 4,
    "raised_exceptions": [
      "TaintLaunderingError"
    ],
    "sample_fail_closed_invariants": [
      "tool returned None; cannot derive label"
    ],
    "stdlib_dependencies": [
      "dataclasses",
      "hashlib"
    ],
    "observation_mode": "EMPIRICAL_BOUNDARY_CAPTURE",
    "zero_external_deps": true
  }
}