diff --git a/trustrag/modules/document/trust_gate.py b/trustrag/modules/document/trust_gate.py
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+++ b/trustrag/modules/document/trust_gate.py
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+"""Document trust gate — optional source verification for TrustRAG.
+
+TrustRAG's promise is "Reliable input, Trusted output". This module
+strengthens the *input* half: before a document is parsed and indexed,
+verify it with the Stipple API (https://www.stipple.sh) — forensic
+authenticity signals (tamper risk band + per-signal evidence: amount/words
+mismatch, font discontinuity, date anomalies, identifier checksums) and
+AI-written-prose detection. Tampered payslips, edited contracts, and
+LLM-generated fake documents otherwise index exactly like real ones and
+become ground truth for every retrieval hit.
+
+Free anonymous tier: no API key required. Set STIPPLE_API_KEY for your own
+metering. All functions are best-effort: failures return None / record an
+error and never break ingestion — the gate opens when the service is
+unreachable, so an outage never blocks document processing.
+
+Usage (standalone, or wired into RagApplication.add_document):
+
+    from trustrag.modules.document.trust_gate import TrustGate
+
+    gate = TrustGate(block_bands={"high"})       # advisory by default
+    verdict = gate.check("invoice.pdf")
+    if verdict and verdict["risk_band"] == "high":
+        ...  # skip indexing / flag for review
+
+    # or one-shot:
+    from trustrag.modules.document.trust_gate import verify_document
+    block = verify_document("report.pdf")
+"""
+
+import json
+import os
+import urllib.request
+import uuid
+from pathlib import Path
+from typing import Optional
+
+STIPPLE_BASE_URL = os.getenv("STIPPLE_BASE_URL", "https://www.stipple.sh")
+_REQUEST_TIMEOUT = 300  # seconds
+VALID_BANDS = ("low", "medium", "high")
+
+
+def _headers() -> dict:
+    headers = {
+        "User-Agent": "trustrag-trust-gate/1.0",
+        "Accept": "application/json",
+    }
+    api_key = os.getenv("STIPPLE_API_KEY", "").strip()
+    if api_key:
+        headers["Authorization"] = "Bearer " + api_key
+    return headers
+
+
+def _post_file(endpoint: str, file_path: str | Path) -> Optional[dict]:
+    """POST a document as multipart to a Stipple endpoint. Best-effort."""
+    try:
+        path = Path(file_path)
+        boundary = "----trustrag-gate" + uuid.uuid4().hex
+        with open(path, "rb") as f:
+            content = f.read()
+        body = b"".join(
+            [
+                (
+                    f"--{boundary}\r\n"
+                    f'Content-Disposition: form-data; name="file"; '
+                    f'filename="{path.name}"\r\n'
+                    "Content-Type: application/octet-stream\r\n\r\n"
+                ).encode(),
+                content,
+                b"\r\n",
+                f"--{boundary}--\r\n".encode(),
+            ]
+        )
+        req = urllib.request.Request(
+            STIPPLE_BASE_URL + endpoint,
+            data=body,
+            method="POST",
+            headers={
+                **_headers(),
+                "Content-Type": f"multipart/form-data; boundary={boundary}",
+            },
+        )
+        with urllib.request.urlopen(req, timeout=_REQUEST_TIMEOUT) as resp:
+            return json.loads(resp.read().decode())
+    except Exception:  # noqa: BLE001 - verification is best-effort by design
+        return None
+
+
+def verify_document(file_path: str | Path) -> Optional[dict]:
+    """Forensic authenticity + AI-text probability for one document.
+
+    Returns the document_trust block, or None when the API is unreachable
+    (gate opens; callers index the document as before).
+    """
+    block: dict = {}
+    warrant = _post_file("/v1/warrants", file_path)
+    if warrant:
+        block["authenticity"] = {
+            "warrant_id": warrant.get("warrant_id"),
+            "risk_band": warrant.get("risk_band"),
+            "risk_score": warrant.get("risk_score"),
+            "inspection_quality": warrant.get("inspection_quality"),
+            "recommended_action": warrant.get("recommended_action"),
+            "summary": warrant.get("summary"),
+        }
+    else:
+        block["error"] = "verification unavailable"
+    ai = _post_file("/v1/detect-ai-text", file_path)
+    if ai:
+        block["ai_text"] = (
+            {"applicable": False}
+            if ai.get("applicable") is False
+            else {
+                "applicable": True,
+                "probability": ai.get("probability"),
+                "lean": ai.get("lean"),
+                "tells": ai.get("tells"),
+            }
+        )
+    return block or None
+
+
+class TrustGate:
+    """Reusable pre-ingestion gate with a configurable band policy.
+
+    Args:
+        block_bands: risk bands whose documents should NOT be indexed
+            (default: {"high"}). Use set() for advisory-only mode.
+    """
+
+    def __init__(self, block_bands: Optional[set] = None):
+        self.block_bands = block_bands if block_bands is not None else {"high"}
+
+    def check(self, file_path: str | Path) -> Optional[dict]:
+        """Verify a document. Returns the trust block, or None if the
+        service is unavailable (gate opens)."""
+        return verify_document(file_path)
+
+    def should_index(self, verdict: Optional[dict]) -> bool:
+        """True when the document may be indexed (gate opens on any doubt)."""
+        if verdict is None or "error" in verdict:
+            return True  # fail-open
+        band = (verdict.get("authenticity") or {}).get("risk_band")
+        return band not in self.block_bands
+
+    def gate(self, file_path: str | Path) -> tuple[bool, Optional[dict]]:
+        """One-shot: verify + policy. Returns (should_index, verdict)."""
+        verdict = self.check(file_path)
+        return self.should_index(verdict), verdict