diff --git a/scripts/check_adjoint_prevalidation.py b/scripts/check_adjoint_prevalidation.py
index e9659b7e6f..7dc9346f0f 100644
--- a/scripts/check_adjoint_prevalidation.py
+++ b/scripts/check_adjoint_prevalidation.py
@@ -732,6 +732,21 @@ def carrying_lane(snapshot: dict[str, Any]) -> str | None:
     return match.group(1) if match else None
 
 
+def is_out_of_fleet(snapshot: dict[str, Any]) -> bool:
+    """True when the pull request is carried by no fleet lane at all.
+
+    Both conditions, mirroring the `tag_required` exemption (#17713/#17715,
+    `variation_tag_required.py`): the head branch starts with ``claude/`` AND
+    the body declares « Hors flotte ». Either alone is not out of fleet -- a
+    fleet lane renaming its branch or copying the marker stays inside the
+    third-party rule (#17791 acceptance 3).
+    """
+    head_ref = snapshot.get("headRefName") or ""
+    return head_ref.startswith("claude/") and "Hors flotte" in (
+        snapshot.get("body") or ""
+    )
+
+
 def validate_dossier(dossier: Dossier, snapshot: dict[str, Any]) -> list[str]:
     """Validate a parsed dossier against one live PR snapshot."""
     f = dossier.fields
@@ -779,6 +794,13 @@ def validate_dossier(dossier: Dossier, snapshot: dict[str, Any]) -> list[str]:
         errors.append(
             f"lane must be one of the qualifying cluster lanes, got {dossier_lane!r}"
         )
+    elif is_out_of_fleet(snapshot):
+        # #17791 -- a maintainer cloud session (`claude/*` + « Hors flotte »)
+        # carries no fleet lane: CLAUDE.md ('À qui ce fichier s'adresse') exempts
+        # it from the Grain tag, so the third-party rule has no carrier to
+        # compare against. Every qualifying lane is third-party by construction,
+        # and the self-attestation check below is unreachable for it.
+        pass
     else:
         carrier = carrying_lane(snapshot)
         if carrier is None:
@@ -982,6 +1004,7 @@ def build_result(
         "ready": verdict == VERDICT_READY,
         "verdict": verdict or "NO_DOSSIER",
         "errors": errors,
+        "out_of_fleet": is_out_of_fleet(snapshot),
     }
     if dossier is not None and verdict:
         result["dossier"] = dossier_payload(dossier)
@@ -1316,6 +1339,11 @@ def main() -> int:
         )
     elif ready:
         print(f"READY -- PR #{args.pr} prevalidated by adjoint at {snapshot['headRefOid']}")
+        if is_out_of_fleet(snapshot):
+            print(
+                "  note: hors-flotte PR (claude/* + 'Hors flotte'): no fleet "
+                "lane carries it, any qualifying lane is third-party (#17791)"
+            )
     elif verdict == VERDICT_BLOCKED:
         print(
             f"BLOCKED-WITH-SUBSTANCE -- PR #{args.pr} has an intact adjoint dossier at "
diff --git a/scripts/tests/test_check_adjoint_prevalidation.py b/scripts/tests/test_check_adjoint_prevalidation.py
index 4abec23834..30ae48d6ab 100644
--- a/scripts/tests/test_check_adjoint_prevalidation.py
+++ b/scripts/tests/test_check_adjoint_prevalidation.py
@@ -283,6 +283,62 @@ def test_non_shared_github_author_cannot_satisfy_gate():
     assert any(error.startswith("comment author must") for error in errors)
 
 
+# --- #17791 : PR hors flotte (session cloud du mainteneur) -------------------
+#
+# Miroir de l'exemption `tag_required` (#17713/#17715, variation_tag_required.py) :
+# branche `claude/*` ET marqueur « Hors flotte » dans le body. Une telle PR n'est
+# porte par AUCUNE lane de la flotte, donc tout dossier d'une lane qualifiante
+# est tiers par construction -- le refus « carrying lane cannot be established »
+# y est une impasse structurelle, pas une garantie.
+
+
+def test_out_of_fleet_pr_accepts_qualifying_dossier_without_grain_tag():
+    """claude/* + « Hors flotte » : un dossier de lane qualifiante est tiers."""
+    snapshot = _snapshot_with_body(
+        "Session cloud du mainteneur. **Hors flotte**", lane="myia-po-2023:CoursIA"
+    )
+    snapshot["headRefName"] = "claude/affectionate-mccarthy-6dvuea"
+    ready, errors = mod.evaluate(snapshot)
+    assert ready, errors
+    assert errors == []
+
+
+def test_claude_branch_without_marker_is_still_refused():
+    """Le prefixe seul n'exempte pas : sans le marqueur, le refus est intact."""
+    snapshot = _snapshot_with_body(
+        "Session cloud du mainteneur.", lane="myia-po-2023:CoursIA"
+    )
+    snapshot["headRefName"] = "claude/affectionate-mccarthy-6dvuea"
+    errors = _errors(snapshot)
+    assert any(
+        error.startswith("carrying lane cannot be established") for error in errors
+    )
+
+
+def test_fleet_branch_with_marker_is_still_refused():
+    """Recopier le marqueur sur une branche de flotte ne sort pas de la regle."""
+    snapshot = _snapshot_with_body(
+        "Session cloud du mainteneur. **Hors flotte**", lane="myia-po-2023:CoursIA"
+    )
+    snapshot["headRefName"] = "feature/renamed-to-escape"
+    errors = _errors(snapshot)
+    assert any(
+        error.startswith("carrying lane cannot be established") for error in errors
+    )
+
+
+def test_self_prevalidation_refusal_survives_the_exemption_next_door():
+    """Une PR de flotte taguee reste refusee en self-attestation : l'exemption
+    hors flotte n'ouvre aucune echappatoire a cote."""
+    carrier = "myia-po-2023:CoursIA"
+    snapshot = _snapshot_with_body(
+        "Grain: DEEP/lean -- lane %s -- prev: MED" % carrier, lane=carrier
+    )
+    snapshot["headRefName"] = "feature/ordinary-fleet-branch"
+    errors = _errors(snapshot)
+    assert any(error.startswith("self-prevalidation refused") for error in errors)
+
+
 def test_blocked_preflight_is_a_valid_dossier_but_never_ready():
     """An honest BLOCKED dossier must be distinguishable from an absent one.