diff --git a/spec/crude_oil_exception_spec.rb b/spec/crude_oil_exception_spec.rb
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+++ b/spec/crude_oil_exception_spec.rb
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+require 'spec_helper'
+require 'set'
+
+# The :emissions_skip_crude_oil_mix edge group is an exception used by ETEngine's
+# direct-emissions calculation. Normally the recursion stops at the first
+# crude_oil edge and uses the crude_oil carrier's intrinsic CO2 value. For the
+# energetic final-demand chain of a sector whose oil demand is a *blend* of
+# biogenic and fossil oil products (diesel + biodiesel, kerosene + bio_kerosene,
+# lpg, bio_oil, ...), that intrinsic value is meaningless: the carrier is only a
+# proxy for the basket. The group forces the recursion to keep traversing so the
+# CO2 content is derived from the actual bio/fossil composition.
+#
+# Rule guarded here: EVERY crude_oil edge downstream of such a blending node must
+# carry the group. Sectors fed by crude_oil alone (e.g. `other`) and all
+# non-energetic feedstock chains (which are not combusted, so the bio/fossil
+# split is irrelevant) are left unmarked.
+#
+# See app/models/qernel/recursive_factor/direct_emissions.rb and
+# app/models/qernel/node_api/direct_emissions.rb in ETEngine.
+module CrudeOilException
+  GROUP   = :emissions_skip_crude_oil_mix
+  CARRIER = :crude_oil
+
+  module_function
+
+  # Biogenic carriers are fully sustainable.
+  def biogenic?(carrier)
+    Atlas::Carrier.find(carrier).sustainable.to_f == 1.0
+  end
+
+  # Fossil carriers are fully unsustainable.
+  def fossil?(carrier)
+    Atlas::Carrier.find(carrier).sustainable.to_f == 0.0
+  end
+
+  # A node blends biogenic and fossil oil when it is energetic, supplies
+  # crude_oil downstream, and is fed by both a biogenic and a fossil carrier.
+  def blending_node?(node)
+    return false unless node.get(:model).use == 'energetic'
+    return false if node.out_edges(CARRIER).none?
+
+    incoming = node.in_edges
+    incoming.any? { |edge| biogenic?(edge.label) } && incoming.any? { |edge| fossil?(edge.label) }
+  end
+
+  # Every crude_oil edge reachable downstream of +node+ by following crude_oil
+  # edges. Returns the underlying Atlas::Edge documents.
+  def downstream_crude_oil_edges(node, seen = Set.new)
+    node.out_edges(CARRIER).each_with_object([]) do |edge, collected|
+      next unless seen.add?(edge)
+
+      collected << edge.get(:model)
+      collected.concat(downstream_crude_oil_edges(edge.to, seen))
+    end
+  end
+end
+
+RSpec.describe ':emissions_skip_crude_oil_mix edge group' do
+  graph = Atlas::GraphBuilder.build
+
+  # All energetic final-demand nodes that blend biogenic and fossil oil.
+  blending_nodes = graph.nodes.select { |node| CrudeOilException.blending_node?(node) }
+
+  # Every crude_oil edge downstream of those nodes.
+  downstream_edges = blending_nodes
+    .flat_map { |node| CrudeOilException.downstream_crude_oil_edges(node) }
+    .uniq(&:key)
+  downstream_keys = downstream_edges.map(&:key).to_set
+
+  describe 'every crude_oil edge downstream of a blending node' do
+    downstream_edges.each do |edge|
+      it "#{edge.key} carries #{CrudeOilException::GROUP}" do
+        expect(edge.groups.include?(CrudeOilException::GROUP)).to be(true),
+          "#{edge.key} should carry #{CrudeOilException::GROUP} but does not"
+      end
+    end
+  end
+
+  describe 'every edge carrying the group' do
+    marked_edges = Atlas::EnergyEdge.all.select { |edge| edge.groups.include?(CrudeOilException::GROUP) }
+
+    marked_edges.each do |edge|
+      it "#{edge.key} is a crude_oil edge downstream of a blending node" do
+        expect(downstream_keys.include?(edge.key)).to be(true),
+          "#{edge.key} carries #{CrudeOilException::GROUP} but should not"
+      end
+    end
+  end
+end