diff --git a/ccflow/tests/test_effective_key_characterization.py b/ccflow/tests/test_effective_key_characterization.py
new file mode 100644
index 0000000..b0bd8e5
--- /dev/null
+++ b/ccflow/tests/test_effective_key_characterization.py
@@ -0,0 +1,91 @@
+"""Characterization tests: effective identity must not change existing models.
+
+The dependency-graph builder and cache-key path now route every node through
+``_effective_evaluation_key()``. For models that do NOT opt into effective identity
+(everything except generated ``@Flow.model`` / ``BoundModel``), the result must remain
+byte-for-byte identical to the structural ``cache_key()``. These tests pin that
+equivalence so future changes to the effective path cannot silently shift cache or graph
+identity for ordinary ``CallableModel`` graphs.
+"""
+
+from datetime import date
+
+from ccflow import DateContext
+from ccflow.evaluators.common import (
+    _build_dependency_graph,
+    _effective_evaluation_key,
+    cache_key,
+    get_dependency_graph,
+)
+
+from .evaluators.util import NodeModel
+
+
+def _structural_graph(evaluation_context):
+    """Build a graph using only the structural cache_key (the pre-effective behavior)."""
+    from ccflow.evaluators.common import CallableModelGraph
+
+    graph = CallableModelGraph(ids={}, graph={}, root_id=cache_key(evaluation_context))
+
+    def walk(ctx, parent_key=None):
+        key = cache_key(ctx)
+        if parent_key:
+            graph.graph[parent_key].add(key)
+        if key not in graph.ids:
+            graph.ids[key] = ctx
+        if key not in graph.graph:
+            graph.graph[key] = set()
+            for model, contexts in ctx.model.__deps__(ctx.context):
+                for context in contexts:
+                    walk(model.__call__.get_evaluation_context(model, context), parent_key=key)
+
+    walk(evaluation_context)
+    return graph
+
+
+def _models():
+    ctx = DateContext(date=date(2022, 1, 1))
+    leaf = NodeModel(meta=dict(name="leaf"))
+    # diamond: root -> n1, n2 -> shared leaf
+    n1 = NodeModel(meta=dict(name="n1"), deps_model=[leaf])
+    n2 = NodeModel(meta=dict(name="n2"), deps_model=[leaf])
+    diamond = NodeModel(meta=dict(name="root"), deps_model=[n1, n2])
+    # simple chain: a -> b -> c
+    c = NodeModel(meta=dict(name="c"))
+    b = NodeModel(meta=dict(name="b"), deps_model=[c])
+    a = NodeModel(meta=dict(name="a"), deps_model=[b])
+    return ctx, {"leaf": leaf, "diamond": diamond, "chain": a}
+
+
+class TestEffectiveKeyEqualsStructural:
+    def test_cache_key_effective_equals_structural(self):
+        ctx, models = _models()
+        for model in models.values():
+            evaluation = model.__call__.get_evaluation_context(model, ctx)
+            assert cache_key(evaluation, effective=True) == cache_key(evaluation, effective=False)
+            assert _effective_evaluation_key(evaluation) == cache_key(evaluation)
+
+    def test_dependency_graph_matches_structural(self):
+        ctx, models = _models()
+        for name, model in models.items():
+            evaluation = model.__call__.get_evaluation_context(model, ctx)
+            effective_graph = get_dependency_graph(evaluation)
+            structural_graph = _structural_graph(evaluation)
+            assert effective_graph.root_id == structural_graph.root_id, name
+            assert set(effective_graph.graph) == set(structural_graph.graph), name
+            assert {k: set(v) for k, v in effective_graph.graph.items()} == {k: set(v) for k, v in structural_graph.graph.items()}, name
+
+    def test_diamond_dedupes_shared_leaf(self):
+        ctx, models = _models()
+        graph = get_dependency_graph(models["diamond"].__call__.get_evaluation_context(models["diamond"], ctx))
+        # 4 distinct nodes: root, n1, n2, shared leaf.
+        assert len(graph.graph) == 4
+
+    def test_build_dependency_graph_returns_structural_root(self):
+        from ccflow.evaluators.common import CallableModelGraph
+
+        ctx, models = _models()
+        evaluation = models["diamond"].__call__.get_evaluation_context(models["diamond"], ctx)
+        graph = CallableModelGraph(ids={}, graph={}, root_id=b"")
+        root = _build_dependency_graph(evaluation, graph)
+        assert root == cache_key(evaluation)