reducer as wf_mcp capability

tests/artifacts/test_factory.py

@@ -33,6 +33,26 @@ def test_create_workflow_artifact_from_plan_derives_boundary_schemas() -> None:
     assert artifact.created_from_catalog_version == "catalog-1"
 
 
+def test_create_workflow_artifact_from_plan_adds_reducer_dependencies() -> None:
+    plan = _plan()
+    plan["state_schema"] = {
+        "fields": {"best_score": {"type": "integer", "reducer": "wf.std.max"}}
+    }
+
+    artifact = create_workflow_artifact_from_plan(
+        artifact_id="score",
+        version=1,
+        title="Score",
+        plan=plan,
+        outcomes=("done",),
+    )
+
+    reducer = artifact.required_capabilities["wf.std.max"]
+    assert reducer.logical_source == "wf.std"
+    assert reducer.capability_name == "max"
+    assert reducer.kind == "reducer"
+
+
 def test_create_workflow_artifact_from_plan_accepts_wrapper_kind() -> None:
     artifact = create_workflow_artifact_from_plan(
         artifact_id="normalize_status",

tests/artifacts/test_validation.py

@@ -176,3 +176,28 @@ def test_validate_deployment_allows_changed_schema_when_policy_allows() -> None:
     )
 
     assert diagnostics == []
+
+
+def test_validate_deployment_accepts_reducer_capability() -> None:
+    reducer = RequiredCapability(
+        logical_source="wf.std",
+        capability_name="set_union",
+        kind="reducer",
+    )
+    diagnostics = validate_deployment_dependencies(
+        artifact=artifact_with(reducer),
+        deployment=deployment(bindings={"wf.std": "wf.std"}),
+        sources=[
+            AvailableSource(
+                id="wf.std",
+                capabilities={
+                    "set_union": AvailableCapability(
+                        name="set_union",
+                        kind="reducer",
+                    )
+                },
+            )
+        ],
+    )
+
+    assert diagnostics == []

tests/core/test_nested_state_paths.py

@@ -40,7 +40,9 @@ def test_state_field_defaults_to_replace_reducer() -> None:
 
 
 def test_unknown_state_reducer_fails_clearly() -> None:
-    workflow = _workflow(fields={"person.tags": StateField(type="array", reducer="x.nope")})
+    workflow = _workflow(
+        fields={"person.tags": StateField(type="array", reducer="x.nope")}
+    )
     state = {"person": {"tags": ["seed"]}}
 
     try:
@@ -51,6 +53,28 @@ def test_unknown_state_reducer_fails_clearly() -> None:
         raise AssertionError("expected unknown reducer to fail")
 
 
+def test_set_union_reducer_preserves_first_seen_order() -> None:
+    workflow = _workflow(
+        fields={"person.tags": StateField(type="array", reducer="wf.std.set_union")}
+    )
+    state = {"person": {"tags": ["alpha", "beta"]}}
+
+    write_state_value(workflow, state, "state.person.tags", ["beta", "gamma"])
+
+    assert state["person"]["tags"] == ["alpha", "beta", "gamma"]
+
+
+def test_max_reducer_keeps_larger_value() -> None:
+    workflow = _workflow(
+        fields={"best_score": StateField(type="integer", reducer="wf.std.max")}
+    )
+    state = {"best_score": 7}
+
+    write_state_value(workflow, state, "state.best_score", 9)
+
+    assert state["best_score"] == 9
+
+
 def _workflow(*, fields: dict[str, StateField]) -> Workflow:
     return Workflow(
         name="nested_state_paths",

tests/rewrite/models.py

@@ -125,9 +125,7 @@ class PartialRates(SophisticatedRates, total=False):
 
 
 class Rates(BaseModel):
-    rates: Annotated[
-        PartialRates, state_field(reducer="wf.std.merge_object")
-    ]  # or_!
+    rates: Annotated[PartialRates, state_field(reducer="wf.std.merge_object")]  # or_!
 
 
 class CurrentPools(BaseModel):

tests/wf_mcp/test_service.py

@@ -113,11 +113,13 @@ def test_service_lists_all_capability_sources_with_owned_capability_names() -> N
     assert "wf.std.runtime_error" in std_source["capabilities"]["node_specs"]
     assert std_source["capabilities"]["reducers"] == [
         "wf.std.append",
+        "wf.std.max",
         "wf.std.merge_object",
         "wf.std.replace",
+        "wf.std.set_union",
     ]
     assert std_source["capabilities"]["tools"] == []
-    assert std_source["reducer_count"] == 3
+    assert std_source["reducer_count"] == 5
 
     mcp_source = sources_by_id["wf.mcp"]
     assert mcp_source["capabilities"]["node_specs"] == ["wf.mcp.call_tool"]
@@ -159,7 +161,9 @@ def test_wf_std_source_contains_builtin_reducers() -> None:
     assert set(reducers) == {
         "wf.std.replace",
         "wf.std.append",
+        "wf.std.max",
         "wf.std.merge_object",
+        "wf.std.set_union",
     }