fix: reject scalar context foreach sources

src/wf_core/schema_navigation.py

@@ -115,11 +115,7 @@ class SchemaView:
         """Return the first declared homogeneous item schema for an array."""
         for cursor in self._cursors:
             for branch, _ in _structural_branches(cursor, frozenset()):
-                schema_type = branch.contents.get("type")
-                is_array = schema_type == "array" or (
-                    isinstance(schema_type, list) and "array" in schema_type
-                )
-                if not is_array:
+                if not _declares_array(branch.contents):
                     continue
                 child = branch.child(branch.contents.get("items"))
                 if child is not None:
@@ -127,8 +123,16 @@ class SchemaView:
         return None
 
     def is_array(self) -> bool:
-        """Return whether any structural branch declares an array."""
-        return self.array_items() is not None
+        """Return whether any structural branch declares an array type.
+
+        JSON Schema permits arrays without an ``items`` keyword; those are
+        still arrays whose element schema is unconstrained.
+        """
+        return any(
+            _declares_array(branch.contents)
+            for cursor in self._cursors
+            for branch, _ in _structural_branches(cursor, frozenset())
+        )
 
     def standalone_schema(self) -> dict[str, Any]:
         """Detach this view as a valid Draft 2020-12 schema resource.
@@ -208,6 +212,13 @@ class SchemaNavigator:
         return None, ""
 
 
+def _declares_array(schema: JsonSchema) -> bool:
+    schema_type = schema.get("type")
+    return schema_type == "array" or (
+        isinstance(schema_type, list) and "array" in schema_type
+    )
+
+
 def _structural_branches(
     cursor: _Cursor,
     active_references: frozenset[ReferenceIdentity],

src/wf_core/validation/context_paths.py

@@ -79,7 +79,7 @@ def validate_context_paths(
             # Context-rooted `over` paths reach this pass; the old
             # input/state-only check stays permissive for them.
             if node.over.root == "context":
-                _validate_one_context_path(
+                _validate_context_foreach_source(
                     node.over, f"nodes[{idx}].over", node.id, schema, report
                 )
         elif isinstance(node, InterruptNode):
@@ -197,6 +197,30 @@ def _validate_one_context_path(
         )
 
 
+def _validate_context_foreach_source(
+    path: GraphSourcePath,
+    location: str,
+    node_id: str,
+    schema: ContextSchema | None,
+    report: ValidationReport,
+) -> None:
+    """Require an existing context path to declare an array source.
+
+    Missing paths keep the more precise ``INVALID_CONTEXT_PATH`` diagnostic;
+    this adds the foreach-specific error only after the path resolves.
+    """
+    _validate_one_context_path(path, location, node_id, schema, report)
+    if schema is None:
+        return
+    source = SchemaNavigator(schema).at_path(path.parts)
+    if source is not None and not source.is_array():
+        report.add(
+            ValidationIssueCode.INVALID_FOREACH_SOURCE,
+            location,
+            f"foreach source {str(path)!r} must resolve to an array",
+        )
+
+
 def _failing_segment(
     schema: Mapping[str, Any], parts: tuple[str, ...]
 ) -> tuple[str | None, str]: