fix: harden canonical output binding projection

src/wf_api/draft_authoring.py

@@ -525,17 +525,11 @@ class WorkflowDraftAuthoringApi:
             spec.output_schema_contract or spec.output_model.model_json_schema()
         )
 
-        targets = [str(binding.target) for binding in bindings]
-        if has_overlapping_paths(targets):
-            raise _overlapping_output_targets_error(bindings)
-
-        projected_state = _draft_schema(workspace.draft, "state_schema")
         for index, binding in enumerate(bindings):
-            source_parts = binding.source.parts
             try:
                 schema_fragment_at_path(
                     capability_schema,
-                    source_parts,
+                    binding.source.parts,
                     label="capability output schema",
                 )
             except ValueError as exc:
@@ -544,6 +538,13 @@ class WorkflowDraftAuthoringApi:
                     f"is not declared by capability {capability_name!r}: {exc}"
                 ) from exc
 
+        targets = [str(binding.target) for binding in bindings]
+        if has_overlapping_paths(targets):
+            raise _overlapping_output_targets_error(bindings)
+
+        projected_state = _draft_schema(workspace.draft, "state_schema")
+        for index, binding in enumerate(bindings):
+            source_parts = binding.source.parts
             target_parts = binding.target.parts
             try:
                 projected_state = project_schema_path_to_schema_path(

src/wf_api/schema_projection.py

@@ -110,8 +110,18 @@ def project_schema_path_to_schema_path(
             raise ValueError(
                 f"schema path {'.'.join(target_parts[: index + 1])!r} is not an object"
             )
-        _ensure_object_schema(child, ".".join(target_parts[: index + 1]))
-        parent = child
+        target_label = ".".join(target_parts[: index + 1])
+        # Mutate the referenced definition rather than adding sibling schema
+        # keywords beside $ref, which path lookup intentionally does not merge.
+        resolved_child = _resolve_local_reference(
+            projected,
+            child,
+            label=target_label,
+        )
+        if not isinstance(resolved_child, dict):
+            raise ValueError(f"schema path {target_label!r} is not mutable")
+        _ensure_object_schema(resolved_child, target_label)
+        parent = resolved_child
 
     properties = _properties_for_object(
         parent, ".".join(target_parts[:-1]) or "target_schema"