diff --git a/CHANGELOG.md b/CHANGELOG.md
index 8a7b3991f..63bd13f25 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,12 @@
 
 ## Unreleased
 
+- Fixed child ID linkage in `DecompositionService._build_hierarchy()` where
+  `children_ids.append(nodes[-1].node_id)` used the last node in the flat
+  list instead of the actual child node ID from each recursive call.
+  Changed return type to `tuple[int, str]` to propagate both `max_depth`
+  and `node_id`, and updated `decompose()` caller to unpack the new
+  return type. (#1225)
 - Added missing `LspServerConfig` model fields per specification:
   `description` (max 1000 chars), `transport` (`LspTransport` enum with
   `stdio`/`tcp`, default `stdio`), `initialization` (dict for LSP
diff --git a/src/cleveragents/application/services/decomposition_service.py b/src/cleveragents/application/services/decomposition_service.py
index fb42fa100..6fb68aa58 100644
--- a/src/cleveragents/application/services/decomposition_service.py
+++ b/src/cleveragents/application/services/decomposition_service.py
@@ -168,7 +168,7 @@ class DecompositionService:
             )
 
         nodes: list[DecompositionNode] = []
-        max_depth = self._build_hierarchy(
+        max_depth, _ = self._build_hierarchy(
             files=unique_files,
             config=cfg,
             depth=0,
@@ -198,10 +198,12 @@ class DecompositionService:
         depth: int,
         parent_id: str | None,
         nodes: list[DecompositionNode],
-    ) -> int:
+    ) -> tuple[int, str]:
         """Recursively partition *files* into a tree.
 
-        Returns the maximum depth reached during recursion.
+        Returns a tuple of ``(max_depth, node_id)`` where *max_depth* is
+        the maximum depth reached during recursion and *node_id* is the
+        ID of the root node created by this call.
         """
         estimated = sum(estimate_tokens_for_path(f) for f in files)
 
@@ -224,7 +226,7 @@ class DecompositionService:
                 strategy=ClusterStrategy.DIRECTORY,
             )
             nodes.append(node)
-            return depth
+            return (depth, node_id)
 
         # Cluster using directory strategy first, fall back to language
         clusters = ClusteringStrategy.cluster_by_directory(
@@ -261,7 +263,7 @@ class DecompositionService:
                 strategy=ClusterStrategy.FALLBACK,
             )
             nodes.append(node)
-            return depth
+            return (depth, node_id)
 
         clusters = ClusteringStrategy.deterministic_sort(
             clusters, config.language_priority
@@ -271,15 +273,14 @@ class DecompositionService:
         max_child_depth = depth
 
         for cluster in clusters:
-            child_depth = self._build_hierarchy(
+            child_depth, child_node_id = self._build_hierarchy(
                 files=cluster,
                 config=config,
                 depth=depth + 1,
                 parent_id=node_id,
                 nodes=nodes,
             )
-            # The last added node is the child root
-            children_ids.append(nodes[-1].node_id)
+            children_ids.append(child_node_id)
             max_child_depth = max(max_child_depth, child_depth)
 
         internal_node = DecompositionNode(
@@ -294,7 +295,7 @@ class DecompositionService:
             children_ids=children_ids,
         )
         nodes.append(internal_node)
-        return max_child_depth
+        return (max_child_depth, node_id)
 
     # ------------------------------------------------------------------
     # Decision recording