diff --git a/tests/cloud/test_harness_app_contract.py b/tests/cloud/test_harness_app_contract.py
index 6e3300e69..524306a6c 100644
--- a/tests/cloud/test_harness_app_contract.py
+++ b/tests/cloud/test_harness_app_contract.py
@@ -1224,12 +1224,21 @@ def tool():
         tool_names = [getattr(tool, "__name__", "") for tool in cloned.tools]
         assert tool_names == ["custom_tool", "link_reader"]
 
-    def test_spawn_replaces_skills(self, monkeypatch):
+    def test_spawn_merges_skills_into_single_toolset(self, monkeypatch):
         from veadk.cloud.harness_app import utils
 
+        class FakeSkill:
+            def __init__(self, name):
+                self.name = name
+
         class FakeSkillToolset:
-            def __init__(self, names):
-                self.names = names
+            def __init__(self, names=None, skills=None, code_executor=None):
+                self.names = names or [skill.name for skill in skills or []]
+                self._skills = [FakeSkill(name) for name in self.names]
+                self._code_executor = code_executor
+
+            def _list_skills(self):
+                return self._skills
 
         old_skill_toolset = FakeSkillToolset(["old"])
 
@@ -1258,7 +1267,7 @@ def fake_build_skill_toolset(skill_ids, download_dir=None):
         ]
         assert old_skill_toolset not in cloned.tools
         assert len(skill_toolsets) == 1
-        assert skill_toolsets[0].names == ["team/new-skill"]
+        assert skill_toolsets[0].names == ["old", "team/new-skill"]
 
     def test_spawn_clears_builtin_tools_and_skills_with_empty_overrides(
         self, monkeypatch
diff --git a/tests/cloud/test_harness_skill_download.py b/tests/cloud/test_harness_skill_download.py
index c38f8fd2e..202a3ed19 100644
--- a/tests/cloud/test_harness_skill_download.py
+++ b/tests/cloud/test_harness_skill_download.py
@@ -152,7 +152,7 @@ def fail_skillhub_download(*args, **kwargs):
     assert isinstance(toolset._code_executor, UnsafeLocalCodeExecutor)
 
 
-def test_replace_skills_replaces_existing_skill_toolset(monkeypatch, tmp_path):
+def test_replace_skills_merges_into_single_skill_toolset(monkeypatch, tmp_path):
     existing_dir = tmp_path / "existing"
     incoming_dir = tmp_path / "incoming"
     _write_adk_skill(existing_dir, name="existing")
@@ -183,8 +183,15 @@ def fake_build_skill_toolset(skill_ids, download_dir=None):
     utils._replace_skills(agent, ["incoming"], download_dir=tmp_path)
 
     assert calls == [(["incoming"], tmp_path)]
-    assert agent.tools == [marker_tool, incoming_toolset]
-    assert [skill.name for skill in incoming_toolset._list_skills()] == ["incoming"]
+    assert marker_tool in agent.tools
+    skill_toolsets = [tool for tool in agent.tools if isinstance(tool, SkillToolset)]
+    assert len(skill_toolsets) == 1
+    merged_toolset = skill_toolsets[0]
+    assert [skill.name for skill in merged_toolset._list_skills()] == [
+        "existing",
+        "incoming",
+    ]
+    assert merged_toolset._code_executor is existing_toolset._code_executor
 
 
 @pytest.mark.asyncio
diff --git a/veadk/cloud/harness_app/utils.py b/veadk/cloud/harness_app/utils.py
index d960f4414..3a5b8a1fb 100644
--- a/veadk/cloud/harness_app/utils.py
+++ b/veadk/cloud/harness_app/utils.py
@@ -20,8 +20,9 @@
   :class:`HarnessConfig` and builds the long-lived agent, downloading its skills
   from the skill hub and mounting them as an ADK skill toolset.
 * :func:`spawn_harness_agent` — temporary, one-off creation that clones the base
-  agent and applies a per-request override (configured tools/skills replace the
-  base harness selection).
+  agent and applies a per-request override (configured tools replace the base
+  harness selection; configured skills are merged into a single ADK skill
+  toolset).
 * :func:`spawn_harness_run_agent` — per-turn clone that also attaches dynamic
   registry-discovered remote A2A tools for the current user message.
 """
@@ -1097,12 +1098,51 @@ def _is_harness_builtin_tool(tool: Any) -> bool:
 def _replace_skills(
     agent: Agent, skill_ids: list[str], download_dir: Path | None = None
 ) -> None:
-    """Replace the harness-selected skill toolset with ``skill_ids``."""
+    """Merge ``skill_ids`` into the agent's existing skills as one toolset.
 
+    ADK ``SkillToolset`` exposes fixed management tools (``list_skills``,
+    ``load_skill``, ``load_skill_resource``, ``run_skill_script``). Mounting two
+    toolsets therefore produces duplicate model tool names, so per-invoke skills
+    must be folded into one replacement toolset instead of appended separately.
+    """
+
+    existing_toolsets = [tool for tool in agent.tools if isinstance(tool, SkillToolset)]
     agent.tools = [tool for tool in agent.tools if not isinstance(tool, SkillToolset)]
+    if not skill_ids:
+        return
+
+    existing_skills = [
+        skill for toolset in existing_toolsets for skill in toolset._list_skills()
+    ]
     toolset = build_skill_toolset(skill_ids, download_dir=download_dir)
-    if toolset is not None:
-        agent.tools.append(toolset)
+    incoming_skills = toolset._list_skills() if toolset is not None else []
+
+    merged_skills = []
+    seen_skill_names = set()
+    for skill in [*existing_skills, *incoming_skills]:
+        if skill.name in seen_skill_names:
+            continue
+        merged_skills.append(skill)
+        seen_skill_names.add(skill.name)
+    if not merged_skills:
+        return
+
+    code_executor = next(
+        (
+            existing_toolset._code_executor
+            for existing_toolset in existing_toolsets
+            if existing_toolset._code_executor is not None
+        ),
+        None,
+    )
+    if code_executor is None and toolset is not None:
+        code_executor = toolset._code_executor
+    agent.tools.append(
+        SkillToolset(
+            skills=merged_skills,
+            code_executor=code_executor,
+        )
+    )
 
 
 def _remove_a2a_registry_tools(agent: Agent) -> None: