diff --git a/haiku_rag_slim/haiku/rag/cli_chat.py b/haiku_rag_slim/haiku/rag/cli_chat.py
index bf67f300..35070708 100644
--- a/haiku_rag_slim/haiku/rag/cli_chat.py
+++ b/haiku_rag_slim/haiku/rag/cli_chat.py
@@ -1,6 +1,7 @@
 """Interactive CLI chat loop for research graph with human-in-the-loop."""
 
 import asyncio
+import json
 
 from pydantic_ai import Agent
 from rich.console import Console
@@ -225,7 +226,8 @@ async def run_interactive_research(
                     pass
 
             elif event_type == "TOOL_CALL_ARGS":
-                args = event.get("delta", {})
+                delta = event.get("delta", "{}")
+                args = json.loads(delta) if isinstance(delta, str) else delta
                 original_question = args.get("original_question", "")
                 sub_questions = list(args.get("sub_questions", []))
                 qa_responses = args.get("qa_responses", [])
diff --git a/haiku_rag_slim/haiku/rag/graph/agui/events.py b/haiku_rag_slim/haiku/rag/graph/agui/events.py
index 49710126..5cb1728b 100644
--- a/haiku_rag_slim/haiku/rag/graph/agui/events.py
+++ b/haiku_rag_slim/haiku/rag/graph/agui/events.py
@@ -289,10 +289,12 @@ def emit_tool_call_args(tool_call_id: str, args: dict[str, Any]) -> dict[str, An
     Returns:
         ToolCallArgs event dict
     """
+    import json
+
     return {
         "type": "TOOL_CALL_ARGS",
         "toolCallId": tool_call_id,
-        "delta": args,
+        "delta": json.dumps(args),
     }
 
 
diff --git a/tests/graph/agui/test_events.py b/tests/graph/agui/test_events.py
index 297229e6..5c8795d2 100644
--- a/tests/graph/agui/test_events.py
+++ b/tests/graph/agui/test_events.py
@@ -160,12 +160,14 @@ def test_emit_tool_call_start_with_parent():
 
 def test_emit_tool_call_args():
     """Test TOOL_CALL_ARGS event creation."""
+    import json
+
     args = {"query": "test query", "limit": 10}
     event = emit_tool_call_args("call-1", args)
 
     assert event["type"] == "TOOL_CALL_ARGS"
     assert event["toolCallId"] == "call-1"
-    assert event["delta"] == args
+    assert event["delta"] == json.dumps(args)
 
 
 def test_emit_tool_call_end():